houston, i think we have a problem...

my 6 inch rod 383 is built and in the car. when we went to install the intake the bolts would not line up on both sides. we can start one side by hand, but then the other side is off slightly. we initially concluded that this was due to the fact that my heads have been milled approximately .008" during their 2 trips to machine shops.

imagine my surprise when i took the intake to the machine shop today to have the bolt holes elongated. i told the machinist what the problem is, and he told me they had to deck my block .012" on one side and .015" on the other to get it square!! funny that he didn't mention this when i got the shortblock built.

anyway...at this point i'm obviously very concerned about the compression ratio of my new high dollar stroker. my math tells me that one side of the engine has .020" less material between the head and the deck, and the other has .023" less. i don't necessarily mind compression (my race car is 12.5:1), but i don't want to waste my time with pump gas if it's going to detonate the first time i go down the block.

assuming i can't get deck height numbers from the machine shop (i'm gonna inquire tomorrow when i pick up the intake), is there a good way to approximate the CR? the combo is as follows: 6" rod LT1 383 (scat crank and rods, srp flat-top pistons), ported stock heads milled .008", 2.00/1.56 valves.

any advice/opinions/suggestions would be appreciated. if i need to start mixing some race gas to put in before initial start up i will, but the engine is going to be fired once the intake is on (to make sure it seals to the block) and will not be coming back out of the car.

You need to find out the chamber CC's and the piston height in the hole at TDC.

chambers are 52cc's. the heads aren't coming back off to get the piston height in the hole. i'll find out if the machine shop measured it tomorrow.

Here are a couple of combinations:

4.03 bore x 3.75 stroke, 52 cc heads, .039 compressed gasket with 4.12 bore:

1) -8 cc in pistons for valve relief, 0 deck = 12.44:1

2) -8 cc pistons, .010 down in the hole = 12.10:1

3) -8 cc pistons, .020 in the hole = 11.78:1

4) -11 cc pistons, 0 deck = 11.96

5) -11 cc pistons, .010 in the hole = 11.65

6) -11 cc pistons, .020 in the hole = 11.36

Interpolate between CRs for different deck heights.

Only at wide open throttle should you need race gas. Part throttle start up and break-in should be fine on 91-94 premium.


Afterthought: Are you sure about 52 cc heads? Could they be 62 ccs?What heads did you start with, and what chamber volume?

i might have got the chamber cc's backwards. i started last year with stock heads (56 cc's, right?) and have had a total of .008" shaved off.

what are your cam specs?

I believe stock is 54CC's if Im not mistaken..

Cody

On the 5 or six lt1's I have worked on the stock pistons when measured all were about .025 down in the hole + or - about .001

When i mocked my block up with the scat 3.750 stroke crank 6 inch rods and pistons they measured .022 down in the hole. So this could change a little with each combo and piston type. But this will get you close.

Based on this you would be .008 to .010 down in the hole on one side and .013 to .015 down in the hole on the other side

All of the stock Aluminum heads that we have cc'd have come out between 58 ccs and 60 ccs per cylinder. I had .005 removed just to true mine up and ended up at 56ccs so you will probably be at 54 or 55 ccs. You can do a little to help your self if you feel compression is to high. You can gain about 2 to 3 ccs by unshrouding the valves. and if you polish the chambers and remove all the rough edges you can get another cc back.

Also I have a real problem with any shop that cuts more off of one side of a performance block than another this leaves you with having to buy 2 sets of different thickness head gaskets to try and even cylinder pressures up. And I know some will say .005 don't matter but it does when your already on the edge of detonating anyway.

You will need to know what dish piston you have to get a final compression ratio there are several different depth dishes.

Hope this helps

Originally posted by TIMEBANDIT


Also I have a real problem with any shop that cuts more off of one side of a performance block than another this leaves you with having to buy 2 sets of different thickness head gaskets to try and even cylinder pressures up. And I know some will say .005 don't matter but it does when your already on the edge of detonating anyway.



Isn't it possiable that the shop had to square the block to the crank centerline? That could be a reason they took more off one side than the other. If that is the case then your piston would end up being at equal depths in the bore.

RED has a point they could have done this to square your block to the centerline of the crank. And in that case your pistons would be equal distance down in the hole with both sides. But if this is not the reason then you will be hunting for different thickness gaskets. Also your machine shop should be able to tell you if this was the case.

And I spoke with Remy our block guru and he said that he has seen very few small blocks that would require any more than a .002 to .003 difference to accomplish this. And he always issues a spec with his blocks stating what factory specs are, what the block measured when he got it, and what he cut the block to. Along with how much it took to true the deck surfaces and how much it took to make the center line to bore the same on both sides.

This is why it is so important to do a mock up of all your parts and measure everything before you put it together.