Inliners International Forums General Discussion Hi Performance Split exhaust

There is something that I don't quite understand about exhaust systems on an inline six. For a 250 Chevy, most people run either headers or twin cast manifolds. That part I can understand from a hi performance standpoint . What I don't get is what I have seen afterward, Y piping into a single exhaust. Is there still a performance gain to be had with only duals part way? Converting to duals all the way back makes sense as does a larger single pipe but to me going to the trouble of split exhaust at the engine into a single pipe seems like a lot of work for no real gain. I am sure that I must be missing something. Any thoughts? Jay 6155

scavenging amplified at a certain rpm range by taking advantage of exhaust pulses. When you have a well designed header, at the collector the pulse from one cylinder blows past the opening of another (adjacent in the collector) pipe and helps "pull" exhaust out because of venturi effect in the collector. Different designs allow tuning for different rpm ranges etc. and every engine designhas different requirements. Some respond well to long primaries going into one collector 6-1, while some work better with shorter primaries, and maybe even an intermediate collector to make 6-2-1. And the further away from the engine you look, the less exhaust pipe diameter you need. The gases cool off and shrink, so what came out of two or three pipes, will soon fit in one pipe. Also increase in diameter increases flow quite dramatically, you don't need twice the diameter for twice the flow.. And TOO MUCH flow is bad too, unless the engine was designed to run with no exhaust

for a collector to work right, the primaries need to be arranged in the firing order so you have a pulse sequence going in a continuous circle pattern. The primaries also need to be equal length and diameter so the timing of the pulses is even at the collector. I would also make a tall cone or pyramid in the middle to take up the "unnecessary" space in the middle of the collector, to reduce turbulence and increase the scavenging effect.

I'm no expert in our inline sixes, but it appears that our engines have firing orders arranged in two separate groups: the front 3 cylinders and the rear 3 cylinders. Makes exhaust design easier when you can treat the engine as two 3-cylinder engines and all 3 cyls are adjacent to each other. It would be more challenging to build a header if you had to combine cylinders 1-3-5 into one collector and 2-4-6 into another.

Because of this 3+3 design you could probably have true dual pipes working just fine. But once you get to the first pair of collectors and get two pipes out, proper secondary pipe lengths before the last collector could probably offer you a second scavenging boost for all 6 cylinders.

I understand how a header would work by using the vacuum behind the first pulse to help pull out the second pulse. That's why primary length and diameter are so important. There is probably some forumala to figure out the ideal length for a given engine speed and size. Would splitting the exhaust 3 in front and 3 in back make the 2 primary pipes act like headers and the Y pipe a collector? Would this make more power or torque than just dual exhaust comming from a split manifold ? After all even the factory got into the act with the 79 to 84 truck engines. They probably went 2 into 1 because at the time the idea of 2 cats was unheard of. Jay6155

I don't know if this helps, but here goes:

A very important factor of exhaust flow is velocity.
That's why the exhaust piping size (diameter) needs to
be matched to the available flow.

The other factor for street exhausts is the overall
volume of the system before the flow meets atmosphere.
The more volume of the proper diameter equals better flow.

The longer exhaust velocity can be maintained within the
system, the more power the engine can develop. This is
for street/muffled systems.

Think of this, a Harley Davidson exhaust diameter and length
vs. the single pipe exhaust of a stock 1960's inline six.

The trick in improving engine performance by exhaust mod's
is tailoring the exhaust to the engine's state of tune....

The only formula I have ever found that helps is this:
Most engines perform well with at least 2.5 times the
cylinder volume in the head pipe before the 1st restriction.
(Baffle).

One last "trick", remember those coffee cans over the
ends of zoomie exhausts on the old dragsters? The open
tube allowed the exhaust flow to enter atmosphere with
the least turbulence.

I hope this helps.

