Backpressure

[Figment412]

I was talking with someone and they told me that 4 stroke motors do not need back-pressure, only 2 strokes do. This person has been work with engines for some 30 years so I take their word on it too. I just want to know what you people think.

[astroracer]

Welll... He's right... to a point. We put on these big 'ole noisey exhaust systems to improve flow. This in essence reduces backpressure. What he is not telling you is that these 4 stroke motors need exhaust "SCAVENGING"... Where the residual exhaust pulses help suck out the spent exhaust gasses from the cylinders and actually pull the fresh intake charge into them.
This is a highly debated topic on this forum... You have your guys in one corner that swear that "true duals" RULE and the ones in the other corner that swear their "big single" systems ROCK...
It's all about " balance"... Balancing the exhaust pulses from one bank with the pulses from the other. This reduces back pressure, improves the "scavenging" and actually improves your engine 's performance and sound.
A Big Single System (or a well designed dual exhaust WITH an "X" pipe) will both do this job very effeciently. The Big Single set-up will do this very well on a stock motor without the added weight and expense of another muffler or the problems inherent (lack of room?) with running true duals and an X pipe under our late model trucks.
There is a lot of info out there on exhaust systems with some very insightful articles and dyno tests where most of this "rhetoric" has been fully proven.

[Figment412]

Do you know where those articles are by chance?

[astroracer]

Talk to FeatherFoot on these boards. He has a lot of dyno info he can share.
Most of my info is taken from actual experience, magazine articles and stuff I find on the web. This is one site that has a lot of data. www.drgas.com/art-sync.html
Car Craft just did a dyno comparison on a Mustang... Open headers vs. an H Pipe vs. an X Pipe. A well designed X Pipe system made 2 or 3 MORE horsepower than open headers and almost 25 more then the H Pipe System. Granted this was on a track only car but ANY motor will benefit to some extent with a good exhaust system.
A year or so ago Car Craft did another dyno test on a Big Block in a truck. I can't remember the exact numbers but a Big Single system put on in place of a set of true duals dramatically improved the low rpm torque and throttle response. This article made me sit up and pay attention...

[astroracer]

Check out this site... http://www.boyleworks.com/ta400/psp/exhaust.html
Lots of good stuff here too. I will continue to add sites when I come across them.

[PtownZ71]

a good book is How to Build Horsepower by david vizard. this guy really kows what he is talking about. the book is excellent all around, but there are a couple of good chapters on exhaust

[FeatherFoot]

I cannot agree with the statement that 4-cycle engines dont need backpressure. We have a customer, a performance shop, that specifies a certain amount of back pressure in his exhausts, for supercharging, which our muffler produces for him.

We have tested several configurations on the same engine to verify his requirement and on naturally aspirated engines the best gains were realized at the same amount of back pressure.

The gains are impressive. My method of getting there was 180 degrees out from his method but both are the same.

Of all the systems I have tested , the gains were in the 1/2% to 2% gains. Mine shows 7 to 11% gains - at that pressure.:thumbup:

[astroracer]

FeatherFoot,
Am I correct in my understanding that cylinder scavenging is created (or enhanced)by the "correct" amount of back pressure in a specific system.
That is what I was trying to put across in my original post here. I just got side tracked and didn't complete the thought.

[FeatherFoot]

Basically that is correct. A properly tuned exhaust will create a negative pulse that reaches the exhaust port just before the valve closes. Basically `kissing' the valve just before it seats.

example: 3 deg of overlap in the cam, intake opens 1 deg BTDC and the exhaust closes 2 deg ATDC, you want that pulse to hit the valve at 1 deg BTDC, or just before, to scavenge or suck the burnt gasses out of the cylinder before being forced out the intake valve.

Compressed air (gasses) will seek the path of least resistance and in an internal combustion engine those gasses are still expanding during the exhaust cycle. So, the negative pulse at the exhaust valve creates less resistance thereby reducing the amount of gasses forced through the intake valve while it's open and the piston hasn't passed TDC. All of this results in less spent gasses in the new charge to the cylinder producing more power.

ATDC = After TDC
BTDC = Before TDC
TDC = Top Dead Center.