Why are some turbo manifolds the collector is symmetrical, whereas the others are asymmetrical (log style)??

Seems to me that the symmetrical style is better? And that the asymmetrical one is just there cause it's maybe cheap to make?? Any reasons to why?



l e o n

Hi Leon!

Nobody else seems to be replying, so here goes:

Turbo exhaust manifold design is not as critical as normally aspirated (non-turbo) applications in terms of equal length primary runners, but there are certain aspects of correct design that should be observed.

Years back, we prototyped for another company a tubular header for the Ford 2.3 liter turbo. The most glaring problem with the stock manifold it replaced is that the turbocharger inlet was directly adjacent to the #4 cylinder. The stock manifold was also of a small cross-section, causing high gas velocities.

What the combination of high velocity and extreme turbo location presented as a problem is as follows:

The ratio of primary runner length (distance from exhaust port to turbo inlet) from the shortest to the longest was about 8:1. This alone created an undesirable situation with different exhaust back pressure from cylinder to cylinder, but that was not the worst of it. The high velocity from the small cross section created cleaving, or excessive back pressure in the cylinders whose ports the exhaust pulses had to pass by on their way to the turbine inlet. The #3 cylinder always ran hotter, since it had to push its exhaust charge into a small log already crowded with the fast moving charges from the #1 and #2 cylinders.

Our header design for that vehicle made instantaneous improvements in turbo spool up and midrange torque, while also adding about 30 HP overall at the same boost pressure. Eliminating the back pressure and cleaving issues made large differences.

Our 420A exhaust manifold is a similar symmetrical design, albeit not tubular. With the turbine inlet in the center, between cylinders 2 and 3, equal exhaust paths are achieved, and the ratio of runner length is an acceptable 2:1. Additionally, we added a large cross section to reduce velocity, which is the only way to prevent cleaving in log type manifolds. The larger cross section also aids in turbo spool-up by providing a more homogenous flow and reducing "pulsing" at lower RPM's.

Does it work well? Len Ayala's 420A car makes 600HP with it, and enjoys excellent durability. We have NEVER seen one of these manifolds crack. And we will likely run the same manifold in this year's iteration of Len's car, with a target of 750-800 HP! We have also made his car a true drag car now, stripping about 500 lbs. of "street weight" from it. We are going Honda hunting in the Pro Import class; wish us luck!

Bill Hahn Jr.
98 RS Turbo: 10.87 @ 136 MPH
HRC Stage V with N20
Nation's Fastest Street FWD 2G DSM
www.turbosystem.com

LAST EDITED ON 01-Mar-01 AT 11:13 AM (PST)

Thanks for the reply HRC!

Hey Wyatt, think you can ask the STAR engineer or STAR what was his reasoning behind the design of their turbo manifold?



l e o n

Wyatt Leras
http://www.turbogs.com
98 Eclipse GS Star Stage 5,000 Turbo kit

I think most of these manufacturers designed their manifolds the way they did is most importantly for "clearance" issues, and then for performance.

dammit..i sure hope the star manifold has "some" performance reasoning...


l e o n

This is the difference in Design between a RevHard manifold which is more performance oriented, and the Star manifold.
Both manifolds are a good design, but this is to show you the
difference.
(Star manifold pix curtesy of Wyatts website..)

(revhard)


(star)


Hope this helps !

Where's the revhard one from?

l e o n

Revhard is a company down in Southern California.
http://www.revhard.com/