Using 10.5:1 Comp Pistons in a 2.7
 

Hi we are re-building a 2.7 (74 model) for use as a "hot" street motor. It will have modern fuel injection and JE pistons with a suitable (not wild) cam profile. Question is has anyone had success (no detonation issues) with using 10.5 over 9.5 JE's and not twin plugging the heads? Costs prevent us from twin plugging.
Fuel with a min 98 RON is used as I am in Australia.

thanks for your help.

98 is (I think) equivalent to 93 in the US. I ran 9.5:1 with 911 S cams and carbs in my 2.7 and was a really nice engine. I ran it on both 98 and 95 without problems - you can run a tad more advance with 98.

10.5:1 would be a problem with 95 and marginal with 98 so I'm told. Really, it's a question of balance - unless you run big cams, big ports and aggressive timing I don't think you'll get much benefit from 10.5 vs 9.5 in a 'milder' engine anyway. 10.5 may also make it run hotter - never a good thing with mag case engines.

Mark

I wouldn't without dual plug. A MegaJolt jr. system can be done on the cheap.

2.7 heads
 

Curious as to the size of your head intakes. American 74 have cis and 32 mm intakes. not much for your macho hot rod? What size are you using? 32mm may cause you more detonation issues

JE pistons have couture issues (sharp edges) that benefit greatly from massaging the dome.
Buy the 10.5:1 massage the dome and manipulate the compression to just under 10:1 using appropriate base shims and you'll be fine given that you carefully watch your timing and head temps.

Use the S cam and larger port heads (35 or 36mm I forget). Larger cam will take the higher compression. I built one like this decades ago and was wonderful.

2.7
 

Thankyou all for the reply it gives me more to think about.
the heads are standard 911S 74 model, however are about to have a full re-build, is it worth modifying these during the re-build or look for others with larger ports?

cheers