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MG Midget and Sprite Technical - Flat top pistons?

It looks like I will be rebuilding the 1971 1275 which is a 8.8:1 compression ratio engine.

Would it be worth installing 9.75:1 pistons to increase performance? The car is for highway and city driving and it would be nice to have more spirited performance.
Glenn Mallory

Glenn
See your camshaft thread as you will want to match the CR to your camshaft choice.
Alan
Alan Anstead

Thank you Alan. I agree that the components should be chosen carefully and would appreciate recommendations. 9.75:1 compression ratio and a mild road cam?

There are several reasons why it would be good to improve performance. First of all, the car is normally run at altitude, 5,200 Ft above sea level with frequent climbs over 10,000 Ft. This mandates a 3 to 5 spark advance. At sea level, performance has always been noticeably improved.

When the car was new, maximum speed limits were lower. Other vehicles generally didn't have as much acceleration as modern cars, especially some of the current model U.S. muscle cars that one must compete with here on city streets. In order to maneuver and to be safe, it is best to have more acceleration potential than was required in the early 1970s.

Speed limits in the southwestern U.S. are 75 MPH with most traffic moving between 80 and 85 MPH or more. The midget has a 5 speed (Rivergate) adapted which is great once 75 MPH is reached but does not allow any discretion to accelerate for a lane change or to overtake a slower vehicle. To drop back to a lower speed on a hill risks insults from the rest of the pack, just as being slow off the line has the other cars passing on the left and passing on the right.
Glenn Mallory

Glenn

Sounds like you are wanting a nice free breathing torquey engine? Others here would be able to advise on whether a 1293cc, 1330cc or 1380cc is what to go for, plus what dollar and legal restrictions are you under for modifications? MInispares shows the variety of available pistons (and maybe MiniMania in USA too?): https://www.minispares.com/search/classic/Pistons.aspx In the old days oversized MG Metro Pistons were a good budget choice IIRC but in piston selection had to make sure the rods were matching in terms of piston attachment method.

I wonder whether twin 1.5 inch on a suitable inlet manifold with a suitable exhaust would be helpful with the higher altitude, whereas twin 1.25 inch would have probably over wise been OK (on suitable manifold, with suitable exhaust, plus suitable carb needles and rolling road set up). Plus suitable modified head and carefully rebuilt bottom end with ARP bolts and centre main strap.

Do you have a suitable rolling road who understands SU carbs and A Series engines? Or can you persuade Peter Burgess to come for a holiday with a load of SU needles and can persuade a local rolling road to let him loose!

Also what diff do you have in? A five speed gearbox may help the higher speed running in the flat.

Guess you have looked at Daniel Stapleton's book on modifying Midgets and David Vizard's book on tuning the A Series engine for inspiration?

Over here, an 1.8 litre K Series engine swap with a Ford type 9 gearbox would be an option. I wonder how well the fuel injection could cope as well as the ECU be tweaked to deal with the altitude change. Then take it to Pike Peak hillclimb!

How are your rear axle halfshafts, cooling system and brakes?

Cheers
Mike
M Wood

Plus modify the distributor to match the camshaft
M Wood

Glenn
I suspect Bob Beaumont & I share a very close spec on our 1275 engines as he described on your camshaft thread.
1.5 twin S.U.
10.5 : 1 CR
Aldon Dizzy
I run an MED High Torque cam whereas Bob has chosen from Kent. I am running 1.5 roller rockers.

My Sebring Rep pulls well at high altitude as seen in the picture of a club run in the Swiss Alps.
Alan


Alan Anstead

This thread was discussed between 15/08/2020 and 17/08/2020

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