tuned equal length header question

[Gambler87]

So I have searched headers for the Dohc motor in this forum and I'm wondering what size primaries and length and size collectors I should run for my 97 3.4 Dohc motor.

My engine 97 3.4 dohc
10.5 to 1 forged pistons
SLR reground cams
The "custom" short runner intake that was posted here
75mm n* throttle body
42.5lb injectors
Ffp underdrive pulley
Lightened rotating assembly
Fidanza aluminum flywheel
Gasket matched lower intake
Gasket matched and mildly ported heads

I'm just wondering what is the formulas everyone is using to figure out the primary sizes and lengths.
Would like to tune the headers for the same rpm as the intake which should be around 6k rpm. Thanks

[Shaun41178(2)]

I am sure others here will chime in with the math to figure it all out. Quick guess would prob be 1-5/8th diam though.

However getting them to fit will be something else. Are you prepared to cut out your trunk? Is the intake you have the one I sold on here?

[The Dark Side of Will]

Can you link info on your manifold?

The same calcs that give you intake primary length as a function of intake valve duration and RPM will give you exhaust primary length for exhaust valve duration & RPM... The difference is that you can assume an air temp of about 100F for the intake, but a temp of about 1000F for the exhaust. This alters the speed of sound, and thus the length of the primary, significantly for other factors being equal.

I'm interested in seeing what math you used for your intake. 6000 RPM is probably a bit on the high side for tuned lengths for a street engine, but that depends on which reflection your calculating. Steven's had very good results with basically bolting a plenum to the lower intake.

I haven't seen anything but rules of thumb for primary diameter. Horsepower per cylinder is the driving factor for primary diameter. A 300 HP V6 and a 400 HP V8 both make 50 HP/cyl and would use the same diameter primaries. I think 1 5/8" would be a good number.

[FieroWanaBe1]

The end goal of a long-tube header is to provide a strong enough exhaust pulse reflection that yeilds a depression and pressure differential across the exhaust valve and overlap period, complimented by a higher pressure pulse at the intake, we can provide a greater gas flow through the combustion chamber.
For diameter, you should be look at curtain area and valve diameter.
We need to provide an area that can support enough flow without adding undue friction, while still promoting enough velocity to provide a strong pulse wave reflection. This would lie somewhere between curtain area and valve diameter for both valves and available flow from the port. The key figure needed to know is max lift here. I have some SLR reground cams that do not match the spec from other people who had their cams reground. Mine is .397, the valve is 1.26" in diameter.
A=pi*D*L*2=3.14*.397*1.26*2= 3.14sqin
total valve area = A=2*pi*d^2/4 = 2.49
1.75 tubing with a .030 wall has an ID of 1.565, which has an open area of 1.565^2/4*pi=2.24in^2
That can easily flow as much as the exhaust ports without too much of a static pressure increase.
Most people will tell you that is too large, and it probably will kill a lot of low speed torque but would help top end a lot.
Length, you need to look at what the cam timing duration is at a lift that still promotes measurable flow across the valve, and this can very head to head, a small depression across a tiny opening will flow very little. My Cams are 286 seat to seat (.006) lift, or 236 at .050 lift, I do not know the duration at .2 lift which is another common point of interest for available flow.
Pressure waves travel at speeds relative to temperature here on earth. The temperature of the exhaust gasses drop dramatically as they travel downstream the primary pipe. But they do peak at around 1200 plus degrees F at the port, a well insulated pipe will drop less temperature than one that sees a lot of airflow across the tubes.
We need to time the length needed for a number of reflections at the specified RPMs to reflect back to the valve at the point of available flow that can draw more charge out of the head. This depends on your cam timing and intake tuning to gain full effect. One reflection requires an unrealistic amount of tubing, and can create a lot of pressure drop as a result of built up friction, lessening the effect of every reflection.
The next step is the collector volume. We need to evacuate the charge with low restriction and enough depression to not negatively influence other primaries in the collector. A safe bet is the bigger you can fit the better. These parts act as plenum resonators, much like an intake plenum would.
And all this needs to fit in the car too.

[Aaron]

Without running the calculations, I'd guess 1.75 and a 32-38" primary. I ran extensive number scenarios when I built the equal length set for my Lumina, which was a pretty close to stock motor, and built them at 1.625 X 32 IIRC.

Shawn is clueless, disregard anything he says. My DOHC has tuned equal length headers, an equal length Y-pipe, a T4 turbocharger, a 4" exhaust, external wastegate that dumps back into the exhaust, and an air to water intercooler all in the stock engine bay and stock trunk. Packaging concerns quite simply aren't.

[Shaun41178(2)]

equal length doesn't equate to equal flow. Bends hurt flow. Aarons equal length headers fit, but with all those bends, thats hurting flow. But hey, EQUAL LENGTH BRO!!!

[ericjon262]

longer sweeping bends will have a lower reduction in flow than sharp 90* turns. I wouldn't be to concerned with exact equal length, no matter what you do, each primary will have a different impedance. Going N/a and trying to hit 300 WHP is going to be tough, I think you'll probably need 1 3/4" primaries to get there.

[Gambler87]

I used this thread to make my short runner intake.http://www.fiero.com/forum/Archives/Arch ... 04156.html

[Aaron]

Interesting I never knew he made a short runner intake. I'd love to see that video though, too bad its set to private.

Series has a similar short runner intake, and combined with a few other basic mods put down like 220whp.

I swapped to the 96-97 intakes on my turbo motor, but didn't change the manifold at all. Maintaining a strong mid range and good turbo response was important.

