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This modification was pioneered by mail list member Tim Hodge. Tim became somewhat of a legend because of his simple modification that opens the exhaust up, allowing a more mellow sound and some midrange performance improvement. I (Dean) did the "Hodge mod" on my own Nighthawk several times - experimenting with different size holes and settled on five 3/8" holes per side (note that on my funnel though, my innermost hole is partially covered by the baffleplate--I should have moved this hole away from the end a little more). The beauty of this mod is that it's nearly invisible from the outside, thus preserving the good looks of the stock exhaust system. As always, the author(s) take no responsibility for any problems, injuries, or damage you may incur during or after the modification...

Exhaust before modification:
Rivets drilled out with 3/16" drill bit:
There may be parts of the rivet left in the hole that need to be punched through before endplate will come loose:
Grab the inner tube with pliers and start pulling:
You may need to use other means (more pliers, hammers, kroil, friends) to remove the funnels for the first time:

[EDIT: see Gilles Amichaud's steel-pipe suggestion Here ]
The exhaust pipe with the "funnel" removed. The center hole is the one the funnel slides back into. Be sure to remove drillings and shavings with shop-vac before re-insertion:
This is what the "funnel" looks like after removal (this one has the holes drilled already though). Also, please note that my innermost hole is partially inside the baffleplate, reducing its effectiveness:
Drill the staggered holes in the narrow portion. Start with 4 or 5 small holes and "tune" by going larger and larger until you get the effect you're after:
Installing the final rivet:


This is the original series of posts relating to Tim's modification

----- Original Message -----

From: Tim Hodge
To: cb-750@egroups.com
Sent: Monday, June 26, 2000 3:01 PM
Subject: [cb-750] Stock Exhaust Modification for 1999 Nighthawk (long)

I purchased my 1999 cb750 last Memorial Day w/e. It wasn't the Harley I wanted, but it did fit my budget. This is my second bike, my first bike (a Yamaha Virago 750) I bought new in 1982. My Virago always had a nice deep thumping note, which is something you lose when you go to an in-line 4 like the Nighthawk.

Well, that's OK. But the 80dbA sound pressure limited OEM exhaust system on the Nighthawk was just "TOO" quiet for me. I checked into the aftermarket pipes, but they were VERY loud, required carb rejetting or the disc type which would put soot on the back of your bike. None of which held any interest for me. Anyway, I liked the way the stock system looked.

So after six months of ownership, an idea began to form in my Electrical Engineering head: "Why not try to modify the stock exhaust system?" It took me an additional six months to come up with a plan and get up the nerve to actually open up the exhaust system. (1 week for the plan, over 5 months for the nerve). Here's why: If I screw up the stock exhaust system, I WILL have to buy an aftermarket system AND the required jet kit. That would have cost me about $600.

I'll start with the end of the story: MISSION SUCCESS!! A nice mellow exhaust note (not too loud, not to soft) with an additional benefit. That annoying soft-spot in the midrange torque curve has improved too! Yes, there is a small top end sacrifice, but I don't typically run the bike up to 7000 rpm anyway.

This actually was a very easy mod to accomplish.

1) The hardest part was drilling out the four steel rivets that hold the end caps on each side of the exhaust system. Make sure you have a good sharp drill bit!
2) Next, using a pair of Vise Grips give a sharp pull to the inner circle of metal where the exhaust gases come out. Have someone hold the bike steady while you do this step.
3) What you now have in your hands looks like a 7 inch long funnel. Now do the other side. Set them down for a while. We'll get back to them later.
4) Important, reach inside with a pair of tongs and remove the remains of the old pop rivets.
5) Clean out the rivet holes, so they will accept the new rivets. (I used aluminum in case I had to open them back up at a later date.)
6) Now back to the 7 inch funnels. A drill press would help here, but it is not mandatory. Drill four 1/4 inch, staggered holes along the length of both funnels in the longer tube portion. Deburr the holes with a file. The larger the holes, the more of the exhaust gases will by-pass the normal baffling path and "escape" to the outside world.
7) Place the funnels back into the system, use four pop rivets on each side and that's it!

As a side note: I did mine in two stages. The first time I drilled four 3/16 holes in each funnel. I tried that for a week, but didn't get the desired effect (wasn't loud enough). So I reopened the system and drilled out the four original holes to 1/4 inch. I figured I had done it once why not a second time.

