Calculations ??

[Scott Coxwell]

How do you figure rod & crank ratios to determine what deck height block you need?

[supernova]

Scott, that's kind of a loaded question. The only reason I say is you left out the third part to equation. The piston! The piston has what is called compression hight. Thatis the distance between the center of the pin hole to the top of the piston (the flat part). Pistons can come in a verity of compression hight distances.
Trying to figure out your way might put you in a block you don't need.

This is how to figure it one way and it's how I do it. Ok hear goes.
Block hight - decking (machine work to the deck) - half of stroke - rod length = piston compression hight.
Exaple:
My block is a tall deck big block, 10.200 hight. Crank is 4.500. Rods are 6.535.
The engine has been decked .015 so the deck is now 10.185.
Now take 10.185 - half of stroke, which is 2.250 = 7.935 - rod length which is 6.535 = 1.400. That is the compression hight of the piston I need to look for. I found a SRP piston with a hight of 1.395 so it left .005. It came out as I wanted it to.
You may already have a block that will work if there is a piston with the right compression hight you don't know until you do the math.

That's how I do it. Hope it helps.

[70 nova]

Hey Supernova, I'm kind of building a motor similar to yours, but it's going to be a tall deck 496(4.25x4.310). I always thought you couldn't get a 4 1/2 stroke crank in a production tall deck block, unless you have a dart block or something like that. Are those SRP pistons nitrous alloy? Just kicking around ideas of maybe building a little bigger motor. By the way, thanks again, a while back you Pm'd me pictures of your rack setup.

[supernova]

Hey 70 nova,

I had an old boat racer give me the logist on building a motor like this. It is sort of an odd combination. It's a lower rpm motor, max rpm is 6800 which is low compaired to today's race motors. Now here is the but, lower rpm's saves on wear and tear, parts and ones pocket book. This started out as a big street peace and went from there.

Your motor will turn more rpm's because of the shorter crank. And the cam shaft will have effect too. I thought about building one like you are but the boat engine builder that helped with my project was very influinchal (spelling) and so in goes the 4.5 crank. It was buit as a TQ. motor and nitrous makes TQ.

As far as pistons. The class the car built for is limited on how much n2o I can run so the piston are not nitrous pistons but are forged 4032 alloy.
J&E rate these pistons for about a 300 shot max but I know some racers that shoot more to the same pistons I have. Remember this motor started out as a street motor. It is a 12.2 to 1 motor and it does have hellfire rings in it. The 4032 alloy is a harder compond alloy so they will ware better and last longer. That's if you don't over do it with n2o. My class allows my size motor to have a single pate and a 73 size nitrous jet. That's about a 240 hp system I will run about 200 hp nitrous system.

Oh and yes you can fit a 4.5" stroke in a factory tall deck. It takes lot of block work.