RS Motorsport :: Classic Speed Restored

Upgrading existing engine for mainly road use, and a few casual trackdays / year. Head hasn't been put together yet, but all is ready for assembly.

Block:
30thou overbore, std rods, std pistons which protrude 0.4mm above block. (block has been machined)
Std Sierra dizzy with elec ignition

Head:
Ported inj head with 44.5 / 38mm big valves (JEM - UK)
48cc combustion chambers after head planing
Newman Ph4 cam (0.474" lift, 275 intake duration)
Newman rockers
40mm twin webers with 33mm chokes.
Matched redline manifold

So this all works out to about a CR of 10.7 with a Felpro HG or 10.2 with a Durapro graphite HG.

So this is all great but when I run the dynamic CR numbers, I end up with a CR of about 8.6:1. This is mainly due to the short duration cam. This seems abit high for cast pistons and pump fuel? Would I be better getting forged pistons, advancing the cam timing, removing more head chamber material or is there a volumetric effiency factor I haven't considered yet and don't need to worry about all this?

Thanks in advance,
Avon

This is beyond my limited knowledge but hopefully someone will have an answer soon.

Hi,
I am not using the dynamic compression method but a 10.2:1 CR with that spec engine should be no problem.
You are helped by a relatively small choke/venturi-size which limits airflow above 6000/6500 rpm plus the fact that the std Ford electronic distributor starts retarding (less advance) the ignitiontiming above 5500 rpm (inherent to the design of the magnetic pickup).
If you have problems think you will need to retard the camtiming not advancing it.
Regards, Leon.

Thanks for that Leon

Have decided to stop the internet science and just start the engine!

Will post results up here soon.

I assume by your post above you've seen sites like this?

http://www.kb-silvolite.com/calc.php?action=comp

This is worth a read....

http://www.classicinlines.com/CompressionRatio.asp

Yes I saw those websites and thats what got me worried in the first place!

Went with the Durapro HG. Absolutely no problems, runs fine - but engine is completely untuned. Off to Dyno!

If you have the std (1.6) Sierra ignition with the 83BB module then run that without the vacuum-advance pipe connected.
Set it at 34 degr BTDC @ 5000 rpm to start.
Use 145 main jets, F11 tubes, 180 air and 50F9 idle jets.
Balance the carbs and it should go well.