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MG TD TF 1500 - Theoretical CR Formula

Can any one post the theoretical compression ratio formula?

I'm trying to work out what mine would be at:

Bore size: 66.5mm +040"
Laystall head height at 75mm.
Combustion chamber volume: UNKNOWN Anybody have an idea on that for a Laystall? Barring that information a stock head is 45.5cc

.045" Head gasket volume: 4.5cc

I know this is figured out as a flat top piston.

I have a spec for a race piston that is a pop up design but the owner was not quite sure what his set up was and the compression tests were around 210 psi

I'm trying to get somewhere near 9.5:1 -10:1 I want to leave the Laystall at the current height and adjust the piston dome accordingly.

Any of you racers/ engineers, mathematicians out there give me a hand? TIA

Cheers

Bill Chasser JR
TD4834
W. A. Chasser Jr

Hi Bill. You have mail. Cheers
Peter TD 5801
P Hehir

Yikes that couldn't be right that would be 14:1cr. I'll have to recheck my notes on that one. but any help computing the formula as a flat top piston would be helpful

Forgot to add the stroke of 90mm

According to a stage one spec the stock head of 76.75mm is shaved 3/32" to a height of 74.37mm
CR 8.6:1

At stage 2 spec the stock head of 76.75 is shaved 1/8" to a height of 73.575
CR 9.3:1

I'm thinking the Laystall at 75mm and a pop up piston will put me in the 10:1 category. Yes I know I will be running premium fuel and maybe even Sunoco/76/VP race fuels or have to carry a can of tolulene everywhere I go. This is the direction I'm taking the car's performance.

The engine is in the machine shop now waiting only on the rods and pistons to do the final machining and balancing. Components of this build are...
Fanelli 236/236 full roller valve train kit
Saenz billet con rods
Arias Pistons
Laystall head
Steve Baker header with integrated intake
SuperTrapp exhaust
SU H-4 carb
Mallory Ign

Shooting for around 80-90hp on the road dyno when completed
W. A. Chasser Jr

Bill
My engine was built in October 1992 for me by Crankshaft Rebuilders in Melbourne.
Specs are:
Head thickness 75mm
Bore 70.5mm (1380cc)
Pistons JP Racing (NSW Aust)
Inlet Valves 37mm
Exhaust Valves 33mm
Camshaft TF1500

I believe the CR was computed to be 9.1:1 - not sure as to the piston configuration as the engine came to me assembled.
Peter TC9536

Peter Malkin

I think you'll find the Laystall heads had variations in thickness and combustion chamber volumes.
Nonetheless, you have one unknown variable and cannot calculate the ratio.
The machine shop could "cc" the head so you can get a respectable estimate.
I'd stick with a conventional flat top piston and then evaluate compression later. With all you have going for you, extra compression won't really add significant horsepower.
Think of it this way, if it is only around 9:1, it'll be ready for a blower!
JRN JIM

Bill, Hal Kramer wrote an article published in the June, 2001 issue of the TSO in which he presented formulae for computing compression ratios of our engines. A formula that he develops for the compression ratio (CR) of an XPAG engine considering Overbore (OB) and Head Thickness (HT), (all dimensions in inches) is:
CR=((2.618+OB)^2)/(2.193(HT-2.522))+1:1

A table in the document shows the combustion chamber volume to be about 43 cc for a head that has had about .070" removed.

Plugging in your parameters of HT-2.953" and OB=.040" would give a CR of about 8.47:1.

You can find Hal's paper on the NEMGTR TSO CD. IMHO, a great buy for $99. Bud
Bud Krueger

With cylinder bores increased by +0.040", each cylinder would be 322.34cc. The TF 1250 cylinder head depth was 75.16mm, with a combustion chamber capacity of 39.5cc, and I can find details of Laystall Lucas heads with a capacity of 39.5cc, although the head depth was not given. The compression ratio for the above combination would be (322.34+39.5+4.5)/(39.5+4.5) = 8.33:1
However, the Laystall Lucas heads were available with smaller combustion chamber capacities, so Jim, Michigan, is correct.
R A WILSON

Thanks everyone for your replies. I contacted the machine shop this morning to cc the chambers before they pulled it all apart. Waiting for a reply. Ordered my Phoenix Rods from B&G this morning and got it's specs for the wrist pin and rod width at the pin for piston clearance. Once I get the CC vol and can compute it all out as a flat top piston I will know where I'm at. The big variable beyond that is the area consumed by the pop up of the work order example I had made available to me.

