Exhaust Tuning


A long time ago I had a motorcycle as a teen-ager. Probably not the best idea then and especially not today. I say this because today’s motorcycles are so much more sophisticated than the old English motorcycle I had when I was sixteen. Today’s sport bikes can propel a young person into the land of bent parts and St. Peters Gate at warp speed. Be that as it may that’s not what this is about. It is about exhaust tuning. I had this point really driven home to me when I started fooling around with the exhaust system on this 1962 BSA 500 cc Catalina Scrambler. Because this motorcycle was a single cylinder, any improvements in its power output showed up more so than with a twin.

Enter the megaphone. The English realize that there’s power lurking in their old single cylinder motorcycles in the form of exhaust tuning. I was also interested in road racing motorcycles, most of which were single cylinder motorcycles in the earlier days before the advent of multi-cylinder Italian invasion with their MV Augustas. Most of the Norton Manx, Matchless, and BSA singles used what’s called an inverse cone megaphone. The inverse part of it comes from having a lip on very end of the megaphone that runs at an inverse angle to the main cone. It’s only about an inch in length but makes a big difference in how well the megaphone works. Anyway I couldnt’ find the standard BSA megaphone for my motorcycle, but I did find one that was close. Because this one had a longer than normal lip on it I cut it back. Installing one of these onto the end of the exhaust system was real easy. Just unbolt the muffler and pull it off and install the megaphone, mine had a slip on pipe welded to it. Because the megaphones produce a much stronger negative pulse they have a tendency to lean out the cylinder’s new intake charge so I would raise the metering needle out of the main jet by lowering the needle clip one notch. This raised the needle out of the main jet about 1/16 of an inch. Sounds like a lot but it just made the motor run better. Doing this was like giving the motorcycle the personality of MR. Hyde. It would pull like crazy just up a little out of the low-speed area in the carb. The motor showed definite improvements in torque and horsepower. It was very noticeable. The megaphone did decrease low-end torque right off of idle but more than made up for it everywhere else in the power band.

So this made me wonder why automotive applications weren’t using megaphones?  When I investigated things a little further I found that formula one cars used long taper megaphones on their engines.  I never saw it on drag race cars ever. Exhaust tuning on top fuel and funny cars running lots of supercharger boost and nitromethane is sort of a waste of time. These cars have more horsepower than they can put to the track anyway. But for engines that are not using forced induction then intake and exhaust tuning looks more interesting.

I ran some dyno runs on my computer with Dynomation 5 software from Comp Cams. This is very sophisticated modeling, using emptying and filling algorithms. I used my Olds 455 engine as a base. First let me say that I think this program is a bit pessimistic on its outputs. This could be because I dont’ have all the information it asks for, so I winged it with what I thought the engine has. One area that it was asking for was minimum port cross-sectional area. Well I never bothered to measure it when I put the heads on because I didn’t have the program. It also want to know intake runner length. Again I just guesses at mine at 14″. The engine has a Offenhauser tunnel ram on it so I used the runner length of the manifold base and it’s plenum depth plus the length of the intake runner in the head. I think my runner length is a bit short. Be that as it may I come up with some interesting results by just playing with the exhaust system and nothing else.

First of all the test was run with standard 1.750″ primary tube headers with a length of 36″ and a 4 into 1- 3″ X 8″ collector. Nothing fancy here just your garden variety  455 Olds headers. Increasing the length of the collector from 8″ to 10″ didn’t really do much but produced a measly .1 of a hp gain and a loss of .9 lbs ft. of torque.  Ok so be that as it may it shows that just changing the dimensions of the collector will affect the way an engine tunes but to a very minute degree. I left the 10″ collectors on the engine and stepped the primaries, starting out at 1.750″ and finished the step at 2.00″.  Just doing this the engine picked up 3.8 lbs ft.of torque and 19.9 horsepower!  That’s 20 hp for just stepping the primaries!

Going to a 2.00″ primary and adding a 5″ X 14″ megaphone boosted the power at 5000 & 5500 rpm 21 horsepower!  The larger 2″ pipes hurt the torque a bit at 4000 & 4500 rpm loosing 35 lbs ft. at 4000 rpm but increases it slightly at 4500 rpm. This is usually seen when the primary flow velocities slow down in a larger diameter primary tube. It picks up but at a higher rpm because the flow velocity is lower at a lower rpm. Simple really.  But for a 500 rpm difference the system recovers very well. Usually seen in 1000 rpm increments in other engine systems.

I found this pretty cool considering we didn’t do anything internally to the engine, just tweaking the exhaust system brought about subtle changes in torque and horsepower.

When you get the little things right on an engine they start making big power. The little things are what makes the difference between a car that runs ok and one that puts your eye balls in the back seat.  The exhaust system on street machines that aren’t encumbered by catalytic converters and the like is just begging for experimentation. If you are  satisfied with bolting on a set of headers to your car and hanging some mufflers behind them and calling the system a party then you are overlooking some power. I have to say that the header folks like Hooker, for example, do a pretty good job of designing headers for your engine. They usually have the primary diameter right, and the length. Most street machines with nice manners don’t need stepped headers, megaphones and the like. Pro Street cars and race cars do. Alright so you can’t run your pro street car around with straight megaphones on it, you still need to pay attention to header diameter, collector length etc.  Pro Street cars with lots of cam timing really need to pay attention here. The better you can make the exhaust system the happier the motor will be with that big fat camshaft you stuck in there because you like the way it sounds. Yes I’m guilty of this too, but I got an excuse. I’m conducting an experiment. That’s all I’ll say. I dont’ want to get into this because it’s involved and off topic for this post. So there’s some free hp out there if you are willing to dig for it a little, best part is it’s more or less free, and it won’t affect the reliability of your car and you can put it back the way it was without going thru bankruptcy court.  FARM OUT.

DAVE

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