help with exhaust size to maximize flow for lowend

[gregamorton]

Well I have decided to run the stock exhaust manifold on my 300 that im swapping out in my 74 f100. It currently is a 240 3 speed with 3.70s and going to the 300 3 speed with 2.75s. Its overall a stock 40 over engine that I will be adding a hogged out intake and a exhaust manifold that I have opened up to around 2 1/8". Stock its a hair under 2" and the stock down pipe is 1.75"!! I always thought it was 2" piping on the I6... I plan on running a flat flange and going to 2" or 2.25" and run single exhaust out back with walker muffler. Im looking for info on flow to rpms that i will be running. With the 2.75 gears it looks like I will be running in the 1600-2300 cruise range and need to keep all the torque I can with out going to big on piping.

[Ranchero50]

For references sake, a 5.9 Cummins turns 1700 rpm efficiently and has two tiny exhaust ports going into the turbo. For low RPM performance you want very long smaller runners. 1.5" long tube headers may net you some gains around 2500 rpm. Going EFI with a late model long runner intake will help as well. It's all about moving chunks of air efficiently through ports at different speeds using fluid dynamics to overfill the cylinder before the valves close.

Hogging out the stock parts is not going to increase the engine's low RPM performance, it'll actually hurt it.

[gregamorton]

I hogged out the outlet on the factory exhaust manifold and took out the ridges and one spot that sticks out and slows things down. I left the ports stock and did not mess with them at all. Im just trying to figure out on the actual exhaust tubing size to use for best flow without killing and low end.

[sargentrs]

I don't know the tech side but, on my '74 Dodge Dart with a 225 slant 6 and auto tranny, I went from 1.875 up to 2.25" and it made a big difference in power across the whole range.

[gregamorton]

Yea mine is 1.75 inside!! I was leaning tword a 2" down pipe to 2.25 exhaust or just 2.25" from manifold all the way back.

[rollercam1]

No backpressure of any kind is a good thing. It's plain hogwash about backpressure. I would fit the biggest I could. 2.5" if possible. In comparison. the fist upgrade in most 302 mustangs is dual 2.5" exhaust. you will be running almost the same cubes and a little less RPM's

[Ranchero50]

No backpressure of any kind is a good thing. It's plain hogwash about backpressure. I would fit the biggest I could. 2.5" if possible. In comparison. the fist upgrade in most 302 mustangs is dual 2.5" exhaust. you will be running almost the same cubes and a little less RPM's

Sorry bud, but on a combustion engine this simply isn't true. Perhaps back pressure isn't the best term but you want to control the pressure pulses of the exhaust stroke to help have a relative vacuum at the valve when it opens. That will help evacuate the cylinder. Do some research on two strokes and how the expansion chamber on the exhaust works to understand what's going on in the exhaust tract.

It's funny, on my '05 Lincoln LS with a 3.9l Jaguar V8 I replaced the stock kinky twin single 2.125" exhaust with mandrel bent 2.25" along with a 'X' pipe and lost torque. Sounds better but lost 2 mpg and some performance...

Per Greg's build, long thin primary tubes will give him the best low RPM torque performance. If he uses a merge style collector that will help the other two pulses pull the third along. There's an absolute poop ton of info out there for the reading. Spending some time with a fluid dynamics book will also help you understand air flow through a port / tube.

[rollercam1]

No backpressure of any kind is a good thing. It's plain hogwash about backpressure. I would fit the biggest I could. 2.5" if possible. In comparison. the fist upgrade in most 302 mustangs is dual 2.5" exhaust. you will be running almost the same cubes and a little less RPM's

Sorry bud, but on a combustion engine this simply isn't true. Perhaps back pressure isn't the best term but you want to control the pressure pulses of the exhaust stroke to help have a relative vacuum at the valve when it opens. That will help evacuate the cylinder. Do some research on two strokes and how the expansion chamber on the exhaust works to understand what's going on in the exhaust tract.