Cheers

Bill Chasser Jr.
TD4834
W. A. Chasser Jr

So lets start over now that I have more information on my specs

Bore: 66mm +.040" =
2.618" +.040" = 2.658"

Cylinder Vol: 322.34cc

Head Thickness: 75mm = 2.952"

CC Vol: 37cc

Hd Gskt Thickness/Vol: .045" @ 4.5cc

Per Mr Wilson's formula with my numbers substituted
Cyl vol CC HGv CR
(322.34 + 37 + 4.5)/(37 + 4.5) = 8.78:1

Bud, could you so kindly plug in my values with reference to my 37cc chambers and give me the value based on the formula you supplied? I'm curious what the differential might be vs above? either way it looks like the pop up will likely put me near the 9.5:1 - 10:1 figure I'm looking for.

Lastly will be to figure the area of the pop up design to figure what will need to be shaved if any to reach my goal

Is there any thing else I'm missing here? I'm going to try to order pistons tomorrow from Arias. My Phoenix billet rods are on their way from B&G today, Steve Baker is bending tubes now, My Laystall is a mess and needs a work, and I need Len Fanelli to contact me...

Thanks for all your help!

Cheers

Bill Chasser Jr
TD4834





W. A. Chasser Jr

doublecheck 66mm; try 2.598 and readjust your calc.
I came up with 8.64:1

"My Laystall is a mess and needs a work" If your head needs some TLC, you might want to make sure it doesn't require milling before you order pistons.

One of my next projects is to mill an XPAG block that is .005" low right between cyls 2 & 3. It burned out head gaskets every few thousand miles.
JRN JIM

JRN, my mistake, the bore is listed as 66.5mm in the WSM and my calcs are based on that number. Reference post #1
W. A. Chasser Jr

I didn't recall stock XPAG bore, as I've been devoting my time to a 72mm XPAG. At this time, I'm boring the head valve spring seat areas to accept the oversize valve springs. Out of curiosity, what approach are you taking for valve springs with the Laystall? Did they come with higher pressure springs?
JRN JIM

JRN contact me directly via Email salsburyrocketman at yahoo dot com
W. A. Chasser Jr

Alright now I have to ask about CR in relation to and compression check reading. As an example Lawries stated compression checks on his engine come out to 215 psi. What does that actually equate to in real terms. I had used the theory that if one Atmosphere at sea level is 14.7 psi then

215psi/14.7psia + 14,2cr

Lawrie said it doesn't work out that way and that the compression ratio is much less. How about it mathematicians and engineers?

What are your thoughts?

Regards

Bill Chasser Jr
TD4834
W. A. Chasser Jr

Boyles Law applies.
Dave H
Dave Hill

Compression ratio is dimensionless. It's merely the ratio of two volumes, the volume of the closed cylinder with the piston at the top of its stroke to the volume of the cylinder with the piston at the bottom of its stroke.

BTW, Bill, the parameters in Hal Kramer's formulae are only applicable to OEM XPAG engines. Bud
Bud Krueger

>Boyles Law applies.
>Dave H

Not quite true.
when I read this I said to myself "Under adiabatic conditions"
(i.e. no heat lost)
The I thought more about it. It would also need to be isothermal. (Constant temperature )

Since Boyles law is P1V1 = P2V2. There is no mention of temperature, so temperature is assumed to be constant.

But in compressing the gas(es) work is done. This adds energy to the closed system and something must happen. The temperature of the gas increases.

Now since (P1V1)/T1 = (P2V2)/T2 then V1/V2 = (P1/P2)T1/T2).

(This derives from the universal gas law PV/T = constant)

You can also see that this is how the diesel engine works and it's what causes pre-ignition (knocking).

The reason that the pressure do not relate directly to the compression ratio V1/V2 is that the temperature of the gas, as well as the pressure) when the piston is down is lower than the temperature (and pressure) of the gas at compression.

To calculate this you would either need to know the work done and the mass of the gas (and assume an adiabatic compression)or delve deeper into the physics.

It been many years since I did this, although I have a degree in Physics, so I did a search and I find that one could expect.

T2/T1 = (V1/V2)^(k-1) Where K 1s around 1.4 for Air. (Of course we may not have air but a mixture of air an petrol unless the carbs are removed.

k has a physical value but by now you probably do not care.

I think the last time I looked at this was when my daughter was taking chemistry and she is 51 now.

Short summery.

"Its complicated)

Jim B.
JA Benjamin

Nice answer! It is complicated. Many factors are at play here. In brief the temp is not constant, it is elevated, from work done, so the measured pressure will be higher than expected. The quality of the seal efficiency also has an effect, which is what the compression test is really measuring.
Dave H.
Dave Hill

This thread was discussed between 09/02/2015 and 12/02/2015

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