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MG MGB Technical - Compression Ratio

Can anyone offer advice/experience on the best Compression Ratio to achieve maximum power for an 1840 cc engine running on a Fast Road Cam (285)?
Richard Tinkler

Richard-
It all depends on the details. With an unmodified cast iron head 9.5:1 is practical for street use, as is 10.5:1 for an alloy head. Combustion chamber modifications and playing with squish clearance can enable you to go higher, but the engine can become cantankerous, especially if you aren't familiar with what goes with what, and which combinations can actually work against you. Contact Master Tuner Peter Burgess to get the right details for your particular engine: peter@burgesstuning.free-online.co.uk
Steve S.

Richard,

I've an 1840cc like you, with HS6s and a fast road big valve head.

Running a 717 cam, I used 9.75:1 with no issues. I changes the cam to a Piper 285 and, with Peter's advice, upped the CR to 10.5:1 - really needs Optimax, but then it did before anyway.

Goes like a train, much better than before. Could be the CR or the cam or both. No issues; no pinking, no run-on, no overheating. Plug choice is only dilemma - BP6 get too hot and crack, BP7 foul in traffic.

Neil
Neil22

Richard,

Doug Jackson has done some research upon this and you can read about it on his page www.mgbmga.com

With the Piper 285 or the 285/2, you will loose some of the geometric cr.
I have tried 11.5 (flat top pistons) for my 1868ccm engine with modified carst iron head and it only works with 100 Octane fuel and the D43 dizzy.

I am planing to built another engine for another car soon. This will also be a 1840 and this time it will have a gcr of 10.25 and the 285/2 cam. It should work with 98 Octane then.

Hope this helps

Ralph
Ralph

Neil,

after trying different plugs, i ended up with BP7EVX and they work very good, although at prohabitive costs for a plug.

Ralph
Ralph

Ralph,

BP7EV and BP7EVX foul in traffic. They're about £7 each here, need to get them from a motorsport specialist or a motorcycle shop. And you can't clean them.

I use BP6ES to drive around, change to the 7s on the open road or on the odd occasion I actually make it to the track!

Neil
Neil22

I am currently running 10,7:1 with dished AE pistons. Engine runs fine on Optimax or similar. Problem is that the pistons don't last long and I have now had 3 with pieces breaking out between the rings. I am looking to go to 11.25:1 with forged pistons but do not know whether to go for flat top or dished. They will have to be specially made so would be interested in any opinions.
Richard Tinkler

Richard,

if there is material breaking away at the top of the pistons (down to the grove of the first ring) it is normally a sign of ignition problems.
With flat top pistons you have a better/larger squish area than with dished ones and do not need to skim down the head that much.
If it is not a problem to get the right fuel just everywhere, then an high CR is fine, when i travel to some areas in europe, it sometimes becomes difficult to find 100 octane fuel. So my next engine will be prepared to except 98 but with many modifications and lightening. I am also planing to get rit of the dizzy on this one and wil try Mat Kimmins version of the EDIS instead.
If you go for (carst) county flat top pistons, only very little grinding of the head will result in a 11.25 CR. Forged pistens are only necessary, i think, when the engine is often used at high revs in upper gears under hard load (racing and at high speed on the motorway). May be you should ask Cambridge Motorsports, if forged oens should be used.
There are some very useful information given in the Special Tuning Manual too. If you need a copy, go to Mike's lowcost page (www.7builder.com/index.html) and click MGB stuff there and you can download it for free. You will find a lot of usefull stuff in this
booklet!

Hope this helps

Ralph
Ralph

Ralph, thanks for the feedback. The engine in question is a race engine and gave some great results this year.
Richard Tinkler

Richard,
My rev limiter is set to 6500rpm. I don't run continuously at that (!) but I'm using 'standard' replacement pistons at +030" and haven't had any problems with the pistons in 10,000 miles.
I've got a hard copy of the Special Tuning Manual and it will not help you one tiny little bit... it's way out of date and even Stage 6 would be considered 'mild' these days.
Neil
Neil22

Richard,

if it is a race engine, do not go to compromises.
Take flat top forged pistons, lightened, and polished AND shotblasted con rod and a lightend crank and fly (if allowed by the rules of the class you race in)and talk to Aldon about a Special Tuning dizzy. They should be able to deliver it when you quote the data out of the ST Manual (curve, wights and springs), but it comes in a D43 housing then. On my engine it has proofed extremely relyable and was a surprising cheap solution too.
For selecting pistons, read on doug Jacksons page www.mgbmga.com what he practises on his engines.
I did not proof it just before writing this, but as far as i remember, there are JR pistons (from the UK) with surplus crown material used, that are custom blueprinted to gain the right hight for the desired CR. It is worth thinking about his information given on this page.
BTW, if you surf there, also have a look upon his report upon lifters an push rods too.

Hope this helps

Ralph
Ralph

Thanks everybody for the information. I attach a horror photo of a piston that suffered detonation as a result of a blocked water way around the no. 4 piston. This was an earlier GB block with which apperas to have had a faulty core when it was cast or the moten iron was too hard causing the core to fuse. So, always check the water jacket for depth and section if you are building a race engine, otherwise it gets very expensive. The engine ran fine on the road and on a rolling road but got too hot very quickly on the track. The piston is destroyed in seconds!

Richard Tinkler

This thread was discussed between 18/10/2007 and 28/10/2007

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