I am very happy with the results. The mod has been in for about a month now. No ill effects have shown up. The plugs are still a nice color which means to me that I did not lean out the motor.

As a warning: I would suggest that you don't open the holes much more than I mentioned. 3/8 might work, but you should then install a jet kit which would richen the fuel/air mixture.

Tim
1999 cb750 (slightly modified)

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Being a sheetmetal mechanic, all this riveting and drilling is well within my means, and I have all the tools and supplies to accomplish this. I would love to have a little more sound, too. However, I have a few questions.

--The factory sets the carbs to be lean due to emission controls. Wouldn't this procedure make the gas mix even leaner?

--Should you drill the four holes in the top and sides only, so rain won't get into the muffler (and so the state inspectors won't see the mod)?

--Does "drill the funnels in the longer tube portion" mean drill them in the front or rear of the installed funnel?

Also, I would love to hear the opinions of others on the long term effect of this procedure. I am seriously considering doing it.

Glenn Stephens
Portsmouth, VA
2000 CB750 Nighthawk (The Black Mamma Jamma)
Only Bikers understand why dogs love to stick their heads out of car windows.

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The holes he talks about drilling are completely internal.

The black covers at the end of the pipes has a internal tube that goes farther into the assembly and into the baffles.

The distance from the baffles and the end of the pipes is about 6 or 8 inches and is just empty. Exhaust does not even enter that void because it is going through that small pipe directly to the outside.

The theory is to put holes in that pipe (or remove it altogether) to allow the exhaust into that extra void so it can resonate more.

If it works, no one would be able to tell without opening them back up again.

Good luck
Jeff Sloan

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It seems to me that this mod would be reversible by using some muffler tape and compound around the holes without any welding, so it is "Less risky" to try.

Any other ideas on easy effective ways to undo it if the results are not to their liking?

Jeff Sloan

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Jeff is absolutely correct about the "holes" being completely internal. However, regarding the theory of operation Jeff is a little off base. (Sorry, Jeff)...

The area that surrounds the "funnel" is part of an overlapping baffle system and is essentially an expansion chamber. It's how you can double the effective length of the baffle by folding it in two. The gases leave the four cylinders and is combined (for path balancing) and is divided into two paths (one for each side). From here we'll concentrate on one side only, since both are the same. The gases are then squeezed into two small openings at the bottom of the "funnel" area. At that point the gases from the two cylinders expand into the "funnel" area. Now for the tricky part. The stock system forces ALL of the gases back toward the front of the bike and into a "baffling" area. After the gases travel thru the baffling they then are allowed to pass thru the center funnel and into the air. What I have done is allow the gases to by-pass the post expansion baffling system. Apparently someone else has tried a similar mod by cutting off the forward end of the funnel. This would probably set up some unwanted resonances were the normal path and the by-pass path would merge all at once. Thereby interrupting the normal flow instead of just merging with the flow.

It is important to place the holes at different places along AND around the funnel tube to minimize the risk of unwanted harmonics (resonances). Finally, regarding mixture issues...the lean settings could be a problem if we opened the bypass path too much. After a month of riding with this mod the plugs remain a very nice gray color. Also, keep in mind the radical aftermarket systems, which are VERY open compared to this mod, normally don't REQUIRE rejetting (at least that's what the manufacturer says).

Just don't go to radical with the size of the holes...1/4 is what I used...but I have thought about trying 3/8 or adding one more 1/4 hole. Once you open the rear of the exhaust system all of the details will become clear. Take your time...

Tim Hodge

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First of all let me apologize for the length of this post.

I got bored today. My company instituted a new rule this year requiring employees to use all of their vacation in the year that it was earned. In the past we could carry over any unused vacation, but now we are "forced" to use it up. Bummer, huh? (wink). The good thing is that I have this week off all by myself. The bad thing is that is has been raining , or threatening to rain, the past two days.

Since it was raining and I was bored I decided to try the Stock Exhaust Modification for 1999 Nighthawk as described by Tim Hodge in his post of 6-26. My bike is a 1999 model as well and I figured since the factory jetting was the same as Timís my results should be the same. I was especially impressed with Timís second post detailing the construction of the stock system and his concerns for creating harmonics. It was obvious that he had done some homework on this project. Well, hereís how it went.

