I am in the Process of re doing some parts on the Ruster Engine.

I would Like to Mill the head for a bit More Compression.

I thought that I had ~12.5:1 Compression Ratio. But after Running the caluclations and entering the values of the Head CC's From Mike when He did the head for me, and with Chaging to a Big Bore Perma seal / Endurotec Head Gasket, that Ratio was actually more Like 11.6:1

My Head cc's at these Numbers.

#1 46.4CC
#2 48.0CC
#3 47.2CC
#4 46.7CC
#5 48.1CC
#6 47.6CC

They average to 47.3CC

The calculator says that I need to get the Head CC's Down to 38.0CC to get to 13.5:1 CR.


The question I have is How do I determine How much to mill off the head to arrive at this Number (which is an average of course)?


Thanks for your help!


Greg

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The calculation can't be done without more info. What's the deck height? Gasket thickness? Flat top pistons? Valve pockets? Bore size?

For example,
The spreadsheet I use for CR calculations says a stock bore motor with a zero deck height and .040" thick head gasket and a 48cc combustion chamber will have a 12.4:1 compression ratio.

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Pro Cycle,

I have all of that Info. I didn't post it here because I didn't feel it was necessary.

I have Done the calculation. I want to know How many Thousandths of an Inch to mill off the head to go from an average Chamber Volume of 47.3CC to get to an average Chamber volume of 38.0CC?

Greg

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The 0.006" per 1cc falls apart once you pass a certain point. Once you are in the sub 45cc realm, use caution. It accelerates from there.

No way to specify at this end. DusterIdiot did a write-up on that some time ago. Maybe he'll chime in.

CJ

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Part of Tyrde-Browne Racing

Doc's formula of .0066" per 1cc is accurate and works great up to the point that you run out of the round/radiused chamber. Since I only have used BL heads for my high SCR mills, about 38cc things start to get hairy and the .0066 stops working...I would measure how much depth you have from the mating surface until you hit the "shelf" in the chamber. After that you will have to cc things...

Going from 47.3 to 38, you will need to cut about .062...if you don't have that much left before hitting the shelf things will become "interesting"....

That wasn't clear to me. I couldn't tell if you were trying to go from 11.6 to to 13.5 of if you are trying to go from 12.5 to 13.5

If all you want is to subtract 9.3cc it's simple to google up a 'volume of a cylinder' calculator like this one --> http://www.onlineconversion.com/object_ ... r_tank.htm

I never trust 'rule of thumb' stuff on this. It can be close but as CR goes up the effect on CR per thousandth increases exponentially. The higher a CR you are working with the more critical it is to have accurate numbers. For instance, if we take the imaginary 12.4:1 motor above and mill .020 we increase the CR by .65 to 13.05:1. If we do the same thing to a stocker with 8.1:1 it only increases by .25 to 8.35:1 I will always take the time to make new measurements and a new calculation. If I had those numbers I could plug them into my spread sheet and give you a precise figure.

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Greg, i'll send you an email an tell you how to figure this out. It's real simple.

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There's no such thing as too much cam....only not enough engine!

If 47.3cc is giving you 11.6:1 then taking off .060" will get you to 13.5:1 and a chamber volume of 38.5cc

That assume you won't run into a shelf at .060. If you will then you'll have to go a little deeper.

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Thanks everyone,

I will be looking into this later this week.
I hope to drop the head off at the machine Shop Friday, and I want to have the Correct Numbers before leaving it with them.
Scott, I got your email and will be printing it to study and fully understand.

Pro cycle, by shelf, do you mean an area of the combustion Chamber that started to be flat upon milling?

Greg

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If you hit the shelf, you will have to mill less, not more, than the 0.0066"/cc to get the desired CR. Personally, I would take the prediction and subtract 0.005-0.010" from it.

You could also try to measure with fluid the difference in cc's and the exact distance to mill. This would be a more complicated measurement.

Lou

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Home of Slant6-powered fun machines since 1988

Yes.
That's not the case. A thought experiment can demonstrate this. Let's say you want to take 2cc out of the chamber. That is going to require milling .014". Now let's imagine you have 2 very big shelves that take up exactly 50% of the chamber area. How much has to be milled to subtract 2cc? You will have to take off .028" in order to decrease the chamber volume by 2cc.

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I had to print this thread, because I needed a real life example for my son's math teacher on how two guys can approach the same subject but be talking about two different parts of the
calculation but not being on the same page but both are correct in their observations...Classic case of setting up the story problem and using the same variables....
This statement is correct, since he is talking total compression Ratio (swept volume and all that)...once you start into the hi compression numbers (and if you start milling into the wedge, per above, a smaller cut on the head will make larger increases in CR (a fraction of a cc can be enough to pop the SCR up .3 to .5....)
This is also a correct statement if we are just talking just about straight up chamber volume before it is applied to the multipliers/divisions of the ratio formula... cutting X" inches off the deck will remove Y amount of volume from the chamber (either by liquid or clay volume/displacement if not so uniform, or, if the chamber is uniform, area x height of cut x measurement translation variable = volume removed with desired cut)



-D.Idiot

"A long time ago a Buddhist master was teaching 3 of his students about observation. He put an elephant in a tent with no light inside and led his students inside and asked them what could be hidden inside the tent. The first student exclaimed that he had a long rough arm that breathed out the end that had left some slimy stuff on him, and it was near some hard pointed objects. The second one exclaimed that it was a large massive wall of rough material that seemed to expand and contract as the thing was breathing... the final student exclaimed it was a horrid creature that stank like dung and it had a whip that smacked the student and knocked him down...."

I just hate it when everyone is right! Can I be right too??

Rick

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12.70 @ 104.6

I don't really want to belabor the point, and I definitely don't want to turn this into an 'argument thread'. But...to clarify for curious folks who might be reading along; If you use that .0066" per CC number, once you mill past the shelf the amount of milling per CC is going to have to go up. When you have cut into the shelf a little bit you will be looking at .007" per CC. as more shelf area is exposed it will require .008" per CC and so on. Distance per CC will have to increase. It can't decrease. This is true regardless of what the desired CR is.

Now if someone was saying 0.0xx" milling per 0.1 of compression ratio then as CR goes higher the required distance to change CR gets smaller. But that isn't what was said. If you mill into the shelf you will still have to mill further to have the same affect on CR than if there was no shelf at all.

It's all a bit counter intuitive. When you are milling a head you are taking off metal but the real point of it is to remove the 'empty space' from the chamber. If we think of it as 'how much empty space are we taking away?' it is easier to visualize why the 0.0xx" per CC increases.

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How about it Scott, care to weigh in? I suppose if the remainder wasn't bathtub shaped, he has a point, but as we are cutting on a smaller area, yes, this may be counter intuitive. I want to know.

Not that I've ever cut that far. I deck my blocks for compression. Cutting the head that deep causes lots of other 'issues." I like to keep the valves as far away from the deck as is practical.

CJ

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