Quicker spool up on 2.7 Turbo

bb · 01-11-2005, 07:02 PM

I am going to change over the pipe that goes from the headers to the BAE Turbo on my 2.7. I am going to make the cross over pipe smaller. This should let the turbo spool up faster. The question is... I have a 2-1/4" pipe now. Should I go down to 2-1/8" or 2". I was going to go to 2" but am just wondering if that will be too much of a change. It is coming on now around 4000 RPM.

Thanks
Oscar

Lukesportsman · 01-11-2005, 08:02 PM

I don't know the BAE setup specifics so this may be ill advised. Often times in a turbo setup you want to keep volume down and velocity up along with temps. This means moderate sizing and short pipes to the turbo. It does seem to me that after the collector and within reason a smaller pipe after collector could result in more backpressure and therefore not necessarily be all "gain" in downsizing. It could help a little down low and hurt a lot up high.

Just concerned me and wanted to make sure you played devils advocate with yourself before cutting.

air-cool-me · 01-12-2005, 12:32 AM

i say go for it and test it out... 4k seems a bit late for it to spool up.. what turbo are you using?

this sounds like a good idea but im not sure were the gains would be offset by the backpresure.

(dont mean to hyjack) but

wouldn't reducing back pressure after the turbo help it hit earlyer?
any chance that you can also make the pipes larger after the turbo?

anyone have the bae instruction's handy to send to me?(email)
like the original manual or paperwork or anything?

RallyJon · 01-12-2005, 05:43 AM

2.25 sounds about right. That's the size of the uppipe on an STI which supports up to about 450hp.

Looking at the design of the BAE exhaust side (from here: 911SC Turbo ) I bet you get huge turbulence where the two sides of the exhaust meet at the turbo. I would think a modern twin-scroll turbo with one bank feeding each inlet would be a huge improvement.

tsuter · 01-12-2005, 06:10 AM

There were two BAE cross over pipes for the turbo kit depenedent on the heat exchangers:

If the car was Federal i.e., late style heat exchangers, the crossover was in the center in the stock location with a Y collector replacing the catalytic converter. It has a good run to the turbo flange. BAE P/N 20-1000-1.

With early heat exchangers or SSI type, the cross over was below the fan, engine tin, parallel the muffler and merged at a flange for the turbo mount. This setup is suspect due to the turbulence at the turbo mounting flange. BAE p/n 20-1000.

tsuter · 01-12-2005, 06:19 AM

Here is a picture of the BAE exhaust manifold(crossover) for the late style heat exchangers. (Federal). The Y collector is well before the turbo mounting flange.

I don't have a picture for the BAE cross over for the SSI style heat exchangers but that one bolts off the back of the SSIs and merges at the mounting turbo flange. Suspect design...

tsuter · 01-12-2005, 06:37 AM

Here is a drawing showing the BAE crossover for the early style heat exchangers where there was no catalytic converter.

Note the collector is at the turbo mounting flange. Maybe not the best design.

Later style with a Federal catalytic coverter (removed) and replaced with the BAE Y pipe collector at the stock crossover upfront did not have this problem to deal with.

bb · 01-12-2005, 07:07 AM

I am not using the stock BAE exhaust. When I had to replace it I ended up using BB headers that I shortenned and made the crossover pipe. I had talked to Turbocity who has worked on my turbo who has told me that I should make the diameter smaller to increase velocity. I have a Rajay turbo. I have takine pic's which I will try and post.

Oscar

sammyg2 · 01-12-2005, 07:10 AM

I have the later style where the Y is at least 16" before the turbo, in that case reducing the diameter would result in a net loss IMO, The change from pressure to velocity is supposed to happen in the turbine scroll kind of like a reverse volute. If you have turbulence at the turbine inlet then a redesign should help.

I also found on mine that at first the boost seemed to hit at around 3400 rpm.
I installed a little thingy (excuse the technical jargon) that acted as a relief valve that only let the wastegate see pressure when it got to a pre-determined setting. That kept the waste gate from gradually opening too early. I felt immediate improvement. Boost seemed to hit earlier, but in reality it just hit harder.
Afterwards if I put it in 4th gear and bogged the motor and watched the boost gauge as the rpm climbed I noticed that the boost went positive at 2900 rpm.
I contribute the boost threshold difference partially to lag and partially to the spring loaded adjustable thingy. Hopefully someone here can help me out and assign the proper name to the mechanism.
EDIT: PS I've dealt with turbo city before and while they seem a bit arrogant I got the impression they know their stuff. If they say go smaller I'd try it.

bb · 01-12-2005, 07:18 AM

We've tried changing part of the turbo to come in earlier, and with the Headers it is 100% better than the stock was. Geary at Turbocity which nows my turbo information said that I can try a smaller turbo or changing this pipe. I figure I can try this first since I can fabricate it in a weekend.

THanks
Oscar

tsuter · 01-12-2005, 07:35 AM

Manual boost controller (MBC) Sam. Its worth the $35. When I put on the Fabspeed 4.5" exhaust - right side outlet. I noticed about a 400-600RPM drop in when the boost began.

Don Z. · 01-12-2005, 08:04 AM

Just a thought from a completely different direction... Ceramic coating the pipes retains heat, and makes the boost come on a lot earlier... Also, depending on the waste gate limitations, may increase overall boost as well.

As an added bonus, the pipe sizes stay the same...

air-cool-me · 01-12-2005, 01:01 PM

or maybe some kind of heat wrap.... IDK?

Don Z. · 01-12-2005, 02:43 PM

Problem with the wrap is that moisture tends to condense inside it, which leads to rust.

On the Corvairs, I've seen people who need new ones after only a year or less.

On the LAV-25, which is a turbo diesel with an asbestos wrap, they last a little longer, but part of our PMs is to get underneath there and clean it out...

bb · 01-12-2005, 02:57 PM

I had wrapped the pipe with some tape that Summit had. I will do it again with the new exhaust. Also, How does the boost controller work to drop the rpm that the boost comes in. Right now I don't have a Wastegate just a pop off valve. I am going to incorporate an intercooler and Wastegate this winter.

tsuter · 01-13-2005, 06:48 AM

Quote:

Originally posted by bb
I Also, How does the boost controller work to drop the rpm that the boost comes in. Right now I don't have a Wastegate just a pop off valve.

It doesn't. It just gives you a sharper rise if the wastegate is cracking. The design of that pop valve in the turbo outlet pipe is better so it may not be cracking. With the MBC, you can set the pop valve spring at say .4 Bar. Then use the MBC to adjust higher. Changing or shimming that pop valve is a nuisance for adjusting boost.