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post #1 of 19 (permalink) Old 12-08-2010, 06:46 PM Thread Starter
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To shave or not to shave?

In the quest for evermore performance through compression ratio increases, has anyone out there machined a set of 3.5 heads more than the FSM specified .008 inch? I'm supposing that bad things happen between the valves and block or pistons if you exceed this limit. If it's been done, what's the max cut you can take?

As an alternative, if you can't cut the heads over the .008 inch, thats not going to make much difference in compression ratio, has anyone ever welded up the combustion chambers to decrease the volume and increase the CR? It's relatively easy to weld extra material in alumunum, and machine it into shape so the valves are not shrouded and flow is maintained. Anyone done it?

Thanks for any input anyone has.

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post #2 of 19 (permalink) Old 12-08-2010, 07:31 PM
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i got to say that i believe that a performance increase probably would not be noticable. would also put extra stress on the head gaskets and other components for that matter. i would also think (but can not say for sure) that the pcm may be programmed to operate the engine on the compression ratio that the engine was originally designed to have. and perhaps to much compression could cause problems. and of course there is the chance for valve and piston damage if it is cut to much. it would totally suck a$$ to fire the engine up for the first time and actuomatically destroy it. IMO, not worth it

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post #3 of 19 (permalink) Old 12-08-2010, 08:58 PM Thread Starter
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i would also think (but can not say for sure) that the pcm may be programmed to operate the engine on the compression ratio that the engine was originally designed to have.
I have heard, and maybe someone on here can substantiate it, of someone running 12:1 CR in a 3.5 or 4.0 without the PCM having any problems.
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post #4 of 19 (permalink) Old 12-10-2010, 10:49 PM Thread Starter
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and of course there is the chance for valve and piston damage if it is cut to much.
Thats the reason too for asking if this has been done, to avoid this scenario. And i plan to clay up the cylinder before assembly to make sure there is clearance on everything.
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post #5 of 19 (permalink) Old 12-10-2010, 11:59 PM
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Thats the reason too for asking if this has been done, to avoid this scenario. And i plan to clay up the cylinder before assembly to make sure there is clearance on everything.
yeah i guess the lack of responses means nobody knows. bummer
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post #6 of 19 (permalink) Old 12-11-2010, 12:29 AM Thread Starter
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I believe you're right. I guess I'll post the results as i go and call it a learning curve.
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post #7 of 19 (permalink) Old 12-11-2010, 01:25 AM
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I believe you're right. I guess I'll post the results as i go and call it a learning curve.
Thats one way to do it. Keep us posted.
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post #8 of 19 (permalink) Old 12-11-2010, 11:51 AM
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I believe you're right. I guess I'll post the results as i go and call it a learning curve.
yeah if you do it let us know what the results are because you got me interested
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post #9 of 19 (permalink) Old 12-17-2010, 01:19 PM
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You can't cut too much off of the heads on overhead cam engines because you won't get your valve timing right. The belt is obviously a fixed length so if you change the distance from the crank to the cams, you can't line up the timing marks (unless you have adjustable cam sprockets). I learned this lesson in the 90's. I had an Isuzu truck with the 2.6 4 cylinder. It developed a crack in the head so I bought a remanufactured head. It had been shaved too much but I didn't figure it out immediately. The timing marks were always a half tooth off and the compression was crazy. I had to run the highest octane gas available and retard the ignition timing so it would run. I was getting check engine lights and it just drove me nuts but I ran it like that for around a year before one day it dawned on me. I did play with that 1/2 tooth leeway to see which way made the engine run better.

I was in the Army at the time so I didn't have a lot of free time so I kept putting off dealing with it. I did get a used head and put it on, solving all the problems but then it felt underpowered. All that compression (around 180-190 lbs) compared to the 130-140 I got with the unshaved head made a difference. I traded the truck off at a car dealership a year later, and a few weeks after that I saw the truck sitting in their maintenance section. I stopped in one day and asked about it, it has developed a bad rod knock a week after I traded it during a test drive. Talk about dodging a bullet.
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post #10 of 19 (permalink) Old 12-17-2010, 09:34 PM
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...you can't line up the timing marks (unless you have adjustable cam sprockets)...
Our cam sprockets are adjustable - takes a special tool to do it right, but they are adjustable.
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post #11 of 19 (permalink) Old 12-17-2010, 11:32 PM
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I think girls should always shave.

Err, whats this thread about?

Why not try the route of using different pistons to alter compression ratio?

Its an interesting theory. Care to do the math to figure out how much a change in compression ratio you could achieve?
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post #12 of 19 (permalink) Old 12-19-2010, 10:15 AM Thread Starter
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I haven't measured the combustion chamber volume of a stock head yet, but will have to do that and do the math to figure out how much material would have to be added to the head to increase the CR. I'm sort of stuck using stock 4.0 liter pistons (it's a stroker motor), not sure if anything else will fit without interference. I have two sets of heads, may use stock ones first and then modify the second set for later installation. I hadn't considered the effect that shaving the heads would have on the timing belt and sprocket alignment, but that makes sense. Mopar Performance used to make a one tooth shorter timing belt for the 2.2/2.5 liter engines for just such a reason, a lot of guys were shaving those heads to get a performance increase through raised CR. Too bad there isn't one available for the 3.5 liter.

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post #13 of 19 (permalink) Old 12-19-2010, 10:15 PM
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Originally Posted by dodgebob68 View Post
Mopar Performance used to make a one tooth shorter timing belt for the 2.2/2.5 liter engines for just such a reason, a lot of guys were shaving those heads to get a performance increase through raised CR...
If you think about it, shortening the belt one tooth would not affect timing one iota. All it does is move the tensioner back closer to its original factory position so you're not close to reaching the end of its extension range. Think about it - I think you'll see what I mean.

Does the spark time off of the crank position sensor or the camshaft position sensor? Again, the cam timing itself (mechanical) is adjustable (D holes in sprockets).

Last edited by peva; 12-19-2010 at 10:23 PM.
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post #14 of 19 (permalink) Old 12-19-2010, 10:21 PM Thread Starter
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I believe its the cam position sensor. How are they adjustable if they only fit the cam snout one way on the D shank?

I mean besides the obvious of installing the belt on the sprocket one tooth off.

Last edited by dodgebob68; 12-19-2010 at 10:24 PM.
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post #15 of 19 (permalink) Old 12-20-2010, 02:39 AM
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I believe its the cam position sensor. How are they adjustable if they only fit the cam snout one way on the D shank?

I mean besides the obvious of installing the belt on the sprocket one tooth off.
Pretty sure our cams don't work that way. If you have the cam lock tools you can rotate the sprockets freely with the bolts loosened, then wherever you tighten them down the cams are still at the TDC position.
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