How much can I mill my '906 heads

Posted By: 70Drop

How much can I mill my '906 heads - 03/14/11 06:23 AM

I'm building a 500" stroker motor for an A-body strip/street car, using a 440 Source rotating assembly with dished pistons and ported '906s. After cc'ing, I've found my compression ratio will be 9.5:1. I'd like to have more. Ideally, I'd like to be around 10:1. The easiest way for me to get there would be to mill the heads. I can't run a thinner gasket, since I'm flush decked and need to leave some safety clearance. Anyhow, I currently have about 0.075-inch between the gasket surface and the quench pad (the chambers are 82 cc's}. I don't think much has been taken off the gasket surface so far. How much can I take off safely without running the risk of head warpage?

Posted By: Kern Dog

Re: How much can I mill my '906 heads - 03/14/11 07:17 AM

It annoys me when people try to steer me away from my intended path, so instead I will just ask: Why is half a point of compression that important? Tests have shown that you gain on average 2-3% of power per point increase.
My machinist has said that a .030 cut will raise the squeeze by one point. If thats what you are after, a .015 surface mill should certainly be safe to do.

Posted By: dOc !

Re: How much can I mill my '906 heads - 03/14/11 07:27 AM

Quote:

It annoys me when people try to steer me away from my intended path, so instead I will just ask: Why is half a point of compression that important?

So THEN Fd ....why are you trying to STEER HIM away from his "path" ? ...

And that 2-3% ... is a bunchOhooey !!

Posted By: Kern Dog

Re: How much can I mill my '906 heads - 03/14/11 07:40 AM

Come on dude... You can READ! I was ASKING a question first, see?
Whats YOUR take on the gain % regarding compression? Got proof? Lets see!

Posted By: dOc !

Re: How much can I mill my '906 heads - 03/14/11 07:54 AM

Come on FRANKENdude ... lighten up !!

...

I have seen your sense-of-humor and style of writing before ... Mildly twisted ... I must say

...

Now the % ? ... again ...LOOK at what you are saying. LOTS of guys have taken smog motors and boosted the compression up 3 points ...and seen A BUNCH of performance increase.

Way WAY more than 6 or 9% as you suggest. Do I have factual numbers and dyno sheets in-front of me now ? ... No.

Posted By: JohnRR

Re: How much can I mill my '906 heads - 03/14/11 01:45 PM

I'll jump on the telling you what you should do instead of answering the question ... With open chamber IRON heads you don't really want to go much higher than your are now with your compression ratio due to the risk of detonation. Your heads have already had 8-10 cc cut out of them , I'm not sure what that .075 measurement is from what to what but if it is piston to head you can close it up to .035 -.040. What bore size ? You want to make sure the edge of the chamber is not overhanging into the cylinders .

Posted By: 70Drop

Re: How much can I mill my '906 heads - 03/14/11 05:02 PM

The bore is 4.365 (.040 over on a 440). And yes, the bore is bigger than the chamber in the head. The 0.075-inch measurement is from the head gasket surface to the quench pad portion of the combustion chamber in the cylinder head. That is measured by putting a straightedge over the gasket surface of the head and sliding feeler gauges underneath.

I figure I can squeeze a little more compression out of this motor, since I'll be running a fairly big cam with a late intake valve closing event. I don't have that spec in front of me right now, but I've run 10:1 on an engine with a cam that had an earlier intake closing and had no detonation problems.

Posted By: JohnRR

Re: How much can I mill my '906 heads - 03/14/11 05:59 PM

Quote:

The bore is 4.365 (.040 over on a 440). And yes, the bore is bigger than the chamber in the head. The 0.075-inch measurement is from the head gasket surface to the quench pad portion of the combustion chamber in the cylinder head. That is measured by putting a straightedge over the gasket surface of the head and sliding feeler gauges underneath.

I figure I can squeeze a little more compression out of this motor, since I'll be running a fairly big cam with a late intake valve closing event. I don't have that spec in front of me right now, but I've run 10:1 on an engine with a cam that had an earlier intake closing and had no detonation problems.

You need to round that edge over or you have the potential for a hot spot that will cause preignition. General rule of thumb is .005 for evry 1cc on a 906 so you need .0025 off to get your half point compression.