13 replies to this topic

[wassupcrew]

I have a non cooper spi. I am getting the head ported and would like to know if anyone could tell me how much I need to skim off to increase the compression ratios from 9.4 to 10.5 to 1. I am only going to quickly clean up the chambers so I would assume standard chamber size. Thanks all

[Tones61]

10 thou off head equals minus 1cc approx in chamber

Edited by Tones61, 06 October 2020 - 07:38 PM.

[wassupcrew]

Thats great thanks

[Cooperman]

I don't think that is accurate.

10 thou down the bore equals one cc. The chambers are not the same cross-sectional area as the bores, thus 10 thou off the head will be less than one cc.

The only way to accurately find the CR is to measure the head volume and the distance the piston top is down from deck level at TDC. Allow 4 cc for gasket volume.

Then calculate what you have, decide on the CR you want and use the correct equation, which you will find on here, to establish the chamber volume you need. 

Then you can put that volume of fluid into the chambers with the head upturned and horizontal and measure how much needs to be removed.

Doing it any other way is guesswork.

Of course. many do use guesswork and simply skim 25 thou off on the basis that this will increase the CR and this improve performance. The old workshop manual says that a maximum of 25 thou may be skimmed as a service repair and that is what has been done many times to ensure a head is flat.

[Spider]

Not to nose in where it may not be wanted, however, if you are running a stock cam or a very tame one, I'd express some caution raising the CR to 10.5:1. The Dynamic CR could easily be too high.

 

Special Tuning give a figure of 0.012" for 1cc on the 1275 heads, however, as Cooperman suggests, it would be wisest to measure.

[dotmatrix]

I made a 998 engine with kent 266 cam and 11:1 in compression and it was pinging. I took it down to 10:1 by removing material from the combustion chambers and it almost cleared the problem, but still has a bit of pinging at low revs+high load. So based on this I think I would keep your compression where it is, at 9.4:1

[ACDodd]

Leave your compression where it is. The engine will perform very well on the 95RON fuel we have in the UK. One of the biggest failures people in the UK still do is set the compression too high for the fuel available.

Ac

[MiNiKiN]

Leave your compression where it is. The engine will perform very well on the 95RON fuel we have in the UK. One of the biggest failures people in the UK still do is set the compression too high for the fuel available.

Ac

He might go to the local airports to re-fill

 

We in Austria seem to be lucky - there is sufficient supply with 98, or even 100RON fuel down here. You just need a capable wallet...

[Midas Mk1]

My daily needs 98 or above, less and it will det. Its just something you get used to, I wouldnt worry about it.

Millers do octane booster to keep in the boot just incase. Only time ive not been able to get 98 was in Italy on the way to the Stelvio. 

[gazza82]

Super unleaded in UK ranges from 97-99 .. so that should be OK.

 

95 RON (normal unleaded) is targeted to get 10% ethanol soon .. most petrol companies have agreed to supply 5% as well for a while to cater for owners of older vehicles that would struggle on 10% (or rubber parts that are constantly being eaten by the ethanol but that might only be in unleaded form!).

 

The FBHVC has done research into octane boosters, etc.  https://www.fbhvc.co.uk/fuels

 

 

[Spider]

 

Leave your compression where it is. The engine will perform very well on the 95RON fuel we have in the UK. One of the biggest failures people in the UK still do is set the compression too high for the fuel available.

Ac

 

He might go to the local airports to re-fill

 

We in Austria seem to be lucky - there is sufficient supply with 98, or even 100RON fuel down here. You just need a capable wallet...

 

Our 98 here in Australia, was really good, while we were refining and blending locally, it was guaranteed to hold it's Octane rating for 8 weeks from delivery (to the Station), however, since the closure of our refineries it's now only guaranteed for 4 weeks. Compounding this further is the disproportionate high price of it, most retailers have had a marked downturn in the sales of it so often what you are actually putting in your tank they have already been sitting on for 3 - 4 weeks.

 

I used to be able to fill up and find up to 3 months later, it still had it's zing. Now, I'm lucky to get 2 weeks before noticing pinging, and often only a week.

 

As I build engines these days, instead of setting them up for 98, I'm now doing them for 95.

 

There are of course Octane Boosters about, but if you look in to just how much they boost the RON by, it's very little, a few only lifting it by less than 2 points and most just over 1 point. It's very expensive too.
 

[Tones61]

Tesco super green is 99

[wassupcrew]

thanks to all your comments. I think i am going to follow ac dodds advice and just leave it as is with a minimal skim. I am going for a sw5i cam so it should perform alot better than it is.

[Cooperman]

My 1964 full-spec rally Cooper 'S' has 11:1 CR. I use 98 RON with some octane booster and it is fine. My dizzy is a custom Aldon as an Aldon 'Red' did not have the right advance curve. It has never been any problem.

I also have a 1990 Cooper 1275 road-rally car which has 10.3:1 and I find that with 98 RON it is OK. It has a standard MG Metro cam and an Aldon 'Yellow' dizzy. Head is mildly flowed and it has 21253 pistons at +0.060" bore (1330 cc).

I always recall what David Vizard says which is that the old A-Series does like a high compression. I do take the point about using 95 RON petrol and, yes, with a CR over about 9.75:1 a better petrol is needed, which can be achieved by using the easily available 98 RON with, if over about 10.8:1, an octane booster such as Valvemaster-Plus.

I am old enough to recall the 1960's when we could get 'Super Shell with ICA' which was a 101 RON petrol. We also used to modify the distributors as shown by Clive Trickey in his 'little yellow book' called 'How to Modify Your Mini'. These days it seems very basic, but back then it worked.