My engine block is at the machinist waiting to be bored out to 83.5 - that's when we can get pistons.
It looks like we may have to use pistons rated at 12:1 CR. This is too high for a road car that does the odd sprint and track day, so it looks like I will have to machine the pistons down to a realistic CR.

The quesion is - what will be the most sensible CR for my engine that has a QED420 head running on 99 octane fuel?

Hi. I had my engine rebuilt by QED, and one of the issues they had was that the cylinder head had been skimmed to the point that the compression ratio would have been too high. The old chap that rebuilt the engine machined some extra volume out of the cylinder head combustion chambers to bring the compression ratio down to 10.9:1. In the process he managed to improve the head and give me some more power. Anyway the point is that maybe it's not the pistons that need to be machined, rather the head that needs looking at. Don't know myself, just relaying my experience.

All the best.

Sean.

Thanks for the reply Sean, so do I assume 10.9:1 is OK for normal running?

The head is fully built-up and unused so I don't fancy doing any work on it. I checked the volume of each chamber at 35cc, so adding 5.5cc for a 1mm thick head gasket, I at least have a starting point for doing the math.

I am more than capable to remove material from the pistons (graduated hacksaw ) - I just have to decide how much for whatever CR I should have.

A little easier than a graduated hacksaw, is to measure it with a yardstick, mark it with chalk, and cut it with an ax.

Rob Walker
26-4669

Would it not be possible to use a thicker head gasket to reduce the compression ratio?
The Burton catalogue (page 38) lists a Cometic gasket for the Twincam, on page 37 it states that these gaskets can be supplied in 17 different thicknesses from .027 thro` to.140 inch.
Hope this can help.
Regards John.

It is part of my notes on this subject - ???Use 2 gaskets???.

I will e-mail Burton when I know the volume data, but I suspect an oddball Cometic gasket will be quite expensive. The standard one is £100 delivered.

But I still need to know what CR I should use.

35cc is small for a combustion chamber and it sounds like your head has lost some thickness over the years. The combustion chamber size for the standard sprint motor was 36cc to 37cc at a 4.60 inch head thickness which produced 10.3 comp ratio in 1558 cc

High compression forged pistons typically have an intruder on the top around 4 to 6 mm high - you may be able to get an acceptable compression ratio just using a thicker gasket depedning on the intrusion volume. I would personally not aim for more than 10.5 for a road car running premium unleaded.

The alternative is remove the intruder or buy flat topped pistons and use a Gasket closer to standard thickness.

cheers
Rohan

Thank's Rohan, I guess I've now got the response I was looking for.

The old BV head is 4.6" thick with a 36cc combusion chamber. The QED head is 4.52" thick with the depth an shape of the combustion chamber exactly the same as the BV head. The only obviouse difference is that the QED head has bigger valves that protrude more than the BV head.
The QED head has 1.625" inlet and 1.4" exhaust valves.

I shall have to wait to see what pistons are available and then do the calcs to target the 10.5:1 CR I had in mind.

The people at Cometic are very easy to work with. Twin Cam builders in the US work with them frequently on specials. I have seen Cometic email a local engine builder an editable CAD file to simplify the process of developing a special from a standard product offering. Cometic does most operations on CNC equipment, so setting up for a special is not a big process. They don't show pricing for specials on their web site but I am sure they can quote you for a thicker gasket without knowing the final thickness provided that you can use one of their standard thickness options. Cometic sells standard thickness head gaskets from their web store for $95 US and additional gasket sets for about $60 US. From this link select "Ford, Lotus, Cosworth" and then "Twin Cam". The also make gaskets for Coventry Climax FW series engines (I always like to throw something nostalgic in Mr. Pelly's direction.)

http://www.cometic.com/sportcompact.aspx

In the US we currently plan for 10.5:1 compression ratio for any normally aspirated Twin Cam for road use.

You're a wind up Merchant Russ; you certainly know which Strings to pull to get me going.
You wait; one of these fine days I'll be announcing that I'm driving an Elite again!
Who whispered, "in your dreams mate"?
Was that you Mark?

John

P.S. John Pelly Esq. if you may