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914World.com _ 914World Garage _ 2.0L heads...
Posted by: jhadler Oct 9 2009, 04:49 PM
I'm looking at getting the heads flycut when they come off. Is there any rule of thumb for 2.0L heads on how much of a cut equals how much of a change in combustion chamber volume?
Just curious.
Thanks!
Posted by: r_towle Oct 9 2009, 04:57 PM
What is your objective? Why do you want to flycut the heads?
What is your target CR?
If you are planning to use race fuel only...ok.
If not, you can get 9.5:1 without any cutting...and that requires good fuel.
Rich
Posted by: jhadler Oct 9 2009, 05:04 PM
I was thinking 9:1. Currently have 59 cc's. With an effective deck height of 0.045 (deck + cylinder base shim), I calculate a CR of 8.6.
No, not planning race gas. But would certainly accept 91 octane as a necessity.
Am I off in my thinking and/or calculations?
Thanks,
-Josh2
Posted by: yeahmag Oct 9 2009, 05:13 PM
I had my just "kissed" as I was in the same boat as you. Brought me up the the CR I wanted and took off almost nothing.
Posted by: r_towle Oct 9 2009, 05:32 PM
I was thinking 9:1. Currently have 59 cc's. With an effective deck height of 0.045 (deck + cylinder base shim), I calculate a CR of 8.6.
No, not planning race gas. But would certainly accept 91 octane as a necessity.
Am I off in my thinking and/or calculations?
Thanks,
-Josh2
How does your math look without the cylinder base shim?
Without the head gasket also.
Rich
Posted by: jhadler Oct 9 2009, 05:40 PM
How does your math look without the cylinder base shim?
Without the head gasket also.
Not enough deck height. See my "it is alive" thread...
-Josh2
Posted by: r_towle Oct 9 2009, 05:55 PM
sorry, what is the deck height?
If you flycut the head, you will still have valve contact...its the same result...you are moving the valve closer to the piston.
A normal 2.0 liter with the shim and gasket removed will not hit.
Rich
Posted by: yeahmag Oct 9 2009, 05:58 PM
He says he has .045" DH. I feel a minimum "safe" is .040". I've run as tight as .037".
Posted by: jhadler Oct 9 2009, 06:54 PM
Numbers from the friend who built the motor scared me into tearing it down with only 2 hours break-in logged.
Claimed 0.016 deck height. If I haven't already introduced the pistons to the heads, it's probably only a few rpm away. So at the VERY least the heads need to come off.
My thought was this: The existing deck height (I will remeasure to confirm) is way too low. Pull the heads, take a few thou off. Pull the jugs and shim them to bring the deck height up to .040. The question is: "how much to take off to reduce the combustion chamber volume by x cc's?". Right now I'm at 59 cc's. By my calculations, I'd want to get the CC down to 56 cc's. (with 0.025 base shim that should give me 9:1).
I will likely go through the bottom end as well to confirm all the bearings have the proper clearances.
-Josh2
Posted by: bam914 Oct 9 2009, 08:47 PM
I am running +.002" deck and the stock head gasket(only because of the rules). So I have about .025" total deck. The pistons are not hitting the heads. You can just see were in some places it is knocking the carbon off. I can only turn the engine 6000rpm because of the stock springs. I would say safe min deck of .035" would be fine. Len knows how to calculate the amount to cc's.
Posted by: r_towle Oct 10 2009, 09:02 AM
Numbers from the friend who built the motor scared me into tearing it down with only 2 hours break-in logged.
Claimed 0.016 deck height. If I haven't already introduced the pistons to the heads, it's probably only a few rpm away. So at the VERY least the heads need to come off.
My thought was this: The existing deck height (I will remeasure to confirm) is way too low. Pull the heads, take a few thou off. Pull the jugs and shim them to bring the deck height up to .040. The question is: "how much to take off to reduce the combustion chamber volume by x cc's?". Right now I'm at 59 cc's. By my calculations, I'd want to get the CC down to 56 cc's. (with 0.025 base shim that should give me 9:1).
I will likely go through the bottom end as well to confirm all the bearings have the proper clearances.
-Josh2
Well, now I am confused. Are you guessing on your real numbers, or have you pulled the heads and measured the deck height?
You may be OK if you just add a shim.
If you deck/flycut the heads, you are moving the valve closer to the piston so any normal advice you get on deck height will be WRONG.
Normal advice assumes stock heads.
If you remove 004 from the heads, you just moved the valve 004 closer to contact with the piston.
I would suggest that you measure the deck height on every piston first.
Then CC every port in the heads.
Match the heads, thats it...do as little decking to the heads as possible and make up your difference with base shims (which are cheap and can be changed easily if you measure out to small.)
Rich
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