I gotta question on compression ratios and your opinions on chamber sizing to achieve effective boost compression ratios

Right now i got a n/a 11 to 1 compression 383. Runs good about 400whp, mid 11's at 119 as it sits with more in it once i set it up to launch right

Over winter i am contemplating a boost build, single turbo to be different or just a procharger

what i wanted to do since the motor is so new, i wanted to get new AFR heads or similar with a larger 75-78 cc chamber which with my flat tops will give me 9.5-9.8 to 1 compression. SHould be good enough to run 8-10psi which is all i want. I want 550whp or more and i think that will get it.


Is that ok to do versus the typical dished pistons and standard 64-65 cc style heads. Kinda a dumb question but i'm not sure if that would make a difference or not.

Quench height will be standard .040-.041 with my zero deck block and gasket.

I dont see a problem running a larger cc chamber and a flat top as long as compression is low enough. But i dont know a whole lot on boosted applications

 

Bigger dished pistons, and smaller chambered heads typically provide a better flame front travel vs the soup dish heads and flat tops.
I'm not sure how much proof there is in that, but that's the theory at least.
Pistons are cheaper then heads anyway, why not look for some big dish pistons?

 

well i'm not sure if i i can get work done to my heads to open up the chambers or else just buy new and sell off the old

i figure i could easily get 900-1000 for these heads, they have 700 miles on them. new heads would only be 500 more and thats easier to swap heads than pistons on a already build shortblock thats balanced and broken in already

 

While that last part is true, I wouldn't expect to get any more than 50% of the value of a used part. That's just the rule.

IF you could get 80% of the value of the heads, and just buy new ones, heck yea, do that. If not, then get some super dished pistons, rings, and try to get the pistons weight matched to the old ones. If not, rebalance time....

 

i got srp flat tops now, all forged. i figured the srp dished version should be just a touch lighter since they are dished but then again i have no idea if they took off any material on the pistons during balancing.

i'm sure barely used AFR 195's with the upgraded springs will fetch 900 bucks atleast. If not i'll take what i can get and sacrifice that loss. i really dont wanna tear the block apart. AFR told me i cant have the chambers opened up to 75cc as they use a different casing for the larger chamber so i'll have to sell them off or do like you said and get new pistons. Maybe i should do that anyway and shoot for even more boost than i planned and more power

This will be over winter probly and possibly over late fall into next spring. I gotta decide if i want to do this for real or not.

 

I thought the chambers on the AFR heads were copies of some LS? type head and don't they use flat top pistons for there cars? One of the reasons I bought my heads with 75cc chambers cause I was thinking they would work better with flat top pistons instead of buying dished for when I rebuild the engine.

 

i never heard of the chamber design being not applicable to flat tops. they do use LSx valvetrain stuff...8mm hollow valves etc

 

Dished pistons don't have any squish.

 

What I mean by work better is as in give more power compared to using dish pistons. I figure GM designed the chambers to match up with flat top pistons to give the best flame front travel so probably should do the same.

 

i guess i may find out. I probly should really look into replacing the pistons just because i know that i may want more power down the road and to turn up the boost on an already high compression setup isnt a good idea. i fear 9.8 to 1 with near 10 psi will be around my limit, but then again i'm not sure

 

I'd go with a reverse dome if I could, meaning that the dish mirrors the combustion chamber and the quench pad is untouched. I don't know if there are any off the shelf pistons like that, you could call JE or somebody.

 

Yea, what 327??? said. He beat me to it. Reverse dome, that's where the money is.

 

any links to tests with these pistons?

 

a tight quench forces the air/fuel to jet out from the quench area at high velocity into the combustion chamber and helps stir the mixture. Some builders grind troughs (however you spell it) that help direct jets of air towards the plug.
Tricks like this can reduce the octane requirement a bit, which means you can run more boost or compression for a given octane of fuel.
All of that means more power.
How much? I dont know, it all depends, but a tight quench is better than none.

