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998 Turbo Compression Ratio


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#1 dyshipfakta

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Posted 07 May 2021 - 02:51 PM

Hi all getting ready to put my 998 turbo together got most everything but I am in 2 minds about compression ratio.

I have an a+ 998 at 9.6-1 with standard head engine and gearbox are completely stock so my intention is to be quite conservative with regards to boost and power.

I have unskimmed and good condition 202 and 295 cylinder heads.
So my question is would the 202 and a slightly higher compression ratio of approx 9.2 be better then the 295 and a lower ratio probably more like 8.8.

These are rough estimates. I am not going for huge power 70-80 hp would be amazing. I will be using a smart car intercooler and fan with t2 turbo on mirage manifold and boost at 7-8 psi.

Any input appreciated

#2 Dannyboolahlah

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Posted 08 May 2021 - 09:47 AM

Mines supercharged with no intercooler, so had to go a bit lower, but I'm using a 202 with the chambers opened up a bit around the valves to get it down to 8.5, even for an intercooled turbo, I'd be a bit worried going for 9.2 on a standard engine to be honest.
Even without the extra compression, I'd still have thought the 295 would be better than a 202 anyway.

#3 dyshipfakta

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Posted 08 May 2021 - 11:09 AM

Perhaps the 295 is the right way to go then it actually runs ok nasp around town but is completely gutless after 2000 rpm which i presume is as a result of the low ratio.

#4 jd24-7

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Posted 22 May 2021 - 08:33 AM

If it's any help, I am running a 100%std 998 A+ from a late 80's mini. It runs boost from 2k and hits 10psi very easily. The ignition is running minimal advance as the boost increases and I have had only slight pinking. Engine is a temporary fit whilst I put the 1275 back together, but if I hadn't had the 1275 machined I would stick with the 998. No idea on power but bags of torque and good acceleration. It's 20 odd years since I ran the 1275 turbo, and the 998 feels equal to it. Maybe that's 20 years of modern performance and traction control!!!!!

#5 dyshipfakta

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Posted 22 May 2021 - 09:55 AM

So if you are pretty safe at 10psi on stock I should be fine with either lower ratio. Might just stick the 295 on can always swap it out later I guess. Thanks for the feedback

Edited by dyshipfakta, 22 May 2021 - 09:56 AM.





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