SOHC intake on DOHC engine

General engine tech -- Drag Racing to Circle Track

Moderator: Team

Post Reply
n2xlr8n
Expert
Expert
Posts: 508
Joined: Tue Jun 28, 2005 9:01 pm
Location: Bama

SOHC intake on DOHC engine

Post by n2xlr8n » Wed Aug 31, 2005 1:31 pm

I read the ram tuning thread, but I'm hoping I can find a few educated opinions.

The engine: A 2.5L Boxer four cylinder

The issue I'm trying to resolve: A 13% intake manifold (call it "A") flow disparity between cylinders 3 and 4...and overall crappy plenum design (small)

I could obviously tune injectors 3 and 4 a little richer....but I'm not in a hurry, and I'd like to fix the problem.

An easy and inexpensive way is to replace the manifold with a factory design (this one, we'll call "B") that comes stock on Japanese market only SOHC, naturally aspirated engines. Intake B has larger I.D. runners, a larger plenum, and significantly more volume, tapering to the same port size as the A manifold. DOHC/SOHC engines share the same displacement, same R/S ratio...different cam profiles. The engine I'm using is a turbo model, with aftermarket camshafts.

Can someone hypothesize on what effects intake B will have on the turbo engine? I'm in the process of having the B manifold flowed...so if you need that info versus the A, it'll be a while. :wink:

Thank you for your time.

S.
He who is in me is greater than he who is in the world.

bill jones
Guru
Guru
Posts: 2518
Joined: Sun Sep 05, 2004 6:38 pm
Location: salt lake city, ut
Contact:

Post by bill jones » Wed Aug 31, 2005 5:25 pm

-are you talking about the subaru engine with the long manifold that has the plenum in the center then splits off and runs over to each bank?
-How are you flowtesting the manifold?
-Are you including the short manifolds when you test?
-Does one of these manifolds have the vanes in it?
-What are you using for the throttle body? and are you using any other parts of the intake system?

n2xlr8n
Expert
Expert
Posts: 508
Joined: Tue Jun 28, 2005 9:01 pm
Location: Bama

Post by n2xlr8n » Thu Sep 01, 2005 1:56 pm

bill jones wrote:-are you talking about the subaru engine with the long manifold that has the plenum in the center then splits off and runs over to each bank?
-How are you flowtesting the manifold?
-Are you including the short manifolds when you test?
-Does one of these manifolds have the vanes in it?
-What are you using for the throttle body? and are you using any other parts of the intake system?
-Yes. I am strictly speaking of replacing the WRX/STi intake manifold with a newer model (2006 JDM-issue SOHC 2.5L) that LOOKS like it would flow incredibly, without Tumble Generator Valves. Straight through. One piece. The US market does not get the trick pieces that the JA market does. Ever. It can seen here, with TGVs:

http://www.subaru.co.jp/legacy/outback/ ... index.html

and here on a nasty buggy (without TGVs):

http://www.glamisdunes.com/invision/ind ... =48338&hl=

Notice that the intake in the pics, the runners face the TB :wink:

-that, I'll leave to my porting / flow guy.
FWIW, the stock WRX intake flowed:

Avg tract flow = 269cfm

Tract 1 - 259cfm, % deviation from avg = -3.72%
Tract 2 - 263cfm, % deviation from avg = +0.76%
Tract 3 - 252cfm, % deviation from avg = -9.01%
Tract 4 - 302cfm, % deviation from avg = +6.82%

Total intake flow = 1076 cfm

This is not my particular manifold, but my numbers were almost identical.

-I'm assuming the short manifolds you are referring to are the TGVs. They were ported (butterflies and throttle plates removed) and included in my testing of the stock WRX intake. The manifold I'm using is one piece...just a smooth transition from the "O" of the runner to the "D" of the port.

-Stock ported TB; there has been >700whp made with this TB; I think it will do nicely

-No other stock intake parts, as my car's EM is stand-alone, utilizing Speed Density...and a 4" compressor inlet.

Thanks for the help, Bill. I owe you a beverage.

S.
He who is in me is greater than he who is in the world.

bill jones
Guru
Guru
Posts: 2518
Joined: Sun Sep 05, 2004 6:38 pm
Location: salt lake city, ut
Contact:

Post by bill jones » Thu Sep 01, 2005 3:31 pm

-I haven't seen that particular manifold yet but it looks pretty good.
-------------------------------------------------------
-I really don't know the answer to your questions but if I had a significant disparity between manifold runners I'd be working on'm to fix the problems.
-But if the jap manifold is available why even mess with the stock one that doesn't flow decent?
-Is it a big deal to change manifolds on the engine---to do back to back testing?

n2xlr8n
Expert
Expert
Posts: 508
Joined: Tue Jun 28, 2005 9:01 pm
Location: Bama

Post by n2xlr8n » Thu Sep 01, 2005 5:41 pm

bill jones wrote:-I haven't seen that particular manifold yet but it looks pretty good.
-------------------------------------------------------
-I really don't know the answer to your questions but if I had a significant disparity between manifold runners I'd be working on'm to fix the problems.
-But if the jap manifold is available why even mess with the stock one that doesn't flow decent?
-Is it a big deal to change manifolds on the engine---to do back to back testing?
-I talked to a USTTC / rally mechanic. I'm going to use this manifold, period.

-That's exactly why I went looking for an alternative.

-It's now a conversation piece, but way back when...folks (like me) using the stock US WRX manifold were breaking ring lands on #3 and #4. Some say it was cooling, others detonation...I flowed my heads and manifold, and this is what I came up with.

-The rumor is that Subaru will introduce the aforementioned "improved" manifold on the 2006 models in the US....naturally aspirated only...go figure.

Thanks again!

S.
He who is in me is greater than he who is in the world.

Post Reply