The whole process is unbelievably easy. After drilling out the pop rivets my "funnels" pulled right out, no twisting or hard pulling. I drilled the ľ inch holes spaced more or less evenly along the straight pipe being careful to drill them in different places around and along the pipe per Timís instructions. Then I put everything back on after cleaning out the filings and rivet pieces from inside the pipe. (Finally found a use for that telescopic magnetic pick-up thingy that I got for Christmas). The whole job took about an hour including stops for nerve enhancement.

Then I held my breath and started her up. Itís not really loud at all but defiantly sounds different, deeper tone and kind of more authoritative, but certainly not offensive. By that time the rain had stopped temporarily so I decided to get in a quick spin to try it out. I noticed no difference in the sound at idle while sitting on the bike with my full face helmet on, only when I blipped the throttle could I hear any difference while sitting still. I rode the short distance out of my sub-division an onto the highway accelerating very easy in respect of the cold engine. I could hear the pipes but still no big difference at that point. Once warm and up to 55-60 MPH in fifth I rolled on the throttle a couple of times to see if there were any flat spots at that engine speed. There werenít any unless you want to count the BIG flat spot I had until I discovered that the fuel petcock was still off. Then I turned onto a secondary road and got behind some traffic. Traveling at a steady 45 MPH in fifth I could detect a very slight flat spot when the throttle was rolled on a little. It didnít stumble, it just wasnít as responsive to a small increase of throttle. This was so slight that if I werenít so familiar with this particular bike I probably wouldnít have noticed it. Then again maybe it wasnít there at all, maybe I was being too sensitive.

At that point it started to sprinkle again so I turned around and headed for home. With the engine now plenty warm I could rev it up a little. This is where the difference in sound is apparent. Accelerating briskly and shifting at approximately 5000 RPM a very sweet growl emits from the pipes. It sounds nothing like a 4 into 1 header system but it sounds very nice, at least in my opinion. Again the sound is not close to being loud enough to be offensive and there is no droning at steady speeds. I like it. Tomorrow promises sunshine so Iíll give it a more through test and report back.

To Jeff Sloanís point, there are several ways to reverse this mod if you found that you didnít like it. You could use muffler tape as he suggested or maybe a hose clamp on the pipe to cover each hole. A piece of 7/8 inch I.D. tube cut to length would probably slip over the pipe and cover all the holes. There is plenty of clearance inside the muffler in that area so most anything will fit. Of course if you had access to someone with a welder it would be real simple to weld the holes shut. Maybe someone would be willing to drill the oversized 3/8 inch hole and experiment with "tuning" the pipe by partially covering the holes with hose clamps.

Many thanks to Tim Hodge for the suggestion.

Ron in VA

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THANK YOU, TIM HODGE!!!!!!!!!!!!!!!!!!!!

I did the exhaust mod yesterday, and put 100 miles on it last night, trying it out in every possible scenario. I am ABSOLUTELY THRILLED with the results. My Y2K seemed to fare better from the mod than most of what I've read in this thread, though. It increased torque all along the tach scale, most notably at the low and mid range. It takes off from a stop a lot more forcefully, and all hesitation is gone. In the first three gears, there is a noticeable increase in high-end torque. I always shift before the red line when accelerating quickly, but last night it lunged into the red zone in third gear very quickly. In fourth gear, it seems to act as it did before the mod at the high end. At first this felt like a power loss, because it does flatten out at about 7.5K in fourth. But I think it's just that I gained no extra torque there, as I noticeably did everywhere else. It doesn't make any sense to have gained high-end torque in the first three gears only, but that's what it feels like. I guess the first three gears don't have as much wind to buck.

I haven't checked the gas mileage yet, but I expect it to increase, since the motor sounds and feels so much happier now. Even if the gas mileage goes down, plugging the holes is not a consideration.

The sound change did not noticeably increase the sound level, but it did give it a deeper sound, like you'd expect from a "non-sewing machine" engine. It feels like a "second sound" has been added to the rear of the bike; but it's not much louder. Just VERY pleasing.

The mod was A LOT easier than I expected. Instead of just drilling the rivets out, I drilled them a little until the drill bit hit the steel pin in the center of the rivet. Then I took a hammer and a small flat punch, and drove the pin back about an eighth of an inch (tapping lightly). Then the rivets drilled out easily. I then took a long pair of needle nose pliers and pulled the drilled rivets out of the muffler, and stuck a vacuum cleaner attachment in there and sucked the shavings out.

