Here's the skinny, hitting a target 9:1 compression with 52.5cc heads proves impossible in a 5.7" rod 383. Even if using -31cc pistons, with a .015" deck height and .039" head gasket, compression still lingers around 9.3:1. Too high for my tastes when shootin' for a boosted 600+ RWHP on 91 octane owl ****.

OTOH, opting for a flat top -5cc piston for a 5.85" rod/3.75" stroke - while retaining a 5.7" rods - seems like a viable option. Although it would sacrafice .14" in compression height and compromise piston strength somewhat, it would also drop the piston an additional .14" down the bore at TDC, yielding enough additional "deck height (.14 + .015)" to lower compression down to an even 9:1. Of course "piston swept" displacement would be slightly less than 383 cubes, but who the hell cares?

While quench would take a poop with that much clearance between the piston and cylinder heads at TDC, it probably doesn't matter that much in a forced induction motor anyhow. Agreed? Likewise, are there any other adverse effects to "dropping the piston down the hole?"

TIA

 

I wouldn't open it up that much.. it will effect it. Remember back in the day when Chuckster posted regarding his trails & tribulations altering quench on a blown 5.0? Higher CR, less quench yielded more power .

You want as little quench as you can get... get the piston just as close to the head as possible. With a steel rod.. depends on how far you turn it.. but it'll stretch a good bit, probably .038-.040" would be plenty, depends.

If it's a blower motor, why not consider building a 355? You dont need the inches, and it may help hook up a lil better to ditch the added mechanical advantage that extra stroke will give it. As far as the thick gasket & piston outta the hole vs. the thin gasket & piston in the hole.. IIRC the guys going faster were running it in the hole with a thinner gasket, but we arent talking to the extent you're talkin aboot.. and youre FI anyway.

Dunno maine, i'd try'n lay the chambers back a lil more methinks... do somethin.. i just wouldnt run half that much quench. Have you called JE/SRP/PSI/etc. about pistons? Whatever you need may be a shelf piston that isnt listed in whatever catalog etc. youre lookin at...

 

I'm sure you've thought of this but I'll mention it anyways

Have you considered some heads with a larger chamber like Skardom mentioned? It just seems sorta futile to get a low CR with chambers that small. I bet if you sold your current heads you could almost pay for a set of AFR's with larger chambers. That seems alot easier (and cheaper) than swapping pistons, rods, and rebalancing. You could also run a tighter quench and have more compression height in the piston (if you stuck with the 5.7 rods).

My $.02

Good Luck!

 

Phil,

Since a 355 and 383 cost the same, save for a bit of block clearancing, why not opt for the extra cubes? Although it will cost a bit more to balance a stroker....... At any rate, a piston in the hole with a thin gasket was the set-up in question.

Swapping out the heads for large chamber AFR's was the initial plan, but it's just out of the budget. The current heads might be worth a little over a grand, which will buy a set of bare AFR's. Throwing in the cost of valves, springs, retainers, locks, blah, blah and some minor clean up work racks up another $1K or so for heads that won't flow as well. Also, even with 74cc chambers, reusing the same bottom end (-4cc flat top pistons) still nets a 9.7:1 CR - too high.

As for checking for off-the-shelf parts, I'm practically on a first name basis with JE at this point

Soma,

Speaking of the bottom end, it just won't cut it under a maxed out T-trim. At least not for long Not only that, the counterweights on the current crank will only work with 6" rods, limiting pistons selection and compression height. Additionally re-using the crank/rods with new slugs would require rebalancing anyhow.

 

Because that's what's gotcha in this pickle in the first place . Honestly.. what's it matter if it makes 675rwhp or 625rwhp on pumpgas.. not gonna go anywhere on street tires anyway & even slicks are gonna be struggling to hang on without some killer suspenders. Dunno.. I dont see the point when you could go just as fast, if not faster, for less $.


$$$.. for my money i'd do a 35X. That and.. yeah.. im talkin bout in the hole.. but i'm not talkin .150" quench either
. I'd want as little quench, and dish as I could get away with. That's moi however... i also like to slather pancakes with peanut butter prior to syruping them

Yer motor maine, I'm sure you'll figure out somethin .

