How do you determine deck clearance in an engine using math?

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Kern Dog

Kern Dog

Build your car to handle.
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I have fuzzy memories about this subject...
There have been guys that post about this or speak of it but I don't recall how it is calculated.
Lets say that you know a published deck height of an engine, the stroke, the rod length and piston compression height. What is the formula to get an estimate on where the piston will be at TDC?
Part of my curiosity stems from my recent experiences shopping for pistons.
The KB 107 has a compression height of 1.675. The Speed Pro H116 CP has a height of 1.660 meaning the KB is .015 taller.
Can you spill the details of how to figure this?
 
Stroke ÷ 2 + conrod centre to centre length + piston compression height.
Subtract that number from the deck height
 
Thanks.
I don't know the actual deck height, the length of the rod either.
This is a 1990 360.
 
I use the United Engine and Machine calculators, because I'm too lazy for all that.
 
Yeah gotta measure to be sure, 1990 block - would that be a magnum block? I read somewhere they have a shorter deck height than the LA
 
Ted265 said:
Stroke ÷ 2 + conrod centre to centre length + piston compression height.
Subtract that number from the deck height
Click to expand...

RustyRatRod said:
LA 360 deck height is 9.6" connecting rod length is 6.123".
Click to expand...

Okay, here we go with the Speed Pro piston that is a 1.660 compression height:

3.58 divided by 2 = 1.79.
1.79 + 6.123 = 7.913.
7.913 + 1.660 = 9.573.
9.573 - 9.6 = -0.027. This theoretically puts the Speed Pro piston .027 in the hole assuming the block hasn't been milled since new and matches the spec.
This does help!
That makes the KB 107 and the 232 pistons approximately .012 in the hole.
Thank you guys!
 
Kern Dog said:
Okay, here we go with the Speed Pro piston that is a 1.660 compression height:

3.58 divided by 2 = 1.79.
1.79 + 6.123 = 7.913.
7.913 + 1.660 = 9.573.
9.573 - 9.6 = -0.027. This theoretically puts the Speed Pro piston .027 in the hole assuming the block hasn't been milled since new and matches the spec.
This does help!
That makes the KB 107 and the 232 pistons approximately .012 in the hole.
Thank you guys!
Click to expand...
Yup, that's what the calculator says too. Easy to achieve zero deck height.
 
RustyRatRod said:
Nope. LA
Click to expand...
Ah ok, i wasn't sure what year the change to the magnum was. Being a late model casting you would hope the factory tolerances had tightened up, but the 70s blocks I've had apart were a good way off the factory quoted 9.6".
 
Ted265 said:
Ah ok, i wasn't sure what year the change to the magnum was. Being a late model casting you would hope the factory tolerances had tightened up, but the 70s blocks I've had apart were a good way off the factory quoted 9.6".
Click to expand...
I wasn't either. I had to look it up. lol
 
You really need to confirm the deck height if you want an accurate number. Not easy to do unless you have the mesauring equipment.
 
Bewy said:
You really need to confirm the deck height if you want an accurate number. Not easy to do unless you have the mesauring equipment.
Click to expand...
Oh, I agree. That will be done. I was just curious about the method that I'd heard people use to get the approximate deck clearance numbers.
The last 2 engines that I built, I had the decks milled to get the deck height where I wanted to get the compression ratio to the target numbers.
Ever try to build a 383 with some decent compression? I found that the short stroke really makes it difficult unless you use domed pistons. Even with the pistons a mere .009 in the hole, I was still only able to get it to 9.2 to 1 with iron heads that also had been milled.
 
There are not enough significant figures in the published deck height measurements to make this calculation even remotely accurate. Everything must be measured to the thousanth.
 
01 pffft.jpg


I'm not looking at the math to be an exact science, just to be a guide.
I am aiming to end up somewhere from 9 to around 9.5 to 1. A few thousandths won't make a difference to me.
 
What is the formula to get an estimate on where the piston will be at TDC?
Click to expand...

I generally use addition and subtraction.
There’s a little bit of division involved too.
 
If you are running alloy heads with flat tops on the street;

some of the guys on FABO are running a tic over 220psi on pumpgas, with a tight squish/quench.
I myself have run up to 195psi, still on 87E10.

The point is, that to get that high pressure you will need to coordinate with the ICA of the cam; and if you start with an Scr of only 9.5, then you could quickly end up with a doggy engine pushing 130psi. I guarantee that 130psi will be a big disappointment.
Once I tasted 195 plus psi, I know I'll never go back to even 160psi.
-----------------------------------------
BTW
my KB 107s dropped in at .012 below the deck on my 1971 block. I decked it to zero, and I ran that with an 028 gasket. But at the end of the season, on teardown, I found that gasket had migrated nearly into the valley.
I cut the decks again for a pop-up of .007, and used the FelPro 039s.
Three years later, my little 223* cam dropped lobes.
So I installed the next bigger cam, (230) and I advanced that cam, in an effort to get more pressure. But it was no use.
I got me a transmission with a lower first gear, and retimed the cam to straight up. Pressure is now around 185, which is NOTHING like 195! But, the top-end rush sortof makes up for it.

Of course if you cannot afford alloy heads, you are sortof stuck down at 165psi max on best pumpgas.
Whereas at up to 195, I was still on 87E10.
At that time my car was a DD, and I estimated that those alloy heads had payed for themselves in about two summers, just in the difference of the cost of the 87E10 versus 91 gas, alone. Not even allowing for the difference in fuel economy, which I have no way of comparing.

Some guys on FABO poo-poo high pressure, which I don't understand. I'm never going back to Iron heads @155, which barely supports 87E10.
 
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Remember that calculating that is based on a crankshaft that theoretically has all the throws indexed perfect. I'll let yall guess if that's allways the case.
 
When the goal is to be between 9.0 and 9.5, there is room for some variance. I'm not referring to bearing clearance here where a rod with .008 will rattle and one at .002. runs quiet.
A few thousandths difference in deck height will not make a POINT in compression. By a few, I mean less than 10.
A few CCs in combustion chamber volume will.
 
Deck heights are all over the place, on an unmolested block the height is probably taller than spec. If the plan is to zero deck the block, your math becomes much more accurate. I have seen the small ends on a set of rods machined not parallel to the crank journal. The biggest unknown is deck height at this point.
 
When I built my 400 I did the calculations and figured the block would need to be milled .025 to get the pistons at zero deck. The machine shop measured and said they only had to remove .010 or .015, I forget exactly. Guess what, the pistons ended up .015 below the deck. So much for precision measuring.
 
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