12G940 / CR

[Dan Tweed grey]

While Ive got my engine out, on my list is to swap my 940 cylinder head with 23cc chambers to one with 17.5cc.
This wil increse my swtatic cr to 11.5 ; 1 . Does enyone run this cr with out any problems? Also I have a MD286 so I reckon the vaves shut when the pistons are already on the way up the bore therfore the cr would be significantly less?
Any experiences welcome.
cheers

[Spider]

Depending on where you set your Cam Timing, a static figure of 11.5:1 maybe OK. You won't have an issue with Valves kissing the Pistons, unless you advance the Cam Timing to around 95 degrees or less.

Coming back to the CR, if you work to Dynamic numbers, you'd be looking for a (safe) figure of 8.4:1 for 98 Octane fuels. You can go a little higher than that, but be sure the fuel is always very fresh, though, I wouldn't be going over 8.6:1 for the street.

[Dan Tweed grey]

OK, thanks, I'll see how it goes and maybe run sone octane booster. I'm planning another set up on the rollers anyway to set up my twin su's I've just rebuilt/
At least I should be able to hear it pinking now I've ditched the sc drop gears.

[Rusty old S]

You can reduce the CR by fitting a commetic head gasket, look at the website they do 30 thou up to 100 thou with many in between . 10.5 Is good 11.1. Max on cast pistons

[timmy201]

My 1100 engine is currently running at 11.4:1

I’ve been working on it for a few years and I think I’ve finally got it to a decent stage:
Went a couple numbers colder on the spark plugs (NGK 7 or 8)
Changed the carb from being hard mounted to the manifold to adding a plastic isolator
Reviewed the timing curve and dwell using a programmable unit and finally got it somewhere I’m happy with it
Added some octane booster to the usual 98 fuel

Heat and timing seemed to be the most critical things causing irregular combustion on my engine

[Dan Tweed grey]

Thanks for the info chaps, I'm not really inteligent enough to work out the dynanic cr so I'll see how it goes on the rolling road and go from there.

[mk1]

I'm happy to cut to the chase.

In my experience eleven point anything to one is FAR to high for reliability on the road unless the car is set up to PERFECTION and a top quality octane booster is used. Even when it is, I'd never take a road car past about 10 : 1 max.

Why do you feel that increasing your CR to over 11:1 is worth thinking about on a road car?

[Spider]

Thanks for the info chaps, I'm not really inteligent enough to work out the dynanic cr so I'll see how it goes on the rolling road and go from there.

If you can work out Static CR numbers, then you can sure do DCR. There's many different ways of going about measuring the DCR, but the most accurate and I feel the easiest is with the the head off the engine, first turn it over by hand to TDC (measure that with a DTI) and measure how far down the bore the crown of the piston is (measure over a gudgon pin). Next, turn the engine over until the Inlet Valve is just closed (you'll be measuring for this off a push rod, so be sure to take in to account rocker clearance). Stop there and take another measurement how far the piston is down the bore. Subtract the first measurement from this one, then do a CR Calculation like you normally would, only for the Stroke value, insert this number you've just calculated from your measurements.

With DCR numbers, you want to be looking for the same number regardless of the combination of parts (8.4:1 on 98 RON).

With SCR, that number changes with the Camshaft. Basically, the later the Inlet closing angle, the higher the static CR is needed to get the same compression pressure.

[Dan Tweed grey]

Thankyou . Spider'

[Dan Tweed grey]

Head chambers are actually 21.4 cc, so drama over.
Static equates to 10.44;1. I did have a quick look at how much stroke was left after the inlet valve was close and to my supprise it's verry little.