compression ratios

BISH V8 · Post by **BISH V8** » Mon Jan 17, 2011 11:50 pm

Having spoke to john eales with regards my compression ration being 11.12 i was looking to get a slighter thicker composite gasket instead of the one purchased which is 0.046 thickness.I was surprised to hear that i wouldn't have pinking if i used the 99 oct fuel as he reckons his compresions can run up as high as 12.1,its only that ive had my block top hat and skimmed at chesman that the compression is higher than i wanted.
Any feedback with regards max compression,current project mgb roadster v8 4.6,stage 3 heads,238 cam,yella terra rockers,4 lt high compression pistons,Fully ballanced,waiting for my custom pushrods from america to finish engine build..

BISH

kiwicar · Post by **kiwicar** » Tue Jan 18, 2011 7:10 am

Hi
I presume that is the RS238 cam, 238 degrees at .05 lift? you should be fine the cam has alot of duration so the Dynamic compression will be muck lower than a standard cam.
Mike

CastleMGBV8 · Post by **CastleMGBV8** » Tue Jan 18, 2011 2:07 pm

Hi,

Are you Mark MGBloke's friend as your engine spec sounds familiar?

Putting thicker head gaskets on will reduce compression but also squish which you need for effective combustion

How did you arrive at 11.12/1 C/Ratio? 4.0 pistons? and was the block height decked/ reduced prior to tophatting?

As said the M238 cam has a fair bit of overlap so will reduce the dynamic CR (real compression) which is the important factor.

If you can confirm the static CR and the cam timing events then the dynamic CR can be calculated.

Kevin.

BISH V8 · Post by **BISH V8** » Tue Jan 18, 2011 2:20 pm

I am indeed Marks mate who has carbon copied the mgb,the only difference is i have his old rover stage 3 heads whilst he has moved to the Merlin Heads,my calulations for my compression are as follows
Bore 3.7''
Stroke 3.20''
Head Gasket thickness 0.046''
Deck Height 0.031''
Piston top Volumn 14.9cc
Combustion Chamber 27.2cc

As for the cam it is indeed real steels RS238

BISH

BISH V8 · Post by **BISH V8** » Tue Jan 18, 2011 2:26 pm

Forgot to mention it is indeed brand new 4 lt high compression pistons with machined cut outs done by chesman motorsport..
Also going back to my stripped head bolt threads,all of them finally torqued to 68 lb with steal helicoils in, heart beat was pounding though at final torque up

BISH

kiwicar · Post by **kiwicar** » Tue Jan 18, 2011 3:04 pm

Hi
I would be inclined to get the squish band tighter, at .077 total it is very wide, if you use tin gaskets then you should get it to a more reasonable .05 ish and it will probably up the CR to the 12.3 region, a bit higher than ideal for a road engine, but that cam makes me thing this is a primerily race engine in which case it should be quite managable. Really you need to work out the resulting dynamic compression from the cam card.
Best regards
Mike

ian.stewart · Post by **ian.stewart** » Tue Jan 18, 2011 4:19 pm

Its possible to run much more static comp, Im running 14:1 on the street with a silly overlap cam, on optimax, it seems to cope with it, without much complaint, however, I did have some minute det marks in the squish area, this may have happened before I set the engine up correctly.
I think full electronic injection is a must, and may be worth using knock sensors just to ensure you are not detonating.

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