Compression ratio advise

[derm.1785]

Hi. I’m about to start rebuilding my 3.0SC, I’ve already got some 98mm cylinders that are going for a re plate and hone , I’ll get the cams reground to suit whatever o need but I’m looking at new 98mm pistons, 70.4mm stroke and 22mm pins. I like the price of Wossner but their 11.1 CR is too high “possibly, Carrillo do a 10.1 and JE have several but are a bit more money. I’m UK based so import costs are steep but petrol octane is high so I can get away with higher CR. I would like some solid advise on piston choice and CR for my needs. Thanks in advance.

[derm.1785]

Anyone ?

[derm.1785]

I’ll be changing to PMO too.

[Dpmulvan]

Bumping up compression will cost you a twin plug set up.

[spuggy]

Originally Posted by derm.1785

I’m UK based so import costs are steep but petrol octane is high so I can get away with higher CR.

*cough* Euro and US use different systems to quote octane ratings.

The reason that European octane ratings seem higher than American ratings is because Europe, Japan and Australian only use the RON numbers. American and Canadian fuel uses an octane number derived from the average of the RON and the MON numbers. American fuel pumps have the designation R+M/2 to designation that the octane rating is RON plus MON divided by 2.

Simple math will show you that because of the 8 to 10 point difference of RON over MON, the octane rating shown on American fuel is approximately 5 points lower than the rating shown in Europe for the same fuel. In essence American 90-octane fuel would be the same as European 95 octane fuel.

For example https://aircooledbug.co.uk/usa-versus-uk-fuel-octane/

With precise fuel/ignition control (and sane values), instrumentation and a knock.sensor, you have more flexibility. My effective compression ratio when boost control is operational is too silly to quote or even think about. But no knock, On pump gas.

Carbs and a distributor? Consider your usage envelope & safety margins. Although hotter cams (more overlap) reduce effective C/R. And general wisdom seems to be that 97 mm bore size (eg stock 930) is right on the edge of "should have been twin-plugged from the factory".

You should probably talk to a good engine builder - or ask in the Engine Rebuilding forum, where the guys who've done it all before and know what works well/reliably hang out.

FWIW, I sourced JE from LNE with a set of their Nickies. I believe JE can/will make whatever you need if it's not already a stock thing. Although you should probably ask yourself "why not" if that happens....

[Tremelune]

Where's the ol' single plug compression limit on California 91 octane gas?

[derm.1785]

Twin plug is a direction that I’m not sure I want to go down in respect of £$, but I’d like to know if the higher end of 10/11 CR is daily driver/street or should I just go safe with 9.

[mikesarge]

I’d think twin plug is mandatory if you’re going much above 10:1, so that may need to be a consideration. Lots of info on that via search.

I went with 9.5:1 on my 3.1 top end rebuild. Single plug. It’s very civil to drive at cold start, and in traffic with a 964 cam grind on CIS and 93 octane (usa).

The induction is more a concern for the cam overlap, not the CR, so I’m not sure carbs give you any additional headroom for 10 or 11:1, if that’s what you were asking about.

[Focker]

I bought the PS98-014 Mahle piston and cylinder set.

Compression is 10.4:1.

Remember cam selection needs to go hand in hand with piston selection. I forgot about this and started down the road of a 964 cam, which would have resulted in lower power and more detonation.

There is static compression and dynamic compression. I went twin plug when my heads were being rebuilt as the difference in cost was inconsequential at that point. I wasn't originally intending to run twin plug right away, but I has already gone so far over budget, that adding a second coil pack and harness to the XDi wasn't too bad.

I know cost is always going to be a consideration, but you don't want to wind up with regrets. You may want to consider getting everything ready for a twin plug then backing off the timing to avoid detonation.

I think it's worth the money to discuss goals etc. with an engine builder/consultant to ensure you don't waste money on parts or having to redo work later on.

I used William Knight (knightrace on here) but I know Steve Weiner from Rennsport also offers this service.

Steve has some good articles as well.

https://rennsportsystems.com/technical-articles/

With respect to brand, I have used Carillo/CP and JE in racing motorcyles and had good results. Wiseco did not seem to have as good a handle on Quality Control as JE and CP.

The quality of the Mahle pistons so far seems excellent. Right out of the box, the difference in weight from the heaviest piston to the lightest was .7 of a gram.

Best of luck.

Ryan

[derm.1785]

Great info guys and the link is an interesting read. The cost of twin spark is probably too much for me, it’s a good 4k addition to what is already a big spend. It looks like I have a couple of options, 1, keep it at around standard CR and run some nice cams to suit, 2, take the CR to around 10.5 and run matched cams but tickle the ignition so I don’t get knock. I could throw the old cylinders back in and keep it 3.0 with a better piston choice ???? Mmmm still not sure.

[Focker]

My honest opinion would be talk to Steve or William or a knowledgeable builder. Tell them what you have, the condition of what you have, your budget and your goals.

One thing motorcycle racing taught me is that there are a lot of ways to make an engine/vehicle feel fast that isn't necessarily involving a high compression, big bore type motor.

There might even be a third direction that us arm chair amateurs haven't even considered yet.