Strick

When building a 383, what pistons are used? I see flat, dome and dish. I want to keep my CR around 10.8 or so (I have LT4 heads), to stay on pump gas. I also see a hyper(something) piston. What are the different type pistons for? What ever happened to just notched or notched pop up pistons?

1963SS

LT4 heads are around 54 cc so to keep your compression where you want, you'll need a -16cc dish piston. The different materials are available based on your horsepower requirements. Many people that build a 383 used forged because they are less prone to catastrophic damage.

Next down the line would be the hypereutectic. They are better than the stock cast pistons for most people. Many times it's best to purchase the entire rotating assembly already balanced and just specify what pistons you want. I like to use JE/SRP, Mahle and Diamond. There are other options out there but these are three of the best. There are also different ways of reaching the -16cc range that you'll need.

Some pistone manufacturers dish the entire top of the piston while others only dish part of it and allow for a good sized quench pad. You'll want the second type and it is usually referred to as a "D-Dish" piston.

Good luck with your 383. It's a lot of fun.

rklessdriver

Unless you have a supercharged or turbocharged engine... at which point you want a full dsih with as little quench as possible.

Don't forget that the headgasket (thickness and bore dia) and deck clearance play a big part in compression ratio as well.

That 16cc piston can have as little as 10.32 SCR if you have a stock deck block (piston .025 in the hole) using a stock .039 thick 4.125 bore headgasket (Fel Pro 1074).

It can have as much as as 11.00 SCR if you zero deck the block and use one of those .040 thick 4.040 MLS headgasket (Mr Gasket 3140, Cometic 5645, Flat Out 921044040).

Static Compression Ratio is just one of the many factors in terms of "running on pump gas" or not. That's a product of the SCR, intake valve closing point which controls your cyl pressure and the chamber design which control the flame front speed once the AF mixture lights off.

Alot of people use a calculation for Dynamic Compression Ratio (DCR) as the end all be all of "pump gas compatibility" but really it's just a number... it gives you a number to compare to other combonations that either did or didn't work.
Will

1963SS

That's all true. I just sometimes assume things. I assumed that he wanted naturally aspirated because of the 10.8 compression range he specified. . I don't know for sure. I generally zero deck my blocks and set quench with a gasket, so I ASSumed that everyone does. That was wrong. I also agree on the DCR. I've ran well over 9:1 and had no knock counts at all. It depends on the pains taken to reduce or eliminate "knock". DCR, quench, hotspots, temperature, sharp edges, cam selection and several other things all must be considered.

That's the great thing about forums. There's always an answer in there but sometimes you just have to sort through a little BS. I was just trying to give a generalization and not a recipe.

To the OP, I do have some recipes for LT4 engines that run great on pump gas and I have the approximate build cost for them as well. I'm a big fan of longevity(Over 100,000 miles on the old 412 RWHP Impala SS) as well as performance. You can have both and it doesn't always cost an arm and a leg. Maybe your first born male child but you can keep your arms and legs.

Thanks for reminding me that we're not all the same and generalizations don't always work.

Strick

I'm looking at balanced assemblies right now. However, there aren't too many assemblies for one piece rear main engines as the LT1 is. I've learned that Scat makes a good assembly and some have had trouble with Eagle. My flywheel, dual mass OEM, doesn't have any weights on the outer holes or on the damper. This engine is an external balanced engine and my question is with a balanced assembly won't I have to rebalance the flywheel, too?

1963SS

Howdy again Strick. I always get the entire rotating assembly balanced, flywheel/flex plate included. The additional cost for the flywheel is only around fifty bucks or so and is worth it for the peace of mind. You haven't said much about your horsepower goals and depending on your budget and power needs you may be able to save a lot of money.

I've personally never had a problem with an Eagle crank and have not personally seen the machining problems that some have experienced. I check every tolerance very closely and have never seen an Eagle crank fail.

For the past few years I have cranks and rods cryo treated. There are many write ups on the benefits. Some see it as a waste of time while I just see it as a why not sort of issue. Some Lunati cranks, some Callies cranks, Eagle, Scat and others are forged overseas. If that's what they want to do, I have no control over that. I do use American products if the customer can afford the extra cost but as far as quality, IMHO they're all very good now days.

What are your power goals? I know you want reliable and certainly pump gas but at what power level? What kind of a budget are you playing with. A lot of times you can save big bucks in some areas and put the money to better use in other areas.

rklessdriver

Yep, there is no reason to count out Eagle's parts, espc in a street car. They have some of the most cost effective parts availiable.

Strick-
Just about everybody is capable of putting one out thats f'd up and you will read about more Eagle/Scat problems because there are more of them out there (due to the low cost).... You also don't know how many of these internet problem children actually bought a "blem" off ebay and then figured out why it was so cheap....

I personally have an Eagle crank in my 92. Scat rods and Manley pistons.... Nothing was wrong with any of it except some of it didn't come from here.

If you can't find what you need in a "mfgrs kit" then buy the parts seperately and build what suits you. You'll probally save money and have a better kit in the end. I can't stress this enough....

I can't tell you how many times a customer has brought a "KIT" in my shop and I spend more time (and they pay for it) modifying other things to work with the parts he bought... If he would have asked I could have saved him $$$ on the parts and there would have been no labor to modify the other stuff.

If you want to do stuff right:

1. Lay out how much you have to spend.
2. Decide what you expect out of the engine. Either HP or 1/4 mile times....
3. Ask and engine builder for recomendations. We do this for a living and we generally know what parts will be compatible with what.

On the balance issue. I always balance the rotating assy with the FW that the customer is going to use. Some LTX's have the factory balance problem that was corrected on the assy line (weights in the FW$/or balancer)... unfortunatly that makes all factory LTX FWs, crankshafts and balancers suspect... When we reuse that stuff I personally check them all.
Will

vetteoz

Sure?

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