I recently got a new cylinder head for my 225 slant and plan on taking it to a machine shop to get extra material shaved off. My engine is currently all stock, never been resurfaced and I plan on trying to achieve a 10:1 compression ratio using 92 octane fuel. How much material would I need to remove in order to achieve that?

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1980 Plymouth Volare Duster w/ 225 Slant 6

Are you sure that 92 fuel is good enough for 10:1 compression ratio? I have milled head and block (both 0.08" class - have forgot the exact numbers) - and it is now good for ethanol fuel.

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1966 Dart. 170 cu.in. 200 rwhp.

The amount to mill to achieve any given compression ratio depends on the current size of the combustion chamber, the piston deck height, the head gasket you intend to use along with the bore and stroke. You probably know the bore and stroke, but the piston deck height and combustion chamber volume will need to be measured. And 10:1 with a stock cam is a recipe for detonation.

The short answer is .0066" per 1cc of head volume.

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Joshua

How would I be able to find out the volume of the cylinders, and why would a 10 to 1 ratio be a ticking time bomb without a new cam?

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1980 Plymouth Volare Duster w/ 225 Slant 6

Others can say it more accurately, but in the short answer, too much cylinder pressure... at low RPM (where the stock cam is designed to work best with stock-ish compression ratio)

The introduction of too much compression, and the stock cam jacks up the cylinder pressure, and you will get detonation.. which is when the gas vapors light off due to hots spots in the chamber. Detonation IS the limiting factor of all engines in producing power.

There are a lot of other ways to get power, and have a motor that does not blow out head gaskets, or hammer the con rod bearings flat in 1000 miles.... (and make more power while doing it) compression is nice, but it is not the only way to make your engine come alive..

The relationship between compression ratio and cam timing establishes cylinder pressure.

When you increase compression ratio you change cam timing to optimize cylinder pressure.

There is a name for this pressure/cam timing/compression ratio relationship. It is called the dynamic compression ratio.....or DCR

Read up about it.

Would anybody be able to tell me how much to shave off of a head without needing a new cam? Pretty much if you look in books as to stock compression ratio and all the measurements to find it, that's what I have currently so it's supposedly at 8.4 to 1

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1980 Plymouth Volare Duster w/ 225 Slant 6

A supposed compression ration isn't necessarily what you really have. Without cc'ing the head, making the other measurements and doing the math you might really have a current CR anywhere between 7.5 and 9. More if someone has already milled the head or decked the block some time in the last 50 years.

Without knowing the real CR nobody is going to be able to tell you how much you can safely mill off and still retain the stock cam.

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Well that's just it thought, the engine has never been even taken apart before. And as for the current CR it will never be able to be acyrately measured until after the head swap on acount of the head gasket us leaking along with my current heads valve seats. That's why I've resulted to whatever the books say is what I've got or where I should be with good gaskets and seals that is.

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1980 Plymouth Volare Duster w/ 225 Slant 6

Also I have already purchased a new gasket(Fel-Pro), and cannot find anything on compressed thickness of it on any sites or forums. What other kinds of things cam I do to the engine that'll get better performance out of it. Please don't say turbo or supercharge it haha.

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1980 Plymouth Volare Duster w/ 225 Slant 6

Well all engines pretty much react to the same mods for performance, so a cam, intake, headers, ignition upgrades all add to the equation.

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

If you can't take the old head off the engine before getting the new head milled, you're going to be guessing no matter what you do. If you can safely assume that the engine has never been disassembled or rebuilt, then you can at least assume it's a stock bore (3.400"). Someone here can give you the compressed Fel-pro gasket thickness.

Luckily you already have the new head you want to get cut, so you can also go ahead and measure the existing volume (in cc's) of the combustion chambers. But you're still missing one very important piece of the puzzle: the deck height of the block above the piston tops. This can vary a lot based on factory machining tolerances. But even if you assume a deck height measurement to calculate your existing SCR, you're still kinda putting the cart before the horse.

What are your performance goals for this motor? Do you just want a little more pep for a street driver, or do you want this thing to turn impressive 1/4 mile times? For the former, it might be as simple as shaving the head for 8.5-9 SCR (It could be below 8 from the factory), leaving the stock cam, and bolting on a 2-barrel intake, a good carb, and a larger exhaust pipe. For the latter, things can get a little crazy: higher compression, longer duration cam, oversized valves, aggressive port work, and an entire world of bolt-ons to choose from. No matter what you do, it needs to be considered as part of an overall system that works together, not just a bunch of random performance parts. No one here can really give you good advice unless they know what your goals are.

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Somehow I ended up owning three 1964 slant six A-bodies. I race one of them.
Escape Velocity Racing

My goals are to get more pep in my slants step but I don't want to go too over the top on account of I'm using my car as a daily driver. That's also why I'm unable to get measurements of the engine's current status. I'm looking for a good amount of material to take off if the new head in order to achieve higher performance, not too high just higher. As for the carb swap and headers you mentoned, I have already done a 1 to 2bbl carb swap and put a 2.25 pipe that splits into dual 2.25 side piped. I don't really have intentions of dropping a more agrees I've cam in.

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1980 Plymouth Volare Duster w/ 225 Slant 6

It's an adder, even stepping up to a stock style cam with slightly more lift will help with what you already have done and will help if you can clean up the ports and bump the compression ratio to say 9:1... but you have to measure it, you haven't posted what year block and head so we have a rough guess-timate of what you are trying to build up...

Fel-pro composite gaskets crush to the .038-.043 range (.040 is a good "average" useable number for compression ratio calc).

Another item to address is cam phasing... degreeing the cam will help make sure the cam isn't retarded and wasting your low end power and mileage....

Cut .060" off the head and call it a day. That will add a little zip to it without having to get into changing cams or getting pushrods made.

Tell everyone it's 11-1, who will know any different?

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