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MG MGB Technical - 1950cc pistons compresion?

I would first like to say hello As I'm new to these boards.
But I was wondering what the compression rating of the 1950cc pistons are for my 76 B I bought them but I think I will need to take about a millimeter of the top of the pistons. I know poeple in the UK run them or used to but you guys have way higher octane fuel then I do here in Canada (94 octane)
Thanks guys
Ross

I used Lotus TC pistons in my 1950. In order to get 9 to 1 comp I had to remove .100 from the block. This was caculated using actual piston height and combustion chamber volumn. Engine was an 1972 18V
gerry masterman

If you are trying to limit detonation, then you need to retain as much squish area as possible, and you must maximize that which you have. If you intend on modifying the pistons to reduce the compression ratio, it is advisable to do so in an area which is "open" to the combustion chamber, preferably on the exhaust side and also near the spark plug.

If you merely face or skim the entire piston top, and/or use a super-thick head gasket, the effectiveness of the squish area will be drastically reduced and it is very possible to end up lowering the compression ratio, while at the same time increasing the engine's tendency to detonate.

I would carefully mount the piston in a vice (preferably by the pin) in a milling machine, and use a ball end mill to cut a dish in the proper location as noted above. When you're done you can hand smooth the sharp edges with some 180 grid sand paper and you are ready to clean and install your pistons.

I'd recommend that you try to arrive at .040" cylinder head to piston clearance, (squish clearance) and always check for valve clearance prior to final assembly. Not all services available in all areas, prices subject to license, tax and docking fees, personal weight loss may be less than advertised, not a member FDIC....

Sean
Sean Brown

Well we (machine shop and myself) did the math and they are going to give me 10.5:1 compression and thats as high as you can go on 94 octane fuel so it works well i think
Ross

Bet you will regret that much compression! Good luck
gerry masterman

Ross-
I don't know what kind of fuel you can buy up there in Canada, but thanks to our benevolent federal government's EPA regulations, down here in the USA we've been standardized on this wonderful stuff called "oxygenated gasolene" since January 1st of 2000. It doesn't like to go over 9:1 in an MGB's antiquated kidney-shaped combustion chamber, even when using 93-94 octane fuel. If we go beyond that we start having problems with preignition. You'd better check on what's available up there. The 1.5 points difference in compression isn't going to make a whale of a difference in power output, so why risk it. Even if you can get fuel that doesn't tend to preignite at 10.5:1, you're going to have to watch the setting of your carburetion and ignition timing like a hawk.
Steve S.

I have an ignition retard system from MSD but I dont know if that will help at all not sure if our gas is oxygenated or not but I know all the street racers use petro-canada's 96 octane fuel.
Ross

Hi,

Here in Holland (Europe) they still sell 98 octane petrol. Nevertheless I had problems with pinking with my 10.5 compression in a MGB. After 2000 miles the engine was 'eating' the pistons. I was using a Aldon diss. specially made to avoid these problems. So I think 10.5 comp. is rather (too) high.

Peter
Peter

Hmm maybe I should deck the head but I have an igintion controll box I can change me timing at the turn of a knob but i feel it may be to high too
Ross

Ross-
Altering the ignition timing is treating the symptom, not the cause. You might reduce the tendency to preignite, but you'll reduce power output and the engine won't run as well, so what's the point? Use less compression. If you can get 96 octane fuel, then 9.5:1 should be OK if your cooling system can handle the heat.
Steve S.

Ross: what camshaft are you going to use? A hot cam will lower net effective compression ratio. Did your shop maybe figure this into their calcs? I wouldn't think that a competent engine builder would go for a straight 10.5:1 on a street engine unless you are planning on hanging around the local airport using airport transfers sevenoaks a lot.
Terry

I'm using a custom cam from a local can grinder here who does many cams for poeple over in europe becuase hes cheaper then kent and others, The place I'm getting the engine done is a racing shop and he assured me that it would run on pump gas and if not he would rebuild it so it would, I have all that on paper. Also I was woundering with the incressed cc's would I need to run a oil cooler now? I have a new custom Rad (was cheaper than stock!) also has anyone tried a heat exchanger to cool oil I may go that way its a little bit more than an oil cooler but I dont need to plumb lines to the fron of the car.

Thanks, Ross Robertson
Ross

Ross-
I consider an oil cooler to be importantfor any enhanced-performance engine. For the type of engine that you're planning, I'd consider it to be essential! If you do decide to mount one, install a F200 thermostatic bypass valve so the oil won't overcool by accident during those cold winters that you get up there.
Steve S.

Thats what I was thinking I know those cover are most likely crap and wont do much in cold weather,
Ross

Ross;

I am running a 1924 cc engine with 10.5:1 compression ratio, using CA premium unleaded and I have no problems with the engine. It starts fine, runs to 6,500 RPM and stops with no run-on. I have not had any detonation problems at all. It sounds like you are on the right track, but let me pass on what I did, just as an example that works.

I replaced the cam with a longer duration, higher lift, cam; basically a full road rally cam. It is a little bit lumpy at idle (1,000 RPM), but pulls like a train from 1,500 to 6,000 RPM with a flat torque curve.

The head was ported and polished and the combustion chambers were lightly polished, slightly reshaped, and equalized for volume. The valves and valve seats were 3 angle cut and I used Rimflow valves with the large intake valves.

I replaced the rocker arms with 1.55 roller rockers. I also replaced the rocker separation springs with machined aluminum spacers. The rocker support tube was tuftrided to increase wear longevity.

The exhaust was replaced with a PECO header and exhaust and the intake system was replaced with HS6 (next size up) SU carbs on a polished and ported manifold. The Weber 45 DCOE set up is fine too; I just happen to like SU's.

The engine was assembled with extreme care, including several dry assemblies to check for fit, deck clearance, etc.

I think the most important thing, when building one of these "2 liter" engines, is that moving the air in and out of the cylinders is absolutely critical and all components have to work together to maximize air flow. Like yourself, I had a race engine builder put together my engine and he paid a lot of attention to making sure that the air flow could be as smooth and as fast as possible.
Ron Kluwe

Great I was scared that I would have to lower the compression, My cams power band is alomst the same as yours 900rpm to 6000rpm cant wait to get her running!
Ross

Ron, and others

What is the actual cylinder compression pressure you read for your compression ratio? Mine concern happens to be a 1500 MGA engine that was rebuilt in a HS auto shop by the PO. No idea what was done, but it pulls nicely. Compression test data is about 185 psi on all cylinders, which is much higher than I have encountered on other MGs. Have been trying to get an idea of what has been done. It does run fine on California high test.

Thanks

Larry
Lawrence Hallanger

Ron-
The reworked combustion chambers of Peter Burgess' Fast Road head can safely handle 9.5:1 with ease. However, a stock combustion chamber has problems going over 9:1. From what you describe it sounds like you built a 1924cc engine the right way. The fact that you had your head reworked to a non-stock configuration may have a lot to do with with your ability to run 10.5:1 compression ratio. Of course, the piston crowns are part of the combustion chamber configuration too, so please share with us the name of their maker. The name and address of the shop that did the heads would also be valuable for a lot of people who have ambitions like yours.
Steve S.

So would the cam maker and specs.
Terry

If any of you americans want to know a canadian shop just tell me and I can give you there info it would be alot cheaper to get it done in cdn cash
Ross

This thread was discussed between 08/02/2002 and 16/02/2002

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