For those of you running 78-79mm pistons what are you doing to keep compression ratio reasonable?

My spare race prepped head has 27cc combustion chambers and combined with a 79mm flat top piston and .8 gasket the compression ratio would be 14-1.

Are people using custom head gaskets (79mm gaskets are .8 & .6) , dished pistons or do you just run 110 octane fuel.

I have easy access to 100 octane and would like to keep the compression at 12-1 or slightly below. The A15 I just blew up was at 11.89 but unfortunately the head on that would need significant work to be fixed (10mm wide passage between combustion chambers) that head had 29cc chamber.

I ran 78.5mm flat-top pistons and it worked with mid-grade gasoline (pinged on regular). Had ported H89 cylinder head and multi-layer steel gasket. As I recall it was 11:1 or 11.5:1

The A14 I just built was 79mm with 22cc chambers.14.5 comp on E85 fuel.Only problem on the weekend it went bang rod out the sump
Did you find out what caused your A15 to go??Greg.

My ported H89 head had near-stock chambers (enlarged to 32cc)



78.5 pistons -- A14 = 1491cc



Gasket 0.020 inch thick (0.50 mm), pistons flush with deck



At that time I figured it to 10.9:1 compression ratio. I had to use mid-grade 89 octane gasoline. It would ping on regular 87 octane RON+MON/2 rating (equivalent to 90 octane RON rating used by some countries). Did not use high-octane pump gas

Dave what pistons are those? The cuts for the valves looked staggered I.E. not for an A-series engine.

On the subject of valve cuts; according to the Nissan prep manual for the Cherry the cuts have a volume of 1.8cc and that would bring the compression down a bit.

Brand unknown -- the pistons were in the A14 when I bought the truck

The valve reliefs measured at 3cc per cylinder

Dave they may have been custom.

Greg we haven't found a definitive cuase; obviously something caused it to overheat. The temp was fine on the warm up laps but after 3 it wasn't. It's also possible that my being glued to the bumper of the car in front coming off every corner didn't allow enough air through the rad. It's possible I was a step to lean on the jetting combined with the 38 degrees of timing I run caused detonation. It's also possible the gasket was on its way out. A couple of the head bolts were loose but that's likely from the motor melt down. Going forward I'll get a wide band air fuel gauge just to be safe.

Tom I have not had any time to see what went wrong with the new motor as I am putting the 1300 back together for our short circuit season in 2 weeks.Let you know when I do.Greg.