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MG MGB GT V8 Factory Originals Technical - increase rover compression ratio

Him guys,

I have a 3.9 rover with a with a 38d engine code. From what I can find the compression ratio is 8:13 to 1.
What is the highest ratio you would recomend on pump gas, and can I shave the heads with out causing too many problems. what are the pros and cons of steel gasket vs. composite? I am guessing I will have to buy different pistons to achieve a higher ratio.Being a efi does this have the larger chamber heads? I will quit.

Tanks Paul

Not sure I think 10.5 to 1 is decent any thing higher may be a little to radical for pump gas.
MG person


I just a great (long) reply for your and my pc crashed. Here's the short version...
Disclainer: the following information is correct to the best of my knowledge but the actual given values should be varified.

your heads are 37cc and the 28cc heads found on engines with higher ratios are 37cc's milled .040". You should be able to mill this much and not have valve interference.
Take these measurments to you machine shop and tell them what you want, the inturn should be able to tell you what you can have.
/Engine deck to edge of piston distance (at TDC)
/maximum valve lift from cam spec
/your heads
/and desired ratio
/gasket you are planning to use

On gaskets; there are generally two types, Metal/composite, or the thicker Felpro composite
the difference being about +/- .35 in your ratio

As far as ratio's and pump gas the general consensus
IMO is stay below 10:1 anything above this will not work on pump gas.

I plan on going with 9.75 with the thinner gasket, this way if I can't the the heads to seal I'll go with the thicker Felpro and still maintain around 9.4:1.

All this info is out there between the internet, D&D, and a good machine shop with experiance in high proformance work.

Good luck,


I find that 10.5:1 is as far as you can go IF--you run premium gas (91 octane)with a cast iron head and block.

Now enter that alluminum dissapates heat better, and is thus considered that 10.75-11:1 is attainable(depending on cam selection)..especially with a car as light as the MGB.
HOWEVER, it seems that the confines of the MGB compartment may offset this (heat) forcing one to stay at 10.5:1 as the safe maximum.


10.25:1 may be warranted with a VERY aggressive/high lift cam..strictly a strip cam and would not apply to most.

Best to ask your builder--be sure to state your vehicle weight and intended gas.

Incidentally, I have read alot of negatives about those steel gaskets primarily due to the poorly placed outer head bolts which they recommend not torquing down as much as the primary ones.

I have read several articles detailing what to do with those outer bolts.

Here is one...

This thread was discussed between 27/07/2004 and 28/07/2004

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