The funnel came out easily. It's actually a funnel at the rear end, with a straight pipe welded to the front of the funnel. I measured one half inch forward of the joint where the pipe is welded to the funnel, and drilled my first .25" hole there, in the top of the pipe (to be sure no one could ever see the holes). Then I went one inch forward of that hole, and rotated around the pipe 90 degrees and drilled the second hole in the left side of the pipe. I measured forward an inch from that hole, rotated around the pipe 90 degrees and drilled the third hole in the bottom of the pipe. Then I went one inch forward of that hole, rotated around the pipe 90 degrees again, and drilled the fourth hole in the right side of the pipe. This left an extra inch in the exposed area of the pipe to drill a fifth hole if I decide to in the future.

Once again, THANK YOU, TIM HODGE! I won't ever be closing those holes up, and am considering adding a fifth set of holes. If anyone does this mod with five holes, please report your results to the group.

Gratefully,
Glenn Stephens
Portsmouth, VA
2000 CB750 Nighthawk (The Black Mamma Jamma)
Only Bikers understand why dogs love to stick their heads out of car windows.

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I COULDN'T HELP IT!!! I COULDN'T STAND IT ANYMORE!!! I JUST HAD TO DO IT!!!!!!!!!!!!!!!!!!!

Yesterday I took my second test drive with the four holes added to my exhaust funnel. I was very pleased, just as with the first test. I posted a long letter about that two days ago, so I won't repeat it here.

Anyway, I just couldn't stand wondering what it would be like to have five of those quarter inch holes in the funnel/pipe assembly, so today I did it. Knowing what tools I needed in advance, it took ten minutes from start to finish to put the last two holes in.

That one extra hole made the noise even more noticeable. Especially when riding through the tunnel today. It seems deeper and a little louder. VERY GOOD SOUND.

It appears that with every hole in the funnel, you lower your powerband. My Y2K has always entered the powerband at around 6K rpms. After drilling the first four holes, I could feel it entering it at about 5.25K. Today, with the fifth set of holes, it consistently entered the powerband at 5K, which is a very welcome change. Now I can just roll on the throttle, in some situations which used to require downshifting.

However, today I did notice a tradeoff in top end power. With four holes, it still went into the redline nicely. But in third gear today, I could feel it losing it's "oomph" just before it touched the redline, at about 8.25K. It still went into the redline; just not as briskly as before. However, when hauling *ss, I ALWAYS shift at 8K, so this new situation is the perfect compromise for me. But I am not considering adding a sixth hole, or making the existing five holes bigger. I'm happy now where it is, and very glad I drilled the fifth set.

As for fuel mileage, I usually get between 37 and 43 mpg, depending on how I drive. I was expecting 37 or less today, because the whole tankful was used in testing this new mod at high rpm's. At the pump today, it worked out to 39.7 mpg. It was a very welcome shock.

One other thing I noticed. My bike has always kind of sputtered when idling in fourth gear. Today I let the bike glide to idle in fifth gear, just to see what it would do. It purred like a cat, and rolled at 18 mph. No bucking, sputtering, or anything. Perfectly smooth.

To anyone contemplating this mod, I would suggest first adding four holes, leaving room for a fifth, as I did. Then test drive it for a few hundred miles. If you feel yourself losing any top end power, stop there. If you can't detect any loss in power, go for the fifth set.

Once again, TIM HODGE, U DA MAN!!!!!!!!!!!!!!!!!!!!!!!!!!!

Glenn Stephens
Portsmouth, VA
2000 CB750 Nighthawk (The Black Mamma Jamma)
Only Bikers understand why dogs love to stick their heads out of car windows.

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----- Original Message -----

From: Tim Hodge
To: cb-750@egroups.com
Sent: Tuesday, July 18, 2000 3:28 PM
Subject: [cb-750] Re: Stock Exhaust Modification for 1999 Nighthawk (long)

Hello to all those who are following this thread:

To Glenn S. You are welcome! I'm happy your are trying out my little science project on your bike AND having success! Hold on to your rivets though, you maybe opening up the exhaust system one more time.

