I'm remembering from high school (way back when) that you need some amount of back pressure for optimum performance. Obviously you want as little as possible, but can you go too far? I'm not talking about a 700hp stock car, I'm talking about your average big block v-8.......360, 390, etc. I was talking to a guy at the local shop about the flat bed I'm rebuilding....it has a 390, with minor mods, and a 2.5" exhaust. I was joking with him about using my friends exhaust pipes off of his semi...6 inch pipe....and he said that it was possible to go too far and actually have a negative effect.
Anyway, this has no created an argument with a friend of mine.....so you guys get the final word. Be careful now, I'm taking your response as gospel! Thanks for your help guys!
Brian

1970 F250 2wd (Nice & 4-sale)
1972 F350 Flatbed
1976 F250 4wd (Huntin rig)
1999 F350 4wd (work truck)

 

Well, don't take my words to the bank, but look at drag racers, they have no backpressure. In my opinion, you need a little, but not much at all. Headers and 2 1/2" straight pipes are all you need buddy.


Primary rig is Green Thunder:
95' F-150 XLT 4x4, 302, 5 spd, MSD 6A, Flowmaster American Thunder Exhaust, Sunroof, Clear corners w/ Diamond headlights, CD player with 2 10" subs and some 32" BFG Muds .

Check out my Gallery for a look-see.

Then theres:
99' Mustang GT 4.6L
88' F-250 Heavyduty 4x4 351/c6
95' Mercury Cougar 4.6L V-8
80' E-350 300/6 with a
3 spd column shifter. Weighs around 7,000lbs w. 65mph top speed, who wants to race me!

 

You don't need backpressure, back pressure is a BAD thing. What you need is a constant velocity exhaust stream. It will sorta suck it's self out, and unfortunatly, backpressure is caused by this...I don't remember all the phyics and stuff to it, but I remember the idea.

 

The only meaningful reason that I can think for needing some back pressure would be the oxygen sensor. Perhaps it need a more steady flow instead of a pulsing exhaust flow? Still I think a straight piped exhaust should do fine. Just a little speculation on my part:7

 

I guess, If I remember correctly.......the only reason I can think of for backpressure is if you use mufflers. Like on dragsters, straight big *** pipes let it exhale. But when you add mufflers you kind of mess up that theory don't you? In other words, I would actually lose power if I went from 2 inches at the manifold to an eight inch pipe (just an extreme example) and then down to 2.5 inches through the exhaust mufflers. Something to do with trying to push all that cold dense air through the larger pipes that obviously would have no back pressure. But if you reduce the size of pipe to say 2.5 inches all the way, then you keep the air (exhaust gases) inside the pipe at a warmer temperature and thus flowing more freely and smoothly, and actually creates a venturi effect sucking the exhaust gases out of the combustion chambers. But by restricting the diameter of pipe to 2.5 inches you are actually creating SOME back pressure......right???

Thanks for the responses, and thanks for settling a stupid argument between two friends!

Brian

1970 F250 2wd (Nice & 4-sale)
1972 F350 Flatbed
1976 F250 4wd (Huntin rig)
1999 F350 4wd (work truck)

 

I don't agree, but I can't come up with a good arguement. Just run 2 1/2 dual straight pipes and your fine, we're talkin 2-3hp difference here.


Primary rig is Green Thunder:
95' F-150 XLT 4x4, 302, 5 spd, MSD 6A, Flowmaster American Thunder Exhaust, Sunroof, Clear corners w/ Diamond headlights, CD player with 2 10" subs and some 32" BFG Muds .

That's it for mods now, saving for a Stang. Check out my Gallery for a look-see.

 

all street engines require some sort of backpressure. I like to use the example of inhaling and exhaling through a straw. if the diameter of the straw is small, the velocity of the moving air is quick. if the diameter of the straw is large, velocity drops considerably. how does all this relate to engines? on a drag car, they operate at an RPM where exhaust velocity is not that important; hence, that is why they have little or no ehausts--called zoomies.
on a street engine, the RPM ceiling is MUCH lower and exhaust velocity is lower. TO have no backpressure and large tubing, performace will actually suffer as exhaust velocity suffers. Smaller tubing or backpressure negates this effect for the most part and helps with performance....
sum: if you are running a street engine, by all means design some backpressure into the system. If you are running a dragster, none is better. If this is still hard to grasp, do the straw trick while taking into consideration operating RPM! just dont let others see you doing this because you might end up getting a date in the looney bin!
HOPE THIS HELPS!
regards
ben

 

I agree that you need some back pressure. I just restored the exhaust system back to almost stock on my truck and it runs smoother and has a lot more low end power.

I was running headers, 2in pipe and hooker Aerochamber mufflers. This setup worked great running down the highway at 70 mph but not so hot pulling a trailer from a dead stop.

I think this is true for most engines. A friend of mine put drag pipes on his Harley and eventually had to go back and weld a washer in the pipe to create some backpressure to make it work right.

One more advantage was that I could not tune the original carburetor with dual exhausts. I had to change to a Holley. Now that I went back to an almost stock exhaust, I can use the original carb again.

With duals, the original carb had too small of idle bleeds to get the idle mixture properly. It also had a tendancy to ping on really hot days. Now the ping has gone away.

I have no scientific reasoning for this. All I know is that it works.

 

Ford Nut-

Thank you for that explanation, I'll try and share that with my friend who doesn't buy it! LOL... Seriously, that all makes sense to me...air in, air out, pretty simple really.

Brian

1970 F250 2wd (60k original new heads w/hard seats $2495 obo)
1972 F350 Flatbed (soon to be new work truck)
1976 F250 4wd (Huntin rig)
1999 F350 4wd (work truck)

 

hey blue oval

glad to have helped ya out some!

regards
benj