How much milled off heads to raise compression

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Brooks James

Brooks James

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360 magnum stock heads
If heads are milled do I do the same on the intake.
I’m trying to go from 9.0 to 10.5
How much do I cut them??? Is there a formula ???
Also need recommendations for thin
Head gaskets ; steel vs composite???
Thanks
 
Brooks James said:
360 magnum stock heads
If heads are milled do I do the same on the intake.
I’m trying to go from 9.0 to 10.5
How much do I cut them??? Is there a formula ???
Also need recommendations for thin
Head gaskets ; steel vs composite???
Thanks
Click to expand...
Just my two cents, but it would be a whole lot easier to get some new pistons to raise the compression rather than milling the heads. To pick up a point and a half of compression from milling the heads, I'd imagine that would be a pretty substantial cut, but I'm no machinist. I'd think that you'd definitely have to have the intake machined, and then the heads and intake are married. Seems like a ton of work for stock Magnum heads.
 
Hi Brooks,

My Moparmate milled '100 thousanths' which is 2.5mm and the chamber is almost flat now!
Probably 10 to 1 compression currently, but the intake face needs to be milled also, for manifold fitment.
Vey expensive way to go and the biggest problem was, it hit the intake valve area so the 2.02 valves had to be sunk down to remake the seat!
This is apparently a No-No indeed...

So be careful
 
Yeah new pistons sounds good but the frig factor is high. Would they have to be custom made or does someone make them already? The cam I want
Calls for 10.5 to one and now have
9 to one, if I’m lucky. BTW does nitrous work better with high or low compression??
 
Hi Brooks,

I would never go over 10 to 1 in a street motor for sure!
If its a race motor then you need forged pistons that are specific for Nitrous.
If not then the ring lands will be destroyed in a heartbeat!

Pump gas is not good for anything over 9.5 so be careful.
Low comp is not a disaster and small cams do work fine with NOS too.
 
Would the cam 512 /286
498/274 and 150 shot nitrous work well?
Specs:
360 magnum stock heads/pistons/block
Rods/crank
Edelbrock knock off rpm intake
Holley 750 double pumper
MP dizzy 36 degrees all in by 2600
3.91 rear gear
904 trans
3000?converter

off topic: I’m thinking the motor will have about 500 hp at the flywheel and the car weight is 3200- with adequate traction do you think it will dip into the 11’s
 
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No particular formula that I know of, and how much the CR will be raised for any given cut will depend on chamber volume and total cubic inches. So it will vary from one motor to another. However, a rule of thumb that will get you kinda close most of the time is one half a point for every .030.

You can typically cut .030 without having to cut the intake surface (unless you're really a perfectionist), but more generally requires a cut on either the intake surface of the heads or on the intake itself. Also, front and rear sealing surfaces of the intake/block interface will need clearancing (on a small block), although substituting RTV for the end gaskets should suffice.

But, you want to go from 9:1 to 10.5:1, which will require close to a .100 cut. I'm not an expert on magnum heads, but I'm thinking they won't take that big a cut. Plus, now we're talking about potential valve to piston clearance issues, as well as rocker arm geometry issues. Probably best to change pistons.
 
65 Cuda 340 said:
No particular formula that I know of, and how much the CR will be raised for any given cut will depend on chamber volume and total cubic inches. So it will vary from one motor to another. However, a rule of thumb that will get you kinda close most of the time is one half a point for every .030.

You can typically cut .030 without having to cut the intake surface (unless you're really a perfectionist), but more generally requires a cut on either the intake surface of the heads or on the intake itself. Also, front and rear sealing surfaces of the intake/block interface will need clearancing (on a small block), although substituting RTV for the end gaskets should suffice.

But, you want to go from 9:1 to 10.5:1, which will require close to a .100 cut. I'm not an expert on magnum heads, but I'm thinking they won't take that big a cut. Plus, now we're talking about potential valve to piston clearance issues, as well as rocker arm geometry issues. Probably best to change pistons.
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Yeah I thought about it and I’m not sure the stock pushrods will work-here I go again opening a can of worms..maybe be too much lifter preload
 
Not sure of how much cut equals cc's but the formula is for every .020 off the block side, take .019 off the intake side of the head. I would not take more than .040 off magnum heads. That leaves new pistons to get to 10.5 :1.
 
If you do mill the heads the machine shop should know how much to get to where you want to be. They will also mill the intake surface a lesser amount. If they don't know you're using the wrong shop. I wouldn't go for 10.5/1 unless you run race fuel all the time. (unless you have aluminum heads)
 
My moparmates heads are 051 77-83 'hipo' with 100 thou. skimmed off!
Not sure which ones are Magnums, but they have little or NO chamber now!
Fitted with 2.02 vlaves and ran real nice without too much porting work.
I am gonna be doing a FULL-ON porting jobbie for him over the next few weeks.
Its a 318ci +30 rebore with ICON forged pistons and reworked 340 rods, being changed out for Eagles.
Beat my other moparmates 440 car with ease lol...

