How important is exhaust diameter as compared to overlap. Clearly there is a relationship between EVC and IVO but what affect does exhaust restriction have on these events?

Does a larger exhaust improve power on a high overlap (10 degrees) cam?

Is there a way to measure exhaust restriction? How do you calculate header diameter for a given overlap?

 

I'll take a stab at some of this:

Not sure how to quantify this. I wont even attempt answering this, but I
would think flow might not have *as much* of an effect on valve timing
as the pipe dimensions will. This is just an assumption based on the relationship
between frequency and amplitude however.

Exhaust primary diameter are sized against exhaust port sizes from the texts
I've been studying. Not sure if there's much benefit of having a 2" primary on
a 1.5" exhaust port (for example).

I wouldn't expect to see this sort of crude transition between intake runner and
intake port.

 

https://ls1tech.com/forums/showpost....93&postcount=3

You could weld a small tube into the exhaust just before the muffler or somewhere behind the header, and attach a pressure gage with a piece of flexible (insulating) tubing.

Perhaps overlap is not the driving force for determining header diameters. HP per cylinder might be the first consideration.

 

The high overlap cams benefit from a good flowing exhaust. There have been a couple posts about poor performance from "big" cams where exhaust restrictions have been fingered as the culprit. You can crutch it a bit by opening the exhaust valve a bit sooner

Patrick G's current set-up is one of the few cars to perform very well with a cam approaching 10* overlap at .05 and cats, so it can be done.

A restrictive exhaust will certainly contribute to poor street manners on a car using a MAF

 

The exhaust article by David Vizard which was posted on the forum a few days ago has a chart for selecting header pipe ID vs. exhaust port CFM at max valve lift. It is based on the formula: ID = square root of [(CFM x 1.27)/FD], where FD is Flow Density, a measure of relative flow rates for different applications, 80 being good for a race engine, 85 for a hot street car and 90 for a strictly street engine. Chosen head flow is usually related to the desired power level and so it seems to pass a reality check. By this formula, a head flowing 200 CFM @ 0.565" would get a pipe of 1.72" ID, or ~1 -7/8" OD for a hot street application.
For what it's worth, after working with my Dynomation engine simulation program, a 7,500 RPM 433 LSx with ETP's 240 cc C5R heads, which I'm speccing out for a road racer, looked best with stepped 2 -1/8" to 2-1/4" pipes, whereas the Vizard formula gives 1.93" ID, or ~ 2 -1/16" OD.

 

Not sure if it is worth mentioning, but the length of tube before the first bend greatly effects velocity.

 

I think I understand the header formulas but not sure how to calculate the exhaust diameter. For a given application how do you determine when to go from a 2.5" exhaust to a 3"? Are there any formulas or is it trial and error?

 

The example numbers you mention are representative of collector/tailpipe sizes, not header primaries. Collectors and tailpipes are a very different story, too long to tell here. That Vizard exhaust article posted somewhere nearby goes in to it quite throughly.

 

Stuff behind the header isn't going to be effected much by overlap or other cam events. Restriction in the system is what costs you power.

Flywheel horsepower is a fairly good guide for exhaust pipe size. Your goals are free flow and minimum back pressure and, for a street car, a sound that you and the law can live with.

Mufflers can be the biggest restriction in the system, and if yours don't have adequate flow, big pipes in front of the muffler(s) don't help much, because the muffler acts like a plug in the system. Good choices are Magnaflow and Dynomax Ultra or welded. All of these are flow-through designs with no baffles. There are some other mufflers which sound great, but don't "flow" like the ones mentioned, inspite of their name.

If you have adequate flowing mufflers, choose pipe size by HP you have at the flywheel. If you are using true duals, 2-1/2 inch pipes will support well over 400 fwhp. Note that the new LS7 has 3 inch true duals. They are oversized for the 505 fwhp the engine now has. They should support 600 fwhp.

If you have a Y-pipe into one exhaust pipe and muffler it gets tougher. You should compare inside area of different diameter tubes to estimate flow.

2 pipes each 2-1/2" outside diameter (OD) have about 35% more flow area than one 3 inch OD pipe. However, because there is about 65% more surface area in the 2 x 2-1/2 pipes, they won't flow 35% better than one 3 incher. Similarly, one 4 incher has about 35% more flow area than two 2-1/2 inchers.

OK, so if you are in the 400-450 fwhp range or about 350 to 400 rwhp, 2-1/2 true duals or one 3 inch after the Y should be adequate.

If you plan to do a bunch more to your GTO, look into a 3-1/2 inch pipe after the Y or maybe even 3 inch true duals. For the GTO as it is or the Yukon, a 3 incher after the Y should be plenty, as long as you have a good muffler. If you want true duals, 2-1/2 should work.

Make sure you have a free-flowing muffler(s)!

My $.02

 

Thanks for the info. I'm currently at 407rwhp, without a good tune, and without the FAST intake I'm about to install. I expect to get around 450rwhp with those mods which would take me over 500 FWHP. I'm currently running the stock 2.5" dual exhaust with an x-pipe and stock mufflers. I guess going to 3" exhaust will become a necessity.

 

Can you tell us the benefit, if any, of an X pipe? I didnt read anywhere were you mentioned its benefit/hindrance.

Wouldn't it be wiser to just send the pipes as straight back as you can get them?

 

Some form of crossover or balance tube behind the headers and ahead of the muffler generally helps both power and sound. I'm not convinced an X pipe with 4 extra bends about 45 degrees and two about 90 degrees helps flow much compared to straight tubes with an H pipe. The X does seem to alter the sound however.

IMO, flow is Job One.

 

I apologize if this is too remidial, but I had always heard the X pipe helped increase flow as opposed to an H pipe. But from what I have read, theoretically, wouldn't they simply just want straight pipes?

At what point does the length of exhaust not matter? And by that I mean at what point will the length stop helping with scavenging and the flow will not benefit from a longer pipe.

Wouldn't a shorter pipe, stopping around mid-car flow best? It would still allow for LT headers for optimum performance, and still give the neccessary flow, without the added weight of extra piping.

Again, I apologize if these question are too remidial for this section.

 

Any bend causes flow restriction. You could think of a bend of X degrees causing the same restriction as Y extra feet of tubing. Let's say a 90 degree bend equals 5 to 10 feet extra straight. If I had head pipes 2 feet apart I probably would not use all those bends to get an X. I'd use an H.

The reflection which determines savenging stops when the pipe stops or reaches something it thinks is "all outdoors". This can either be the end of the system (with a straight thru muffler) or the muffler if it's an expansion box type. Look for a discussion on "terminator boxes" for more info.

Every engine combination will have a best header extension length, depending on rpm range and where you want it to perform best. That could be less than 18 inches from the header to maybe 4 feet.

Dumping exhaust at mid car is VERY noisy on a street car.

Your questions need no apology, IMO.