I've been staring at piston choices for the past two weeks and have come up with a few likely candidates. Clevite alone has over 10 different pistons for the 300, and specs listed in this catalog and that sometimes conflict, so figuring out which one to use has been a chore... at least I'm 98% of the way there now.

Our three finalists are:

Speed Pro H519P: cast hypereutectic alloy, 1.767" compression height, 8.5:1 CR, offset round recess

Clevite 2581: same specs/ material as the 519 but it had a "D" shaped recess

Clevite 2039: 1.763" compression height, 8.8:1 CR, offset round recess like the 519, cast aluminum [not Hypereutectic]

The first two are almost identical save for the recess confuguration. The 519 has the off center round "plug" style cup common in most 300 pistons up thru '82. The 2581 has the "D" shaped cup like you find in most '83-up models. Anyone have thoughts as to which configuration is better overall? My gut says the "D" shaped recess would provide better quench, but that's about it.

The draw of the 2039 is the higher [8.8:1] CR without having to shave the head or block too much. It does have a slightly shorter compression height so might have to get the block decked to get it zeroed out. It has the plug style recess like the 519, but its volume is a bit less netting a higher CR. This piston is plain cast, not hypereutectic. I've read the pro's and con's of both and lean more toward the hypereutectics from a wear/ durability standpoint, bur "hear" they're more likely to crack than plain cast. Any thoughts?

Yes I'd love to drop in a set of custom forged pistons, but I've sunk WAY too much $$ into this rebuild already. Started out just doing a re-gasket job to stem the leaks, but have wound up doing a total performance rebuild. Oh well... better than giving the $$ to the crack dealer

 

Go with hypereutectics and don't look back. They only thing they won't tolerate is alot of detonation. But then, no piston will. Otherwise you can't kill them. Not even with nitrous oxide. The D-dish is better for quench especially if you zero deck the block. Which hapens to be a win-win situation. It raises compression and greatly increases quench which allows for more advanced timing while reducing the chance for detonation all at the same time. If you're concerned with affecting valve train geometry by decking the block, no fear. You can shim your rockers with shims that match the thickness of material remove from the block. Cheers.

 

I feel your pain on piston choices. There is a good selection of them on the KB-Silvolite website, but they only list half of the specs. I don't really care what shape the dish is nearly as much as I want to know what the volume of the dish is. The factory spec is useless once you have strayed from stock very far. There are enough different chamber volumes listed to write a book about. It just depends on which manufacturer you believe. If the round plug dish is the one I'm thinking of it is the one I would use with an EFI head. The dish is offset well over to one side and IIRC is directly under the intake valve which makes the dish and chamber line up very well in several places. Don't forget that you don't have to use a 300 piston. The 352, 360 and 390 are all good candidates to look at. Your block will need to be thick enough to take a 0.050 bore, but it's worth a look. There are flat tops available for each of those three engines.

 

OK... I'm going for the Clevite 2581. They're 8.5:1 CR as is, but by the time I get the block decked to zero and head cleaned up [maybe .020"] I'll probably be looking at 8.8 to 9.0. Want to make sure I can run safe on regular... gas is too $$ for premium. They're only $50/set surplus from Northern so if I don't like 'em it's not like I'm aout a ton of $$.

 

The way gas prices are right now you might be ahead financially to build an engine for premium fuel. Adding compression is one of the best ways to improve mileage. Premium is about 10% more expensive than regular, but you might gain 15-20% more mileage with an added point of compression.

 

Streak you are correct, however that only applies to engines with high volumetic efficiency. The only way a 300 will achieve high volumetric efficiency is with boost. I believe that it would not be cost effective to raise compression in a naturally aspirated 300 and expect an economic advatage by using the more expensive fuel. If you feel the desire to run premium in the future, you can always advance your timing and enjoy slightly better performance without the commitment of higher compression.

 

My car ['86 Mercedes 560SEL] is a good example of octane rating vs. MPG. The 5.6 all aluminum V8 has a 9.5:1 CR and specs premium unleaded of 91+ octane. It will run OK on 87 octane regular, the computer just dials back the timing a bit and you lose some power [mostly top end]. On a tank of premium [we have 93 octane here in TN] I'll get about 16.5 mpg, on a tank of regular I'll get about 15.5.

Do the math out and it's an extra $4.80 per tank to fill up with premium [.20 cents/gal X 24 gal]. I'll get about 24 extra miles out of a tank of premium which = $4.33 worth or regular @ $2.80 gal. So.... about .50 cents per tank difference to use premium vs regular at today's gas prices.

I would certainly build up the CR more in the van's 300 but when I tow cross country much of the upper midwest only has 90/91 octane premium. I'd be concerned about getting gas "low" enough that detonation might occur, and I don't want to have to carry around gallons of octane boost. It's scary enough driving across parts of Utah and Nevada where you can go 100+ miles between gas stations and a full tank of gas is only carrying you 220 miles. Miss the wrong fill-up stop and you could be out MANY miles before the next opportunity . I always carry 10 gallons of gas in reserve, and have had to tap into it more than once.

 

At part throttle all engines have low VE. I haven't seen an engine yet that didn't run more efficiently with a higher compression ratio and a corresponding increase in octane if needed. Advancing timing isn't necessarily going to give you more power. An engine only needs as much timing as it needs, adding more won't make any more power. Mine makes best power at about 12-14 degrees. I can add timing up to 25 degrees with no hint of detonation, but I'm down 14 hp on the top end. Adding premium fuel and trying it again doesn't result in any additional power.