Growth of Forged Rods and Pistons

[TTipe]

What is worst case of forged rod & forged piston growth. I have a new engine going together. My pistons are 0.005 below the deck and I currently have a set of 0.040 Cometics. I would like to go to 0.027 Cometics to increase my CR from 8.51 : 1 to 8.74 to 1 to make better use of my GN1's.

Thanks for the help.

 

[earlbrown]

Not enough room to run .027"s. The general rule is .040" is the closest you want to run. In real life 0.038" or so is starting to push it with steel rods. When you start getting that close you really have to keep piston rock at a minimum and don't overrev the engine EVER. You could definitely use some more compression with those aluminum heads.

What pistons are you running that give you 8.51 @ .045" with GN1s? Aren't those heads 45cc's out of the box?

 

[TTipe]

I have diamond pistons and 46cc chambers out of the box. Bore is 3.820, stroke is 3.400. Its a good thing that I requested 0.005 below the deck (from Weber) which gets me about as close as I dare go. I wanted the engine to run pump gas with no alcohol solidly into the elevens. I have a 212/206, 111, 1.6 rockers at 0.503 int and 0.523 exhaust. The heads are ported. I'm an engineer so I would be curious to see how rod/piston growth was computed to support a general rule. I'm sure you are correct, but I'm just surprised at the amount of growth.

 

[earlbrown]

It's not just growth. It's crank flex, compressing the hydrodynamic wedge in the main and rod bearings, big end deflection, rod strectch, wrist pin deflection, piston rock, thermal expansion of alum being right at double that of iron, etc... (man, with that laundry list I'm surprised it's not .400" )

I don't think those general rules are based on any hardcore engineering. I think they're more of a general rule that's been proven over and over as times went by. I rarely buy into a 'rule' that applies to all engines as most of those are way too overlysimplistic and repeated as gospel(and get on my nerves). Example.. shaving .005" off a head is 1CC... it doesn't matter if your combustion chamber is 2 square inches or an ocean liner with a 8 foot bore... 5thou = 1cc!

With automotive engines the .040" thing is kinda accurate as the bore sizes don't vary in the big grand scheme of things, and piston to cyl clearances don't vary that much (also in the big scheme of things). I'm pretty sure if you put a 2" tall slipper piston in an ocean liner with a 9 foot bore, you'd need more than .040" or if you made our Buick engine zing up to 50,000, you'd definitely need more than .040" too


I believe it was Bison that posted a couple weeks ago about running .035". I said something about it and he mentioned he creeped up on that number with head deckings and was starting to see first signs of pistons making kissy face with the head.


I'm not an engineer but I did attend a sex convention at a Holiday Inn once... I would guess the math would be thermal expansion of the piston due to operating temps and it's alloy. Big end deflection of the rod coupled with the modulus of elasticity of the rod material (and wrist pin) from the weight of the piston and it's kinetic energy at the top. (plus any rocking motion that might come from an asymmetrical crown moving the CofG somewhere other than the centerline of the wristpin pivot point)... oh, and some other stuff too. I haven't started drinking yet


We do get a little bit of help with the inertial stuff at TDC exhaust. Since we run a pressurized exhaust system, the piston hits an air cushion that helps absorb the piston to keep heading north. That's the reason aluminium rods last so much longer in a turbo engine than a non turbo.

 

[TTipe]

It's not just growth. It's crank flex, compressing the hydrodynamic wedge in the main and rod bearings, big end deflection, rod strectch, wrist pin deflection, piston rock, thermal expansion of alum being right at double that of iron, etc... (man, with that laundry list I'm surprised it's not .400" )

I don't think those general rules are based on any hardcore engineering. I think they're more of a general rule that's been proven over and over as times went by. I rarely buy into a 'rule' that applies to all engines as most of those are way too overlysimplistic and repeated as gospel(and get on my nerves). Example.. shaving .005" off a head is 1CC... it doesn't matter if your combustion chamber is 2 square inches or an ocean liner with a 8 foot bore... 5thou = 1cc!

With automotive engines the .040" thing is kinda accurate as the bore sizes don't vary in the big grand scheme of things, and piston to cyl clearances don't vary that much (also in the big scheme of things). I'm pretty sure if you put a 2" tall slipper piston in an ocean liner with a 9 foot bore, you'd need more than .040" or if you made our Buick engine zing up to 50,000, you'd definitely need more than .040" too


I believe it was Bison that posted a couple weeks ago about running .035". I said something about it and he mentioned he creeped up on that number with head deckings and was starting to see first signs of pistons making kissy face with the head.


