Twin turbo configuration questions: Theory & Application

[sammyg2]

The dashed line is the surge line IIRC. Anything above or to the left of that line means the compressor will be in surge which is hard on the compressor, results in loss of positive flow, and if sustained for a period of time can result in extremely high compressor discharge temperatures.

[BURN-BROS]

Originally posted by beepbeep

For laymans like you and me, it's best done by comparing boost treshold for other cars with same turbocharger and compensating for engine volume accordningly.

[/B]

Thats exactly what I did. I used a calculator and placed the displacement of a 3.6 and 3.3. It gave me the same turbo with the same trim(but it was a garrett). But the association was established so I think I am o.k. But I would still like to see.

[beepbeep]

Originally posted by sammyg2
The dashed line is the surge line IIRC. Anything above or to the left of that line means the compressor will be in surge which is hard on the compressor, results in loss of positive flow, and if sustained for a period of time can result in extremely high compressor discharge temperatures.

Yepp. Dashed curve is compressor-stall limit. Push it left of curve and air flow over compressor vane will loose laminar flow and start to swirl with total loss of efficiency. It can even damage the blades (they are actually quite soft). Any jet jockey will tell you what happends when compressor stalls Big no-no.

[BURN-BROS]

Thanks Sammy, got it.

[beepbeep]

RarelyL8:

It's very mature approach. I'm really glad to see people doing "moderate" turbo stuff. Most of the time you just get carried away and turn that boost knob just a little bit more (it's an addiction ) and then something grenades. Very few keep their heads cool and tweek turbo engines for maximum driveability and reliability. 930 with two small turbos and 0.8 bar of boost equals instant 340hp, no broken parts and "real world" usability.

[Natchamp]

Burn, here is a K16 compressor map;

[BURN-BROS]

Thanks Mark. BTW, nice ride! What is the boost level you are running? Since you have the same turbos as me, how do they behave?

[Natchamp]

I don't have the same turbos as you, at least I don't think so. I have K16/24 hybrids which are K16 turbines with K24 compressors. I'm currently running two boost levels, 15 and 18lbs. When they hit, sheet happens quickly. Now I'm working on getting the boost to come on sooner and contimplating switching to Garretts.

[BURN-BROS]

Well I might, They are autothority stage 3s if I dare believe what the seller said. What rpm do yours come on at?

[Natchamp]

I haven't datalog'd it since the major dyno tuning so the following is by the seat of the pants; boost begins at @ 2500, slowly builds to @3500 and hits hard and full by 4000. I might do some data logging this weekend and I'll post the results. One important thing to point out is the dramatic effect ignition timing has on the spool up time. Frankly I wasn't a believer until the esteemed Dr G, on C2turbo, helped me out with tuning. When you advance, how much, for how long, when you retard, how much, for how long, etc. dramatically effects when and how the turbos spool, hence give you boost.

[Mr Beau]

Mark,

What are you using for boost control? Is the Haltech handling it? You may be able bring in the boost sooner by changing how the wastegates are controlled (assuming the are). With the appropriate boost control, you can also soften the hit of the turbos if desired, but it's a balancing act with bringing the boost in quickly.

Is there by chance a smaller A/R you can use on the exhaust side? That would most effectively bring the boost in sooner with some cost to high end power.

Also, don't get caught up in the stroke vs bore argument. Engines (turbos included) love displacement, and they really have no idea if it is obtained through bore or stroke...

[67nin11]

beep beep, any idea how a t3 off an 85 saab 900 would match a 2.4E engine? i sometimes fantasize about mixing some of my saab parts into the 911 (non watercooled turbo,intercoolerand apc) to make a low boost (5-6psi?) system. the goal would early boost and maybe 230 or so hp. any input?

[BURN-BROS]

Thanks for the info, one day when money falls from the sky I will go efi and have more control over fuel and spark!

[RarlyL8]

The Garret GT17s sound like a good candidate (thanks!)
Is the 340HP quoted at the crank or wheels?
I'd like to target 400HP at the crank with absolutely no less than 320HP at the wheels. I started out with a 1.0 bar spring and backed off to 0.7 bar. The power was nuts when boost kicked. I could just see the side plate of the 915 exploding. I think the tranny can live under 400 crank HP given my driving habbits.

So is the GT17 water cooled? Used cost / availability?

[beepbeep]

Originally posted by 67nin11
beep beep, any idea how a t3 off an 85 saab 900 would match a 2.4E engine? i sometimes fantasize about mixing some of my saab parts into the 911 (non watercooled turbo,intercoolerand apc) to make a low boost (5-6psi?) system. the goal would early boost and maybe 230 or so hp. any input?

T3's were a tad too big for 2.0's and were often used on Volvo 2.3L engines too. I believe it will be just right on 2.4E. I'm a bit sceptic about using APC on aircooled engine. It's tuned to listen for pinging in B2x2 engine block and not inherently noisy 911 engine. Turbo itself will flow air for 230-250hp but i wouldn't expect that much if used in low-boost installation.

RarelyL8: non watercooled GT17 should sit on certain diesel cars: BMW 540 TD, Renault Laguna 2.2 TD, Renault Espace 2.2 TD etc. They are pretty common and should be available from any turbocharger dealer.

Here's interesting page where you can choose the type of turbo ans see which cars it sits in:
http://cossie.adsl.dk/~powerpage/turbo.php

[RarlyL8]

I'm not familiar with any of those cars. Do they sell them in the US?

What about the 993tt units? They come up from time to time when owners upgrade. I believe they have internal wastegates but can't swear to it. The books I've read state external WG as being superior, again I have no experience. True?

[Natchamp]

Rarly, what liters is your motor?

[RarlyL8]

'78 930 Engine
3.3L
SC Cams
Euro Exhaust
B&B Intercooler
K27-7200 Turbo
Modded Stock Muffler
0.7 bar WasteGate w/ Dump (formerly 1.0 bar)
Stock bare-bones CIS tuned to 3% CO
ROW Dizzy
915 Tranny

[RickM]

Rarly, Your engine bay looks really "clean and simple".

Looks like you can double up on the length of that intercooler and bump you boost back up to 1.0 bar.

[RarlyL8]

You're right about having room for a full bay cooler.
Trouble is having money for a full bay cooler. Have you ever priced those things? Good lord the're expensive!

Actually the B&B unit can easily handle 1.0 bar.
The reason I went back to the stock wastegate was the tranny - I had terrible nightmares of blowing the side plate off when the boost hits in second gear. There is significant difference in torque produced at 1.0 bar vs 0.7bar, at least it sure felt that way!