Calculating compression ratio...

[Batwing-8]

Does anyone know...........

When auto manufacturers during the '30s through the '50s calculated gas engine compression ratios (you know, the 5.5/1 or 6.0/1 values printed in owner's manuals and shop manuals) did they include head gasket volume in the calculation; or was the value taken only from combustion chamber and cylinder volumes? I ask because some of the earlier motors used fairly thick asbestos gaskets that if included, or left out of the compression ratio calculation, could make a meaningful difference in the compression ratio number.

 

I also ask because in a stock motor on which I'm working, including the head gasket volume along with the cylinder and combustion chamber volumes, calculates to a half-point below the compression ratio noted in printed text.

 

Thanks in advance for any wisdom here! Bill.

 

[Rusty_OToole]

The quoted compression ratio from the manual, was often rather higher than actual on a new motor. They did this knowing carbon buildup would reduce combustion chamber volume, and wishing their customers to err on the side of too high octane rather than too low. Since the latter could have serious effects on performance and engine life while the former only wastes a little money.

 

One clue that the published compression figures are approximate, is that they always happen to be an even number. Always 7:1 or 6.5:1 never 6.792:1.

 

While we are on the subject don't be surprised if your car weighs more than the published shipping weight and the speedo reads higher than your actual speed. Or that a substantial part of your horsepower is a figment of the advertising department's imagination.

Edited August 2, 2018 by Rusty_OToole (see edit history)

[playswithbrass]

Batwing the more important factor is the valve timing.Compression ratio is the volume of the cylinder at bottom dead center to top dead center then squeezed into the combustion chamber.Right?   But what about the valve timing are both valves closed when the piston starts upward on the compression stroke?  No the intake valve depending on the engine will be still open so you must subtract the amount the piston moves upward from your stroke length.Example a 4 inch stroke of piston movement may only be 3. 3/4 of piston 

movement with both valves closed. This is tricky and difficult to find understand at first but a good head scratcher. Cheers

[W_Higgins]

15 minutes ago, Rusty_OToole said:

The quoted compression ratio from the manual, was often rather higher than actual on a new motor. They did this knowing carbon buildup would reduce combustion chamber volume, and wishing their customers to err on the side of too high octane rather than too low.

 

Interesting.  Can you document this practice on the part of the manufacturers?

[Rusty_OToole]

 I have a secret memo from Henry Ford to Walter Chrysler, dated 1927, in which they agree to phony up their compression ratio, shipping weight and speedometers but if I show it to you I will have to kill you. (Joke)

 

What I know is based on 50 years of working on cars and reading about them. This phenomenon of published compression ratios being on the high side is well known. In fact if you take a piece of plexiglass and a burrette and check the volume of each individual combustion chamber on a  cylinder head you may well find that each cylinder has a slightly different compression ratio to the next on the same motor. In other words these published figures are approximations, and always rounded off on the high side.

 

If you know anyone who builds racing motors or blueprints engines you can ask them. They do these measurements all the time.

 

 

Edited August 2, 2018 by Rusty_OToole (see edit history)

[W_Higgins]

3 minutes ago, Rusty_OToole said:

How in the hell am I supposed to do that? Do you think I have some secret memo from Alfred Sloan to Walter Chrysler, dated 1927, in which they agree to phony up their compression ratio, shipping weight and speedometers? What I know is based on 50 years of working on cars and reading about them. This phenomenon of published compression ratios being on the high side is well known. In fact if you take a piece of plexiglass and a burrette and check the volume of each individual combustion chamber on a  cylinder head you may well find that each cylinder has a slightly different compression ratio to the next on the same motor. In other words these published figures are approximations, and always rounded off on the high side.

 

You list a total of 5 collector cars from 1911 to 1960. I suggest you take all their engines apart, measure the combustion chambers and see if I am right. If every combustion chamber is exactly the same on every engine, and none of the compression ratios exaggerated, you can come back and call me a jackass.

 

I don't know how you're supposed to know that.  You're the one who made a statement of fact regarding manufacturer practice.  That's why I asked you and not somebody else. 

[Rusty_OToole]

Mr Higgins let me apologize for my rant. I went too far which is why I went back and modified it. I did not mean for you to see the original for that I apologize.

