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MG TD TF 1500 - Compression Ratio for supercharged XPAG engines

What is the maximum compression ratio that can be ran in XPAG engine with supercharger running 6 lbs boost at 5500 rpm.
J.W. Herbert

I don't know what the maximum would be. I suppose it would be determined by how close to the piston tops the valves came on the upoward strokes, or until the head thundered into orbit.

I run a Shorrock blown XPAG with about 8.9 CR and it goes very well. However, I had increased the CR prior to bolting on the blower.

Remember, one of the basic tenets about supercharging was to offer a quick and easy increase in performance on standard CR engines without having to take it apart.

Think about this. If you have a lower compression motor, that would give a greater volumetric capacity at TDC than a higher compression ratio head would allow. If you still blow at 6psi into a head which has a greater volumetric capacity than a shaved head, there would be a larger charge induced into the cylinders, resulting in a longer and therefore more powerful downstroke.
Geoff Love

The head is currently decked 1/8 in. and the cylinders have been bored to 60 over. According to my calculations this will give a 9.5 to 9.8 compression ratio. I am wondering if I should be installing a thicker head gasket to reduce this.
J.W. Herbert

I have a TD with a shorrock and the compression ratio is about 9.1-9.3. I don't think the compression ratio will make too much of a difference. unless you are racing. The biggest improvement will come with your choice of cam and the shorrock itself. Choose your metering pins carefully when you start up the engine for the first time with the blower. Too little
oil = expensive noises.
Cheers, Rob
Rob Silverman

Rob is right. The metering pin's diameter is all important. As he correctly says, too large a pin and too little oil, VERY expensive noises. Unfortunately the alternative also has a price, Too small a diameter, too much oil and very unpleasant remarks from the bystanders who, up until the moment of firing the beast up had been clustered around lost in awe and wonder.

However, too small a pin can be of help under certain conditions such as when requiring obscuration of one's licence plate when fleeing from an over enthusiastic policeman.

I hope J.W.Herbert has a sound crankshaft. With a 9.5 CR and possibly a blower added, the bottom end is going to be under a lot of stress.
Geoff Love


The maximum compression ratio (C:) you can probably run in a 6 psi blown T-motor is about 10:1. The limiting factor is the poor grade of fuel we currently enjoy. Around these parts, premium runs about 91-92 Ron/Mon octane. If your cylinder head has had lot of material removed to raise the C: you can lower it by stacking up a couple of the solid copper head gaskets available, I think Moss offers these.

"Carburation is nice, but I'd rather be blown!"

Terry Peddicord

I am rebuiding my XPAG to go vintage racing. Against the current wisdom, I am going to run with a supercharger. I am interested in any advice, particularly cam profile and ignition set up and timing. For the reasons outlined above I will keep the CR at 8:1 and strengthen the bottom end. I am using a Judson unit so boost will not be more than 7-8 psi.
Tony Cove

Tony, I recently sold a 1962 MGA MKII with a judson supercharger. The judson is a whole different animal than the shorrock. There were several different options for carbs on the judson. They originally came with a holley carb that was developed for a tractor and then used on several cars in the 50's. It actually is not a bad carb, it has an accellerator pump and everything. It is fairly bulletproof, but it is sensitive to fuel pressure increases above 5#, which tends to pop the seat, resulting in an interesting pool of gas under your car. You may be more interested in putting in a weber DGV if it will fit. With the huge choice of jets etc.. this would probably bettter suited for racing and the weber will give a better idle than the holley, (which is not saying much). Another thing to keep in mind is the possibility of getting rid of the SU fuel pump and switching to a slightly higher flow rate pump used in conjunction with an adjustable pressure regulator. On the MGA, at about 90-95 mph, you will overcome the ability of the SU pump to supply enough gas to get you going faster, so either a double ended pump or a higher rated pressure pump can be used. I doubt this will be a problem with the TD, since 75-80 mph should be the top-end,but it is something to keep in mind. Remember to set up the oiler with Marvel mystery oil or 10 wt whichever you can find easier. If you switch back and forth, remember to adjust the drip rate each time, since it will not stay the same due to viscosity differences. Unlike the shorrock that uses a metering pin to regulate oil flow to the supercharger, the judson uses a manually set drip rate that must be checked periodically since the screw will rotate with the vibration of the engine.
Good Luck,
Rob Silverman

I doubt this will be a problem with the TD, since 75-80 mph should be the top-end,

That's really anemic as the stock TD was timed to 77 mph. My stage II 1250 with a 4.875 rear axle ratio has seen and
honest 95.

I agree with the top end of 90-95 but I'm not sure if you could sustain that for more than a short time. With a 4.3 diff, you would be at about 5200 rpm and that is pretty close to redline. I will confess my ignorance about racing with a TD. I have had my TD up to about 80mph and it scared the hell out of me, but you racing folks have a higher testosterone level than me!! I have a shorrock and a 4.3 diff so I can actually drive up the hills of central NY and not be going 25mph, and it is nice to cruise at 65mph at about 3800rpm.
Rob Silverman

The 95 MPH was soon after the car saw a complete rebuild of the mechanicals, suspension, brakes, and tires. It was done with two on board, windscreen down and motorcycle helmets. I wouldn't do it now as the suspension rubbers could use a rebuild.

The stage II engine has a redline in the 6500+ RPM range (I shifted when the tach needle points to the clock!) due to lightened lifters, lightweight pushrods, alluminum spring retainers, and no oil shield cups.

This thread was discussed between 17/04/2003 and 04/05/2003

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