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911st 911st is offline
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Join Date: Jan 2004
Location: Sacramento
Posts: 7,269
My two cents:

A well developed 3.0T with a 915 could give many 3.3 Turbos fits.

As it is 10% smaller, by running 10% higher boost or moving the power band up 10% should alow you to match a 3.3T's power.

As you are using a 915, using RPM's is a better approach.

On top of that a 3.0 is well suited to extend the RPM range and with the right valve train (cams, springs & retainers) can go up to around 7500rpm without issue. As most 3.3's are done by 6500rpm, 7200rpm would for a 3.0 would put you on equal footing.

Further, most 3.3T's make there peak HP near 6000rpm. This would put your goal to make peak HP at around 6600rpm. This mostly comes from the cam as long as the ports are not to small. Your SC cams are good turbo cams. You might try timing them to the 1977 euro 3.0 specs. Most go the other way and time there SC cams for low end. Alternatly, you could look in to a custom cam that would match your goals. Kind of a moderan turbo Grupe B cam.

To put this another way. A 3.3 that makes 330 ft/lb of TQ at 6000rpm will make 380hp. A 3.0 that makes 300 ft/lb at 6600 will also make 380hp.

Your 915 will limit you to about 300 to 325 ft/lb of TQ. You might want to talk to Kenedy Enginering about what you are doing and if they can make you a disk that has springs of the right strength to help reduce the pounding load on your transmission. This is what Porsche did with the 944 Turbo S.

HP comes with boost. Pre-boost drivability and spool comes with compression. EFI reduces drag on the turbo and it spools faster. EFI also alows for closer to ideal AFR's and timing that also increases response. Most report that around 8/1 CR drivability seems to be imporved.

Proven boost to compression combonations are: 7/1 CR w 1 bar boost, 8/1 CR w .8 bar, and 8.5 should be good with about .7 boost.

A well built normaly asperated 3.0 with SC cams and carbs should make around 200-220 normaly asperated HP. With 1 bar boost it should make up to 400-440hp, with .8 bar it should make around 360 to 400, .7 bar should be about 340 to 375. Cams that move the rpm's up 10% should approach 10% more HP than these numbers. Do note that these might be ideals and effecencies will fall off to some degree.

You will need to check if you have cylinders that can be re-used. Also, you should up grade your rod bolts and head studs.

JE makes good Turbo pistons if you want to lower compression. You should be able to use your 8.5/CR cast pistons also and might even be able to shim them or have the small end of the rod offset bushed and get near 8/1. The JE's are around $1k and will stand up to the heat of a turbo motor better.

It would be a shame to use a stock Inter-cooler. Go with a 3.2 Carrera manfold. You can prety much use an off the shelf IC with this and it dose not have to cost much but will offer a lot better performance.

Good luck.

Last edited by 911st; 10-27-2009 at 09:04 AM..
Old 10-27-2009, 08:59 AM
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