possible power gains from milling the head?

[Guest]

Hey, a few nights ago while i was drinking and staring at the bike thought of milling the head down to up compression and increase power output. I have done this before with 2 stroke engines and was wondering if this would be wise on a 4 stroke. What do you all think?

[Danaconda7]

Does anyone have any thoughts at all?

[Steve]

It might work, but you will have to do some other stuff, too.

You will be changing your cam timing in the process, so you will have to slot your cam sprockets to adjust them.

Probably more stuff, but I am not a high-performance engine builder.

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[Danaconda7]

Thank you Steve, i did not even think about the timing.

[stetracer]

Before you pull the head check your valve to piston clearance I like to see min clearance of .060 intake and .080 on the exhaust. If you have .100 on the intake and .115 on the exhaust then you can safely take .040 off your head. But as stated you will have to degree the cams in and unless your already are running slightly lean you might need to rejet to a size or two smaller main jets.

[81GS850]

Just food for thought, found this on a hotrod site a long time ago and always refer to it before deciding to mill heads.

New CR	8:1	9:1	10:1	11:1	12:1	13:1	14:1	15:1
9:1	3.5
10:1	6.5	2.9
11:1	9.2	5.5	2.5
12:1	11.5	7.7	4.7	2.1
13:1	13.6	9.7	6.6	4.0	1.9
14:1	15.4	11.5	8.3	5.7	3.5	1.6
15:1	17.0	13.0	9.8	7.1	4.9	3.0	1.4
16:1	18.6	14.5	11.3	8.6	6.4	4.4	2.8	1.4

Read more: http://www.popularhotrodding.com/tec...#ixzz3AL9bVxzE

This is a graph published that shows original vs new compression ratio. The numbers show are theoretical percentage increase. You'll also notice this graph shows a rate of diminishing returns. EX going from 8:1 to 9:1 is more profitable than going from 10:1 to 11:1. etc.

[posplayr]

What are the units on the table?

[Steve]

What are the units on the table?

From the source cited:

This is the percentage increase that can theoretically be had by raising the compression from a lower to a higher level.

Read more: http://www.popularhotrodding.com/tec...#ixzz3AMXNmwMW

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[Danaconda7]

Stetracer, Is the valve to piston clearance somthing that would be written down somewhere or would I need to measure it myself? Also, what direction would i need to adjust the timing? I have not had to fool with anything like that in the past.

[Danaconda7]

Thank you for the graph, looks to be some good information! I assume you have some experiance is this? Any pit falls i should be aware of?

Just food for thought, found this on a hotrod site a long time ago and always refer to it before deciding to mill heads.

New CR	8:1	9:1	10:1	11:1	12:1	13:1	14:1	15:1
9:1	3.5
10:1	6.5	2.9
11:1	9.2	5.5	2.5
12:1	11.5	7.7	4.7	2.1
13:1	13.6	9.7	6.6	4.0	1.9
14:1	15.4	11.5	8.3	5.7	3.5	1.6
15:1	17.0	13.0	9.8	7.1	4.9	3.0	1.4
16:1	18.6	14.5	11.3	8.6	6.4	4.4	2.8	1.4

Read more: http://www.popularhotrodding.com/tec...#ixzz3AL9bVxzE

This is a graph published that shows original vs new compression ratio. The numbers show are theoretical percentage increase. You'll also notice this graph shows a rate of diminishing returns. EX going from 8:1 to 9:1 is more profitable than going from 10:1 to 11:1. etc.

[81GS850]

Some experience, mostly with car motors. I just don't think you're gonna see a lot of power gains with it, and it's gonna be a lot more work than what it's worth. My bikes rated at 70hp, a 4% increase would be 72.8

[stetracer]

Stetracer, Is the valve to piston clearance somthing that would be written down somewhere or would I need to measure it myself? Also, what direction would i need to adjust the timing? I have not had to fool with anything like that in the past.

it it is something you have to measure and it is at certain spots. I measure it at 10 and 5 degree btdc then at tdc then 5 and 10 atdc you should be able to find a video on utile on how to do this. For what you are doing I would not go more than .040 off the head. If your motor is all stock now then you should be safe. But I would still measure. .040 should raise the compression about 1.5 so if you have a 9 to1 it would make it about 10.5 to1. Which derection to move the cam is unknown at this point. I would set them at 105 intake and 107 exhaust. When you mill the head it will advance the intake and retard the exhaust cams some but the stock cam timing can be way off from the factory. So it could put one of the cams where you want it to where you only have to move one but it not certian. You have to check first

[stetracer]

Some experience, mostly with car motors. I just don't think you're gonna see a lot of power gains with it, and it's gonna be a lot more work than what it's worth. My bikes rated at 70hp, a 4% increase would be 72.8

If he was to match it up with a set of cams then he would be a happy camper. I think you get more from milling a motorcycle head because the chamber is so much smaller. But I am no car guy I only build bikes

[Steve]

When you mill the head it will advance the intake and retard the exhaust cams some but the stock cam timing can be way off from the factory.

Huh??? Since the cams are connected by a chain, why would one be retarded and the other one advanced?

I would think that with a milled head (or block), the cam gets a little closer to the crank, but the chain does not shrink, so the cam rotates backward to take up the slack. In my mind, that "retards" the cams. Both of them. Am I mistaken in my thinking?

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[stetracer]

Huh??? Since the cams are connected by a chain, why would one be retarded and the other one advanced?

I would think that with a milled head (or block), the cam gets a little closer to the crank, but the chain does not shrink, so the cam rotates backward to take up the slack. In my mind, that "retards" the cams. Both of them. Am I mistaken in my thinking?

You right I am not sure what I was thinking. In my defense I am sitting or lying in a hospital bed waiting for test to find out what's wrong with me now. Heart rate keeps going into the 30's must not be getting enough blood to my brain
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