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911st 911st is offline
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Join Date: Jan 2004
Location: Sacramento
Posts: 7,269
Just a thought, it is a great idea to start with the cams but most of us are working withing some type of constraint.

For many that is the transmission. How strong it is and more importantly what the gear ratios are. Often someone puts high rpm/hp cams in a motor that makes more HP but narrows the power band. Then with shifts if falls out of the juice and the average HP per gear is not that much higher considering all the effort. Might not be that noticeable on the street but on the track it could be.

Starting by matching the cam / power band to the transmission might be something to consider.

If the motor is going behind a stock geared 930 4 speed a strong mid range and 5500rpm HP peak might not make the most peak HP but it might make for a much more drivable car on the track. If it goes behind a closer range N/A G50 5 speed it might be different.

If the pistons are cut down and attention is paid to the valve pockets, the stock 993 cam might be advanced some and work well. Then fitting a turbo to match a strong mid range would be a fit.

One can make a lot of HP but if it is for a track car and reliability is important, might want to reference the factory 993TT and RUF 993 conversion. Most variations ran about 8/1 compression with about .8 bar boost for 400-450hp. Thus a turbo that is in its efficiency sweet spot at 400-500hp or so might be something to think about.

Once you have your goals inline with what makes sense get with the best turbo supplier or Porsche builder you can and have them help you.

I have a lot to learn about turbos but have some thoughts. The GT 35 compressor is nice and at .8 bar is looks like it should work well in the 300-450hp range at .8 bar. Past that and it will starts to loose some efficiency. Going up to a GT40 or GT42 dose not seem to look like a fit or much improvement with making power in the .8 to 1 bar range. They look like they are designed to make power best in the 1.5 bar range.

For the track, there is power and some reliability to be found on the hot side. A small hot side on the street makes for a very drivable car but on the track it makes for increased back pressure and heat at the exhaust valves. A larger turbine might be good for 20-30hp and lower temps but will delay boost some. The GT 35 hot side is not much different than the small K27 turbos and on a 3.6 might hit .8 bar boost as early as 2500rpm. Great for the street bit probably not the best for a track car. I suspect a turbo that hits you boost target in the mid to late 3000's is going to be a better track turbo. Thus a hot side from one of the larger frame turbos like a T4 or the GT40's might be something to consider. The K27 turbos mostly use a T3 sized hot side.

I am not an expert but it might be worth looking at all your options.

I would do some research as there might be something that is a better fit depending on you goals.

Last edited by 911st; 02-06-2010 at 08:41 AM..
Old 02-06-2010, 08:25 AM
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