CR Ratio question

[quickv6]

I know I am wearing you guys out. My project is in full bloom and ready to install engine in truck. I have installed splayed caps on my block have it ready to go. I am using the stock CR right now guess thats 21-1. How much would I need to mill off the pistons to get 18-1? Is it really worth doing?

[Hubert]

Don't know the dimension to mill off but I don't think its worth it if you are not: towing heavy or going to add an intercooler and maybe a bigger turbo. You should be able to run 14-15 psi boost intercooled with stock compression. If no intercooler ~12 psi is max recommended due to compression heating of intake air anyway. Maybe a good idea to list plan in signature I may have missed a related comment.

[quickv6]

Have new engine with splayed caps, Intercooler, HX35w Holeset turbo, 3" downpipe and exhaust, Marine injectors, Lightly ported heads and intake, Cooling upgrades.

[rustyk]

I can't think of any reason to mill pistons down to lower CR. If it were I, the bullet would be bitten, and aftermarket 18:1 pistons installed. But then, I don't like overhauling engines I need.

[quickv6]

I can't think of any reason to mill pistons down to lower CR. If it were I, the bullet would be bitten, and aftermarket 18:1 pistons installed. But then, I don't like overhauling engines I need.

Just cat see spending a grand when I can put the same piston they are going to sell me in the piston vise of the bridgeport and cut it myself. . The truck is running. This is the new motor dont need it yet. :)

[quickv6]

anyone know the Head CC?? CC of stock piston??

[ogrice]

I run stock pistons with .030 shaved off of the tops. Been running hard for the last 4 years and 40k+ miles. Final calculated CR was 18.7:1 assuming .030 compressed head gasket thickness. Using bore x stroke dimensions and displacement for one cyl at BDC, you can calculate cubic ml volume for a 21:1 CR engine at TDC. When you have those numbers, you can calculate the amount of volume required in the combustion chamber at TDC to make whatever CR you desire...................




And yes, this will save you a lot of money if you are serious about going this route. Don't forget to heat treat the piston crowns after your done cutting.

[quickv6]

Thank you for that info. That confirms what I am coming up with. I figured about .035-.040 would be about 17.8- 18.2 range. Any idea of where to have the pistons heat treated. I was thinking of having them coated at Calico coatings. Do the crowns need to be heat treated?

[Robyn]

I would be reluctant to cut these pistons.
The 18:1 purpose built pistons are made by moving the pin height up and not cutting the crown.
If you cut you have reduced the weight and then your ballance goes out the window.
To do it right you would need to redo the hard coat on the crowns plus rebalance the rotating assembly. $$$$$$$$$$
If you really want 18:1, bite the bullet and get a set of Mahle 18:1 pistons.
If your not planning on boosting it clean to the moon stick with the stockers and keep the boost around 10 PSI

Best

Robyn

[quickv6]

I would be reluctant to cut these pistons.
The 18:1 purpose built pistons are made by moving the pin height up and not cutting the crown.
If you cut you have reduced the weight and then your ballance goes out the window.
To do it right you would need to redo the hard coat on the crowns plus rebalance the rotating assembly. $$$$$$$$$$
If you really want 18:1, bite the bullet and get a set of Mahle 18:1 pistons.
If your not planning on boosting it clean to the moon stick with the stockers and keep the boost around 10 PSI

Best

Robyn

Thanks for the info Robyn. I can balance in house so not a problem there. The Mahle 18.1 are the 21cr pistons cut down and re-coated. Some have dish cut in them others are straight cut. I can also do this in house. I can not of course coat or harden in house. Are the pistons Hard anodized? What is actually done to the pistons?

[Hubert]

I thought it was a ceramic coating. To help piston crown deflect some heat and improve its tolerance to thermal stresses. Anodizing only improves surface hardness doesn't it? Not sure who does it for pistons though.

[quickv6]

I thought it was a ceramic coating. To help piston crown deflect some heat and improve its tolerance to thermal stresses. Anodizing only improves surface hardness doesn't it? Not sure who does it for pistons though.

I would aSSume that they are coating them. The top of the piston is hard for aluminum. Hoping someone knows what needs to be done. If nothing else I will have them heat reflective coated and hope for the best.

[Robyn]

I am not sure of the coating if its hard anodized or ceramic.

For a turbo engine it does need to be there for that added protection from the extra heat.
If you can do the ballancing inhouse as well as the other work your all set.

Most guys here are not set up to do any of the machine work themselves.
Seems that the 18:1's are .060 further in the hole than the 21.3:1 stockers.
Do a little more asking around here and see what crown profile is most popular.
I have heard pros and cons but have no personal hands on with either of the 18:1 pistons.
Mahle makes them as well as JK and Im sure others too.

Email John Kennedy and pick his brain a little to see what the trick fix is.

All the shops that advertise here are good folks and will offer some advice to forum members.

Go for it and let us in all all the little things. :D

Robyn