The Stovebolt Site Forums The Shop Area Engine Shop Lost all the torque

We build a new exhaust setup for the 235 (typical dual Offy/holley-weber/split exhaust/m4F/ .06 over).

The pipes are 2 1/4 inch diameter with 2 flowmasters just before the rearend.

I was kind of disapointed when I drove it the first time. It feels like it lost all of the low end torque.

Is it possible to loose so much torque just by installling bigger diameter pipes? I thought that the lower backpressure the better it is.

Thanks, Frank

I think 2 1/4" pipes will handle up to 300-400 hp. Your 6 cylinder is making less than 200 (?). Lower back pressure is better up to a point. I am not positive, but for some reason I don't think you should go bigger than 1 3/4". Somebody on this board WILL correct me if I am misleading you.

Sounds like a lot more exhaust system than needed but it shouldn,t hurt anything although I,ll bet it,s really loud. You may be running too lean. With better exhaust flow you need a richer mixture. Also you may not have enough heat to the manifold. The timing could also be advanced a little.

I go and check timing and mixture.

By the way, I saw in an vintage hot rod magacin a J.C. Withneys ad for a dashboard device to adjust timing "ont the fly" (I think it was made by Almquist) and similar Do-Dads.

Did anyone ever came up with the idea of an exhaust throttle?

I mean something like a choke plate that sits in the exhaust pipe near the header and can be adjustet from the dashboard. Open for high rev performance and more restriction for low end torque?

....probably just an other of mine silly ideas but they offerd all kin`s a stuff back thaen.

Frank

Yeah Stovebold, and don't forget the magnets on the fuel line and the Turbo air inlet. That out to make another 50 HP, huh?

open up those lake plugs

seems there was a big argument over on Inliners site about a year ago about exhaust diameter, but the gen. consensus was duals bigger than 2" was too much for a six, I believe. brian

Exhaust pressure is very essential on 2 stroke engines but I think a little tinkering with mixture and timing will correct your problem. Don't think you have created a problem with the oversize pipe

The word back pressure is used a couple of times in this thread.My opinion is there should
be little to no back pressure in the exhaust system. The pipe should be small enough to maintain exhaust velocity to help evacuate the
cylinder. I think when you went to the larger pipe it probably slowed the exhaust gases down enough to effect low end torque by not evacuating
the cylinders as well at low RPM.
When the exhaust valve opens the gases escape at
a high rate of speed as the piston rises to near TDC the gases start to slow down. If the pipe is
small enough or large enough to maintain velocity it will help
to pull the remaining gases from the cyl.
I also believe that a cam with a lot of overlap
and an exhaust system piping to small will lead
to over scavaging the cyl.(pull to much of the
incoming charge thru the exhaust valve)leading
to poor performance.
I guess what I am trying to say is the pipe dia.
should match the RPM range you want run.

My opinions only
Arnie

Amen on that, Arnie. I don't know why so many people think it is the backpressure that improves the torque when it really is the velocity. Sometimes improving the velocity can create a little more backpressure, but the benefit comes from velocity, not backpressure.

Arnie and Baldy are right. A close friend of mine has one of the top pull tractors in the country and spends not hours but days on his dyno. He sometimes takes my humble advice although the student is now much smarter than the teacher. Here is a description of exhaust function in very common and hopefully easy to understand terms. To visualize scavaging think of the old suction guns used to suck up oil out of the old cannister type filters, when you pulled up the piston in the barrel of the gun it created a vacuum that sucked up the oil. In the exhaust tube on an engine the piston is replaced by a pulse(ball) of exhaust and the faster it moves through the tube the greater the vacuum is behind it. At this point when the intake valve opens and this vacuum from the exhaust helps pull(scavage) the burned gasses from the cylinder and pull in the new air/fuel mixture. Now, if you increase the size of the tube the ball doesn't fit as tight and loses some to much of it's suction so the scavaging effect drops. If the engine can run at a very high rpm then the large tube gives less restriction and though each pulse has less draw there are so many more of them moving at much higher velocity that there is a net gain in power. In a low rpm engine big exhaust tubes will kill low end. I know I've used "vacuum" with license but it is easier to visualize. Now if all this back to school stuff hasn't put you to sleep you can go on to the next thread.

Thanks guys, that makes perfect sense....

.... BOOOO HOOOO! All that work and money for the birds.

Well at least a chance to try my new pipe bending machine :rolleyes:

would you say that I can keep the bigger pipes from the mufflers to the bumper or would they influence the torque too because the gasses "shrink" when they cool off?

Frank

I think keeping the pipes after the muffler would have minimum influence since the gas has already expanded and slowed in the muffler.

You can make a temporary baffle to block off about 1/3 the exhaust pipe outlet to see if there's any improvement,a piece of sheetmetal and a hose clamp will do it.
When you change one thing,it may screw up the engine tuning.Been my experience that the old 6's,once modified,especially with a hotter cam,need more spark advance at mid speeds,like a recurved distributer advance.For a test,simply advance the timing about 10 degrees,road test it.Temporarily over advancing the timing won't hurt anything,just don't run it at full throttle and maxiumum revs.

my truck has 2" exhaust. but it is a single pipe, with standard muffler. I notice no bad effects from 2" pipe, it is the same diameter as the old pipe that was on it. I am not sure what to tell ya, sounds like a wierd one to me. maybe you have it over muffled? too much pipe diamter? But I am not really sure. Good Luck.

Kyle

ft knox, I think he is running TWO 2 1/4 pipes on a 235,brian