My motor came with dished pistons, and it seems to be causing me so much problems with getting my compression up even with new heads.

Why in the world do they make these pistons? Is it for boosted engines?

 

Since they're cast, no its not for boosted engines. They keep the compression low enough you can run the crappiest gas you can find. I would wager that a lot of those engines end up in trucks, not performance cars.

 

Yeah, I was told it's basically a stock truck motor, but still 8.3:1 compression when all said and done, is crap.

You can run 9:1 compression easy on 87 octane can't you?

 

Dished pistons started getting used in almost all production engines around the same time that all the emissions laws and all that came along (1973). They were used to lower comperssion ratios to around 8 to 1. Dished pistons were to cheapest way for the factory to lower the compression ratios.

 

also with the dish design you get a lot better flame travel

 

Easy as long as your car is in tune.

 

As in with the quench area? Because, that seems to be causing me problems as well, someone told me with these pistons, it's going to be hard to stay in the quench area.

 

Exactly. Lots of power can be made with small chamber heads and dished pistons! The placement of the flash point is in a much better position relative to the piston. There are plenty of high powered crate engines to prove that. As with the quench height it should be right within .045" if you use the steel shim. Sorry Mark, I still haven't found that catalog. I'll see if I can find that company online or I'll just call a friend.

 

i've already decided my next preformance engine i build for myself will have dished pistons and very small chambers in the heads. in the past i had always tried to get away from dished pistons, guess as i get older i'm getting smarter

 

My next motor will have dished pistons too. Of course, my next motor will have 15+lbs of forced induction.

 

Arnt the LT-1 and LS-1 ,11:1 and 10.5:1 compression engines? I know those run on 87 octane.

 

But does having the aluminum heads help with that?

Also, I believe they're around 10.5:1

 

so r u saying that the 77 350 from a truck that I just dropped in has the dish pistons? and If so I could put my 305 heads on there and have a decent motor? thnks I know it is off subject a bit but anyway..... thnks

 

Shagme, there are more variables than just that.....but in essence yes. Dished pistons and small chambered heads is the ideal route to go. The byproduct is a more efficient burn and transfer in energy (heat to mechanical). Look up Sitting bull. He can give you a few hints on those 305 heads!

 

Mark, maybe you should have gotten 305 heads...

I'm running 9.5:1 compression, 10 degrees base timing, and burn 85 octane with absolutely no problems. Of course, that's at 5800' elevation, and my chambers are polished. Functioning EGR valve also very helpful in this regard.

If/when I upgrade the shortblock, a dished-piston 350 under my current heads is my plan.

Back to the low octane question: My 396 has 9.7:1 TRW pistons, but that's with 106cc chambers. I have 98cc chambers, mounted with composition gaskets, so I'm probably pretty close to 10:1 on it. With the HI-6 ignition and 40 degrees total mechanical timing, I burn - yep, 85 octane.

 

cool thnks man. been wondering what I should do with the 305 sitting in mky driveway I really need to take it aprat and move it. my garage is stone not concrete. yet anyway Ill lookup sittingbull I hope he isnt in a bad mood. he might scalp me l8tr

 

If your budget can take it, get a set of reverse dome pistons. They have a larger quench area than a dished piston. Dishes on aftermarket pistons are usually only used for very large "-" cc's that cannot be machined using a reverse dome pattern due to dome thickness.

Also, someone mentioned an ideal quench area of .045. With a quality 4340 steel rod, you can run .036-.038 quench on a street engine without any worries.

 

what pistons does the 93 9C1 LO5 have in it? Noway they are dished since it has 9.6:1 compression. Im not sure what CC the heads are tho but im assuming 64cc but I could be wrong. Im thinking about going with alluminum heads so i can go 10.6:1 with a lt4 hotcam. so much for the warrenty on the motor

 

Man, don't put 305 heads on a 350.. Just because someone can take a set of 305 heads and make big ponies doesn't mean they last on a street car for a long time. Read and study and make the right desision....

 

Don't let F-bird88 hear you say that.

 

Even the iron-headed LT1s run at 10.58:1, with flat-topped pistons and four valve reliefs. Their big advantage is the reverse flow cooling and vastly improved PCM. That enables them to run 85 octane camel-urine gasoline without detonation, albeit at reduced power.

Cast dished pistons are usually considerably heavier than their flat-top counterparts, and the masses vary widely. Attention to detail in the stock cast pistons just didn't exist when most of the smogger engines were built. I've seen as much as 23 grams difference in the stock cast pistons when trying to balance one. It's scary trying to remove that much metal, wondering when you're getting too thin.

As for the original question, dished pistons were manufactured simply to **** you off, Mark.

Actually, Greasemonkey and A4 covered the reasons pretty well, but I just couldn't resist...

