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Small displacement twin turbo for pca gt4

I was at Mosport this weekend racing in GT4s.

One of my competitors was there with a very nice motor setup that I would like to consider replicating for myself.

He had a twin turbo setup using a 3.6L case (I crawled under and looked)

Now the limits of gt4 is a displacement of 2.8L and a turbo adds a multification factor of 1.3, so the largest displacement turbo that you can build and still be in gt4 is 2.15.

I have toyed with the idea of building myself a nice turbo motor for the class. I had never thought about starting with a 3.6L though. I have plans for a sleeved down 3.0 to give the 2.8L displacement.

How hard is this, what would I have to do. De-stroke, custom pistons, what else.

This motor seemed to do extreemly well for him. Last year his fastest time was 1:37 with a 2.8L na, now he was able to hit 1:31.

Thanks

Ed
Old 08-01-2005, 05:54 AM
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Must be fairly common, as i've done quite a few 930 cylinders undersized from the stock 95mm to 92 or 93mm. It wouldn't be a huge stretch to go down to a 90mm or smaller. You could probably easily run over 2 bar reliably with such thick walls. It has been suggested that they would hold to 3 bar, but I would be more concerned with the rest of the components at that point and you'd better have some real good race fuel to keep it together.
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Old 08-01-2005, 03:08 PM
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Hi,

What I am thinking of is more of the heads and the valve placement. The 3.6L is 105mm in diameter, and for a reasonable 2.1 you would need to go down to something more like a 83mm piston. That would be 22mm smaller than stock.

Thanks

Ed
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Old 08-01-2005, 06:41 PM
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Those twin turbo 2.1+ liter motors that ran at Mosport are both nicely done. The other one I know of you can see on Pat William's website. I hear there is one out on the West Coast too.

There are at least two different approaches, as exemplified in the two cars with these motors at Mosport. One is to use a 3.0 case (the normal 930 case, not the early one) and:

a) Machine up custom cylinders and pistons so the spigots fit

b) Machine (calls for some welding too) 2.7 or earlier heads to fit the stud spacing

c) have a billet crank made (using the Euro/Turbo '76-7 930 case would allow use of a 66mm crank).

This way you can manipulate all the displacement variables as you wish and have really thick cylinder and spigot walls.

The other is to use an early sandcast case, offset grind a 66mm crank, and use stock 84mm Mahle Ps and Cs. I think that all the new Ps and Cs that size are Nikasil. This was the route I was headed before I decided I couldn't afford it, but from the performance of the motor made that way it works.

993 TT intercoolers seem to work, and by simply changing the flanges from 3 to 2 bolts the 993 TT manifolds bolt up to early heads, and use of the 3.2 manifold and throttle body further simplifies things. I'm hearing that the big HP numbers are coming with only one bar boost. And the motors don't seem to be blowing up due to too much stress (missed shifts will still bend valves, of course).

I don't know why anyone would want to use a 964 case for this small displacement motor when simpler alternatives are available (though with guys liking the challenge of custom Ps, Cs, and cranks anything is possible I suppose), and I haven't seen one used.

Walt Fricke
(now trying to upgrade from 2.7 to 2.8)
Old 08-02-2005, 10:49 AM
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What are the turbo's that are favored for this application.

Thanks

Ed
Old 08-02-2005, 12:43 PM
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One engine builder just used the 993 twin turbos (along with most of the plumbing) and said they worked just fine despite the difference in displacement. The other two I know about used Garretts of one flavor or another (but I don't know which of the many combinations available).

Walt
Old 08-02-2005, 05:56 PM
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Shoot Charles.....I bet a set of 98's would hold over 2 bar no problem wimpy little 90's

Eric Hood
Old 08-02-2005, 09:25 PM
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Hello.

For maximum power and reliability, I would suggest using single turbo instead of dual ones. On such small displacement, twin snails also mean twin sets of piping + additional hassle with oiling pipes etc. It's often more effective to use one medium size turbocharger than two teeny-weeny ones. Spin-up improvements with two small GT17's (or such) are neglible compared to one GT27Rs or GT30R and boost-treshold can even get worse with dual ones, thanx to their lower efficiency. Bigger turbo will be more efficient and is often available with ball bearings centre section. Only drawback is that headers will get more complicated.

If I was building 2.15L turbocharged engine, I would ditch twin approach and build with single turbo. It's just more simple, efficient and cleaner.
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Old 08-03-2005, 06:37 AM
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I'm not really familiar with PCA rules, but how can it be that the displacement factor is only 1.3??? Seems like a bunch of guys are going to build fast turbo cars, and then the NA guys will complain, the rules will change, and everyone will be unhappy...
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Old 08-03-2005, 07:06 AM
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This is a huge risk with anything that you can do to your car that is much faster than anyone else.

Probably the biggest risk is that they will change the ratio of 1.3 to something more like 1.5. So even at this, a person would be bumped to GT3, which you can still be competitve.

Ed
Old 08-03-2005, 09:45 AM
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