Well, the story continues...after two months of riding with the 4 quarter inch holes drilled in the "funnels", I decided to pump up my nerve bag and open up the holes to 3/8 inch each. As expected, the sound has once again increased and deepened, but thankfully not too much. The torque curve moved down the RPM scale a little more! :) Which is what I want it to do! However, this is as far as I'm willing to go with this mod! I believe if I go any further it will require re-jetting.

Now for a little analysis:

Hopefully, the chart I imported from Excel will appear properly below. In summary, with four quarter inch holes drilled, the effective opening is less than a quarter of a square inch (0.20 sq. in). By adding a fifth quarter inch hole the area increases to exactly one quarter of a square inch (0.25 sq. in).

As it turned out, I really took a LARGE leap of faith by drilling out my four holes to 3/8 of an inch. Because, the resulting area increased to almost a half square inch (0.44 sq. in)! That's OVER double the area of 4 quarter inch holes. You would have to drill nine quarter inch holes to equal four 3/8 holes.

I should have done the analysis BEFORE I grabbed my drill, but alas I didn't. As I always say, "I'd rather be lucky, than good" certainly applies here. I've checked the plugs after each ride to ensure the motor is not too lean. I'm getting great mid-range performance AND a much nicer exhaust note.

But as I stated above, that's it for me! No more exhaust mods unless I buy a jet kit. And I don't plan on doing that any time soon.

(NOTE!!! The following chart has been ADDED TO AND EXPANDED from Tim's original chart by Glenn S. If any mistakes are found, please notify me at GlennS1956@erols.com)

Effective Area in Square Inches

   # of           Hole Diameters 
   holes    1/4      5/16     11\32      3/8
   
     1      0.05     0.08      0.09      0.11
     2      0.10     0.15      0.19      0.22
     3      0.15     0.23      0.28      0.33
     4      0.20     0.31      0.37      0.44
     5      0.25     0.38      0.46      0.55
     6      0.29     0.46      0.56      0.66
     7      0.34     0.54      0.65      0.77
     8      0.39     0.61      0.74      0.88
     9      0.44     0.69      0.83      0.99
    10      0.49     0.77      0.93      1.10

If anyone opens up their system further, please post the results and either the size and number of holes or the equivalent area.

Best Wishes
Tim Hodge
1999 cb750 (slightly modified, again)

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----- Original Message -----

From: jrhannon@zoomnet.net
To: cb-750@egroups.com
Sent: Tuesday, July 18, 2000 8:54 PM
Subject: [cb-750] Re: Stock Exhaust Modification for 1999 Nighthawk

I have drilled my mufflers, too, but I did it a little differently. I drilled four holes through the end of the mufflers at the opposite locations of the rivets. I did this in case I didn't like the way it sounded, I could just put some screws into the holes to plug everything back up, paint the screws black, and it would look like I had 8 rivets instead of four. I started small, and ended up with 1/4" holes. The sound is deeper and louder, but not too loud. I like the sound the way it is now, and will probably leave it alone. Also, it runs no better or worse than before. This is a cheap and easy way to change the sound of the stock exhaust, and you can easily change it back if you don't like it. Jim.

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From: Glenn Stephens
To: nighthawk-750@excite.com
Sent: Wednesday, August 16, 2000 2:13 PM
Subject: [cb-750] Exhaust Mod Question

:::I've been reading with interest about the exhaust mod performed by
:::several people. Very nice info - I will be doing it soon. A local dealer
:::told me that they could open up the exhausts, and remove a baffle or
:::two to achieve the same effect.

I absolutely would not remove the baffles, nor would I let anyone else do it. That would take a lot more work or expense than the Tim Hodge mod, which is free. And the Tim Hodge method bypasses the baffles, making their removal unnecessary. Basically, it consists of drilling out the four rivets in back of the muffler, pulling out the funnel/pipe assembly, drilling four or five holes in the pipe, and re-popping the rivets. In other words, it costs next to nothing, and takes about ten minutes per muffler. The beauty of it is that, if you don't like the results, it is a simple matter to either drill more or bigger holes, or plug the holes, until you get the desired effect.

:::Has anyone who opened their mufflers seen what the baffles look like,
:::how many there are, and how they are attached? I'm wondering if one
:::could be removed and replaced if the effects are undesirable.

When I opened my system and looked in there, modifying the baffles was not an option for me. Especially since I could bypass them a lot easier.

:::I'll be digging in there either way - and will be seeing it myself, but it's
:::nice to have some information in advance. If this was already covered,
:::I sure couldn't find the post and would appreciate being pointed in the
:::right direction.

