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Unread 08-16-2009   #1 (permalink)
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Default Milling heads to lower chamber volume

I have a set of TFS Track Heat heads that have 64cc chambers. My new rotating assembly has 16cc dished pistons (3.25 stroke, 4.030 bore). I'm running boost, so I'd like a lower compression ratio, but I think this will be a bit too low.

How much would I need to mill from the deck of the heads to achieve 61cc chambers (or ~8.5:1 CR)? And is this too much milling?

BTW, I'm inclined to deck the heads anyway because they have some pitting.

Thanks.
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Unread 08-16-2009   #2 (permalink)
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I don't have an answer for you, but thought I would add some questions for those who are more experienced than I with this.
What impact will this have on pushrod length? How about piston to valve clearance? How much can he mill before he has to worry about problems with the intake manifold sealing up against the heads.

On page 208 of the Mustang 5.0 Technical Reference and Performance Handbook there is a chart for the Mustang's E7TE heads that shows it requires shaving those heads 0.20 inch to go from a 64cc to 60.92cc chamber. For that same change, the compression ratio would increase by 0.32 (8.80 to 9.12).

I'd talk to your machine shop and Trick Flow.
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Unread 08-16-2009   #3 (permalink)
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How much you planning to run the pistons in the hole? If you zero deck you get better quench and compression.

I figure you are at about 0.010" now which puts you at ~8.5:1 with the 64 cc. Drop to 61 and with 0.010" in the hole and you get to 8.75:1. Drop to 61 AND zero deck you get to ~9.0:1. If my calcs are right.

I would surely think you would need to check your pushrods and intake alignment if you do both.

Not sure how much needs to be taken off.
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Unread 08-16-2009   #4 (permalink)
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you could also look into a thin head gasket from cometic.
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Unread 08-16-2009   #5 (permalink)
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Milling the heads raises compression, it doesnt lower it.

Milling the heads has a direct effect on Push Rod and PtV Clearance. What ever is milled, brings the PtV clearance that much closer.

Milling makes the Push Rods longer than what would be optimal to achieve the proper rocker sweep on the valve stem.

Its hard to say on the intake alignment. It does have an effect, but it all depends on how well the intake fit before hand.
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Unread 08-17-2009   #6 (permalink)
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What he said... ^^^
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Unread 08-17-2009   #7 (permalink)
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Id deal w/ the low compression just add a smaller pulley on the boost!
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Unread 08-17-2009   #8 (permalink)
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Yeah, I'd either change the pistons or heads rather than trying to modify everything, sell what you have on Ebay to finance the transition
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Unread 08-17-2009   #9 (permalink)
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Here's an article you may find interesting if you have time to read it. At the very end of it is a calculator you can download and put in your specs and it will calculate static and dynamic cr, which are drastically different. It will help you determine in fact how much to run it in the hole, how much to deck it, etc and see your end result a lot easier and cheaper.

This is a good read for anyone building an engine, and the calculator is a must have.

Dynamic CR
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Unread 08-17-2009   #10 (permalink)
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Thanks for the responses......

For those that misunderstood, I am looking to raise compression. After thinking on this a bit, I figure that I may be able to use some simple geometry to figure how much deck needs removed to lower chamber volume by ~3 cc's....

Vol = Pi * R^2*H

3cc = Pi * (4.030/2 * 2.54)^2*H

H = 0.0365 centimeters or 0.014"

This would be the amount milled if the chamber had the same cross-sectional area as the cylinder bore, but just eye-balling it, the chamber looks to cover about 75% of the cylinder bore; so H would need to increase by 1/0.75. This gives about 0.019" required to be milled from the heads to reduce the chamber volume by 3cc. This seems pretty close to what was stated in an earlier post by vetprowanab.

Assuming this is correct, I can see how the P2V clearance will reduce by exactly the same amount milled, but with my cam and piston geometry this isn't a problem.

As for the pushrods, I also see how they will become relatively longer by the same amount, but is 0.020" significant? In looking over some pushrod lengths offered, many of them increase in length by increments larger than 0.020".

Thanks again for the input.
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Unread 08-18-2009   #11 (permalink)
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When talking pushrod length, .020" is HUGE. Many rod makers can make rods in the exact length you need, you're not held to the preset lengths you see in catalogs.
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Unread 08-18-2009   #12 (permalink)
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I try to stay away from milling heads (except on flathead type motors) because it just creates fitment problems with intake manifolds. I fixed a motor for a guy that had milled the heads and then didnt take the correct amount off the intake manifold (took to much) and the water outlet from the heads to the manifold didnt seal and filled the motor with water. He brought it in because it just stoped running after filling it with gallons of water. Turned out the water was going inside the crankcase filling the entire motor with a oil colored mayonase substance (about 5 gallons worth of emulsifyed oil and water). We were able to save the motor by taking the heads off and draining the gu, flushing the oil system with gas and putting it back together with a correctly milled manifold. Once it was back together we ran it for 15 min with atf in it at a little above idle, then changed the oil to regular oil. That was several thousand miles agao and it still runs tip top!
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Unread 08-19-2009   #13 (permalink)
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Again, thanks for the thoughts. I've called the machine shop to stop them from milling. I'm just going to turn up the boost.
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