Engine compression : Spitfire & GT6 Forum : The Triumph Experience (2024)

As a general reference what should the cylinder compression readings be? Someone told me ~120lbs, is that correct?

1980 TR7 Spider, summer driver
1973 Spitfire 1500, "Barney" w/overdrive, Triumph Color code #92, Magenta, summer driver.

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Burns,
That is a bit low generally speaking. The problem is there are a lot of variables: battery condition, engine compression ratio, camshaft, oil viscosity, temperature, open throttle VS closed throttle, accuracy of the gauge....... A guide is there should be less than 10% variation from highest to lowest.
All the best,
Paul

TRF# 10423

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I'd say a spitfire engine with 120psi would need rings.

'S all for now
Vic

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try it again with the throttles open and a good battery and warm if you can get it running. I would be looking for about 150-160

Douglas Hansen
New Parts; Engine Rebuilds; Sheet Metal work and Advice.
http://www.1147cc.com

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If my memory serves me right, the US version of the 1973 Spitfire engine had a compression ratio of 7:5 to 1. That would show around 110 lbs on a compression gauge, if accurate, so 120 seems alright to me. I might be wrong on the compression ratio though!

To calculate the compression ratio, try this formula: compression gauge reading divided by 14.7 (atmospheric pressure) i.e 140/ 14.7 = 9.5; 110 / 14.7 = 7.5.

As Paul M. mentioned, there should not be more than 10% difference between the cylinders, and many factors mean your readings may not be the same. I always take my readings with the throttle wide open and a warm engine, and I am lucky in using a gauge which seems quite accurate.

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Gauges very, as does valve overlap. So actual compression readings are not as important as how close they are to each other. The spread from the highest to the lowest should not be more than 10% of the highest number, as Paul says.

The actual formula for comparing pressures and CR is P = Po X CR^g where P is the achieved pressure, Po is the ambient pressure, CR is the Compression Ratio, and g (darn iPad) is gamma, or the specific heat of the gas, for air 1.4 may be used. The ^ means the CR is raised to the gamma power. In practice this doesn't work anyway because of the aforementioned valve timing and the following relationships.

Since CR is derived from volumes, not pressure, CR is V1 / V2. Pressure Ratio PR is P2 / P1. Assuming an adiabatic compression, and air being a perfect gas, we can define the change in volume and pressure as follows:

P1V1^g = P2V2^g implies P2 / P1 = (V1 / V2)^g.

Because, in reality, the specific heat will change with temperature and significant deviations from adiabatic behavior will occur, there is no good way to relate CR to the compression readings.

warmly,
Dave

Member Services:

dbraun99 LLC no longer offers Carburetor Bench Services on SU Carburetors. See www.dbraun99.com for helpful pictures of carburetor rebuilds.

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next thing you guys will say is use a slide rule to figure it out.

Douglas Hansen
New Parts; Engine Rebuilds; Sheet Metal work and Advice.
http://www.1147cc.com

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In reply to # 931649 by Dave Braun Gauges very, as does valve overlap. So actual compression readings are not as important as how close they are to each other. The spread from the highest to the lowest should not be more than 10% of the highest number, as Paul says.

The actual formula for comparing pressures and CR is P = Po X CR^g where P is the achieved pressure, Po is the ambient pressure, CR is the Compression Ratio, and g (darn iPad) is gamma, or the specific heat of the gas, for air 1.4 may be used. The ^ means the CR is raised to the gamma power. In practice this doesn't work anyway because of the aforementioned valve timing and the following relationships.

Since CR is derived from volumes, not pressure, CR is V1 / V2. Pressure Ratio PR is P2 / P1. Assuming an adiabatic compression, and air being a perfect gas, we can define the change in volume and pressure as follows:

P1V1^g = P2V2^g implies P2 / P1 = (V1 / V2)^g.

Because, in reality, the specific heat will change with temperature and significant deviations from adiabatic behavior will occur, there is no good way to relate CR to the compression readings.

warmly,
Dave

Just funny I was thinking that Dave must be an Engineer or a Physics Teacher or just really smart which is not all that bad.
Lost me when he started to explain the formula which is no surprise.
My rule of thumb is if the cylinder pressure is below 110 psi on all the cylinders and no one cylinder is 10% lower or higher that the others when I check it then I take it to someone who knows more about it than me to determine why and the best route to fix the problem. To my limited knowledge the cylinder pressure for a 1500cc Triumph US Spec should be between 125 psi and 165 psi when doing a wet test of the cylinders (tea spoon of oil in the cylinder be tested for lube and sealing).
Hope this helps.

Martin Keller
Ventura, CA.
Caretaker of a 1974 TR6

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Nope, not a physics teacher, although I considered that as a kid. I am an engineering consultant, mostly for engines.

The interesting thing is that higher compression will help an engine run more efficiently and develop more power (higher compression contributes to higher thermal efficiency, or more bang for the buck) but with gasoline, a limit is reached with the pre-ignition. Actually, engines can run quite well with low compression, and the loss in efficiency and power is sometimes hardly noticeable. Uneven compression is more noticeable, both from a fuel economy standard and a smoothness of the engine. You have to balance the cost of the rebuild with the savings in fuel, need for more power and the smoothness one desires.

Warmly,
dave

Member Services:

dbraun99 LLC no longer offers Carburetor Bench Services on SU Carburetors. See www.dbraun99.com for helpful pictures of carburetor rebuilds.

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" a wet test of the cylinders (tea spoon of oil in the cylinder be tested for lube and sealing"

Isn't adding some oil to the cylinder a quick test to determine is low compression is due to rings or some other issue, like head gasket or valves? The theory I heard was the oil will temporary seal the piston but not effect the top side. If the compression goes up after a shot of oil in the cylinder, you have bad rings, no impact and the air is leaking out the top via a valve or the head (bad gasket, warped head, etc.

mike h

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In reply to # 931798 by OFRacer " a wet test of the cylinders (tea spoon of oil in the cylinder be tested for lube and sealing"

Isn't adding some oil to the cylinder a quick test to determine is low compression is due to rings or some other issue, like head gasket or valves? The theory I heard was the oil will temporary seal the piston but not effect the top side. If the compression goes up after a shot of oil in the cylinder, you have bad rings, no impact and the air is leaking out the top via a valve or the head (bad gasket, warped head, etc.

mike h

That's right Mike. No need for oil if the compression test dry comes out OK. If dry compression is low then a wet test can sometimes tell you whether the trouble is rings or valves. Sometimes it is both. That is where the leak-down test helps, you can hear where the air is getting out.
All the best,
Paul

TRF# 10423

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