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911nut 911nut is offline
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Join Date: May 2001
Location: Michigan
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Originally Posted by 911st View Post
Paul,
Good suggestion, on the Google. I am very much in the learning stage as to cams and will do so.
My biggest question is why should an SC cam improve off idle, pre-boost, or even boost onset on a 930 when a 930 cam seems better suited to be a better choice in those areas?
I think that the reason that the intake ports were made small is due to the low static compression. Small ports keep the mass velocity up at low rpm. In the case of the 930 engine it made the engine less sluggish off boost. Once the blower kicked in it didn't matter all that much and the car was fast.
The no overlap of the 930 cam was for the same reason; no reversion of exhaust gases which dilute the air/fuel charge and make for poor low speed response. Remember that a turbo engine has greater backpressure than a normally aspirated engine so reversion is a big problem. Smart tuners will raise static compression when running big ports and aggressive cams to try to get back some low speed efficiency, on both n/a and turbo engines.
Based on this, I don't think that a cam with overlap will improve off boost performance. The SC cam would improve mid range performance but will not be as good as a 930 cam at low engine speeds. By rights the static compression should be raised but now the price goes way up.
This all gets back to what characteristics we want out of our turbos; how we want them to drive. Some want the instant jolt much like a large displacement n/a engine. Others like the "light the JATO" feeling of big boost.
As for me, I've been kicking around what my car would feel like with 964 cams and a lightweight flywheel. Would better acceleration get through the "dead spot" of low rpm, off boost running? Or maybe the answer is stock cams and a Garrett GT35R. Good low speed performance and more compressor efficiency up top (the K27 is right up at the choke point at .9 bar)?
Ain't bench racing great?
__________________
Paul B.
'91 964 Turbo
Port matched, SC cams, K27/K29 turbo, Roush Performance custom headers w/Tial MV-S dual wastegates, Rarlyl8 muffler, LWFW, GT2 clutch & PP, BL wur, factory RS shifter, RS mounts, FVD timing mod, Big Reds, - 210 lb
Old 02-01-2010, 06:20 PM
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