Exhaust Manifold Primary Diameter Sizing

[MichaelPGriffith]

Gents, the plethora of exhaust diameter calculators available online are giving me a wide variety of ideal primary diameter, and that's a problem; there's only one ideal.

I'm working on cam and exhaust manifold designs for a 3.5 V6. In a recent thread there are some equations but they seem to be specifically for 4 pots and 4-2-1's attached to them.

Does anyone have a reference to a good reliable equation that will help me keep my exhaust gasses moving in the 250-280fps range?

Or, if it's simpler; the engine is a 3500cc V6, 4-valve, 30mm exhaust valves, and I would like to keep the primaries in the 21-24" range. I'll buy shelf 304SS bends for the manifold construction, so there is a limit to available variety. I'll either go stepped 1 1/2" to 1 5/8", 1 5/8" to 1 3/4", or 1 3/4" to 1 7/8". I want them to flow respectably up to 8200rpm.

Thanks in advance for any thoughts and input.

 

[patprimmer]

I would initially make them from mild steel, then make the pretty one after the development work and all its cutting and shutting was done.

Appart from that I also would like to know any software that mere mortals can afford or even a reliable formula for modeling exhausts system possibilities.

Regards
Pat
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[GregLocock]

maybe there's a plethora of solutions because the tradeoffs are pretty lax?

Cheers

Greg Locock


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[MichaelPGriffith]

Alright then, I'll go with my gut and some simple cross sectional area considerations.

The exhaust ports are 26mm at the valve seat (~1062sqmm cross section), which wil step up to 1 5/8" (~1233sqmm cross section), then primaries stepping to 1 3/4" (~1439sqmm), then a 13* merge collector transition to 2" (~1897sqmm) and straight to atmosphere not far from there.

I'll be designing the layout of everything involved using a thick gauge wire. This should cover the routing and centerline radii for the bends; hopefully minimizing waste and time spent trimming everything to fit.

 

[Peter7307]

Michael,
You might find flex tube or even vacuum cleaner flex plastic tube a more accurate substitute.

This tends to give a better approximation of the space required.

Peter.

 

[inline6]

pipemax

http://www.maxracesoftware.com/pipemax36xp2.htm

 

[MichaelPGriffith]

Peter,
Thanks for the input but I think I would rather stick with my simpler method. With the wire frame I won't have to trouble with mitering the flex tubing and my space constraints are likely more lax than most; 25cm+ for each manifold from cylinderhead to chassis bulkhead.

Inline, Pipemax is a familiar name but I hadn't taken the time to research it much because I assumed it would be cost prohibitive. At $45, however, that certainly isn't the case and looking over it's PDF input and solution documents gives me an acceptable degree of confidence in it.

Pat, does Pipemax enjoy your blessing?

 

[patprimmer]

Never used it, but just ordered it.

Larry Meaux? has a very good reputation and is strongly recommended by someone whos judgement I trust implicitly.

Having said that, I believe all exhaust systems should be designed to our best estimates, then proved and tuned with a dyno, gas axe and bits of pipe. Hence my comment of starting with mild steel.


Regards
Pat
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[MichaelPGriffith]

Any input on the effect of advancing the catalytic converters position by ~12" on the torque curve?

 

[patprimmer]

How do you mount the cat on the torque curve ;-)

Regards
Pat
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[MichaelPGriffith]

You got me.

Any input on the effect on the torque curve of advancing the catalytic converters position by ~12"?

 

[pwildfire]

From what distance? If you are advancing it from 18" to 6", then yes, If you are moving it from 60" to 48", then likely not.

In any case moving it closer means less accumulation, meaning that the backpressure at the exhaust valve will vary more, but not necessarily have a higher average pressure. It also depends on the efficiency of combustion and stroke of the engine, if the gases are still expanding in the first section of exhaust, this could make a big difference vs if they are cooling.

One more thing is the temperature. Catalytic converters perform best in a certain temperature range depending on the substrate and type/concentration of sites, so it can be important to place the converter within the appropriate distance from the head to keep it near that temperature.

 

[e21jps]

I like to back up my estimations with a few real world examples because its easy to get carried away with the "little bit is good more is better" trap

Take into account the horspower targets and look at similar size engines known for punching well above their weight, in your case the best example might be 3.2l M3 BMW easy 220Kw at the wheels in near standard trim and quite high state of tune, it has 3 primary pipes of only 1.5" merging to a 2" secondary, they would have chosen that size very carefully

 

[MichaelPGriffith]

Excellent. Thank you all for the input.

If I may steer the discussion to materials. I'm considering building the exhaust manifold flange and the first 4" of primary tube out of aluminum. Then attaching slip fit primaries.

The rational is that this will aid in dissipating combustion heat through the cylinder head, make the whole assembly lighter, and allow me to CNC the transition to round from port shape to primary ID.

Sealing would possibly be an issue. I would have to look at the amount of expansion that will occur at operating temp of the part. I'm inclined to think though that this would seal itself quite nicely with that very expansion.

Any Thoughts on other cons?

 

[patprimmer]

Exhaust pipes get red hot near the head, Aluminium melts below red heat.

Regards
Pat
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[izzmus]

Pontiac tried aluminum exhaust manifolds in the early 60's in a homologation model. The racers would qualify with iron manifolds and only put the aluminum ones on for the actual races, as erosion was a problem.

This is manifolds bulky enough that the weight savings was on the order of 50-80lb depending on who you talk to. I can't imagine aluminum headers lasting more than a few glorious seconds of WOT before collapsing.