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MG MGA - Punched out 1500

Does any one know what a 1500 becomes with the various options of over borring the block? ie: 0.030, 0.040, etc. Any idea on what the over sizes should produce in terms of horsepower changes. How is the new displacement figured? (formula?)Any reference for this?

Thanks
Garland
GD Glenn

Garland

+0.010 gives a volume of 1502cc
+0.020 gives a volume of 1512cc
+0.030 gives a volume of 1523cc
+0.040 gives a volume of 1533cc
+0.050 gives a volume of 1544cc
+0.060 gives a volume of 1554cc

Calculation is piston surface area X stroke X number of cylinders.

In the case of the 0.10 oversize this gives 3.665cms (bore radius) squared X 3.1416 (pi) X 8.9cms (stroke) X 4 (number of cylinders).

I will have to mug up up on horse power unless someone else can enlighten us.

Steve
Steve Gyles

I thought the increase would have been a little more but am willing to go with Steve's arithmatic. :-))

To consider a power increase you could determine the percentage the extra cc are equal to and then multiply your present power by that percentage to get a ball park figure, certainly it would not be more except for one very important factor!! If you keep the same head volume then the C/R will be increased due to more swept volume. Now there are those of us who believe C/R is very important in increasing Horse power and naturally this increase in C/R will increase the horse power by more than the suggested amount so is normally worth doing.
Bob (robert) Midget Turbo

Bob

You have me worried now! I took the bore data out of the Workshop Manual Section A.25. and the stroke out of the general data section. I hope my calculator is not on the blink!

I followed and understood the logic of your 'horsepower' calculations. The only horsepower I could do on my calculator was the old RAC Horsepower for taxation purposes (see Wilkipedia). The MGA 1500 worked out at 13.2HP: Bore diameter in inches squared X number of cylinders, divided by 2.5. - Nothing whatsoever to do with real power.

Steve
Steve Gyles

No Steve I have just worked it out for myself and now definately agree, My instincts were optomistic..
Bob (robert) Midget Turbo

In stock form these engines produce about 0.8 HP per cubic inch. Increase of bore size may result in about 0.5 HP increase for each 0.010" increase of bore diameter. I would rather do minimal overbore and save the metal for a future re-bore. That might save the cost of sleeving next time around.
Barney Gaylord

So...looks like your not really gaining enough horsepower to warrant reboring beyond what it would take to clean the block up. Only 2-3 if go all the way to 0.060". If this is the case than Barney is right to not go anymore than required. Everyone concur?
GD Glenn

Agreed, the only thing I could think of that might alter the situation is some slight gain from the bigger bore unshrouding the valves a bit and resulting in some breathing improvement, but that would take successive dyno runs to prove or disprove. Perhaps one of the guys racing might have done that type of research, but for a street engine I doubt if you'd notice the difference.
Bill Young

This thread was discussed between 11/08/2008 and 12/08/2008

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