I have been saving all the detailed posts about this mod, and have been posting them in our group's "Files" section. At the bottom of this and every other 750 eGroup letter, there is a link to the Group website. Click it, and then click on the link to the Files section. There you will see the link to the "Free Muffler Mod" file, in both Word format and generic format. If you would prefer, let me know which format you want, and I'll email directly to you. Or I could copy and paste it into one LONG email letter. Either way is easy for me, and only takes a few seconds to do.

The file is extremely long and goes into great detail. But when you're planning on operating on your baby, that is probably exactly what you want!

:::Thanks for any help in advance!

You're welcome. Have fun, and report your experiences back to us for the files.

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From: W Favre/C Hellwich
To: cb-750@egroups.com
Sent: Friday, August 18, 2000 2:45 PM
Subject: [CB-750] Exhaust Modification

Raining in Seattle so it seemed like a good day to try the exhaust modification on my '91 Nighthawk. Got the rivets drill out of one exhaust, grabbed ahold with the vise grips, yanked, pulled, grunted, scratch my head, tapped on the muffler with a dead blow hammer and it didn't budge one bit. Guess on one this old the inside has rusted solid . So going to get some rivets and secure it and forget about the modification.

Wayne Favre
Bothell, WA
'89 Transalp
'91 Nighthawk

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From: Glenn Stephens

WAYNE, DO NOT START THAT MOTOR!!!

Damn, I hope I'm replying to this post in time! I just got home and read it.

When you drilled out the rivets, the back 2/3 of the rivet fell into the inside of your muffler. This stuff MUST BE CLEANED OUT before starting your motor. If you're lucky, the aluminum rivets and the steel rivet shanks may blow on through. If they don't, they will block up the internal passages of the muffler..

Now that you've gone this far, you'd better finish it. Spray some good ole' WD-40 all around on the inside of your tail pipe, every 20 minutes or so, in hopes that it will loosen it up. Some Nut Buster would be better, if you can spray it or brush it all the way around the pipe, top and bottom, on the inside at the joint you've got to break. This joint is about 8 inches inside the tailpipe.

Take large Vice Grips, and grab the very ass end of the tailpipe at it's top. Don't pinch it too hard, or the jaws will leave an indentation in the pipe. Take a hammer and strike the hammer, swinging the hammer from the front of the bike, just above the top of the muffler, down and rearward against the hammer, using the hammer blows to try to drive the pipe backwards. After a few hits, move the vice grips to the bottom of the end of the tailpipe, and hit it with rearward strokes again. Then do the same with the left side and right side. And keep soaking it in nut buster. When you get it out, vacuum all that stuff out of your muffler.

While you have the funnel/pipe assembly out, you may as well drill the holes in it as described in the "files section". But if you don't, at least you'll have gotten the scrap metal out.

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From: Glenn Stephens
Date: September 11, 2000
Subj: Tim Hodge Muffler Mod...

Once again, I couldn't resist the temptation, and just had to go back into those mufflers. I had previously drilled five 1/4" (.250") holes in each of the funnels inside my mufflers (ten holes, total). I was amazed at the resulting sound and power changes, and I drove it like that for four thousand miles. I was absolutely satisfied with the current evolution of my muffler modifications. The spark plugs still looked brand new, with a hint of tan/gray deposit, and were still gapped perfectly. But the curiosity was killing me. "How would it change performance, if I enlarged the holes?". "If I screw up, I could always plug one of the holes!"

My Y2K Black Mamma Jamma reached eight thousand miles on the odometer four days ago. After the 8,000 mile check-up at the local Honda Shop, my curiosity finally won, and I drilled the ten holes from 1/4" diameter to 5/16" (.312"). The combined surface area of the ten holes that I have drilled into the "innards" of my muffler system is now 0.77 square inches; which is exactly equal to seven 3/8" diameter holes, for those of you who have been experimenting with 3/8" holes. I have driven it for three days since then, testing the heck out of it. Here's what I foundÖ

I filled the tank soon after the new mod, and checked the mileage when I refilled. Exactly 40 mph, which is 1mpg higher than my normal 39 mph. And I RAN THE HECK out of it during that tankful; a lot harder than normal. A VERY welcome change.