 

You asked a legitimate question for which I do not have a hard and fast answer. Maybe some experienced engine builders will chip in. But, that leaves us back where we started as there is no reason you should take their word for it either.

Edited August 2, 2018 by Rusty_OToole (see edit history)

[nzcarnerd]

I would have thought that in an antique engine with a compression ratio of under 5:1 the thickness of the gasket would have made little difference. I think that bringing the compression ratio of an engine of that era up to 6 - or maybe a bit more - to one, should improve its performance with no loss of low end driveability. Fuels today are much higher octane than they were in 1930. The pic shows a Studebaker head with .080" removed from it. Unfortunately other projects have got in the way and the engine has only a had a few short runs.

[padgett]

" you must subtract the amount the piston moves upward " errr no. Might be true if the intake charge was massless but it has inertia (velocity times mass as long as you don't go sonic). In a "High Speed Internal Combustion Engine" this translates in a continuedl cyl filling even though the piston has passed BDC. Further most modern cams are measured not at full closed but at .050" lift. Can spend all day on the math but using a flow bench is much faster. It is an art form (one carb was even designed to run at the edge of choked flow. Also need to understand laminar vs turbulent flow And then there are stacked head gaskets and milled heads...

 

And it all changes with direct injection, boost, and variable valve timing. Just static compression ratio is only the start. See Boss Kettering and Tom Midgley.

Edited August 2, 2018 by padgett (see edit history)

[Ben Bruce aka First Born]

50 minutes ago, Rusty_OToole said:

Mr Higgins let me apologize for my rant. I went too far which is why I went back and modified it. I did not mean for you to see the original for that I apologize.

 

You asked a legitimate question for which I do not have a hard and fast answer. Maybe some experienced engine builders will chip in. But, that leaves us back where we started as there is no reason you should take their word for it either.

 

 Heck, Rusty, I thought it a CLASSIC.

 

  Ben

[Rusty_OToole]

I hate it when I give a good answer to a question and someone goes "citation please" or "can you document that". This isn't a PhD thesis and I don't have to justify myself to anybody. If you want further particulars Google it like anybody else. If you want me to go into the Library of Congress or Detroit Public Library and document everything I say, my rate for research is $100 per hour with a $10,000 minimum.

 

If what you really mean is "you are a God Damn Liar and I don't believe anything you say" why not come out and say so.

Edited August 2, 2018 by Rusty_OToole (see edit history)

[C Carl]

57 minutes ago, Ben Bruce aka First Born said:

 

 Heck, Rusty, I thought it a CLASSIC.

 

  Ben

 

Yup ! Rusty sure is a precious one-of-a-kind on our forum here. We are very fortunate he gives so much of his time to us.  I particularly like to have long-held misconceptions set straight. Feisty but trusty Rusty' gave me my first AACA reality check, this having to do with octane of kerosene. This led me to research, which gave me a vastly greater understanding of our ancient autos, the combustion process, and the mechanics of knock suppression by octane rating. From all this, I have greatly reduced the intake temperature on my Nickel Era cars, and substantially advanced the timing. Thanks not only to Rusty', but to each and every one of you. I am so grateful to have found, and joined up with you all. So many of us have problems with varying degrees of severity. The AACA universe is the most satisfying universe I inhabit. Every now and again I have the opportunity to add a little something, and it keeps me relevant to a satisfying degree. Keeps me from going nuts. Seriously !  ? !   -  Carl 

 

P.S.  Oh yeah  :  And compression ratio is a fixed mathematical/mechanical expression of swept volume over combustion chamber volume. Not to be confused by compression pressure. That is even more complex than what is written here so far. I will be late if I don't sign off right now. I expect more will have been revealed by the time I get back. Over, and out.  -  CC 

[maok]

If the manufacturers did take into account the gasket thickness, how much did they estimate it to be? 0.020", 0.030", 0.040" etc

[W_Higgins]

2 hours ago, Rusty_OToole said:

Mr Higgins let me apologize for my rant. I went too far which is why I went back and modified it. I did not mean for you to see the original for that I apologize.

 

You asked a legitimate question for which I do not have a hard and fast answer. Maybe some experienced engine builders will chip in. But, that leaves us back where we started as there is no reason you should take their word for it either.

