360 piston selection advice

Anonymous · Post by **Anonymous** » Sat Sep 18, 04 9:47 am

i'm intending to rebore my 360 and fit higher compression pistons. To the unwary (me!) there is a huge variety of pistons out there. Looking at the Hughes engine site a set of Keith Black hypercutical pistons giving approx 9.5 compression seems a reasonable choice but, are they for racers only?
Advice requested! I want a tractable street engine that will pull 3.23 gears and perform well.

Pete · Post by **Pete** » Sat Sep 18, 04 10:45 am

Careful. Depending on what material you select you have to bore the block to different tollerances......

There are Forged (considered the best), Hyper-wotsit (Considered the best value), and Cast (the cheapest). These all require different tollerances.

You can run up to 10:1 on the street OK and maybe a little higher, but I would call a halt at 10.5:1 personally......You can run higher with Alloy heads. Cam selection also plays a part (I believe) due to varing amounts of overlap, so you (once again) need to consider the whole package.

Enter Stage Right Mr Robson for the definitive statement....

Anonymous · Post by **Anonymous** » Sat Sep 18, 04 10:48 am

Thanks, Pete. The problem for us newer guys is that the yank sites tend to assume a large degree of prior knowledge which is why this site is so helful! I have a decent machine shop to use who should be able to take account of the diffeent tolerances required.

Dave-R · Post by **Dave-R** » Sat Sep 18, 04 11:13 am

The KB pistons are great for a street/strip car that will never see more than maybe a touch of NOS. If you want to use a big shot of NOS at some point go for forged pistons.

It is the pressure in your cylinders that makes the power. But compression ratio does not determin the pressure. If for some reason you have a valve open all the time you would not have any pressure no matter what the compression ratio was.

So cylinder pressure is as much dependant on valve timing and piston speed as it is on compression ratio.

In general the more duration your cam has the higher the static compression ratio needs to be to maintain cylinder pressure. So for a milder cam 9.5:1 is fine with iron heads. In fact i would not go lower than that in the UK. A hotter cam will need around 10:1.

Too much pressure and it will 'Pink'.

If cylinder pressure ends up slightly lower that is OK because you can use more advance timing. If it is slightly on the high side you can back the timing off to stop 'pinking'.

Aluminium heads cool the combustion gasses in the cylinder and pressure is lower. So with ally heads you need to run higher compression to make up the difference.

Anonymous · Post by **Anonymous** » Sat Sep 18, 04 11:18 am

Thanks Dave. the engine came with a recent edelbrock performer cam (mild, I believe) so i'll head for a 9.5 comp piston once i,ve checked the combustion chamber cc. 1976+heads, 66cc I think.

Dave-R · Post by **Dave-R** » Sat Sep 18, 04 12:24 pm

http://www.nhra.com/tech_specs/engine/b ... HRY-76.rtf

Jon · Post by **Jon** » Sat Sep 18, 04 12:26 pm

I've heard you shouldn't use NOS with hypereucalyptus pistons as they can shatter, which would be worse than melting a cast one
should be fine if you dont plan to use gas

Dave-R · Post by **Dave-R** » Sat Sep 18, 04 1:53 pm

They will take small shots of NOS but for anything serious you need forged.

Piston rings are a bigger worry with NOS than the pistons themselves. The top ring should have a NOS spec if you intend to use the stuff.

Anonymous · Post by **Anonymous** » Sat Sep 18, 04 3:26 pm

Thanks for the link to spec sheet Dave. I was interested to note that the comp rate is listed at 9.0. I thought 360 comp rates had dropped into the 8s at that time.
No intention of using NOS just yet guys!