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Old 04-23-2007, 07:39 PM   #1
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Default L36 Heads; cost/benefit for machine shop work...

As a follow-up to this thread:
http://www.bonnevilleclub.com/forum/...ic.php?t=76453

I'm contemplating some head work, since I can't shake the mod monkey off my back.

Preamble: I've got a front PEM and 1.8/1.85 split ratio modded stock rockers, and am scoping LS6 springs/retainers and viton seals. Those are either going on my current heads, OR some "worked" heads...

I've got a set of used L36 heads coming that I bought for $50 shipped. I figure that'* a low ante, and even if they turn out to be trash I can use them as a template for a new and improved, "v2" ported LIM, and the rockers/PEM, etc. can go on the current heads.

I plan to port and polish these heads myself, using the guide on the standard abrasives website, et.al....heck, even if I wind up trashing them, no huge lost investment (see above). I will have plenty of questions when the heads get here, believe me...no "secret work"!

Assuming P&P goes well, I've called around and here'* the best "all from one shop" quotes I've gotten for local machine work (this guy seemed familiar with GM 3800'*...he wanted to sell me an "abandoned" ported M90 */C and ported L67 LIM he'* got in the shop!). After a free pre-inspection for cracks, valveguide play, etc.:

1) Three-angle valve job, including "standard" valve machining: $125
2) Plus additional machining on valves (25 degree undercut on all 12 valves): $25
3) Plus mill off 0.020": $125

Assuming it gets this far, #1 is a given. For $25 more, #2 is a safe bet (assuming stock valves are safe to undercut...anyone know?).

I wonder about #3...let'* just say the heads are flat enough not to need any milling at all (how likely is that, anyway?); is $125 to get 0.020" shaved off worth it?
(Primary purpose of milling that much off would be to increase compression from 9.4:1 to a calculated 9.75:1...with the other mods, what kind of gain might we be talking from milling?)

P.*., How are these prices? I called 5 different places and these were the best I got, as well as the only place that seemed familiar with anything GM besides SBC/BBC...
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Old 04-23-2007, 08:49 PM   #2
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i think you got the mod bog going full steam in youre head .that ok because i have about the same question going in mine good luck whit youre mod
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Old 04-23-2007, 10:44 PM   #3
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Your probably looking at less than 10 whp for the milling but you should be able to get better overall VE.
Most of the price for milling is for setting it up. Once it'* on there, there won't be much of a price difference.
Cast Iron heads don't usually warp very much. My Milzy Motorsports heads were milled less than 0.010" to get them flat.
Not real sure on the pricing though..
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Old 04-24-2007, 02:16 AM   #4
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Quote:
Originally Posted by 95naSTA
Your probably looking at less than 10 whp for the milling but you should be able to get better overall VE.
Most of the price for milling is for setting it up. Once it'* on there, there won't be much of a price difference.
Cast Iron heads don't usually warp very much. My Milzy Motorsports heads were milled less than 0.010" to get them flat.
Not real sure on the pricing though..
"Less than 10 hp" could be 9.5, or it could be 0.5

If I could expect 5+ hp on top of all the other mods, I'd call the milling potentially worth considering...of course there would never be any way of separating out the effect unless I was willing to test the P&P'd, valve-jobbed heads both before and after milling...
(anyone got a spare set of FelPro head gaskets and a spare set of tty head bolts sitting around? )

So, I could expect to pay about the same whether I'm milling off "alot," or just enough to get flat. We'll just have to see about the condition of these heads when they show up.
I fully expect the shop to tell me they need milling, no matter what my straightedge and feeler gauge say.

Also, any comment on whether stock valves can be safely undercut?