No worries.  I don't bruise as easy as some.

[Rusty_OToole]

Mr C Carl thank you for the kind words. I would love to know what your research revealed, and what you learned about getting the best out of your nickel era cars. I take it that means 1915 - 1928 models, a time when gasoline quality was at a low ebb.  An ounce of experience is worth a ton of theory as they say.

[playswithbrass]

http://members.uia.net/pkelley2/DynamicCR.html

Here is some interesting reading on compression ratio,s hopefully it explaines it better then I can.  Cheers

[Bush Mechanic]

Batwing-eight, in reply to your original query, yes, the head gasket is included in the calculations. 

 

And yes, there are commonly variations in the chamber volumes from cylinder to cylinder on factory heads. This is mainly due to variations in the sand casting surface. 

 

And I don't remember ever finding a standard head which quite achieved it's advertised compression ratio. It's called gilding the lily.

[keithb7]

Does this offer any insight?

 

https://www.wikihow.com/Calculate-Compression-Ratio

[tripwire]

A quick Google search brought up several webpages discussing true vs. measured engine specs. Including Displacement, Compression ratios, factory stated horsepower vs. measured horsepower.  I found it interesting.  I was unable to find a copy of Rusty's letter.  It probably got "lost" when Lee Iacocca was doing his housecleaning.

 

Engine Displacement: http://www.csgnetwork.com/cubicinchdispcalc.html

Static Engine Compression: http://www.csgnetwork.com/compcalc.html

Dynamic Engine Compression Ratios explained: http://kennedysdynotune.com/dynamic-compression-ratio/

Mopar Forum Discussion re: true vs. actual compression ratios: https://www.forabodiesonly.com/mopar/threads/true-factory-compression-ratios.320123/

Understated Factory Horsepower ratings vs. measured: https://driving.ca/auto-news/news/five-of-the-most-underrated-engines-and-their-true-power

 

Wes in VT

[60FlatTop]

I have never seen an automotive compression ratio extended greater than two decimal points. Not enough accuracy for that level of detail. Maybe a k factor here or there. I'd bet my pocket protector they generalize at that input of the equation.

Bernie

 

Note: Not my good pocket protector, one from a vendor I don't care much for.

[padgett]

" Factory Horsepower ratings" are simple: pure advertising in the US. Good dyno is the only way to know. Don't forget for HP we has SAE gross, DIN, SAE net, and wherever the Toledo needle swung to when they dropped the clutch.

 

"Plastic sheet and burette" tells you what the chamber volume is but says nothing about the piston dome shape. Have seen every thing from pop-ups to bowl and everything in the muddle.

 

Finally actual compression ratio says nothing about the head design and chamber shape - center spark can take more advance than an L-head. Straight plug SBCs are when indexing came from which led to the slant plug heads.

 

It is not simple and more an art form than scientific even 100 years on. The method is to come up with a scientific design and then test/modify/test/modify and on many times.

 

Today for street use I prefer a 3.25-3.50" stroke, 3.75-4.0" bore, with a DOHC  pent roof 4-valve center spark head. Vary the number of cyl to hit the displacement target. I personally like a 3-4 liter V6 ( which gets me back to my '59 3.4 Jag was 250 hp then, is 300 hp now.)

 

[Rusty_OToole]

The manufacturer knew how thick their gaskets were and allowed for them in their calculations. Then rounded off the figures to the high side. I have seen this justified on the grounds that carbon would build up in the heads and on the piston tops raising the compression when the engine was in service.

[60FlatTop]

I think the labs monkey around with a bunch of options and when one works they paint it gold, list the specs, and send to to production with a note "Make them like this".

[Marty Roth]

Simplistic approach?

engine properly assembled with access to top (spark plug hole or valve?,

or lay the engine over if the access is lower than on top)-

Bring the piston to top dead center-

using a syringe with delineated increments, use a liquid to fill to brim-

rotate engine to bring piston to bottom dead center-

add fluid to fill again

measure and compute the difference-

 

In simple terms,

if it took one pint to fill at TDC

and seven pints to fill at Bottom of stroke,

then compression would logically be 7:1

 

Of course this is from a non-engineer, less-educated reference point,

and would depend on having access

[Grimy]

14 minutes ago, Marty Roth said:

Of course this is from a non-engineer, less-educated reference point,

and would depend on having access

....and a fluid which will reliably not leak past rings and which can somehow be almost completely removed without spurting all over your nice clean engine compartment, and the residue of which will not emit a mosquito-killing fog over a two square mile area when the engine is next fired up.

