Compression ratio calculations and considerations

[Alan_Myers]

Hi all,

Following up on discussion that got started under another thread...

I'm trying to estimate compression ratio, but have a few "unknowns" making it pretty difficult.

Here's the situation... My TR4's head was milled .125" by the previous owner. In addition, I'll be installing 87mm liners and piston. I might need to rein in compression ratio to help the car run decently on currently available gas.

The displacement of a stock engine is 2138cc (534.5cc per cylinder), the bore is 86mm, stroke is 92mm and it's rated at 9:1 compression ratio originally, with stock head gasket.

Both 86mm and 87mm pistons are flat top (ISTR, some domed and dished pistons were tried in the 4-cylinder TRs in years past, but none seemed to improve performance). So, that's a non-issue. Also, I believe stroke will remain the same.

Here are the problems.... /ubbthreads/images/graemlins/confused.gif

- Don't know combustion chamber volume (i.e., compressed volume at TDC), with either 86mm or 87mm bore.
- Don't know thickness of a standard head gasket (compressed)
- Don't know thickness of optional head gaskets (compressed)
- Don't know deck-to-piston clearance.

Can anyone fill in some of these blanks?

What do you consider a "safe" compression ratio with today's premium, unleaded fuel?

The final point of this all is to make a decision about which head gasket to install:

There is the stock gasket, a shim steel gasket and various thickness of copper gaskets: 1.2mm, 1.6mm and 2.0mm.

Thanks in advance!

Alan Myers
San Jose, Calif.
'62 TR4 CT17602L

 

[Dave Russell]

Re: Compression ratio calculations and considerati

Hi Alan,
The basic formula for compression ratio is (clearance volume plus cylinder volume) divided by clearance volume.

Clearance volume is composed of:
1- combustion chamber volume
2- head gasket volume
3- any volume created by the piston top being below deck level.

Head gasket volume is calculated as actual gasket (hole diameter X hole diameter X .7854) x gasket thickness.

Piston below deck volume is calculated as actual (bore diameter x bore diameter X .7854) x distance that the piston top is below deck.

Cylinder volume is calculated as (bore diameter x bore diameter x .7854) x piston stroke.

The numbers from a guess at combustion chamber volume & assuming a 87 mm bore.
Assuming a combustion chamber volume of 60 cc:
Assuming a head gasket 8.8 cm bore x .15 cm thick:
Assuming a below deck distance of .05 cm:
Displacement = (8.7 x 8.7 x .7854) X 9.2 = 546.9 cc
Gasket volume = (8.8 X 8.8 X .7854) x .15 = 9.12 cc
Below deck volume = (8.7 x 8.7 x .7854) x .05 = 2.97 cc
Clearance volume = 60 + 9.12+2.97 = 72.10 cc
Compression ratio = (clearance volume + cylinder volume) / clearance volume
= (72.10 + 546.9) / 72.10 = 8.59 to 1

Since you really have no idea of present combustion chamber volume it should be measured. One way to measure chamber volume is to fit a small plexiglas plate over the combustion chamber. Drill a small hole in the plastic near one edge of the combustion chamber. Use a light layer of grease around the chamber to seal the plate. With valves installed & the plate in place, use a burette graduated in cc,s to fill the chamber with a thin liquid. Red kerosene works well. Fill the chamber as full as possible, the transparent cover allows you to see & remove any bubbles. Gage the amount of liquid dispensed from the burette. This will be the actual combustion chamber volume in cc's. This is known as cc ing the chamber.

If the ratio comes out too high, you can plug various gasket thicknesses into the formula to see what effect it would have. You could select lower top pistons. You could enlarge the combustion chambers & maybe also get some gas flow advantage. Some pistons could have the tops milled down as long as they don't end up less than about .200" thick.

Maybe you would prefer to take a guess & hope for the best. A safe compression ratio with todays gas would be 9 to one or possibly 9.5 to one.
Good luck,
D

 

[martx-5]

Re: Compression ratio calculations and considerati

Taking Daves formulas and assuming that you are starting out with a 9:1 compression ratio, we can figure out the combustion chamber volume as that is the only unknown. I came up with 66.87 cc for the 86mm bore. If we use that figure for the new calculation, the compression ratio will increase to 9.17:1. However, since I disregarded the below deck volume increase in my calculations, as we don't know for sure what the distance is, the actual figure will be slightly less then the 9.17:1 figure. Either way, whatever the compression ratio was when you first started, the increase will be small enough that I don't think you have to worry about it. If you were going from 83mm to 87mm bore as I am in my TR3 project, then there would be more concern on how much the head was milled.

 

[Dugger]

Re: Compression ratio calculations and considerati

It is a moot point, as long as the car has sufficient compression to effectively ignite and burn the fuel/air mixture. Static compression ratio never actually comes into play during motor operation anyway due to the duration of the cam lobes. Intake valves seldom if ever completely close at exact BDC and at higher rpms the engine relies on the inertial movement of fuel to load the cylinders on the compression stroke.

I really don't think that you will have a problem with the higher grades of gas, but if you do, sometimes a slight adjustment in the timing will allow for the use of todays fuels and eliminate octane knock. I know when my timing is correct when I load the TR in high gear and WOT at low speed, if she pings then I adjust the timing a bit.