Since I've gone crazy already, let'* discuss an even crazier idea; just how much CAN be milled off these heads? How far until valve to piston clearance becomes a concern (at a calculated 0.498" max lift, not accounting for rocker deflection)? As I understand it, any more than the 0.015 to 0.020" milling will require shorter pushrods...can stockers be shortened? At what point does the valvetrain geometry get too stressed?
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Old 04-24-2007, 11:33 AM   #5
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Quote:
Originally Posted by agrazela
"Less than 10 hp" could be 9.5, or it could be 0.5
lol. I was just trying to put a ceiling on it.
I know it'* only a 2-3% increase or so per full point of static compression.
Try this calculator. It actually shows even less hp.
Popular Hot Rodding has a interesting article on compression ratio changes too.

If your interested in how the bump in efficiency occurs, just take a look at a p-v diagram for the Otto cycle. When you raise the compression ratio, the compressed volume is smaller and the end point for the compression process is shifted further to the left. This adds area between the curves which is work. (not sure if you know all this already)

Quote:
Originally Posted by agrazela
Also, any comment on whether stock valves can be safely undercut?
Not really sure. I don't think I've read about anyone doing it on these engines.

Quote:
Originally Posted by agrazela
Since I've gone crazy already, let'* discuss an even crazier idea; just how much CAN be milled off these heads? How far until valve to piston clearance becomes a concern (at a calculated 0.498" max lift, not accounting for rocker deflection)? As I understand it, any more than the 0.015 to 0.020" milling will require shorter pushrods...can stockers be shortened? At what point does the valvetrain geometry get too stressed?
ZZP milled 0.10" off L36 heads in conjunction with a GT3 cam but they had to mill the top of the pistons for clearance. I can't say for sure how far you can go without hitting the piston with stock duration and that lift. I do know that 0.050" has been done with a GT2 cam though.
I wouldn't shorten the stock pushrods. I can't imagine the headache you would have to go through to get that done and you would end up with something significantly weakened.
Another option for making up milling differences is shimming the rocker pedestals. I don't really like this idea since it changes how close the tip of the rocker comes to the edge of the top of the valve.. Higher ratio rockers are doing this too and I would rather not add to it. Maybe getting a shim that is 0.005" if your milling 0.020" would be worth it to get it closer to stock.. I really can't say for sure. Just throwing that out there.
Always take care to make sure the tip of the rocker is contacting the top of the valve and not coming close to the retainer.
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Old 04-24-2007, 12:35 PM   #6
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95naSTA,

Thanks for the calculators. That paints a pretty grim picture regarding what can be achieved through increasing compression by milling (without breaking the bank, anyway).

I think the plan'* gotta be this: If the heads don't need milling just to make them flat, then let 'em be. If they do need milling, go ahead and get a max of 0.020" taken off, so I don't have to go nuts with concerns like over-stressing the geometry, hitting the pistons, buying new pushrods, etc.

Here'* the used modded stocker split-ratio rockers I bought:


The "shiny bit" on the tips of the higher-ratio ones (bottom row) does come perilously close to the edge already.

As for undercutting the valves, I'll let the machine shop guy make that call. He did say all his work was certified and guaranteed, so I guess that'* something. On the other hand, maybe the $125 I might not spend on milling could be put to better use with upgraded (maybe larger) valves...?
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Old 04-24-2007, 01:36 PM   #7
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I would go with the rise in compression over valves if I had to choose. Some new undercut swirl polished valves will flow pretty nice at first but they will get all carboned up eventually and not flow much better than stock.
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Old 04-24-2007, 10:05 PM   #8
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Quote:
Originally Posted by 95naSTA
I would go with the rise in compression over valves if I had to choose. Some new undercut swirl polished valves will flow pretty nice at first but they will get all carboned up eventually and not flow much better than stock.
Yeah, I think you're right. I've been running some calculators, and it doesn't seem like larger valves will do me any good without a cam (which I'm NOT doing!). If the machinist thinks undercutting is safe for the stock valves, it'* only gonna run me $25.

Interestingly, the calculators also tell me my best bet on the rockers is to go either 1.80 or 1.85 ratio on the intake, and stick with the unmodded 1.6 on exhaust...but that is a topic for another thread (as well as an awesome chance to collect and share data on various rocker configurations )
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