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MG MGB GT V8 Factory Originals Technical - Pistons

I purchased a piston Federal Mogul #71P +.020. As soon as I saw them (dished piston) I contacted the local store from whence I had purchased them to inquire about the dished piston being a lower compression piston. I was assured that they would be the same compression, or slightly higher, than my current 10.5:1 ratio on my Rover 3.5 lt. So I installed them. Through this bulletin board I have been in touch with a fellow from eastern Canada. He too purchased the same pistons, but unlike me had a friend of his work out the compression ratio (CR) of the piston and came up with 8.3:1. Needless to say this got me a little worried. Worried enough that I sought out a formula for working out the compression ratio. I removed the head and using oil in a syringe to measure the volume of the cylinder and head, I came up with a ratio of 7.9:1. I contacted the store where I had purchased the pistons and he seems as if he will refund my money and pay the cost of having new pistons re-pressed onto the pins. However he was unable to offer me another piston.
Does any one know of a decent flat top piston for this rebuild? He also mentioned that with a high CR such as 10.5:1 that i would have to constantly add an octane additive. Does any one have any knowledge of this?

Bruce

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Bruce

I have no knowledge of the piston you refer too, but the scenario in the UK has followed a similar route where standard replacement pistons for the early 10.5 to 1 engines were supplied with a piston dish that reduced the compression to about 10 to 1. UK fuels being able to cope with this then and now.

My own engine also uses the SD1 head casting which has a very slightly larger chamber volume, which equates my engine to be about 9.8 to 1 in reality. This is with the metal shim head gaskets and not the composite which add to the unswept volume and thus reduce compression further.

I can only suggest that you may find some suitable pistons from a UK supplier, but knowing the usual difference in advised compression ratios between UK and North America I think that anything over 10 to 1 may be unwise. However on this point comment from someone with direct experience in your part of the world is far better than my speculation from here.

Also keep in mind that theoretical and actual compression ratios vary depending on the volumetric efficiency of the engine. In simple terms is you tune the engine by way of modified heads or a higher lift/duration cam then the actual compression ratio rises.

This is simply because the theoretical ratio is based on 100% volumetric efficiency and so a 100cc cylinder with a 10cc chamber gives a 10 to 1 ratio. If you have a volumetric efficiency of 50% then the real compression rationachieved with be 5 to 1. If you then improve the engine to 75% efficiency the actual compression ratio rises to 7.5 to 1.

The base line is that you want to keep a margin for safety and for any changes in the actual operating efficiency of the engine.

Rog

Roger Parker

Hi Bruce ,

I too have looked all over for higher CR pistons without any luck in the US. However, RPI in the UK can provide what you are looking for. Now the big BUT, what do you want to use it for. Street, stay with the lower compression pistons. You can run on our lower grade fuel without any worries. If you want the max power , then go for the 10-1s.

For what it's worth,

Kelly
Kelly

Thanks for the response guys. I still haven't figured out what to do.
I can get a set of pistons out of England 9.75:1 (+.020) for @381.00cdn/rings/wrist pins. or
I can get a set of pistons out of the states 9.60:1 (+.020) for @ 505.00cdn/rings/wrist pins or
I can get a set of pistons out of Canada 10.5:1 (+.040) for 200.00/rings/wrist pins + 100.00 to rebore.
The big question, what am I going to use the car for? Just to cruise around. I don't have any plans on using it at the drags, but I might occasionally. After reading the info from this bulletin board I am now concerned about getting too high a compression ratio. I don't want to have to fight this car through all the stop lights or worry about it overheating on the hi-way. Looks like the ones from England might be the best bet. Not too radical but definitely on the upper edge. Maybe I have just answered my own question. Input still more than welcome.

Bruce


Bruce

Hi Guys,

Just for the record. The SD1s in the North America and Oz had the dished 8.3 pistons as standard.

My engine also was a 3.5 litre SD1 with 8.3/1 compression. I plan to go up to 3.9 litre with ideally around 9-9.5/1. Anyone know of a good source of pistons at a good price?

Regards,

David
David

For Bruce,
There is a guy close to you, (Oregon, USA) that has good prices on Rover pistons in the lower CR.
Look up Yarwood Engineering. He makes a complete V8 change over kit too. I've bought a few things from him and he's a big help.

For David in Melbourne. RPI has a great deal on 3.9 pistons with rods for 200 pounds. Look in their overstock section.

Kelly

Kelly

This thread was discussed between 09/04/2000 and 02/05/2000

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