It's funny, on my '05 Lincoln LS with a 3.9l Jaguar V8 I replaced the stock kinky twin single 2.125" exhaust with mandrel bent 2.25" along with a 'X' pipe and lost torque. Sounds better but lost 2 mpg and some performance...

Per Greg's build, long thin primary tubes will give him the best low RPM torque performance. If he uses a merge style collector that will help the other two pulses pull the third along. There's an absolute poop ton of info out there for the reading. Spending some time with a fluid dynamics book will also help you understand air flow through a port / tube.

Agree with you on most of it. This will have a common manifold that will not tune nor time exhaust pulses. A V8 engine is a very different animal as well as a 2 stroke. What you are saying is very true. If he were to use a header / headers it would give him the ability to tune all of the things to which you refer. In his case, the exhaust manifold / cylinder head is more than enough restriction. IMO

[colnago]

No backpressure of any kind is a good thing. It's plain hogwash about backpressure.

So, from a purely theoretical viewpoint, would we see better performance if we removed any type of exhaust manifolds and/or headers, and just had an engine "exhaust" into the engine compartment? I know, this is impractical, and would probably create other problems. But would this be "best" for reducing backpressure to a minimum, and maximize engine performance?

Joseph

[Ranchero50]

No backpressure of any kind is a good thing. It's plain hogwash about backpressure.

So, from a purely theoretical viewpoint, would we see better performance if we removed any type of exhaust manifolds and/or headers, and just had an engine "exhaust" into the engine compartment? I know, this is impractical, and would probably create other problems. But would this be "best" for reducing backpressure to a minimum, and maximize engine performance?

Joseph

Did you read the other posts? Here's a post I put up XLforum talking about how pipes effect the efficiency of the engine on a Sporty. What you want is the engine as efficient as possible where you run it most.

It's all about controlling the pressure pulses through the engine so it flows efficiently. Your exhaust pulse goes down the pipe and then a pressure 'echo' reverberates back up to the exhaust valve and bounces back towards the pipe end. As the pulse is heading back down the pipe the pressure at the valve will go lower than natural and as it opens the low pressure will help draw the spent mixture out of the cylinder. If you have enough overlap in the cam profiles the exhaust pulse will help pull the intake mixture into the cylinder. Similar to how a 2 stroke with a good pipe will overfill the cylinders and really make power 'on the pipe' by sucking the intake charge into the engine. What you are seeing with raw fuel at the air cleaner is the exhaust pulse going through both valves and out the intake tract. Your pipe is out of 'tune' and changing the length will change the pulse timing. There's a reason most long tube headers on cars are 34-36" long. With a 2-1 you gain pulse pressure at the collector. Similarly with a crossover pipe you lose pressure, but an X increases it because one path pulls on the other as the pulses go past where they join. Oh yeah, smaller pipe diameter increases the pulse strength, over sized pipes are lazy. A good street pipe will pull hard from 3-4k where the engine lives.

Dumping at the head would hurt efficiency in comparison to a good working pipe. That being said, most of the FE 1 3/4" pipe as too large for a stock low compression 360 and most 390's.

[colnago]

Ranchero50,

Thanks for the clarification. Your earlier post now makes a lot more sense. I've never understood exhaust flow, and how/why it affects engine performance. Can't say I'm an expert, but it sounds like I have a bit of research to do.

Joseph

[Ranchero50]

Find a PDF of the two stroke tuners handbook and read it. Oh look, here it is. http://www.amrca.com/tech/tuners.pdf

It's from the late 70's and still relevant. A 4 stroke is a little different but still benefits from the flow control.

[WarEagle]

Found a good article from Borla. Don't confuse backpressure, exhaust velocity and restriction. Any restriction robs power. The exhaust can be tuned for maximum sound control and power.

http://www.hotrod.com/articles/borla-ex ... o-systems/

http://www.gomog.com/allmorgan/exhaustbackpressure.html