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MG MGB Technical - Compression Ratio

Can anyone suggest the optimum CR for a B GT running at +.060, on a fast road cam, gas flowed head and SU carbs?
R L Tinkler

With unmodified combustion chambers in a cast iron head, I'd suggest 9.5:1. If it's an alloy head with unmodified combustion chambers, I'd suggest 10.5:1.
Steve S.


if there is 100 octane fuel on the market and you go for the right distributer and needles, 10.5 gCR works with the ralley style head on the 1868ccm engine without problems (in my car).

If you can only get 98 octane fuel everything above 9,75 gCR becomes critical and Steve is right in his hint.

I checked it out a few years ago and i am lucy that we have 100 octane sold here (Shell V-Power).

In my car i installed the BL Special Tuning distributer (6 deg. static advance) and BDR needles.


+0.060", Kent 717, HS6s, Burgess Big Valve, Peco, Aldon distributor, CR 9.75:1

Pinks like hell on ordinary unleaded unless I put in octane booster. Works okay on Optimax (didn't have V Power here in the summer).

Fouls plugs (BP7EV) on Optimax, I think it runs cooler. No problem with Octane Booster. Eats BP6 plugs whatever fuel I use.

Neil Lock

I have a .060 over crossflow engine running 10.65:1 with no problems using 96 octane. I'm using Mike Pierce's X-Flow cam and dual 45DCOE's.
David Gable

I mean 93 octane. Still stuck in the '70's I guess.
David Gable

Last year we ran the engine at 10.6:1 on Shell V Power with octane booster and performance was pretty good. At Snetterton, we were lapping at 1.32, just 2 seconds off the pace for the class but with a car 60kg too heavy.
I have another MGB running a full race (320) cam with a CR of 12:1, with 1950cc and a single DCOE45 This has produced 170 bhp at the wheels on a rolling road but has never been raced, only track days. Engine runs perfectly on Shell Vpower (Optimax) without octane booster (Car is built To FIA spec except for the engine and is currently for sale c. 18,000)

R L Tinkler

The hotter the cam (longer duration valves open and the more overlap between exhaust closing and intake opening) the lower the effective compression ratio and the greater need for a higher geometric compression ratio.

A hot cam reduces cylinder pressure at low to mid rpms and NEEDS a higher compression ratio to perform.

Comparing effective compression ratios between a stock cam and a racing cam is not meaningful.

A stock engine at 9:1 with an iron head would probably require premium fuel.

An engine with an alloy head (it runs cooler) and a long duration cam could probably run on the same fuel at 11:1 compression ratio.

There are a couple web sites that include interactive calculators where you can input valve timing and geometric compression ratio and compute the "effective" compression ratio.

Barry Parkinson

Could you post some of those URLs?
Carmine Oblongata

Try this site:
Barry Parkinson


You may want to try running a colder plug to try to cut down in pinging
greg fast

Barry I/m not having a shot just trying to straighten out our thoughts on cams and compression.
We hear a lot of people that do know better, just have not thought enough, talking about overlap and comp. Overlap effects compression very little, a bit but not much. The effective compression is mostly controlled by the degrees after BDC that the inlet valve closes as the engine cannot produce compression until the all valves are shut. Overlap does not even take place on the compression stroke so its effect is minimal. I know when we look at a timing graph it looks like we have overlap top and bottom but the bottom ones are close to a rotation apart.
I agree that the silvolite sight is an excellent one and if you read the part about the old American muscle cars and how they got away with higher compression ratios than thought possible at the time on pump gas, you will see what I mean. Denis

You get the good filling due to the scavenge effect. This only happens when you "get onto the cam".
Stan Best

I think your comment is accurate. Part of the problem is that the distinctions between overlap and intake valve timing are often ignored. Part of the reason is that cams with substantial timing overlap, often have long duration intake valve timing.

I'm running a Piper 270/2 cam which has very late intake valve closing. Running .040 flat top hi deck pistons with a 5 port alloy head, I can run on midgrade fuel with little or no pinging. If I were running a more conventional cam, it would be a different story.

I'm installing a 280+ cam soon. According to my calculations, there will only be a relatively small decrease in effective compression ratio with the hotter cam even though the total valve open period is greater.

An offsett timing key or adjustable timing gear can allow you to adjust your cam to put the power band up or down and to reduce or increase effective compression ratio.

Barry Parkinson

This thread was discussed between 08/02/2007 and 19/02/2007

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