conv90

I posted this in the tech section without luck......
Pistons: Compression ratio differences.
I‘m searching the right piston kit (stock like) to use for my rebuild. My car is a convertible 1990 A/T .
Stock pistons are flat with 4 v. relief no dish .
Searching for catalogs I see the Sealed Power Hypereutectic H345P listed as Flat w/ 4valve relief.
In all catalogs the price range for the set is $75/ $90 and the compression ratio (with 64cc combustion chambers volume – the 64 cc data is allways present on pistons data, the 58 cc it’s a rare data…- ) is allways some thenth lower comparing with others flat pistons w/4 valve relief. H669 P (Fedral Mogul group also.. I think they are Speed Pro) are also Hypereutectic machine worked flat top pistons w/4 valve relief, but are rated 0.2 / 0.3 higher on compression and are $100 / ख़$120 higher in price!
The Question: H345P are lower in compression ratio for larger valve relief? Or these pistons are lower in C.R. beacause they (maybe) are deked 0.025” / 0.030” under cylinder deck? I don’t want pistons with deck gap if stock pistons are not…. (I don’t know if stock pistons are deked or not).. Can I consider H345P a standard replacement of my stock pistons?? Someone in the forum used thhese pistons?
Another technical question: If I use for my rebuild a Forged piston kit ( TRW /Sped Pro forged, Flat with 4 valve relief) which is the advantage instead a cast /hypereutectic??
Finally…. Last question… Could I utilize a flat top w/ 2 valve relief??

Thanks
Beppe

gkull

http://www.smokemup.com/auto_math/compression_ratio.php

This site will help you with figuring compression ratio.

The Sealed power H345p are like a stock replacement piston. The H669 p and the price difference is material and machining quality.

No different brand of 2 or 4 valve relief pistons measure the same. The size of the valve cuts vary in the number of CC's Most valve cuts on none dished piston are in the 4 - 8 cc range. Your also right that different brands of pistons have small variations in the pin to top height.

In my opinion - I had great luck with KB 2 valve relief Hypereutectic pistons. They have tighter cylinder to wall clearance for more overall power. The TRW forged are the bottom of line in forged pistons. Forged pistons have some strength advantages only needed in modified motors or induction systems.

I never use 4 valve relief pistons. I use the least cc I can to clear the valves for a given valve lift.

CFI-EFI

First, you need ALL the specs. If you look in the fine print of the piston catelog, they may tell you the deck height, and gasket thickness used in there computations. YOU have to know what YOU have in order to compare. The spec you may be lacking for the pistons is the "compression distance. This is the distance from the center of the pin to the flat part of the top of the piston. Many, many, stock replacement pistons have less compression height than the design spec. This is done by the manufacturer to reduce complaints and cost to rebuilders who have had their blocks decked. Also, it prevents the piston from being the cause of a spark knock complaint due to an excessive compression ratio. You have to know what you have. You have little choice, but to trust that the crank has the correct stroke. Add half of the stroke to the rod center to center length, and subtract that sum from the deck height of the block. The design block height on all the Gen I small blocks of interest here is 9.025". Your never machined block is probably a little taller. The numer you get will be the compression distance of the piston you require for a "zero" deck height. Other numbers you derive along the way will help you determine the deck height you want. Knowing what height it takes to get, zero, will help you select the pistons. It's difficult to make a chioce, until you know what it takes. Possibly the easiest way, is to mock up an assembly, without rings, and measure the deck clearence, before you have the block surfaced. You should cc your heads to see if your 58cc heads have more or less than 58ccs. I'm betting they are bigger. Many gasket mfgs. will publish the compressed height of their gasket, along with the id of the gasket bore and ccs they contribute to the conbination. You can start by looking at the specs for the components, but you will have to measure the parts YOU have before you will be able to truely determine your compression ratio. Adjustments are most easily and effectively made to the deck height and head volume. Check the compression height spec for the various pistons you're looking at. That will get you started off on the right foot. Good luck and happy building.

conv90

Thanks, two very good technical answers.

Thanks
Beppe.

Dryseals

Go tp the KB-silvolite.com site and you'll find your pistons.