From the horses mouth:
http://www.jepistons.com/dept/tech/d...instrc4032.pdf
Bob

I think he's right. So does JE.

The internal thermodynamics of NO TWO engines are precisely the same... Much less the internal thermodynamics of two different applications, loading profiles, and cooling profiles.

The thermodynamic snapshot of a combustion event is pretty much apples to apples when observing a spark triggered cycle using a similar fuel in a similarly tuned motor, but differences such as one being carbureted and roots supercharged, versus another being normally aspirated with port fuel injection will alter the thermodynmic profile of the event significantly.

But an automobile motor runs in an environment where it gets its coolant up to stable design temp quickly and holds it there. It runs under very light consistent loading, and only short bursts of moderate loading. It is designed, and is successful, to eliminate thermal hotspots internally during normal operation, and also runs a temperature profile that encourages complete, emissions free combustion events. Additionally, consumer NVH (noise, vibration, harshness) characteristics MUST maintain a very high priority to minimize people griping about cold start piton slap, etc. This is why most auto motors run cast (or hyper EU) pistons - they have a lower thermal expansion than forged.
Boats depend on raw water cooling, and HIGH volumes of it. A boat's cooling system MUST maintain stable temps at extended WOT operation, so the end result is that it passes a LOT of water at lower speeds as well. Thermostats (when they are used) are lower temps that favor power production over emissions optimization. Higher output motors can't even make do with high flow thermostats, and usually run without any. This is a drawback due to running a motor designed around passenger car specs and put in an application where it puts out twice the max design HP for extended periods of time.

A far as combustion heat, each event at WOT produces basically the same thermo snapshot, but piston temps are not the byproduct of ONE combustion event. After each event, a cooling process takes place until the next combustion event. A piston cools by conduction, convection, and radiation. Heat is transferred off of a piston through oil, through ring contact to the liner, into incoming intake charge air, down the connecting rod, and into crankcase air. Each heat transfer medium must eventually release it heat load into the environment. The eventual environment is the "planet" in that all of the internal heat has to get out through oil coolers (into cooling water), through block radiation into the engine compartment, and out into the atmosphere, through crankcase venting blowby (usually back into the intake, but partially into the atmosphere as well), through the exhaust into the atmosphere, and out through the cooling water, also into the atmosphere and into the seawater. Since there is no creation or destruction of energy, our motors are all "planet cooled". But as you can visualize, ANY motor running under a heavier load than another similar one will have components intimately involved in the combustion process that are running at higher temps due to the inability to cool completely between combustion events.

A marine motor WILL run its pistons hotter than an auto application. This makes the pistons expand more, and close up the cleaances. A marine motor WILL have cooler water in the block than an auto application. This will (minimally) make the bores stay snugger (near the bottom of the stroke) than those in a car application, closing up clearances.

Only if you planned to pin the block on the end of a great big connecting rod and make a piston out of it. On this planet, a cold block equals smaller bores.

Yes, it is certainly a dumb thing to do. But given the parameters of what we start with (a passenger car motor designed for 300horsepower), it is a necessary concession that has to be made in order for it to not eat itself at WOT. If we started with 600hp continuous duty industrial diesels, we could run them exactly as their designers intended. But then all our 35' performance boats would run 20mph WOT due to the weight, we would have no room for passengers (or adult beverages) since the motors would be 10' long, and we would have to learn how to dock quickly and leap out as the boats rapidly sink tail first when stopped..

there's nothing "simple" about piston to wall clearances. It also has nothing to do with a piston's MASS except for a partial relationship between THICK HEAVY pistons being tougher, stronger, and a lot harder on rod bolts. You also left out the cylinder liner expansion characteristics, the loading and cooling curves of the application, the duty cycle, and the thermal window in which it will be required to operate. There's nothing SIMPLE about the complex math that goes into determining these things, and nothing simple about the trial and error and R&D that goes into fine tuning the theoretical numbers. The only SIMPLE part of it is that the piston Mfrs have done all this for us and all we have to do is SIMPLY follow their hard-earned recommendations.

There are NO MAGIC SPEED SECRETS. The ways to make big block marine horsepower have been established a long time ago and everybody has access to them. The ways to make these motor dependable and lon lasting are also well established and everybody has access to them. No secrets here. Bad ring life? Do you expect that you will get GOOD ring life with your pistons scuffing the cylinder walls, smearing them with surface aluminum? Rings don't seal very well with aluminum scrapings on their faces and in the grooves. Why do you say that running PROPER cold clearances will hurt your ring life. RUNNING clearances will end up being CORRECT for piston stability and ring geometry. Hello. Hello. Can you hear me now?

Normal High performance MARINE temperatures are, exactly as stated in the above text. As tight a piston to wall clearance that will be tolerated in a high performance MARINE application is exactly the marine clearance numbers that JE recommends.