I gained a lot more sound, at a deeper tone, at EVERY speed, during both acceleration and deceleration. But still not too loud. Not enough to annoy the neighbors. This is also a very welcome change. The new sound could be described as "the one I was looking for" when I started these mods.

The most dramatic change was in low-end torque. I gained A LOT more of it. Now she ZIPS through the first three gears A LOT faster than before. This is also a VERY welcome change. I did a wheelie for my wife last night. She ABSOLUTELY loved it! NOT!!!!! It's hard to tell from one wheelie, but it felt at least as easy as before, if not easier.

Now for the "not-so-wonderful" effects. Concerning mid-range torque, I posted after the last mod (five 1/4" holes in each funnel) that, while cruising in fifth gear, "Now I can just roll on the throttle, in some situations which used to require downshifting". That has changed. I lost a small amount of mid-range torque (which, for my own testing purposes, is the force used when rolling on the throttle in high gear at 65 mph). Now, when I really want to accelerate, I have to downshift to fourth EXACTLY as I had to when the muffler was stock (without any modifications). I seem to have come full circle in my mid-range performance.

There is also no doubt that I lost top-end power this time. The bike zips through the first three gears. It starts out fine in fourth, and goes to 95 mph pretty quickly, though not quite as quickly as before the newest mod. But after that, she starts running out of steam, and is noticeably slower getting to 100 mph. I have no doubt that I could get it to 115 or 120 mph, but it would surely take a lot longer to get there now.

In the past, I have performed this modification in small increments, and checked the changes closely over a variety of driving conditions. I have always reported that I was VERY glad that I had performed each step of the mod. But not this time. I'm still "glad", but not "VERY" glad. I feel that what I gained with the 5/16" holes in sound and low-end torque far outweighs my small losses elsewhere, and I'm NOT considering plugging any of the holes back up. But I'm not so sure I'd do it again, knowing what I know now. I know that I should never be riding with the speedo in triple digits, but there's something about losing that top-end power that I'm not comfortable with. I miss knowing that it's there. And downshifting slightly more often is a pain; though not an unbearable one. Since this is the first time I've had any doubts whatsoever about the effects of the mod, I'm done. No more exhaust mods for me. Iím happy with the current condition of my muffler, which is many times better than when it was stock. But I believe that to increase the total surface area of the holes again would not be beneficial in overall performance.

Thanks, again, Tim Hodge for designing this mod. You have done many members of this list a great service by sharing this with us. And experimenting with it was FUN AS HECK!!!

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From: Glenn Stephens
Date: October 2, 2000
Subj: Tim Hodge Muffler Mod...

Remember about three weeks ago, after I drilled out the ten 1/4" holes in my muffler funnel/pipe assemblies to 5/16"; and I said, "I'm done. No more exhaust mods for me"? Well, I lied. It wasn't an intentional lie. It just turned out that way.

I went back into the mufflers over the weekend. I had another idea of a "non-destructive" method of testing the Tim Hodge muffler mod. I decided to take the funnel/pipe assemblies out of the back of the mufflers and LEAVE THEM OUT, and ride around like that for a while. That way, I would know what the worst case scenario could possibly be, if someone was to drill too much surface area into their funnel/pipe assemblies. It wasn't a pleasant ride. The noise was so loud that it hurt my ears. I was afraid the whole time that I was going to get a ticket. Especially when going through the tunnel. It was a horribly loud noise. But performance didn't deteriorate nearly as badly as I expected. Low end torque seemed slightly diminished, and high end torque suffered only slightly, too.

After I got home, I researched the archives of these drilling experiments (stored in the "Files" section of our eGroup website), and found that Tim Hodge last reported that he had drilled eight 3/8" holes in his mufflers (four in each funnel/pipe assembly), and said that " However, this is as far as I'm willing to go with this mod! I believe if I go any further it will require re-jetting". I highly respect his assessment, so I consulted our "Total Surface Area" chart (reproduced at the end of this letter), and found that Tim had a total surface area of 0.88 sq.in. drilled into his system. In my own experimentation, I had drilled a total of ten 5/16" holes in my system, which gave me a total surface area of 0.77 sq.in. If I had drilled my ten holes out to 3/8", my total area would have jumped to 1.10 sq.in.! That would have been 25% more surface area than Tim had stopped at, and would have increased my own surface area by 43%! I wasn't willing to go that far. But, since riding completely without the funnels wasn't "that bad" performance wise, I decided to drill my ten holes out to 11/32". That gave me 0.93 sq.in. total, which is very close to what Tim has. I thereby increased my own surface area by 21%.

