Pistons and compression

volospian · Post by **volospian** » Mon Mar 10, 2008 9:03 am

Just to double check... on a 3.5 engine it's the pistons that determine the compression ratio. The heads are the same, yes? (I have two sets of heads, but not sure of age yet, both 3 row head bolts, one set looks to be 3/4 pattern, the other 3/3, however, both, at first glance anyway, appear to have a similar size combustion chamber).

I have read somewhere that the HC pistons can be problematic. That you have to run it on high RON fuel, and even then retard the timing. Is this something that has been experienced, or just hearsay and assumption?

Also, does anybody know what affect the composite gasket has on final compression ratio? I'm assuming the figures of 10.5:1 are based upon the standard tin gasket. My guess is that it drops by around 0.5:1 but that's just a figure I plucked out of thin air, based on the rimmer figure of 3cc for tin and 10cc for composite.

PS. sorry for what probably sounds like the most basic of questions, but we all have to start somewhere

ppyvabw · Post by **ppyvabw** » Mon Mar 10, 2008 10:42 am

Normally the choice of piston determines the cr, but ofcourse you can alter the compression with headwork aswell, which is what I intend to do with my new 3.9.

Yes you need high ron fuel for high compressions. Generally you would advance the ignition I think.

Composite gaskets lower the compression by about 0.6

ChrisJC · Post by **ChrisJC** » Mon Mar 10, 2008 12:29 pm

There are two head types, the earlier type intended for use with the tin gasket, and the later type intended for use with the composite gasket. If you use the right gasket with the right head, then as you say compression ratio is entirely determined by piston bowl size.

Chris.

volospian · Post by **volospian** » Mon Mar 10, 2008 12:36 pm

ppyvabw wrote:Yes you need high ron fuel for high compressions. Generally you would advance the ignition I think.

Yes, I know you would need higher ron fuel for higher compressions, but the articles I have read have basically said that going HC is pointless because even using 97RON or better fuel, you still need to retard the ignition so much that the power output that could be gained by going HC is lost anyway.

For example, this is off the RPi website:

Please note ------ The reason the compression has to be lowered is that it would be almost impossible to avoid 'Pinking' (pre-ignition) on a 10.5-1 compression ratio.

However, that doesn't mention under what conditions this impossibility arises. Are they talking about using standard 95RON fuel, or is it impossible no matter what pump fuel you use. Seems a bit of a sweeping statement to me.

volospian · Post by **volospian** » Mon Mar 10, 2008 12:40 pm

ChrisJC wrote:There are two head types, the earlier type intended for use with the tin gasket, and the later type intended for use with the composite gasket. If you use the right gasket with the right head, then as you say compression ratio is entirely determined by piston bowl size.

Chris.

I'd assume the heads I have are all "earlier" types. I assume the later types are the series 2 4.0 plus 28cc heads?

Post by **katanaman** » Mon Mar 10, 2008 1:44 pm

Why not compromise and go for the 9.75:1 pistons which many recon to be the ultimate for road engines. All this is theoretical anyway as you dont state what cam your using. if you have a wild cam you can go beyond 11:1 on pump fuel and not have any problems. Its all just theoretical untill you actually sit down with actual measurements and work out the maths.

volospian · Post by **volospian** » Mon Mar 10, 2008 2:02 pm

katanaman wrote:Its all just theoretical untill you actually sit down with actual measurements and work out the maths.

Yeah, I know. I also need to decide if I'm going to go FI with it, or not as that will have a bearing on what I use. The reason I asked about 10.5 and low comps is that Ihave a set of both, but no "mid range" pistons. I was thinking I may go for the HC pistons with the composite gasket, getting around 9.9:1, or stick with the 8.13:1's for FI.

I do have a cam at the moment, however, it's used and I don't actually know what it is as I haven't really looked at it yet, but I'd be surprised if it's anything other than standard. It looks OK, but I've read about used cams and followers and excesive wear, so I may just go for a new set of both.

Post by **katanaman** » Mon Mar 10, 2008 2:28 pm

If that's what you have I would go for the HC and comps then the LC you will probably find a bit lacking.