 

Stephen, what headgasket are you planning on using? I would give quench more weight in the deciding factor....i think that is more important. .042-.039" is a good place to be (not factoring in rod strech). If I were in your shoes I would serously look into a nice set of CNC competition ported AFR 195s. Of course they are more costly, but the intake and exhaust flow are great (especially the exhaust which would be great for you application), thicker deck, and large chambers.

Either that or look at getting a few more cc's out of your chambers. How much have your heads been milled? I would look at that first...mabye get them opened up to ~58cc or so. Custom pistons are also an option, but I would'nt do that until the last resort.

Jason

 

Not really following you here, Phir. Biggest dish JE offers in a 4.03/3.48/5.7 piston is 24cc. Biggest dish for a 4.03/3.75/5.7 piston is 31cc. Either way, compression ratio is the exact same.

Are you suggesting retaining the current bottom end with bigger chamber heads? If so:

1) It's too weak
2) Don't really trust a 1.125" compression height off-the-shelf Ross slugs under alot of boost
3) It will cost quite a bit more money buying/cleaning up a set of AFR's opposed to keeping the stock heads and stepping up to forged crank/H-beams/blower pistons

Just don't see how going with the AFR's will be cheaper.

"Maine" and "aboot?" You comin' out of the closet as an undercover Canadian or somethin'? Stop hanging out with Gord!

Jason,

Going with a stock head gasket (.049") and the proposed bottome end (-31cc pistons) drops CR to a friendly 9.1:1. However, how well the stockers can seal under big boost is questionable. A 1074 Fel Pro (.039") increases CR to 9.3:1, but seals better. I'm opting for the latter.

The stock heads measured 55cc after porting and have been milled another .010" since.

Welp, so much for this in the hole business

 

Duuuuude..

4.030 x 3.48

0 deck height, .039" gasket & 56cc chambers with those 24cc dish pistons

es perfecto... .38-.40" quench, 9.25:1 CR

You can have the chambers on those heads laid back.. isnt hard.. we've gone as big as 58cc on lt4's for some NA stuff, cake.

Whatever you do, dont go over .050" quench. .050-.060, or .070 is the 'dead zone'.. where it's more prone to detonation than it is below that, or above that. Still, over .070.. it's still gonna be a ***** comparitively. If you wanna save money, go 35X, or spend $ on custom pistons if you're hellbent on a certain compression ratio. Whatever ya do though.. no more talk of this way down in the hole business. You'll go just as fast with a 355 as ya will with a 383.. I mean really, it's primarily a streetcar right? So the difference is having to limit yourself to half throttle on street tires or 60% throttle? Whatsit matter? Blower is the limiting factor.. not the shortblock IMO.

So yeh.. spend less.. go just as fast.. 35X.. have stock chambers laid back a lil more.. .038-.040 quench (0 deck height with a felpro), puppies, daisies and all that horse****. It's a good thing.

 

Hey Mr. Cheesesteak,

Sure, a cast crank and I-beams may be fine in the 600 RWHP range, what happens when the boost is cranked WAY up?

And those -24 pistons are for 5.7" rods. Off-the-shelf selection for 3.48"/6" pistons is near non-existant. So, new rods kinda undermines the whole "save cash" plan. Not only that, a 35x requires a purchasing new crank - just like with a 383. Now if you have a spare laying around the shop you'd like to donate, please drop me a line!

Custom pistons? For the extra cost of a custom ($150) over an off-the-shelf, plus money made from selling the current rods ($100?) can damn near pay for a new set of H-beams. So, I can either save and entire $50 or "shell" it out and end up with a much stronger bottom end, with the exact same CR as a 6" rod/-24cc piston combo, and favorable quench to boot. Now you see why I'm asking how sticking with a marginal bottom end will save a significant - if any - amount of cash?

Oh yeah, the counterweights on this current crank will only work with a 6" rod anyhow.

How about once strapping on these only-got-two-passes-on-'em 28" Hoosiers? The extra inches will definitely show up them. And since the car isn't a daily driver anymore, some weekend street cruising with the big meats would be even more entertaining with the extra cubes

Welp, it won't go as fast on tires, and I'll only save an entire $50

It probably would have helped to mention this a bit earlier, but the 5.7" rods and the big dish pistons are already collecting dust at home. Just pondering some crackpot options, so thanks for setting me straight