I drove it like that for about 100 miles today, and was very surprised with the results. Predictably, my sound and low range torque increased very noticeably. What I had not expected was that my mid range and high end torque also seemed to increase! Don't ask me why, because I don't understand it either. Today, after the mod, my bike ran better than ever, all across the tach scale. Maybe it was just "one of those days", when our Nighthawks run better than others. It was very windy, but performance improved both with and against the wind.

Since I've only ridden if for one day with the 11/32" holes, I am not yet declaring total victory. But it surely fooled me this time. I'll report back in a week or two, and let yall know if it's still doing as well as it did today.

One thing is for sure. If you experiment with this mod, get plenty of rivets, start with small holes, and work your way up. There seems to be no way to predict how our carbs will react to this gradual increasing of the holes in our muffler systems. But I had a set of happy carbs today!

(NOTE!!! The following chart has been ADDED TO AND EXPANDED from Tim's original chart by Glenn S. If any mistakes are found, please notify me at GlennS1956@erols.com)

Effective Area in Square Inches

   # of           Hole Diameters 
   holes    1/4      5/16     11\32      3/8
   
     1      0.05     0.08      0.09      0.11
     2      0.10     0.15      0.19      0.22
     3      0.15     0.23      0.28      0.33
     4      0.20     0.31      0.37      0.44
     5      0.25     0.38      0.46      0.55
     6      0.29     0.46      0.56      0.66
     7      0.34     0.54      0.65      0.77
     8      0.39     0.61      0.74      0.88
     9      0.44     0.69      0.83      0.99
    10      0.49     0.77      0.93      1.10

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From: Glenn Stephens
Date: October 2, 2000
Subj: Tim Hodge Muffler Mod...

Three weeks ago, I posted that I had drilled the ten holes in my muffler's funnel/pipe assemblies to 11/32" each. Now, over a thousand miles later (10.5K total miles on bike), I am still absolutely totally 100% satisfied with the mod. I have a lot more power all across the tach now, and the sound is better than I had hoped for when I started this experiment. I know that I've said this many times before, but THIS TIME, I'm through experimenting, because I am totally satisfied with the results.

Gee. I wonder how long it will be before I drill them all out to 3/8"???



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July 2010 - an email sent in by a reader named George. I guess it was easier to get the funnels out 10 years ago (when this mod was first written) than it is now after they have 10 more years of accumulated crud on them... ;)

It took a combination of the Hodge write-up plus the Giles pipe method to get the funnels out. I thought it would be worth going through what I did step by step:

1. I sprayed an aerosol product called PB Blaster, a corrosion dissolver found at Advance Auto Parts, around the edges of the funnel to start to loosen it up.

2. Drilled out the 3/16" rivets and punched the shafts into the pipe.

3. Over the course of the next 6 hours I latched onto the bottom of the funnel and tried to pull or twist it. I reapplied the aerosol 2 more times and patiently waited.

4. After no success I got out the iron water pipe. (by the way, your guess on the pipe size is incorrect, it is the smaller and harder to find 3/8" pipe) I had previously flattened one end as in the Giles method.

5. However the method of pounding it into the funnel, attaching vise grips and pounding the grips straight back away from the pipe did not achieve results for me.

6. Next I struck around the muffler with a rubber mallet, pounding on the chrome toward the center of the funnel to vibrate the funnel loose. All this time the vise grips clamped to the funnel as in the Hodge mod did me no good, pulling or twisting.

7. This time I pounded in the pipe, attached the vise grips and ROTATED the pipe clockwise and counter-clockwise and the funnel finally came loose. I really did have to pound that iron pipe into the funnel a few hard whacks before it would lodge in there tightly.

8. Lastly I used the mallet to remove the freed funnel from the pipe.

The four rivet pins were laying in the bottom of the exhaust, but 1 tried to slip through the small hole at the bottom of the exposed baffle plate. I had to use a grabber tool to rescue it. If it falls through that hole I can't imagine how you would get it out short of removing the pipes, inverting them and trying to shake it out.

Now I can finally start to drill my holes and fine tuning. Thanks for all the great mods!


Thanks for the tips, George. It may help future readers of this mod.

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