 

[Dave Russell]

Re: Compression ratio calculations and considerati

[ QUOTE ]
Taking Daves formulas and assuming that you are starting out with a 9:1 compression ratio, we can figure out the combustion chamber volume as that is the only unknown. I came up with 66.87 cc for the 86mm bore.

[/ QUOTE ]
I don't think you can disregard the effect of the head already being milled .125" as he stated earlier. The combustion chamber volume is likely to be considerably less than original, like maybe 25 to 30 percent less. This is not a good place to be making assumptions or guesses.

I guess I'm saying that the original compression ratio, nominally 9:1, with the head now milled .125", may already be much higher. You can't assume that it is starting out at 9:1.

It isn't THAT hard to measure combustion chamber volume & be certain what it actually is. It isn't hard to find the proposed new piston compression height & compare it to the existing to find below deck volume changes.

Successful engine builds are based on knowing exactly what these specs are & adjusting as necessary. A few come out lucky without knowing, many don't.
D

 

[Dave Russell]

Re: Compression ratio calculations and considerati

[ QUOTE ]
It is a moot point, as long as the car has sufficient compression to effectively ignite and burn the fuel/air mixture. Static compression ratio never actually comes into play during motor operation anyway due to the duration of the cam lobes. Intake valves seldom if ever completely close at exact BDC and at higher rpms the engine relies on the inertial movement of fuel to load the cylinders on the compression stroke.


[/ QUOTE ]
Very true about intake valve closing vs design compression ratio. But at some rpm the charge WILL achieve enough inertial cylinder filling to approach it's actual design compression ratio. Higher rpm detonation is even more destructive & you cannot usually hear it.
D

 

[martx-5]

Re: Compression ratio calculations and considerati

[ QUOTE ]
[ QUOTE ]
Taking Daves formulas and assuming that you are starting out with a 9:1 compression ratio, we can figure out the combustion chamber volume as that is the only unknown. I came up with 66.87 cc for the 86mm bore.

[/ QUOTE ]
I don't think you can disregard the effect of the head already being milled .125" as he stated earlier. The combustion chamber volume is likely to be considerably less than original, like maybe 25 to 30 percent less. This is not a good place to be making assumptions or guesses.

I guess I'm saying that the original compression ratio, nominally 9:1, with the head now milled .125", may already be much higher. You can't assume that it is starting out at 9:1.

D

[/ QUOTE ]

That is true, but Alan was the one that stated his compression ratio was 9:1. Whether or not it is is anyone's guess. But, I was also under the assumption that this was a running engine with the head that was previously milled the .125", and that all he was doing was increasing the bore by 1mm. My point was to show that the 1mm increase would have a very small effect on compression ratio increase.

If, in fact Alan has just had the head milled, then there would be cause for concern and the proper steps as you have outlined would be the best way to proceed.

 

[Alan_Myers]

Re: Compression ratio calculations and considerati

Hi,

Just to clarify, I mentioned 9:1 as the original compression ratio for a 4-cylinder, 86mm bore TRactor motor.

However, I suspect my car's CR is higher, the previous owner *supposedly* had the head milled .125". Once it's disassembled I'll try to determine if this is accurate and will check the "squish" area volume, as suggested. The head might need some gas flowing work, anyway. And, I'm still shopping for valves. (Have brand new sets of OS stellite intake and exahust, but they are likely too large for the cam and certainly won't work if we install valve seats.)

Further, while it's apart I'll check the deck clearance of the new pistons. And, will measure head gasket thickness.

It was my understanding that two other performance items planned for the car will effectively reduce dynamic compression ratio. One is a longer duration cam (282 degrees, compared to 244 degrees stock. This assumes I currently have a stock cam in there, too. I don't know for certain.)

The other is the 87mm pistons and liners, which I had been led to believe would slightly reduce CR. However, our discussion on that other thread has me wondering if that's true.

Heck, I'm not 100% certain what pistons are in the car now. Pretty sure they are original, so most likely 86mm.

A lot of unanswered questions about this particular car, that will only become clear once the motor is taken apart. I'm just trying to get as many replacement parts on hand as possible before the real work starts. It would be nice to have a head gasket ready to install. But, I may end up scrambling for one after it's all apart, in case CR seems to be too high.

This car ran well, at 6000 ft. Colorado altitudes, with de-tuned Weber DCOEs (30mm chokes) and headers, and supposedly .125" milled head. Cam unknown, pistons unknown, head gasket unknown. I hope I can set it up to run as well now that the car and I are living close to sea level!

Cheers!

