StealDads67

not cause anyloss in bottom end performance, and he thinks that there is no such that has to little back pressure..... any imput from anybody would be great

PeteL

Probably absolutely true on a race built engine.

On a "Detroit" engine there is no telling how they set it up and it may compensate for the stock back-pressure. Case-in-point is the EGR system which depends upon a certain amount of back pressure to cause a certain flow into the combustion chamber. Cam design and timing also consider back-pressure.

There is little doubt that back-pressure decreases torque, but the shape of the torque curve can be effected dramatically. It all depends upon the design of the intake / cam / heads/ timing / exhaust manifold.

[edit] forgot programming! Stock programming may "lean out" with exhaust mods and needs to be upgraded to match on a fuel injected car. This is much like changing the timing curve and jets on a carb car.

Pete


[Modified by PeteL, 8:05 PM 3/30/2002]

StealDads67

and im talking on a Daily Driver Car

Dr. Evil

So much of this depends on the cam profile. Look at my exhaust. Pretty freeflowing. I did before and after dyno tests and I lost no low end torque. I gained at every rpm, from off idle to redline.

65Z01

Consider the setup when most dyno pulls are made on an engine: headers into short open pipes. On a properly designed engine on a test stand it shouldn't matter after the header collectors.

As was pointed out though, for a street engine with full pollution dress it may make a difference on some systems.

scorp508

L98 no like back pressure. :)

AquaMetallic94LT1

Its not loss of back pressure that causes a reduction of torque with some open headers or no exhaust. Lower back pressure means the exhaust gases have an easier time to get out of the combustion chamber. Open exhaust will reduce the exhuast flow velocity. Higher exhaust velocity helps empty the cylinder of exhaust gases. Open headers can also allow more air back into the chamber through the exhuast valve resulting in a leaner mixture at lower rpm and part throttle.

Headers are designed to tune the exhuast so that the pressure just outside the exhuast valve is lower than the pressure in the exhuast system and even the outside atmospheric pressure over a narrow rpm range. That is the same as creating lower back pressure or negative back pressure and it increases torque over that rpm.

EGR is designed to reduce combustion temperatures which reduces emissions by introducing exhaust into the intake. Spark knock is result from no egr. This will reduce power and make the motor knock under the right conditions.

Dryseals

The thing you lose with open headers on a stock setup is the drafting of the exhaust pulse. Each pulse has mass and as it exits the cylinder if its allowed to run smoothly will create an area of low pressure behind it. Like drafting behind an 18 wheeler. The lower pressure behind it allows the next pulse to exit quicker. On a well tuned motor that low pressure created from the exhaust pulse will help pull in the intake charge during valve overlap. lots of other factors involved, just a little food for thought.

Mr Mojo

On my 78 Chevy Truck, the entire exhaust system fell off(except for the the Y pipe, so I was basically running open exhaust. All I can tell you was that the truck was a pure slug after that.

BiZ

I put headers on my 71 Camaro some years ago and had to drive it to the exhaust shop to have the rest of the exhaust put on. It was a total dog, loud as hell too.

bogus

there are BOOKS written about exhaust design and theory...

The science is "scavaging". and yes, good quality headers will improve scavaging... but you must tune the motor to match the headers.

That is why I laff at all these import guys... they bolt headers on and expect 20hp... they won't get it without a cam change.

what is boils down to is this: headers need cams. cams need headers... it is so very important to make sure they are compatiable (I don't want to say matched).

you will pick up some power from an LT1 with headers... but that is a result of the compromises of the factory cam. but to really get power, a hot cam with headers will really produce some HP.