Thread: is back pres. good 4 NE thing?

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Thanks to all the great info from the members of this board i've gotten my engine running better than I ever could have on my own. I'm getting ready to take it down to the local muffler shop and get the best flowing exaust I possibly can, But I have heard some people say that some back pressure on a system is good. I am wondering what I can expect (besides noise reduction) from installing a true dual and say a pair of flowmasters as compared to the way the engine would run with just the open headers. Will there be any part of the torque curve that will benifit by having slightly more back pressure on the engine? If so where?

the vehicle is a 94 s10, the engine is a mild 355 with edlebrock performer intake, l82 cam, holley 650 double pumper and 195/160 heads. the tranny is a new turbo 350 running through (as of right now) the original 273 rear end.

any thoughts would be greatly appreciated

you do need a little back preassure, it helps with cylinder scavengeing, you gain a little low end tq by having it there, you would loose it otherwise. run 2 flowmasters ( or any other chambered muffler ) and an H pipe, you'll have a well balanced system that will give what you want.

The 650 DP with mech secondaries will give you problems running an automatic tranny.

what type and can they be overcome???

Since they don't work off of engine vacumn they come in too quick, especially at WOT or better said when you punch it, which leads to a very rich condition and the engine will bog. It can be tuned for a stickly drag race application, but it sucks on the street.

I have no idea how much backpressure is required, or evn a way to determine home much is too much other then on a chassis dyno and playing with different exhuast systems. I do know that I once had a V-8 powered Pinto that was a bit more then a 10th faster through the exhaust then with the headers open!!!

The only motors I have seen that don't respond to a bit of backpressure is hi-rpm motors running mechanical fuel injection. I suppose on the latter it's a matter of runner velocity overcoming the need for scavenging??? I'm sure one of the engine therorists has some numbers on the back pressure amounts.

As Matt said, a good 2 1/4" or 2 1/2" exhaust system with chambered mufflers and either an H or an X pipe will give you good power and give it sound rather then noise!!!!

Not to mention that unless your at WOT all the time you run the risk the engine will pull the cold air back thru and warp your exhaust valves, or crack your heads.
I would run an "H" pipe to help keep the balance on the exhaust side for flow.

This may be controversal but here goes:

An engine needs absolutely NO back pressure to run right. If an engine runs better with mufflers, the primaries are too large/small or the collectors are too large/short. The engine is all about scavenging and when the pulse hits a pressure difference meaning larger pipe or atmosphere, it bounces a wave back to the exhaust creating a pressure difference promoting scavenging. If the exhaust isn't "tuned" to the engine's specific needs, it could use the back pressure to help low end waves but it will cost top end hp. If you tune the exhaust to the engine, it will run better all the way around without back pressure.

So your saying that the overlap of the cam has nothing to do with back pressure then- it's determined by the carb. primary's??

I agree, back pressure is not required. If it runs better through the mufflers, it needs a jetting change, or as stated, there's something else not optimum, like header pipe size, or length.

O.K., let's take the case of the Super classes motors that ran better with a muffler on them. Could it be possible that the little bit of back pressure prevented some of the intake charge from being sucked out the exhaust valve at overlap, allowing the motor to make a little bit more power?

I'm not much of a theorist on engines, but we played with that old Windsor powered Pinto on a chassis dyno for quite a spell, the best numbers we got were with the exhaust on.....

With chambered pipes the overlap will pull heated exhaust instead of cold air, it's going to scavage unless you run a no overlap cam, and I think the only one I have found is made by Federal Mougal for Ford small blocks.
Without backpressure you change the balance of flow thru the engine - i.e. over porting on the exhaust side will kill flow into your heads, same same with open headers except when at WOT.

Ok, so now let's throw in a set of 4 into 2 into 1 merge collectors.....I know they help low end torque and top end horsepower, just don't have a clue WHY!!!!!!

Originally posted by techinspector1
O.K., let's take the case of the Super classes motors that ran better with a muffler on them. Could it be possible that the little bit of back pressure prevented some of the intake charge from being sucked out the exhaust valve at overlap, allowing the motor to make a little bit more power?

Could have also been the overlooked problem of having too large of primary TUBES(for the post further up) and too large/short collectors? We all know that bigger isn't always better and I don't see why that would not stand with collectors. Also with such long overlap, it wouldn't take much back pressure to cause some awfully bad reversion IMO. Just a thought.....