Please call me when you prepare to run your 600 horse motor for the first time in your boat at passenger car piston clearances. I will fly down with gas money and suntan lotion to ride with you in support. I'll even bring money for the guy who has to tow you back to the ramp.

mc

well... youll notice i was careful to say apples to apples... and having built an awful lot of engines over the 25 years i was doing it... including a fair number of marine, my experience doesn't suggest any material difference. your notion of combination and circumstance is, of course correct... but i made no "one size fits all " suggestion...

and if you can;t find a way to keep your thermal balance correct so that your motors operate at essentially nominal temps... well then build them as you see fit to compensate.. that a decision and a compromise you make when you start. one that i was unwilling to make. i have used the je pistons in the cars we ran at daytona... for 24 hrs straight... at 600 hp... all day... and all night. and never stuck a one... or blew one up for that matter... an environment that makes the marine seem tame by comparison.

and the mass makes all the difference in the world as far as the total growth per delta t. the total volume of the material determines the total growth. pistons w/ more mass grow more than ones with less. that is the simple physics of it. i ran 4" je at .007 because they were thick and soft and 4 " cosworths at .004 because they were thin and WONDERFUL material...

the cosworths were able to use 1 mm rings because the stability was so much better... and made significantly more power because of that... on motors that ran in conditions where the operating temps were, in fact less than we wanted, i ran them tighter... and they were better because the block isn't the only element in the equation. and they were ALWAYS better than the loose motors. and lasted much longer.

now.. if you want to perpertuate the myth that marine service is somehow more mystical than " dry service... in the SAME load conditions... well, simply... it makes no difference to me... but in the real world, the marine environment is EXACTLY the same dyno as the the dry environment... SAME loads , and , if done properly, the same cooling. the motor doesn't care if its in a boat, on a dyno or in somebodies camaro. it will make the egts as per the load demands over time.

in an apples to apples there is zero difference... except in what people do to them.

but hey you want to run them cold and combine to compensate... thats up to you. you want to run them loose enough to rattle and lose ring seal, knock yourself out. ill still be running while you are on your third set.

you still want to apply an apples to apples for a marine versus an auto. Or even an auto race situation.

Then you want to try to tell me that a 24 hour race engine is the same application as a marine performance motor?

ANY motor that runs under a 24 hour load situation can be optimized - temps, cooling system, oiling system, you NAME it. And exactly how many start up and reheat cycles did that motor have to deal with? Maybe 1? And in the infield, if you were able to somehow tell me that you didn't have reasonable periods of throttle off time and cycling rpm thru the gears for heat refresh, then I'm all ears.

And when did EGT's become the only thing that contributed to piston expansion over extended loaded runs?

Thick piston with more mass expanding more than the SAME ALLOY in a similarly designed thinner, less mass, piston? Only because you are able to pull more heat out of the thin one on the dead stroke. YOU take a piece of 3" tall 3" diameter aluminum round bar and a piece of 3" tall 3" OD x 2.75" ID aluminum hollow bar of the same identical alloy, throw em in your oven at 500 degrees for 2 hours and measure em. THEN you tell me that more mass equals more expansion.

Material differences in the Cosworth piston versus the JE don't count.

Auto racing make marine environment tame? Depends on what characteristic you are talking about. If you are talking about mechanical fatigue, maybe maybe not. A marine setup typically runs a particular rpm for long periods where an auto race motor cycles up and down. Maine engines are more susceptible to harmonic frequency issues due to this. Valvesprings are a particular concern here. We can get into a buncha diffrent areas where one is "worse" than the other, but a marine application IS AN EXTENDED DYNO RUN. And a car race has periods of thermal recovery due to throttle lifting for braking and modulation in the corners.

As far as running them hot or cold? I'd love to run a 900 horse BBC at 160 degrees stable with rapid warmup and the ability to hold it there at WOT as well as not overcool it at slow cruise.

I'm happy that you've built 25 years worth of marine motors that are better than most in many ways. Ever thought about becoming a consultant for JE?

You can tell them to put a new note in their catalog tech section under "recommended clearances". You can tell them to put "Go buy Cosworth Pistons".

And then send them your consulting bill.

I like rattly pistons on startup that fit real nice at WOT in my cold, clearance compensated underengineered (and certainly underfunded) motors.

Maybe when you get finished with set of pistons you can donate em to me. Your trash should work just fine in my motors. Heck, in mine, they'll never touch the bores anyway...

Dam I forgot how good you were at this stuff.

Welcome back!

HPJUNKIE stick to what JE tells ya.
If you think thats aggresive take a look at what
Ross pistons recommends.

Go ahead and build it with normal car clearances, you'll figure it out when your running a 100lbs of oil pressure and pumping oil out the dipstick, breathers, mains and anywhere else it can find it's way out of the motor.
Been there done that. You need the extra clearance to run high rpms for extended periods of time. You don't run a car motor at 5k for 30 minutes under a full load. Cross over, oil cooler, what ever, you still create too much heat in the cylinders to make it last. It's one of the reasons you don't just buy GM crate motors and stuff them in a boat with brass freeze plugs.

.0055 ?

Wow.

How about ring end gap?

Actually my motor has a crossover ,runs about 110 degrees. I run it mostly 28-3500 rpm with an occassional burst to 6200 under 7psi boost. Very seldom do I cruise at 45-5000 or so rpms. Seldom not never. So.0065 on the pistons and 050 per inch on the top ring and 055 per inch on the second ring per JE . I will look at the ross piston page for comparision. Greg