I am getting things together for my new shortblock. My heads are 74cc and I am aiming for ~8.5:1 static CR. So far, I have spoken to JE and Ross about pistons. They have very different philosophies regarding how to achieve low CR for a "blower" application.

JE's blower piston is a typical dished design. Ross recommends a flat top (with valve reliefs). To get the desired CR they say to simply put the piston deeper in the hole. Even with my big combustion chambers and a 0.039" gasket this would put the piston at least 0.050" down the hole. This goes against the principles that I understand regarding squish, though I also understand that a flat top is probably strongest, everything else being equal.

Is the Ross piston intended for simply to be as strong as possible without regard to squish? I guess under full boost, squish might not be much of an issue. But it sounded bizarre to me. I think I will get another set of dish pistons, as I have had good results with them. But I am interested to hear what other people think about this.

Rich Krause

 

The JE makes sense for exactly what you stated, the Ross doesn't.

Even in a blown motor you will run into quench and squish, just becasue there is more in there it doesn't matter.

I would do with a reverse dome or dish piston to make the CR what you want it to be. JE can make it as strong as a Flat Top, in the blown engien you need to fight against detonation and making the squish/quench area larger will hurt that even more.

Bret

 

I am suprised that Ross said that. ??

I would not sacrifice quench for compression via dropping the piston in the hole. I would go with dish since your heads have chambers that are already quite large. I am no pro engine builder tho...

just my thought
Jason

 

Thanks guys, it just sounded wierd. Maybe I will call Ross back next week and hope I get someone who knows something.

Rich Krause

 

That sounds like a good plan. I agree with Jason and Bret.

Rich, is this also a 383? What's the rod length?

Jon

 

Low rev 383 with heavy duty "blower" pistons. That was another thing, the Ross piston was very heavy. Comparable JE and Ross blower pistons are 508 v. 525gms. Not a big difference, but if the strength is there, lighter is better. I have had good luck with 5.7" rods, but I may try a slightly longer rod this time, maybe a 5.85", to cut down on piston weight.

Rich Krause

 

When you guys look at piston strength you should be aware that there are two different alloys 4032 wich is the weaker of the two and the 2618 wich is far stronger..I think people take way too much consideration in a few grams these days..Think back when these trends didnt mean much when engine made this power a decade ago..

Mahle or diamond pistons would be the better choice vursus the above companys..Never had a problem with mahle and the diamond slugs that im planning to use look very good..Theres more than ross or je out there..They just seem to spend more on advertising

 

Knowing Rich, I dont think he is too concerned with weight of the piston and trading off a few grams for strength. He is not talking about a few grams here.....

I would just go with a good set of JE's. There is a reason why companies like JE have been around for a while. I disagree that that the above mentioned piston companies are "better". Easily as strong.

Jason

 

Rich,

Have you considered Wiseco? My Wiseco pistons are the 2618 alloy, which LT1 Malibu stated, are much stronger than traditional 4032 alloys. I would think that most pistons specifically intended for blown or nitrous applications would be 2618 though. JMHO. Even still, you may want to find this out before placing your order for them.

FWIW, I'm using a 5.85" rod, and a 1.27" compression height with a -20cc dish for my blown app.

Mike

 

Mike: the 5.85" seems like a good compromise between the various configurations. I had a set of Wiseco's in a previous motor and had a detonation problem that I eventually traced to the design of the crown. I don't remember the model number of the piston, but it was also a -20cc reverse dome. Anyway, the there were two areas on the crown that came to very sharp "points", where the squish area joins the valve reliefs.

Take a good look at these before you install them. If I had I would have done some grinding and polishing and had a lot better luck with them. OTOH, they did stand up to all that detonation!

Rich Krause

 

I read the other day, on the quickest F-body that he used V-shaped dished pistons... He said it added some horsepower... I can see why since it is directing all the power right to the center of the piston moving it down at the rod and thus eliminating some friction since it will move straigt down and the exploding charge may be more even since it is directed.. I would try and give them a call and see what they have to say... sound interesting...

Wouldn't a bigger deck hight reduce the transfer in heat through the heads which are the easiest cooled? I think that it would raise the engine's temp to run it with such a deck height, I would prefer to dissipate a bit more through the head rather than the block, its my opinion, if it where an LS1 and I dont think it mattered, but just a thought...

I think that setting the squish correctly will result in a more powrefull engine regardless of its compresion ratio.

a 5.7 rod will result in a faster reving engine although adding more side loading....

 

I saw here in PR a very good respectable engine builder that he always took pistons and removed almost all sharp edges... then putting them to the same weight... The engine looked nice and ready to be beated on.... He said that it reduced the probability of breaking since it was a smooth edge, he also said that engine ran stronger like that. Now I can see why... no detonation!! Interesting...

 

Rich,

Thanks for the heads up. Although these slugs I own don't have any bad "hot" spots that I've noticed, I was already going to take a die grinder and a cartridge roll to the edges of the valve reliefs before I send them off for TBC coating. Maybe Wiseco has since fixed that problem you encountered?

BTW, how far down is the first ring land of those JE's? These Wiseco's are .230 from the top of the crown, but I would have liked a bit more. Somewhere around .250 - .260 would be great I would think, but then again, I'm not a piston designer.

Good luck with your piston decison.

Mike

 

i definitely shouldn't even try to answer this especially since i have not clue what i about to say, but do you think that ross recomended the flat pistons with valve reliefs for you to use with a different cam(keep the valves open longer on compression stroke), that is the only reasoning i can make out of why they would say what they did. but i always thought dished were better for boost type of app.

 

I doubt that is why. Rich runs cams with very little duration anyways.

It just sounds like a tech guy giving the wrong advice.

I am using Diamond pistons on my LS1 engines, they are not a bad company to deal with.

Sanding down the valve reliefs and edges on the dome/dish is always a good idea. I always do that and the ultrasonicly clean them just prior to piston install.

Rich I would imagine that a 6.00" rod would reduce the piston weight even more and still leave you with enough deck thickness on the piston for a large dish. What rods are you planing on running anyways? I have always run long ods in my motors, a 6.25" one would not give you enough room for the dish you need but a 6.00" one would. I have talked with some guys who build 9:1 Nascar engines and they moved to a 6.125" from a 6.25" rod to give the piston more room for a thicker deck. I'm sure that even with all that power you can get enough thickness in there to run a 6.00" rod. That would give you a 1.125" compression height, the 5.85" rod would give you another .150 which would help though, but make the pistons a few grams heavier.


Bret