lowering compression: shaving pistons or shortening stroke, or thicker gaskets...

[turbonator]

what is your preferred method? all input is appreciated.... we are thinking to reduce stroke, and balancing the rotating assembly..... tomorrow is the machining day, we will do a good video at the shop and post it up tomorrow afternoon.... also we will ask the question.... do you guys machine the new 6.5liter blocks here? if not, do you know where? anyways it should be fun... also we are picking up our Moroso block filler tomorrow... cant seem to order Hardblok, summit wont ship it to canada...:mad2:

 

[ak diesel driver]

you could probably find someone on the site who would ship it to you. I'd volunteer but I'm in just about the same boat as you.

 

[turbonator]

you could probably find someone on the site who would ship it to you. I'd volunteer but I'm in just about the same boat as you.

thanks, but we have already decided on Moroso, the 2 are the same as far as we can tell, it is just the wording in the description that is slightly different.... the same product, just a different sales pitch...LOL and both of them would not offer up the recipes, go figure....

 

[buddy]

I am not inclinded to shave pistons flat, simply because it reduces the ring height and can increase fatigue on the rings and reduce depth of the Ricardo swirl bowls that can reduce mixing efficiency. Extending the ricardo bowls, or dishing the piston top would be a better option. Something like Kennedy's custom pistons pictured below. This keeps your ring height and keeps combustion forces in middle of the piston.


By lowering stoke do you mean offsetting the crankshaft and using some undersized bearings?

I think dishing the piston is better than thicker gaskets, just because less gasket material the better right? At least if wanting to run high boost.

 

[ak diesel driver]

I think he meant making the rods shorter, which is what I would lean towards.

 

[buddy]

shortening the rods would not change the stroke but it will lower compression ratio having the piston drop lower in the cylinder. A small change I guess woulnt make a big difference on the rod angle. To get down to 18:1 just need to take off 0.04" if using standard gaskets and starting with approximately 21.5:1 CR. Thats .04" or 40 thousands shorter connecting rod, or shaving .04" off the whole piston top. If dishing the piston would likely have to go .07"-.08" deep with something like Kennedy's design extending the swirl bowls.

 

[LtEng5]

move the wrist pin up on the piston......order a piston with a raised pin bore is the best way so as not to be messing with gasket thicknesses

 

[buddy]

move the wrist pin up on the piston......order a piston with a raised pin bore is the best way so as not to be messing with gasket thicknesses

Thats not something available to order. There are apparantly 18:1 Mahle pistons available or custom machined. Or standard -0.01" compression height pistons

 

[turbonator]

we have already used cometic multi layered stainless .066" thickness to drop the compression down a couple points... we have boosted upwards of 25psi and they are still there, also we tied it all down with ARP head studs... seems good... hey buddy, how much lower C/R are those Kennedy pistons? 17.5:1? also to add one more thing, there was no problems with the intake bolting on the heads, although the water x-over bolt holes needed to be ovaled some to bolt down correctly......

 

[buddy]

Kennedy makes them any CR you want. If you already got .066" gaskets you increased .02", which has you down to about 19.5:1 CR. Getting OEM -.01" annodized top pistons may be the best way to go rather than machining something and degrading the surfaces. Assuming you keep that thickness gasket that will get you close to 18:1 CR and low enough to run 25psi boost. Otherwise if you still have a standard set of pistons to play with, just extend those swirl bowls out a little bit, they are about .07" deep. If you drill down two 1.25" diameter circles at .07" deep and then blend it into the bowls you will end up with about 18:1CR, or a bit little less.

 

[DZZ71]

Here is a photo of mine shaved down. I %100 agree with buddy how shaving them flat will affect the rings. This will definatly be the olny engine i do this to. Mine had .07 taken off.

 

[turbonator]

Kennedy makes them any CR you want. If you already got .066" gaskets you increased .02", which has you down to about 19.5:1 CR. Getting OEM -.01" annodized top pistons may be the best way to go rather than machining something and degrading the surfaces. Assing you keep that thickness gasket that will get you close to 18:1 CR and low enough to run 25psi boost. Otherwise if you still have a standard set of pistons to play with, just extend those swirl bowls out a little bit, they are about .07" deep. If you drill down two 1.25" diameter circles at .07" deep and then blend it into the bowls you will end up with about 18:1CR, or a bit little less.

the motor we are building now is going to get .070" cometics.... thanks for the info on the swirl bowls... we have been discussing that already...... i think it is the way to go...

 

[buddy]

You already get your custom Cometics at .07"? How much is the deck being cut?

I might get the minimum thickness required to get the 5 layer steel vice 3, which is .056" thick, which is basically +0.01''. If it was because of decking the block more than .01" then perhaps shaving the piston top .01 or .02 and then additional dishing may be better than thicker gaskets. It may just be a gut feeling, but I prefer to have as little gasket material as possible, but increasing number of metal layers makes me feel better about the plus sized Cometics.

Did you put your original Cometics on dry? Their technician told me nothing should be applied between it and the bonding surface. And its probably good to retorque nuts after a while of use, if using head studs.

 

[buddy]

Here is a photo of mine shaved down. I %100 agree with buddy how shaving them flat will affect the rings. This will definatly be the olny engine i do this to. Mine had .07 taken off.

That looks pretty cool with the swirl bowls all gone, almost an actual flat top piston.

 

[robzombie4551]

Ah the performance thread. I feel like I'm home.

Can't wait to take mine for a ride tomorrow, and see if mine runs any better.

Btw way are you saying I shouldn't have used blue rtv on my cometic gaskets?

 

[turbonator]

You already get your custom Cometics at .07"? How much is the deck being cut?

I might get the minimum thickness required to get the 5 layer steel vice 3, which is .056" thick, which is basically +0.01''. If it was because of decking the block more than .01" then perhaps shaving the piston top .01 or .02 and then additional dishing may be better than thicker gaskets. It may just be a gut feeling, but I prefer to have as little gasket material as possible, but increasing number of metal layers makes me feel better about the plus sized Cometics.

Did you put your original Cometics on dry? Their technician told me nothing should be applied between it and the bonding surface. And its probably good to retorque nuts after a while of use, if using head studs.

yes we put them on dry, pulled 14,000lbs on the trailer plus the weight of the truck(17000lbs) a few times and she is still running great, boosted up to 25 psi a couple times and had no ill effect...( around 3500km on it now) jim has said several times that we will re-tourque the head studs to +10lbs over the ARP torque specs... here is a cold start vid of the "pugg" starting at -1C (30F)....http://www.youtube.com/watch?v=yy2JlFJY9fg

we had our decks cut to .0115".....

 

[buddy]

The Pugg sounds good and powerful, I love the orange fleet of different truck types. I ran my exhaust out before the wheel like your 3500, after the cross members I upped to 5" pipe and used a 6" tip, and I enjoy watching the backpressure gauge.

 

[Matt Bachand]

Wow the Pugg revv's so silky smooth! That thing is awesome!

As long as gauges in check, and you're not in a rush the 6.5 will pull strong I think. I'm sure that thing is geared up the ass too.

 

[Acesneights1]

Why does everyone lower the CR on these ? What is the difference of Higher CR and lower boost versus Lower CR and higher Boost. I asked Bill H about this once and he had told me it was unessicary??

 

[6.2 turbo]

Not everyone, mine's going to be 22.6-1. Guess I'll try and figure it out by trial and error . Going from one extreme to the other should give me a pretty good clue. I know some people that work on farm tractors and they often up the compression,with +.010 pistons or wrist pins,or lower the valves.