Just seen this a 360 bored +100 thou and destroked to 376ci and making 770hp...:wtf:


You would have thought they got the oilpan 'the right way around'...:thankyou:

2hp per cc 360.jpg
 
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I've never heard so much hearsay in one thread in quite some time.
First off aluminum heads won't cure your 10.5-1 race fuel issue LOL...
Second you can hit your factory Pistons with 150 shot all day long and not do any damage (IF DONE CORRECTLY!!)...
Nitrous favors lower compression..
 
With a stock stroke 360, to go from 9:1 to 10.5:1 will require a reduction of about 14.5cc’s to the unswept area volume(the area above the piston at TDC).
 
PRH said:
With a stock stroke 360, to go from 9:1 to 10.5:1 will require a reduction of about 14.5cc’s to the unswept area volume(the area above the piston at TDC).
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With his eyes set on 11 seconds what's that going to get him?...
 
j par said:
I've never heard so much hearsay in one thread in quite some time.
First off aluminum heads won't cure your 10.5-1 race fuel issue LOL...
Second you can hit your factory Pistons with 150 shot all day long and not do any damage (IF DONE CORRECTLY!!)...
Nitrous favors lower compression..
Click to expand...
Just what I want to hear I’ve built a lot of safety into the nitrous. Engine shutdown if fuel stops, from dash adjustable timing, 2 stage nitrous controller,fuel pump for nitrous only. I read that nitrous prefers low compression.
At this point it is a no brainer.
Nitrous is almost free horsepower vs
New pistons at 400 a pop not to mention pulling the engine and basically doing a rebuild. !!!!!!!!
 
j par said:
I've never heard so much hearsay in one thread in quite some time.
First off aluminum heads won't cure your 10.5-1 race fuel issue LOL...
Second you can hit your factory Pistons with 150 shot all day long and not do any damage (IF DONE CORRECTLY!!)...
Nitrous favors lower compression..
Click to expand...
Iron heads and 10.5/1 compression will probably detonate on 87 octane. 10.5 with aluminum heads won't or shouldn't. 9.5/1 I as about the limit with pump gas and iron heads on the street. These statements are to err on the cautious side. Too many newbies read the high compression blather and get themselves in trouble.
 
toolmanmike said:
Iron heads and 10.5/1 compression will probably detonate on 87 octane. 10.5 with aluminum heads won't or shouldn't. 9.5/1 I as about the limit with pump gas and iron heads on the street. These statements are to err on the cautious side. Too many newbies read the high compression blather and get themselves in trouble.
Click to expand...


For the umpteenth time, aluminum heads do not reduce detonation. Doesn't happen, unless you do to the engine what Engine Masters did in their test and you force the engine into massive heat load. That’s should NEVER happen.
 
yellow rose said:
For the umpteenth time, aluminum heads do not reduce detonation. Doesn't happen, unless you do to the engine what Engine Masters did in their test and you force the engine into massive heat load. That’s should NEVER happen.
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They do not reduce detonation but you can run more compression without detonation issues. WTF do I have to do, write a AJ long thread with all the disclaimers?
 
toolmanmike said:
They do not reduce detonation but you can run more compression without detonation issues. WTF do I have to do, write a AJ long thread with all the disclaimers?
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You can run the same compression with both. WTF, I’ve typed that out a hundred times...head materiel does NOT affect what fuel you can run or what compression ratio you can use.

The coolant can’t absorb the spike in heat fast enough to make a difference. Like I said, the EM test only showed what happens when you heat load an engine way beyond what it should be. That’s why we have transmissions...downshift on a long pull and don’t lug the engine.
 
I'll take that as gospel then and consider all the information out there as hogwash. Why the heck does everybody waste time and money to use aluminum heads if only to save a little weight and may be easier to modify?
 
toolmanmike said:
Iron heads and 10.5/1 compression will probably detonate on 87 octane. 10.5 with aluminum heads won't or shouldn't. 9.5/1 I as about the limit with pump gas and iron heads on the street. These statements are to err on the cautious side. Too many newbies read the high compression blather and get themselves in trouble.
Click to expand...
This hasn't been my physical experience... $800 Speedmaster heads, $95 to have them checked out, $200 plus cemetic head gaskets (extra thick to lower compression).. intake manifold gaskets exhaust gaskets valve cover gaskets new coolant new oil... $$$..
SAME DETONATION PROBLEM!...
I love my new aluminum heads all I have to do is just add a little race gas so it doesn't run on when I turn it off...
Or you can just throw this real world experience out the window and make something up..
 
You are both right, TMM and YR. You can run more compression on a aluminum head engine because the aluminum head flex's more, causing 3 cc's more of chamber. I put my go-pro hero 8 under my hood and when floored under load, you can see the head bulge up under the compression stroke above the cylinder of power stroke if you play the camera in super slow motion to the measurable of 3cc's. That is why aluminum heads allow more initial compression, though they give it right back throughout the RPM.

P.S. - I always run a .050 head gasket to accommodate the flexing so no leaks


:D
 
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toolmanmike said:
I'll take that as gospel then and consider all the information out there as hogwash. Why the heck does everybody waste time and money to use aluminum heads if only to save a little weight and may be easier to modify?
Click to expand...
Because I keep listening to this crazy hearsay that gets perpetuated on and on I know I did...
 
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