Thoughts on larger exhaust manifolds

[CTsupra]

I was watching funny car drag racing on espn the other day and a light went off in my head. The announcer said these guys are using 3" and larger exhaust manifolds for the turbo cars. I heard that and I was like uhh ok - these guys are making 2700hp, so it must be beneficial.

Would there be any decrease in spooling time on large turbos (or any turbo, but mainly large) using a 2" or larger exhaust manifold instead of the normal 1 1/4" - 1 3/4"?

Or would this increase spooling time and/or do nothing at all on the Supra?

Let me know if I'm on to something.


edit: if this has been discussed before here or on sf - you can neg me, yell at me, and then give me a link to the thread

 

[Supra Blues]

I would think that the speed of the exhaust gasses would slow down because they have a larger exit tube.



Ehhh....forget it. I dunno crap about this stuff.

 

[CTsupra]

^ i was thinking the same. didn't want to sound like a dumbass if i was wrong, lol.

also, now that i think about it. they're also running larger displacement motors...... sigh... duh kenny...

oh well, could this still work? anymore input?

 

[mcpcola]

Bottom line is your intake has to do as much work as the exhaust to make that work. with limited intake, cam, and stroke you would not build up enough air flow to work that size exhaust

 

[MDCmotorsports]

To a point it would be beneficial, but the velocity and the pulse of the exhaust to the turbo would fall.

I would think it would only be good for huge, balls to the wall full throttle runs.

 

[Stretch]

Well if too big is bad, then go very small. Try some 1 inch or smalelr tubes to furce air out at high velocity. Make then out of some sort of titanium alloy, or steel atleast stronger than 321 ss and maybe it'l hold together. Then your turbo will spool at 1700 rpms. Think about it.
eric

 

[IJ.]

Well if too big is bad, then go very small. Try some 1 inch or smalelr tubes to furce air out at high velocity. Make then out of some sort of titanium alloy, or steel atleast stronger than 321 ss and maybe it'l hold together. Then your turbo will spool at 1700 rpms. Think about it.
eric

But only rev to 5000 !

It's all a trade off Flow v's Gas speed.

Tight AR great spool low Top end HP

Loose AR Lag but lots of Top end HP

The Exhaust runners follow this to an extent.

 

[Slow66]

IMHO, on a turbo motor, whatever the port size is is what the primary tubes should be. N/A , juice, and s/c motors dont work on the same principle.

 

[CTsupra]

thanks for the info guys!

 

[Adjuster]

One more thing to think about is the built in anti-reversion dam that Toyota put on the 7MGTE engine.
The exhaust ports are smaller than the cast stock exhaust manifold ports for a reason. (Not just slipping cast molds although that can move things around alittle bit.)

I think, and it has been somewhat proven that removing the dam by port matching the exhaust runners with the exhaust manifold actually costs you low and mid range tourqe.

I left a 1mm or so ridge/dam around my exhaust runners when I ported the head, and further opened up the exhaust manifold just after the gasket area to enhance that dam/ridge effect. I exposed about 1mm of gasket, or ported the exhaust manifold 2mm larger than the gasket hole. This stepped area should do the trick, and I still get better than stock CFM out the exhaust runners.

Theory is that exhaust gas trying to reverse back up the manifold would hit that dam and the lip combined with turbulence would decrease it's chances of being pushed back into the combustion chamber.

Stepped header designs do the same thing, and have proven tourqe gains as a result.

My tourqe numbers were very good, but untill I can get some dyno pulls in w/o any fuel cut issues, I will not know for sure. (The curve builds tourqe better than any other dyno chart I've done before, by a large margin. I think the stroker motor helps, put so does the ridge I left behind.)

Let's see if Dr. Jonez has any comments on this as he port matched his exhaust, and IIRC regrets it due to lost tourqe down low.