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MG TD TF 1500 - Desired Compression Ratio

Good day to all,
My engine is back from the machine shop. The block was sonic tested and +.100" pistons added. Head thickness is 2.985". Len Fanelli's fast street roller cam and lifters are added. I did not enlarge the valves. Carbs are original 1 1/4" rebuilt by Joe Curto. Rear end is 4.3:1
Machine shop and internet calculated C.R. is 8.1:1. I had originally intended to increase C.R. to between 8.5:1 and 9.0:1. I really didn't want to go above 9.0:1.

Machine shop rebuilt head, but did not do any more than skim the surface, hence the 8.1:1 C.R.

My question is, Should I send this back and have the additional milled from head to increase C.R. to under 9.0:1? Will this higher C.R. warrant the return to shop for re-milling? What is the expert advice here?

Thanks for any help you can give,

Jim TD 17744
JV Smith TD17744

No answer to your question but I'm surprised having the engine rebuilt you didn't hace the bores resleved back to standard.
JK Mazgaj

JK,
I certainly understand the desire to sleeve back to standard. My thought process was with a 4.3:1 rear end the standard engine is underpowered. Sleeving back to a medium overbore seemed costly to me. With the sonic testing of the block to check for sufficient block thickness for the +.100" overbore, I felt confident that the overbore would be safe. While the +.100" is beyond the original manual specs, it has been done many times without issue. I hope mine is the same.
Jim
JV Smith TD17744

Hi Jim,

Precise compression ratio computations (C.R.) for XPAG engines are prone to debate. It's the combustion chamber that causes most of the debates. There is an excellent article on the subject in the June, 2001 issue of the TSO, written by Hal Kramer. In it he derives a series of equations for the relationships of compression ratio as a function of XPAG and XPEG engine head thicknesses and bore diameters.

Plugging your engine parameter values (.060" overbore and head thickness of 2.985")into his equations indicate a compression ratio of about 8.3:1. A compression ratio of 9.00:1 with an overbore of .060" would be obtained with a head thickness of 2.943". That would mean the milling of another 0.042" from the head.

The Hal Kramer document is printed in the newest "T Series Handbook of the NEMGTR. BUT, I just discovered that there is a significant series of misprints in the Handbook's printing. Many of the C.R. formula calculations end up having to add 1 to the result of the mathematics (a common practice for C.R. calculations). Someone changed the plus sign (+) into a minus sign (-). What a shame.

Bud
Bud Krueger

Don't mill the head just to raise compression. Do it with pistons, or buy a head that has already been skimmed. These things aren't growing on trees anymore.

Your compression is fine for general driving, although you aren't taking full advantage of a fast road cam at 8.1:1. If you want more power, you can easily go to 9:1 or higher without any issue. I run 9.1:1 and I'm about to go slightly higher. Runs fine on 89 or 91 pump gas (a little better on 91). Since the engine is already built, I'd leave it alone and see how you like it.
Steve Simmons

Thanks Steve, Your advice seems to be shared by most others. I think I will just leave it as it is. I can always remove more later if I want, but I can't add it back.

Thanks for the help,
Jim
JV Smith TD17744

Jim, The figures that I did above are based upon an overbore of .100", not .060". Sorry, I was looking at calculations for my engine, .060" overbore and .100" off of head, for a thickness of 2.922". Bud
Bud Krueger

With variations in how far valves are pocketed back in the head, engine capacity etc, I would say the only sure way to determine CR is to measure the head chamber using a burette. Put a piece of flat, thick (6mm?) clear plastic over the upturned chamber. Plastic must have a hole (10mm?) in it. Seal the valves and the plastic surface with a thin smear of thick grease. Measure the amount of liquid it takes to fill the chamber through the hole. I use kerosene but water might be ok if you don't have trouble with air bubbles.

To find CR you need some maths. Add the capacity of one chamber, capacity of one cylinder capacity of gasket (4.5cc). If the pistons protrude above the block deck, measure how much and subtract that volume. When you have an answer, divide it by the sum of head chamber capacity plus gasket capacity minus piston protrusion volume. That will give you the CR.

While you are at it, measure each chamber and match their capacities if you want a smooth motor.

The best CR to run depends on the octane rating of the fuel you want to use.

Cheers,
Bob Schapel
Bob Schapel

This thread was discussed between 19/03/2022 and 20/03/2022

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