Tech Article: Paying for less performance--exhaust manifolds

[Captain Moai]

THis week's tech article, originally posted by Ricky Benitez [www.mrcontrols.com] on www.mr2oc.com

Exhaust Manifold Mythology

We've all seen the big, blingy exhaust manifolds on some of the expensive turbo kits with long pipes running up and over and under and down. The manufacturer says it provides equal length tuned runners to scavenge the exhaust away from the cylinders. Worth the money? Usually not.

Turbines run on heat. The hotter the exhaust, the faster they spool and the more boost they can make. Need proof? Try this for an experiment on an EMS: take out several degrees of timing in the spoolup and boost region of your maps. The EGTs climb and you'll notice faster spooling and, if your turbo setup is right on the edge of boost creep, you'll get more boost creep.

The problem is, those beautiful, long, shiny pipes act as radiators. They lengthen the distance that the hot exhaust must travel between the port and the turbine and radiate that heat off into the engine bay where it becomes a liability rather than a benefit.

What about the scavenging effects? Bogus! As you start to push your turbine to make big power (whatever big power is for your turbo) exhaust manifold pressure will increase substantially and the scavenging effects of the returning finite pressure waves is greatly minimized. So much for all those beautiful pipes snaking all around the engine bay.

What works best? Short, straight runners resulting in minimal energy loss, coated with a thermal coating or wrapped in thermal wrap.
In fact for many applications, the stock exhaust manifold coated and ported is probably going to outperform just about anything you can get out there. Tubular manifolds can be made to work, but their design should be simple and with an eye towards retaining and transferring as much exhaust gas heat into the turbine, not radiating it out into the engine bay.


Comments, Questions, Concerns? email Eric at mofongomr2@yahoo.com

[nysulli]

exactly right, another thing to keep in mind, turbo's run off heat, but also are effected by pressure, not gas velocity, the idea is to create maximum pressure right at the inlet of the turbo, and a minimum right at the outlet on the turbo, again, reinforcing the idea of the shortest pipes possible, since, pv=nRT for an ideal gas (lets just leave it at that to keep things simple), hence higher temp gas gives higher pressure, not a properly designed exhaust manifold could provide some pressure gains, thus, better performance

just something to add to the discussion

[mikecentola]

yeah, i plan to wrap my header and uppipe in addition to the thermal ceramic coating on there now...

[nysulli]

well that might be overkill, i read an article awhile back describing temps of coated exhaust, the difference was huge, temps taken 1 inch from the header was something like 700 degrees or maybe even 900, after coating it was 300, i really don't know how much more insulated you'll really be able to make your header then that

http://www.centuryperformance.com/heatwraps.asp

[mikecentola]

when you're spooling a very large turbo....every little bit helps...

[nysulli]

exactly, now on the otherhand, scavaging is key for me, but at the same time long convoluted pipes like those offered on the techworksengineering header are a bit much, although it does have the benefit of terminating at the stock location, allowing it to work with turbo setups on rs's (and gives a nice 15 whp gain with 6.5 psi setups, 160 to 175 whp)

which shows that sometimes longer piping can still help, as they come coated, and are equal length, and flow a ton better then the stock manifold

[mikecentola]

TWE...i'll prolly get my tranny cryo'd there....

[nysulli]

I've only heard good things about them, lots of rs's have built heads from them with great results, as well as full exhausts that people say are as good as they get