I'm not an engineer but I did attend a sex convention at a Holiday Inn once... I would guess the math would be thermal expansion of the piston due to operating temps and it's alloy. Big end deflection of the rod coupled with the modulus of elasticity of the rod material (and wrist pin) from the weight of the piston and it's kinetic energy at the top. (plus any rocking motion that might come from an asymmetrical crown moving the CofG somewhere other than the centerline of the wristpin pivot point)... oh, and some other stuff too. I haven't started drinking yet


We do get a little bit of help with the inertial stuff at TDC exhaust. Since we run a pressurized exhaust system, the piston hits an air cushion that helps absorb the piston to keep heading north. That's the reason aluminium rods last so much longer in a turbo engine than a non turbo.

Thanks for the input. This is kinda what I figured. I'm at 0.045 deck to deck now which is about perfect.There are two alternatives, 1) different pistons which ain't happening now and 2) a little most air volume. With these heads there is a lot more mixture motion and a lot of other good things.

 

[earlbrown]

you do have better flowing heads. The downside if the thermal conductivity of the aluminum. A lot of heat that used to push the piston down is being wicked away by the head. With only 5 or 6 thou left with piston to head it would be nowhere near cost effective to tear the block down to deck it that little.

What are the CCs of the pistons?

 

[TTipe]

you do have better flowing heads. The downside if the thermal conductivity of the aluminum. A lot of heat that used to push the piston down is being wicked away by the head. With only 5 or 6 thou left with piston to head it would be nowhere near cost effective to tear the block down to deck it that little.

What are the CCs of the pistons?

The piston dish volume is 29.5cc (Diamonds-scallops are mis-aligned because valves are rotated in the GN1 chamber-the spark plug is more to the center of the chamber which means less octane required and possibly a little more boost on 93 pump gas.)

 

[turbodave231]

To help you answer your original question:
The coefficient of expansion for steel is .000006" per inch per degree and aluminum is .000012"

A 6" rod measured at 70 degrees and running at 250 degrees will grow 6 X 180 x .000006"= .0065"
A Piston measured at 70 degrees and running at 536 degrees with Compression height at 1.822 x 466 x .000012= .010

So the whole package will grow .016-.017" at temp.

FYI, I just grabbed an average piston temp. the whole piston doesn't run at 536. This just represents a worst case.

The Buick Motorsports book lists .045 to .065 as the max/min for piston to cylinder head clearance. Rher and Morrison say .040-.050 for steel rods and .055-.070 for aluminum rods.
These are both measured at room temp.......so thermal expansion is already figured into these numbers!!

Dave

 

[earlbrown]

Sounds about right. Throw in 10 or 12 thou of unloaded piston rock and that pretty much uses up the .040" minimum. Even with the .005" or so of block growth.


what's odd is I always thought the thermal expansion rate of aluminum was right at double that of steel. Turns out it's exactly double. Maybe I was thinking of cast iron.

 

[turbodave231]

Cast iron and steel are the same. Yes aluminum is exactly double

 

[TTipe]

Sounds about right. Throw in 10 or 12 thou of unloaded piston rock and that pretty much uses up the .040" minimum. Even with the .005" or so of block growth.


what's odd is I always thought the thermal expansion rate of aluminum was right at double that of steel. Turns out it's exactly double. Maybe I was thinking of cast iron.

Thanks for the info. There are one or two plug ins which are almost straight forward. Piston growth is a function of the energy released to the crown.pin & ring thermal conductivity and a bunch of other stuff . This kind of stuff is usually modelled (using GT Power or Star ccd) because there is just a lot going on. I was just trying to see where people were getting these clearance numbers from when they don't initially add up or make sense. I'm fortunate that I set my clearance to 0.045" a while back. I'm still reluctant to shave my new cylinder heads 0.003" to get me to 8.75 : 1

 

[earlbrown]

Shaving the head won't close up the piston to head gap. That .003" won't really do anything as far as power at the tires you can quantify. It might take .003" to make them flat and smooth though. Seems like one of my heads took 3 and he other took 5.

 

[TTipe]

Shaving the head won't close up the piston to head gap. That .003" won't really do anything as far as power at the tires you can quantify. It might take .003" to make them flat and smooth though. Seems like one of my heads took 3 and he other took 5.

It will reduce chamber volume which would slightly increase compression.

 

[ijames]

Don't forget the block growth, which will be about the same as the rod growth for steel rods and similar temps, so the hot clearance is about 0.006" more than you calculated.

 

[turbodave231]

Thanks James,

.000006 x 9.522 x 180= .010 Which pretty much offsets the growth of the piston.