Interesting. I know somewhat about maintaining proper velocity in the exhaust system. I had a 350 in a Surburban that I put dual 3 inch exhaust on a stock engine. Way too big! You could put your hand on the tailpipe after driving 100 miles in the summer. In the winter as soon as you got off the freeway the pipes would steam like when you first start out with a cold engine. The overall view here is that for a near stock 250 a single 2.5 inch pipe is about right or dual 2 inch pipes. I was thinking of either running twin 2 inch pipes for s certin distance into a 2.5 single or just starting out with the single 2.5 pipe. I just didn't know how much advantage the the twin head pipes would give. Thanks for the replies. Jay 6155

If you don't drive in the narrow RPM range that the primaries tune to, or your cam doesn't have enough overlap for extraction, the next best feature is the largest exhaust valve and header you can fit to reduce pumping losses.
Major work was done on tuning L6 motors 80 years ago, and keeping the 2 groups separate all the way to the atmosphere has been the most successful for a road car: Jaguar, Aston-Martin, Healey, Zodiac, etc.

Like say 2 inch dual pipes all the way back to 2 mufflers and 2 tailpipes would be better due to less pumping losses. Keeping the system at 2 inches would keep up the velocity up and not let the gasses cool too much. That is easy for me to understand. Thanks .Jay 6155

I'm running Langdon headers to a 2.5" exhaust, Y-pipe about 2' back, into a single 2.5" pipe, through a 2.5" Flowmaster 10 series, and out the back. I have a LOT more power than stock now and it sounds darn good.

The only way to tell if there's a difference would be for me to change it to a dual exhaust (which I might eventually do) and compare it with hard numbers. My gut is telling me there won't be a darn bit of difference because any minor gains I'd see would be lost with the additional weight.

Gbauer, you are running the exact kind of exhaust system that I am talking about. I think that you are right, at least for the street. Do you think that you have gained power with split exhaust even if it dumps into 1 pipe in about 2' or so? I think that it would be less restrictive than a single 2.5 inch pipe comming off say a 292 manifold . It would be cool to know how what you gain. For myself, I am going to go one of 2 ways: 1. A single 2.5 inch pipe hooked up to the one bbl manifold all the way back to a super turbo muffler for a Camaro. 2.5 single in twin 2.5 out. When I put on the 2bbl manifold I would run 2 " pipes for a short distance and hook up to the single pipe. OR 2. Run the Z28 twin pipes with twin mufflers and tails. Use the Y pipe to hook to the single pipe for now and run 2 2 inch pipes to the mufflers when I go with the 2bbl manifold. I am leaning toward the single pipe. Thanks again for the replies and advice. Jay 6155

If going dual I'd go with 2". Single: 2.5"

Pi*r^2...

2" cross sectional area is 3.14". 2.5" is 4.9" A 2.5" on one of these inlines is too much if you're running dual pipes.

http://www.magnaflow.com/07techtips/faq/question10.asp

Handy chart for your use.

I have experimented with different exhausts on my 40 Chevy/ 302 GMC. These included: a 4 into 1 header with 2 1/2" single pipe system, a larger 4 into 1 header with 3" single exhaust and finally dual 2 into 1 headers with 1 3/4" pipes. My impressions of the three systems on the road with mufflers and during 1/4 mile drag races without mufflers and pipes were as follows. The 4 into 1 with a single 2 1/2 pipe worked well one the road and at the drags. The 4 into 1 with 3" single pipe was poor on the road and while racing especially at low rpm. The dual 1 3/4 headers work well in both cases but I think the dual headers seem better than the 4 into 1 single 2 1/2" on the road especially at low rpm. Drag racing results were about equal. I went to the smaller dual system after laying under a late model Jaguar XJ6 in my local junk yard. It was a 4.2 liter (252 ci) with a 12 port head, dual OH cams, dual headers and dual 1 3/4" pipes and mufflers. I figured my old GMC will never put out the power of the Jag, so I took the whole piping system and adapted it to my car. I works great and sounds good too.

Thanks for the input radar. Sounds like the 3 inch system was simply too big for the 302. I would imagine that a mild 250 would flow no better than your 302. Probably not as much. I bet the engine sounds good. If I do go with a dual system I will go 2 inch all the way back. It seems like from the posts here a person doesn't lose much by going with a single 2.5 inch system. Keep in mind that I will have manifolds not headers. I really like the way people on this site force me to think and are so helpful. Jay6155