[Marty Roth]

47 minutes ago, Grimy said:

....and a fluid which will reliably not leak past rings and which can somehow be almost completely removed without spurting all over your nice clean engine compartment, and the residue of which will not emit a mosquito-killing fog over a two square mile area when the engine is next fired up.

 

... and da oily boid get da woim...?

[trimacar]

7 hours ago, Grimy said:

....and a fluid which will reliably not leak past rings and which can somehow be almost completely removed without spurting all over your nice clean engine compartment, and the residue of which will not emit a mosquito-killing fog over a two square mile area when the engine is next fired up.

 

I prefer molasses when using Marty's test method.  I'm sure everyone has their fluid of choice.  A high alcoholic content liquid might be useful, too.

 

The other method I like is to take the bottom off a compression tester, and attach a tube to the top over which a balloon can be placed.  Then, insert tester with balloon attached, rotate engine to blow up balloon on up stroke.

 

Then, it's simply a matter of calculating the volume of the oblate spheroid that the balloon has now become (don't use those long, skinny balloons, that's for making balloon poodles).  Once you have the volume, It's intuitive how one then arrives at the compression ratio.

 

The formula for the volume of an oblate spheroid is:

              v = 4/3 π•b²•c

where:

• V is the volume of the oblate-spheroid
• b is the length of the semi-axis rotated
• c is the length of the semi-axis of rotation

I actually had this post in reserve, and it was on my schedule for the first of next April, but since the topic came up....

[Restorer32]

We have a client for whom we rebuilt a '48 Lincoln V 12 engine using repro finned aluminum heads.. He wanted all 12 cylinder volumes to not vary by more than 1/2 cc. Only way to do it was to measure the volume of each combustion chamber to determine which one had the largest volume. Each chamber was then measured and a determination was made as to how much material had to be removed to bring it to the same volume as the largest chamber. Came down to grind a little. measure, grind a little, measure x 12. Very time consuming but in the end we got all 12 cylinders within 1/4 cc of each other.

[Batwing-8]

First, thanks to those who responded! Perhaps I need to clarify: I'm attempting to make a new cylinder head for a vintage auto. Serviceable replacement heads just not available, so this is the only option. The old head is off the car and has been milled, perhaps several times over its life, but I don't know how much material has been removed, so don't know the original combustion chamber volume to assign to the new head.  (Manufacturer's technical drawings also no longer exist). My plan was to calculate current cylinder volume and using the  published compression ratio, work backwards to determine what the volume of the combustion chamber would have been. The compressed gasket is .080", but, as you might guess, the numbers don't "jibe."  If I include the combustion chamber volume (wet measure) plus the gasket volume, the  C/R is too low; without the gasket, it's too high. 

 

Judging by the responses, published data on vintage compression ratios is of questionable accuracy. In addition, a slightly-open intake valve during 25% of the compression stroke likely also plays havoc on the "numbers." The static mathematical compression ratio probably varies from the actual C/R within some running motors. So I'll make a "best guess," erring on the side of caution. It's relatively easy to remove more metal if needed; a lot harder to add it on. 

 

 

 

 

 

 

 

 

 

 

 

c

[Mike Macartney]

Hi,

I'll try and do a spreadsheet for you with the calculations. You can then put your own known measurements in to come up with the volume of the combustion chambers for different compression ratios. I have done it before when we converted a fleet of 15 London taxi cabs from diesel to run on liquid petroleum gas (spark ignition).

