It's really hard to say, without looking at the details of the engine. I'll
tell you how to do the job kind of right, in a round about kind of way, and
you can probably have a good time flailing about, and you'll learn a bit, too.
Different stock engines had different pistons from time to time. They also
were installed with different piston heights, due to machining variances in
the deck of the block. So one thing you want to do is measure the distance
that the piston is down the bore when it's at TDC. You can do this by
putting a straight edge (a ruler is fine) across the bore and using your
feeler gauges to see what the distance from the outer edge of the piston, in
line with the center of the block, is. Your pistons have a dish in them -
you want to measure at the flat ring at the outer circumference, not in the
dish. this gives you the deck height of the piston in the bore, which goes
into calculating the total comustion chamber volume.
Next, you want to know how big the dish is. Most race pistons for 1275
engines have 6cc dishes. I think stock pistons have 11cc dishes. Perhaps
Vizards books can tell you.
Now, you need to figure out how big the new combustion chamber should be.
The right way to measure is to fill the chamber up with liquid using a
calibrated bottle of some sort. I've done it using my wife's smallest
measuring cup, which happend to have a cc scale, and using a medicine
dropper to dispense the liquid. You fill up the chamber with liquid to the
volume that you want, measure how far down from the top the liquid is, and
that's how much you whack off the head.
But there's another way. The stock chamber is 21.3cc (check in Vizards
book). Take a piece of 1/4" squared graph paper and trace the outline of
one of the head chambers on to it. count up all the squares (estimate the
ones on edge that are partially in the chamber outline) and get the surface
area of the chamber in square inches. convert to square centimeters by
multiplying by 6.45, and divide that by the chamber volume to get the
average depth of the chamber. Assume that you got 3.2 square inches. That's
20.6 sq. centimters. the average depth of the chamber is therefore 21.3 /
20.6 = 1.03 c
So, you want to calculate the new average depth, and the difference is the
amount you want to remove.
Compression ratio is the ratio between the combustion chamber and the swept
volume of the piston. Your total combustion chamber is: 21.3 in the head,
4 in the ring land and the gasket plus a little for the deck height (which
you should check using the method above - if the deck height is more than
say .050", add anothr half cc) plus say 11cc for the dish in your piston.
The bore is 318.75cc (assuming you're at stock bore).
so total chamber volume is 36.3. CR is 318.75 + 36.3 / 36.3 = 9.78:1
That's pretty close to stock.
To get 10.5:1, the total chamber has to be 33.5cc, so the combustion chamer
has to be reduced by 2.8cc to give 18.5cc. The new average depth of the
chamber will be 18.5 / 21.3 * 1.03 = .895. The difference is .135
centimeters, or .053"
so you need to mill .053" off the head to get a new compression ratio of 10.5:1
You need to check out the actual area of your head, etc, to get the actual
average depth, but this method should work. It's kind of fun, too.
Race engines are a "Junior Mr Science" kit. some times...
Let me know if this helps at all. If 'twas me, I'd do the above calculation
and be pretty confident. This is how I actually did calculate the amount to
have my very first race head decked, and it worked out just fine.
Brian
At 10:00 AM 8/6/99 -0400, you wrote:
>
>Hi Brian.
>
>Missed you at the Meadowbrook Historics last weekend but did chat with Mr.
>Watson and get a chance to watch your former Midget deport itself well on
>the track.
>
>I'm hoping I can bother you with a question that you might be able to answer
>right off the top of your head. I've got my head off and about to go to the
>machine shop for a valve job. I thought I would have the head skimmed a
>bit, but am not sure how much to take off to remain streetable on pump gas.
> I now have a Piper "285" cam. The specs for the cam are 292' duration,
>0.396" of lift and 107' of full lift (ATDC/BTDC). I think that's quite
>similar to the "649" Works profile. My pistons are stock, and as far as I
>know, my head has not been milled, so my effective compression ratio is low
>with this much cam.
>
>Do you have any idea how much I should have the shop take off the head? I
>guess if I could run in 10:1 nominal CR I'd be ok....maybe a little higher.
> People have told me to go .030-- others to go .050. I know you've built
>these engines before and I am interested in your recommendation. The
>suggestion that I measure the combustion area goes beyond what I know how to
>do or have the instruments for.
>
>Just for background: I still run stock carbs and manifold and the head has
>had no race prep at all. I do have a header on it.
>
>Also, with this small amount of milling, do you recommend I shim the rocker
>pedestals?
>
>Thanks in advance for any help you might give.
>
>Regards-
> --John Deikis
>
Brian Evans
Director, Global Sales
UUNET, An MCI WorldCom Company
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