Header/ Exhaust flow question
 

Lets say you had two identical sets of quality equal length headers, one header I.D. is the same as the exhaust port, and one is 8mm larger. What would be the effects on performance between the two? If any?

Just for fun i am going to put this to a real world test next week, between coats of POR-15 high temp Black Velvet. By real world i mean seat-of-pants test. I would like to do two dynos runs but don't see the point.

If the SSI guy, that hangs around some time, reads this and would like to lend me a set of HE's for 3 comparing dyno runs i would spring for that. The header sizes are 1 7/8 and 1.5, from AJ USA.

Paul

51 looks but no one wants to hazard an educated guess?

PT

you did not mention what motor you are doing it to and the mods that the motor has. you might see a loss in power in a smaller displacement motor due the exhaust being to large especially on the torque that the motor puts out. on a larger motor like a 3-3.2 i think you would see a modest 15-20 hp gain over stock style heatexchangers with the larger headers.

I'd think that if anything, you'd be taking a step backwards by going with larger header, especially if the small ones are matched to the exhaust port. I don't know enough about this (yet), but I think you will lose some of the scavenging effects (of the cooling exhaust) if you go larger.
Gary

I thought some of the Engineers with fluid dynamics exp and old racers turned automotive engineers would jump in and give you more info than you needed, but since they haven't...

Here's my .02 thats worth maybe a quarter of that price.

I think the port matched pipes will perform better all other things being equal. I think they'll give more power in the lower bands than the others, and the others may only surpass them at very high rpm's. I think the larger pipes will slow the gases as they exit the engine, and it's my understanding that you want to keep the gas velocity up as it exits the engine.

It goes back to the statement about an engine being an air pump, so you want to get the air in and out of the engine as fast as possible.

Larger ID or shorter length headers have a higher RPM HP peak and are generally more suited to racing while smaller ID or longer tubes are better for low to midrange power. That said it has ALWAYS been my experience when dealing with headers that a ID that is larger than the port ID will perform better due to the restrictions caused by bends and such. A best of both world would be large ID and long tubes which widens the torque and HP curves with a slight loss at peak HP. The area under the curve is what you want to increase not peak HP. For example lets say we have two motors with peak HP at 6000 rpms of 500. Motor "A" makes 200 HP at 5000 and 200 HP at 7000 while motor "B" makes 450 HP at 5000 and 450 HP at 7000 unless you had a variable speed transmission that kept the motor at peak HP motor "B" will always be faster. This is why drag racers running automatics use "high stall" torque converters effectively holding the motor near peak HP through each gear.

I'm running 1.5" headers on my 3.0 and they are definately on the mild side, with more cam and compression I'd opt for 1.625" or 1.75" pipes. My 600 HP smallblock drag car had the biggest pipes I could fit, 2" mandrel bent custom headers these gave me nearly 75 HP over the 1.75" headers I was previously running.

It cannot be predicted with the info available. You would have to try each one out -- and do a full dyno run on each. The reason is the superposition of pressure waves in the exhaust flow. It's non-trivial in application although the general idea is in all the Soph. physics texts. Most car people call it "scavenging."

All the basics are above. Some more info here:

http://www.elephantracing.com/techtopic/headers.htm

If you want to play with some formulae on optimizing header length / diameter check the following:

http://www.prestage.com/carmath/calc_headerlength.asp

My prediction; unless you have a 3.4 and are running it to 8000 rpm you will lose power across the board. Going from a small port to a large ID header is not a problem, a header that is too big is.

A couple of other things to consider...

What cam are you using: Cams with a lot of overlap I suspect will be happier with higher exhaust velocities (slightly smaller headers) since the extra inertia will help scavenge the combustion chambers. But cams with a fair amount of overlap also tend to be higher rev'ing cams, so now you're moving more air through the headers, and the pulses are at higher frequencies, then with a comparabily sized engine with a milder cam and the same sized headers. So in abstract terms its hard to describe which header really is "smaller" when compared to the engine that it is attached to.

How does the header match the exhaust port? I've also heard that cams with overlap like to have a mis-match between the port and the manifold where the manifold is slightly larger then the port (especially on the outside of any bend in the exhaust shape). This mis-match tends to echo back positive pressure pulses and allow negative pressure pulses to continue into the port. This is the concept that allows Anti-Reversion headers work.

This is one of those situations where a carefully crafted set of experiments combined with some solid DOE analysis would go a long way.