Mike

[C Carl]

Bill, a couple of quick questions, as I think some specificity would help at this point. #1  : what is the car you are working on ? You mentioned in your first posting a range of from the '30s through the '50s. That is quite a spread.  #2 :  are you reluctant to raise the compression as a result of your new replacement cylinder head ? I guess I should add question #3 : what is the current compression ratio ?   -  Carl 

[PFitz]

Race engine builders measure combustion chamber volume whenever they are blue-printing an engine. The usual method is to make a Plexiglas cover that bolts over the combustion chamber with a hole toward one edge. A burette tube - a tall graduated tube with a shutoff at the bottom - is used to measure the amount of liquid used to fill the combustion chamber. Same tool is used to measure compression ratio very accurately.  This is the burette I use.  https://goodson.com/products/ccb-100-glass-buret?variant=42307755078

 

You can usually get small pieces of 1/4 inch plexi at a  glass shop, sheet plastic supplier, or a display builder.

 

The valve faces and seats are greased to seal them, a spark plug is inserted, and the plexi cover edges are also greased to seal.  Then the plexi is bolted on over the combustion chamber.

 

 Then set the head up so it's slightly tilted so that the fill hole in the plexi is the highest point.  Fill the burette tube to it's top mark with a liquid (I use Marvel Mystery Oil). Open the valve until the liquid fills the entire chamber up to the bottom edge of the hole.  Tap the head to move any air bubbles up and out the fill hole. Read the graduations on the burette tube for the amount of  liquid it took to fill the chamber.  Usually graduated in CC's. That's your combustion chamber volume.

 

Paul 

Edited August 19, 2018 by PFitz (see edit history)

[Mike Macartney]

HI,

I will try and attach the spread sheet calculation for your compression ratio calculations if it does not attach let me know your email address with a private message and I'll send it to you by email.

Any queries, just ask.

Mike

PS: It wont let me attach the file. You will have to send me your email address.

[Restorer32]

22 hours ago, PFitz said:

Race engine builders measure combustion chamber volume whenever they are blue-printing an engine. The usual method is to make a Plexiglas cover that bolts over the combustion chamber with a hole toward one edge. A burette tube - a tall graduated tube with a shutoff at the bottom - is used to measure the amount of liquid used to fill the combustion chamber. Same tool is used to measure compression ratio very accurately.  This is the burette I use.  https://goodson.com/products/ccb-100-glass-buret?variant=42307755078

 

You can usually get small pieces of 1/4 inch plexi at a  glass shop, sheet plastic supplier, or a display builder.

 

The valve faces and seats are greased to seal them, a spark plug is inserted, and the plexi cover edges are also greased to seal.  Then the plexi is bolted on over the combustion chamber.

 

 Then set the head up so it's slightly tilted so that the fill hole in the plexi is the highest point.  Fill the burette tube to it's top mark with a liquid (I use Marvel Mystery Oil). Open the valve until the liquid fills the entire chamber up to the bottom edge of the hole.  Tap the head to move any air bubbles up and out the fill hole. Read the graduations on the burette tube for the amount of  liquid it took to fill the chamber.  Usually graduated in CC's. That's your combustion chamber volume.

 

Paul 

 

Exactly the method we use. Hardest part is finding a place to store that fragile burette when not using it.

 

[PFitz]

Yeah, you do have to be careful with them. 

 

Mine (the model in my link) hangs on two pegboard hooks, upside down by it's valve handle - right behind the valve grinder, where it can't be bumped into.

 

Some PVC pipe would make a decent case to keep it from being damaged.

 

Paul 

[Rusty_OToole]

Batwing your calculation with gasket (too low) is probably the more accurate. As I said they typically rounded off the number to the high side, or gave out a number higher than it actually was. Their excuse was, that in service carbon would build up in the combustion chamber and then the number would be accurate.

 

You might put out a request for a stock, unmilled cylinder head and see if someone could give you an accurate measurement. Even if they could just measure the thickness of the head, or the depth of the combustion chamber, that would allow a more definite calculation.

 

In any case I don't see why it matters. You could easily make a more efficient, higher compression combustion chamber and no one would know.

 

A useful rule of thumb is your compression should look like the octane of your gas. So if you plan on using 87 octane regular keep below 8.7:1 compression. If it is a flathead you need to go a little lower than a more modern engine, as they are at a thermodynamic disadvantage to an OHV. In any case, it is practically impossible to get a CR that high on a flathead.

 

To put it another way, on an engine of that age, it is practically impossible to go wrong no matter what size you make the combustion chamber.

Edited August 20, 2018 by Rusty_OToole (see edit history)