Alan Myers
San Jose, Calif.
'62 TR4 CT17602L

 

[TRTEL]

Re: Compression ratio calculations and considerati

Hi Allen,
I second the recommendation from Dave Russell on measuring your 'bathtub' with a burette. If you can't locate one call a drug or vet store, etc. and find one that sells large capacity (marked of course in cc's) syringes and have at it. The note of caution I have to dispense is that .125" is not an insignificant removal on a TR head. My old racecar has .145" off, but has had the surface stress pinned to keep it from flexing. .100" can usually be removed without problems, but over that you typically remove (depending on which model head 3A, 4, or 4A you have) that lip that sticks out around the intake. Usually people trim it back when it gets too thin for obvious reasons. But that becomes a source of volume error for the chamber if they were just eyeballing it. But it also exposes one surface of the stock copper, inert mineral, sandwich gasket on one side. One of the reasons performance engines usually have one of the shim type gaskets steel or copper. A stock head is about 3.330+or- from the valve cover surface to the head gasket side. I suspect when your done you'll find that with the stock headgasket your CR is going to be close to ten or slightly over with that head. But stick with the low/no advance and you'll be OK.
Tom Lains

 

[Dave Russell]

Re: Compression ratio calculations and considerati

[ QUOTE ]
Hi,

Just to clarify, I mentioned 9:1 as the original compression ratio for a 4-cylinder, 86mm bore TRactor motor.

However, I suspect my car's CR is higher, the previous owner *supposedly* had the head milled .125".

It was my understanding that two other performance items planned for the car will effectively reduce dynamic compression ratio. One is a longer duration cam (282 degrees, compared to 244 degrees stock. This assumes I currently have a stock cam in there, too. I don't know for certain.)

This car ran well, at 6000 ft. Colorado altitudes, with de-tuned Weber DCOEs (30mm chokes) and headers, and supposedly .125" milled head. Cam unknown, pistons unknown, head gasket unknown. I hope I can set it up to run as well now that the car and I are living close to sea level!

Cheers!
Alan Myers
San Jose, Calif.
'62 TR4 CT17602L

[/ QUOTE ]
Hi Alan,
It's true that later intake valve closing will reduce "dynamic" compression ratio. However, at some rpm & above, where inertial charge filling is effective, the dynamic compression ratio will approach the design ratio. This is why longer duration cams require higher rpm to be effective. Higher rpm detonation is just as destructive as low rpm pinging. Usually you can't hear detonation at higher rpm, but the problem is still there.

Since air density increases about 3.4% for each 1000 ft. drop in altitude, a drop from 6000 ft to sea level will increase effective compression ratio by about 20%. An engine that is running well with a CR of 11 to 1 at 6000 ft could well be beyond reasonable limits at lower altitude. All the more reason to actually measure & calculate everything.

Also, carb jetting needs to be about 20% richer at the lower altitude. Assuming it was correct previously. One or two steps colder spark plugs may well be needed also.
D

 

[Simon TR4a]

Re: Compression ratio calculations and considerati

Alan, your machine shop can and must cc the combustion chamber as we do not know how much material has already been removed from the squish area or by removing the shrouding from round the inlet, and to a lesser degree the exhaust valve.
Keep us posted as to how things work out!
Simon.
Also, the deck height of the pistons must be measured with the new pistons installed; I suspect you will have to increase the volume of the combustion chamber or skim the tops of the pistons to get a safe ratio.
Just for comparison, most of my driving is around 1,000 feet above sea level, using 94 octane Sunoco, and a 9.75:1 compression ratio with a 290 degree Kent Supersprint cam. I have an extra row of tubes in the radiator to keep cylinder head temps under control.

 

[Alan_Myers]

Re: Compression ratio calculation & considerations

Thanks to everyone for your excellent feedback!

It's clear that a number of factors will have to be carefully confirmed prior to making final decisions on the head gasket, any additional shaping of the head, etc.

However, this discussion has been very helpful giving me some ballpark figures to work with and ideas what to shoot for.

For example, assuming the info I've got regarding the motor is correct, CR is most likely around 10.5:1 or 11:1 right now. This was okay when driving at high altitudes, but is very likely too extreme now that the car will be operating near sea level, with today's commonly available fuels.

Assuming .125" has been milled from this head, it's quite likely a special head gasket will be needed, to pull CR back down to somewhere in the range between 9.5:1 and 10:1.

Alternatively the pistons might be faced, the shape and volume of the head/combustion chamber could be changed, etc. I'm leaning toward using the head gasket as the means of adjustment, simply because it's fairly easily reversed should I ever need to do so. (one possible source is https://www.headgasket.com/)

I've seen all kinds of specifications milling the TR head, ranging from .060" on up. For street performance, .060 to .090 appears to be the typical range. Beyond that we seem to be talking about progressively more tempermental "race car" setups.

Yes, I've recalculated the jetting of the Weber carbs, re-tuning to operate near sea level, plus taking into account the new cam and other recent modifications. This can only be an estimated setup initially, used to get the car started. Final carb tuning will need to be done with a dyno and/or exhaust gas analyzer. In fact, carb setup will be checked again and tweaked as needed, once the motor has some hours on it and is fully broken in.

Colder plugs are definitely an option, too, depending upon the engine performs once back on the road. For now it will have a dual point distributor and hotter coil, which gives some flexibility in terms of plugs. Eventually I plan to convert to a fully electronic, multi-spark ignition.

Yes, the head will need to be carefully cc'ed. That also needs to be done to confirm all the combustion chambers are equal.

Thanks again for all the input!

Alan Myers
San Jose, Calif.
'62 TR4 CT17602L