Too bad you aren't using stock heads that you could hack up. I've made high compression with dished pistons, but the heads are basically useless on anything else. All those wonderful, cheap, stock castings are great for experimenting. The 355 I have in the stand for my truck has TRW cast dishes and tight heads that run it to alomst 11:1 (so close it isn't worth arguing). After being milled, ported, welded, ported again, the heads are trashed for any other purpose, and I had to hack down an Edelbrock Performer to fit those heads only, so it's useless too - on anything else. That's why I'm keeping my eyes open for different heads to set up for the LT1 intake and that case. A carb isn't going to get it for my S-truck.

In your case. I don't believe the Pro Action heads were designed to run as open chambers, which is a little like what the dish does for chamber design and quench area. Depending on your cam selection, having more chamber volume under the deck might not be the best situation on the exhaust stroke, when you really want to scavenge the cylinder. That initial pulse of gasses out the valve as it cracks open may not be as sharp and forceful as desired with the dished pistons, since more of the gas is on the far side of the chamber and "around the corner" from the valve area. But it might not matter a lot, either. You can always open up the chamber volumes more later (to an extent) if you change pistons.

 

Well, they did a very good job then.

 

Yes, you have a bit of a dilemma. I'm not trying to make fun, just to lighten it up a bit. the 50cc heads might work out for you, but you'll have to watch the gasket thickness (or, "thinness", in your case) so that you can maintain a decent seal.

I'm a little leary of gaskets under .030" myself, but I'm just chicken. Did you get an actual measurement of the dish volume yet? That could make a substantial difference in your selection. It's probably worth the cost of a plastic syringe and quart of transmission oil.

 

No still not home yet.

Well, I was thinkin of the 50cc ones and a .040 gasket or so. But, I'm just worried about when I stroke the motor, that will sky rocket my compression, but you were talkin about some things that could be done to keep it down, correct? is this something that a fairly knowledgable person could do? not myself of course, lol.

But then again, I'm not too worried about a thin gasket, as I will prolly have 5k miles max on this engine, before I tear into it again.

 

According to people like Lingenfelter and others who are into the study of asperation and air flow, aluminum disapates heat faster than cast iron and because of this it allows an engine to run a full point higher on compression on the same pump gas as a cast iron head engine would with out detination problems.

G.M.'s found by changing the head to the smaller combustion chamber, moving the spark plug over towards the exhaust valve (kinda the angle plug deal ) and placing the piston pin closer to the top of the piston they are able to get a better burn in the quinch area and better performance from a stock small block because it actually gives it a bigger flow potential and less scrowding in the combustion chamber as a whole. that's another reason why the stock 305's ran so strong.

And Mark, I've read what Sitting Bill has done and what Fbird-88 does and think it's absolutly wonderfull. I know he knows how to build great power with castings maybe like the 083 and 081's ect... and I have the ut most respect for that however my feelings are a bit diffrent in the aspect that if your not a pro with a flow bench then you should try to reach the full potential of your capibilites and do it with something that will be there to last. The potential of doing this is far greater and easier if you start with something that flows good from the start and there are G.M. cast iron heads that do this. I believe that the same effect with the dome piston about the unschrowding of the combustion chamber can be done with big valve smogger heads of the early 70's. I researched this massivly and decided that If I could I would do this and sure enough I found the 993's Lingenfelter recommends except I went up a compression point and ported them bigger than most of the books recommended and my car just flat rips I think about 10.3 to 1 . The car has more throat a better top end and even a lope in the idle ( 1.6 rockers hepled there too.. ).. I know suckers talk but I will bring a time slip to the table here soon. Anyway, It's my B-day tommarrow and my wonderful girlfrinds taking me to Santa Cruse for 3 days. First time I've ever had a woman I loved more than my car..LOL.. gotta run.. Later

Rick

 

Why would 305 heads not last long on a 350 block? asadaily driver anyway? oh nevfer mind u said its in the tech articles Ill rezad them thnks

 

ok there isnt a tech article I guess Ill search alittle thnks

 

I dont know what the deal is with the guy giving you a hard time about the 305 heads. On most 350s the 305 heads are ab ad idea because the compression jumps up so much. For you that's not a problem. You may want to get some bigger valves put in them, but I think it would work out pretty well for you.

 

With your stock dished piston short block and 64cc heads, if you were to mill those heads down to 58cc (approx .060") and reassemble with a thin .015" shim head gasket you would have 9.80:1 compression.
Because of the full dish on the piston you do not have any effective quench effect on your motor.
You'll need to run it on 92 octane gas after raising the compression ratio to 9.80:1.
If/when you shave the heads, the intake manifold mating flange on the heads will also need corrective milling to restore intake port alignment. The intake bolt holes in the manifold will/may need to be elongated a bit.

 

My 355 made 310 RWHP last year with 8.6:1 CR and 87 octane fuel, XR270HR cam, TrickFlow G1 heads, Performer RPM, and a street/strip prepped QJet. Nothing wrong with low compression.