What compression ratio can you run on normal 98 pump fuel?
I typically run 11.5:1 with a street cam (RE83) on 98 with no problems. Is just over 12:1 a tad high with a similar street cam?

Engine is 1310, the head has a slight skim and the block has been decked a bit (piston protrudes about 5 thou)
Pistons are 6 cc dished.

Measuring volumes and doing the calcs gives me 12.1:1 which sounds about right to me.

Markings on the cam are a bit vauge but look like K?MD 276B. No idea what this is but was told a street cam??

So, is 12.1:1 too high for 98 pump fuel?

Its not really as simple as that...

I found 10.5 was too much for my engine with a Kent 286 cam...

Cheers, Ian

I have 10-1 compression, no idea what cam should i be running 98?

I have 11:1 compression ratio and use 98 but can get away with lower octane fuel. I think it depends s bit on the cam's characteristics (overlap) and of course what advance curve you are using. I didn't have a problem using Indonesian fuel either though I did carry octane booster if needed.

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67 Deluxe
Dry suspension
1293cc, 11:1 comp, RE 282 camshaft, 45 Weber, extractors etc
4 synchro close ratio gbox
Cooper S brakes, no booster
Stealth box, as Doc says "goes harder and idles!"
RH tank cos I needed it crossing the Nullarbor etc..

When I was building my Cooper S engine the people I was working with made it quite clear not to take the compression ratio over 10:1 as this would result in an engine which would not be pleasant to drive in traffic. This engine is running at + 0.060", ported and polished AEG163 head, 1.3 roller tipped rockers, Mini Spares fast road cam which is very similar to Russell RE13, LCB extractors and electronic ignition and while slightly lumpy, is quite driveable in traffic.
I am currently doing the engine for my 997 Cooper and the same criteria is being adopted.

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John Sneddon
0408 431 807
1963 Morris Cooper
1965 Morris 850
1968 Morris Cooper S

I am running 11.0:1 on 98 with an RE282 cam and 1.5 rockers. When I had a Camtech VP3 (619) cam in it and flat top pistons I found 12:1 was too high. So for a while I ran 2x BK450 head gaskets.
The more cam duration you have the higher you can go in C/R within limits. I think 12:1 is pushing it on a road car.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Thanks Guys,
I have done a bit more checking and come up with the same conclusions - 12:1 is too high.
I will aim for between 10 and 10.5.
Will probably do this by taking a bit (50 thou) of the top of the pistons (they currently protrude above the block by 5 thou).
This is once I check the pistons have enough meat, otherwise I will attack the head.

A engine, for road use, I have been working on has a calculated CR of over 11:1. Have purchased an 80 thou thick Cometic head gasket to get CR down to 9.5:1. Hope it's a solution, gasket wasn't cheap but cheaper than having head reworked.

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Clive G
1969 Mk II Cooper S
1970 MK II Cooper S
1963 Triumph Spitfire
1982 RX-7 Series 2

Hi cgo, thanks for your comment. The 2mm head gasket may be an option for me.

I will look at this if I can't get the pistons machined.

We're running 10.6:1 with a SW8 Swiftune Camshaft and No:13 advance curve on a CSI Distrubutor on 98 Octane with no detonation at all.