I still have a set of 48mm ITB's I'd love to incorporate sometime, I think the motor would absolutely love them.

[Gambler87]

I have a set of 52/50mm extrudabody itbs that I'm going to incorporate once I get everything up and running and make sure the runner length and volume is right for my goal and then I will be making custom head plates to go straight to the heads to have a better angle of airflow to the valves.

[The Dark Side of Will]

equal length doesn't equate to equal flow. Bends hurt flow. Aarons equal length headers fit, but with all those bends, thats hurting flow. But hey, EQUAL LENGTH BRO!!!

BMW clearly prioritizes equal length over straight runs:


Yes, there is an effect on flow friction from bends. I haven't looked hard at analysis methods that would adjust primary diameter based on the number of bends.

The propagation of pulses and reflections should be largely unencumbered by bends, as the waves move at the speed of sound and move ENERGY, not material. IOW, bends should have a fairly minimal effect on primary length tuning.

[The Dark Side of Will]

The end goal of a long-tube header is to provide a strong enough exhaust pulse reflection that yeilds a depression and pressure differential across the exhaust valve and overlap period, complimented by a higher pressure pulse at the intake, we can provide a greater gas flow through the combustion chamber.
For diameter, you should be look at curtain area and valve diameter.
We need to provide an area that can support enough flow without adding undue friction, while still promoting enough velocity to provide a strong pulse wave reflection. This would lie somewhere between curtain area and valve diameter for both valves and available flow from the port. The key figure needed to know is max lift here. I have some SLR reground cams that do not match the spec from other people who had their cams reground. Mine is .397, the valve is 1.26" in diameter.
A=pi*D*L*2=3.14*.397*1.26*2= 3.14sqin
total valve area = A=2*pi*d^2/4 = 2.49
1.75 tubing with a .030 wall has an ID of 1.565, which has an open area of 1.565^2/4*pi=2.24in^2
That can easily flow as much as the exhaust ports without too much of a static pressure increase.
Most people will tell you that is too large, and it probably will kill a lot of low speed torque but would help top end a lot.
Length, you need to look at what the cam timing duration is at a lift that still promotes measurable flow across the valve, and this can very head to head, a small depression across a tiny opening will flow very little. My Cams are 286 seat to seat (.006) lift, or 236 at .050 lift, I do not know the duration at .2 lift which is another common point of interest for available flow.
Pressure waves travel at speeds relative to temperature here on earth. The temperature of the exhaust gasses drop dramatically as they travel downstream the primary pipe. But they do peak at around 1200 plus degrees F at the port, a well insulated pipe will drop less temperature than one that sees a lot of airflow across the tubes.
We need to time the length needed for a number of reflections at the specified RPMs to reflect back to the valve at the point of available flow that can draw more charge out of the head. This depends on your cam timing and intake tuning to gain full effect. One reflection requires an unrealistic amount of tubing, and can create a lot of pressure drop as a result of built up friction, lessening the effect of every reflection.
The next step is the collector volume. We need to evacuate the charge with low restriction and enough depression to not negatively influence other primaries in the collector. A safe bet is the bigger you can fit the better. These parts act as plenum resonators, much like an intake plenum would.
And all this needs to fit in the car too.

Good point about curtain area vs. valve area.
Is your curtain area really that much greater than your valve area? Wow. I'm not sure that extra lift benefits you, then.

If you plug in the intake cam duration and runner length of an L98 TPI and assume that IAT is 100F, then you get the 4th reflection operating at 3000 RPM, 3rd reflection at 4000 RPM and 2nd reflection at 6000 RPM. While the 2nd reflection is the strongest, the duration of the intake lobe and flow capabilities of the TPI runners are such that the engine can't breathe at that RPM anyway.

Typically the 3rd and 4th reflections are the most practical to use, but 2nd can be used with enough RPM. The first reflection is the province of superbikes and F1 engines.

Pulses travel at the speed of sound in the gas through which they are travelling. They travel faster through 1000 degree exhaust gas than through 100 degree intake air. Chemical composition does not affect speed, as speed is largely determined by the ideal gas law.

Momentum tuning based on primary diameter affects a much broader RPM range than pulse tuning, but isn't as strong an effect.

[Gambler87]

I have got a hold of mrsleeve from this website and I'm trading him some 97 3.4 Dohc parts for him building me a set of custom headers. But thanks for the info

[draven]

I have got a hold of mrsleeve from this website and I'm trading him some 97 3.4 Dohc parts for him building me a set of custom headers. But thanks for the info

Gambler87,
Just sent you a pm

[Aaron]

equal length doesn't equate to equal flow. Bends hurt flow. Aarons equal length headers fit, but with all those bends, thats hurting flow. But hey, EQUAL LENGTH BRO!!!

I didn't think mine had all that many bends, but your manifolds are obviously the best things ever. Less bends right? By the way, a back to back dyno comparison on a turbo DSM showed quite a radical improvement with equal length headers over a shorty header and a manifold.

[ericjon262]

equal length doesn't equate to equal flow. Bends hurt flow. Aarons equal length headers fit, but with all those bends, thats hurting flow. But hey, EQUAL LENGTH BRO!!!

I didn't think mine had all that many bends, but your manifolds are obviously the best things ever. Less bends right? By the way, a back to back dyno comparison on a turbo DSM showed quite a radical improvement with equal length headers over a shorty header and a manifold.

Aaron, those look nice, did you make them?

[Aaron]

Thanks, and yes I did.