 

With a 58cc head, you have to use the JE -31cc dished blower piston or a custom piston to get 9.1 compression.

 

AFR doesnt have 58cc heads so i'll use the 65 or 75. 75 if i dont change pistons, 65, hell i'll keep my current heads and change pistons if i go that way. its a long way off yet (several months before winter) so i have time to think. I may just shoot for 10's all motor but that may still require heads

 

after looking around i dont see any pistons available that are close to the same weight as mine. they are advertised at 419 grams, and most vary between 400-419. I would think plus or minus 2 grams is ok, but 5-10 is too much difference. I'd have to rebalance

on that note, i have found no pistons giving me the desired compression i wanted. srp 16cc dished gives me 9.74 to 1 with my current heads. If i ran 75 cc heads and my flat tops 5cc reliefs, i'll be at 9.8 so i dont think .1 compression is worth the swapping of those srp's

JE makes a 28cc reverse dome but that pushes me way down below what i wanted. These are at 8.7 to 1 and i wanted 9-9.5 i guess for my goals

i'll be running a nitrous shot in the next few weeks. Should be around 520-550whp. if i like it, i want to be around 550-600whp on the turbo next year.
I was thinking 9.8 to 1 with new heads and 7 psi or so from a turbo with meth injection should get me the power and keep it safe

I dont wanna go too low on compression for just 7-10psi and 600whp. I hear you dont wanna go to much over 600-650hp on stock GM block anyway, so i'll get away with the current setup on low boost and 550-600 whp for now and build a bigger/better motor for boost on the side for years down the road.

Thoughts?

 

if the block was already zero-decked, run a fat heat gasket, 65cc chamber and the 16cc piston. dropping from a .041 head gasket to a .061 will drop 3-4 tenths of a point of compression, and will still put you in the 'quench' zone, and put you less than 9.5:1

if you need to trim weight out of the piston, grind it out from around the wrist pin bosses, for a few grams, it's not much grinding/no loss of strength. alternatively, you could massage the piston crown, by removing sharp edges and gaining the weight match, and cc at the same time.

you can also trim a cc or 2 out of the chamber, more than likely, by doing the same thing. in any case, you are probably going to need to break out the die grinder to make some subtle changes to hit your target.

 

i thought good quench zone was .035-.045 with .039-.041 being perfect? I dont really wanna run more gasket as that may lead to more detonation prone activity that i'm trying to avoid. I should be running meth injection so that will help some.

I really dont want to take apart the shortblock but it may be best to just shoot for new balanced assembly with JE's 28cc dish since i cant find anything else between 16 and 28. Maybe custom?

 

well, that's the generally accepted range, although they claim anything over .060" can cause problems. so the way i read that is your optimum quench range is therefore .035" to .060". Shooting for .045" puts you right in the middle of range, to account for tolerances.....so I doubt you'd have a problem. And shooting for a few thousands either way is waaay to small a change to impact anything not on the cutting edge of HP production. You'll get that much in bearing clearance, and piston rock so it's a moot point.....

anyway, that's what I'd do, trim a little on the piston and head, and run it that way. I'd run it like that, and if that breaks, THEN spend more money on bottom end parts.(doubt you'd have issues though) I'd be more nervous running nitrous, and running lean, that tends to melt everything. a turbo will usually pop a head gasket or knock a corner off a piston, which is 'usually' less carnage.

probably not too helpful, sorry 'bout that, just throwin' out my opinion, fwiw....

 

Any opinion is worth it to me. I probly will want to stay with head swap with larger cc chambers and keep the flat tops for awhile to see how the car likes that.

Plan was to run the turbo 383 next year looking for 550-600whp on whatever psi i can run with 9.8 to 1 compression and get away with on 93 octane.
On the side i'll build a dart block for boost or something. That will be another year or two atleast

 

I'd go a bit closer to 9.0:1 for a greater factor of safety. I would think that a piston swap would be cheaper than a new set of AFR's. I've heard from a pretty reliable source(many people on TTF.com) that GM blocks are good until 800-900.

My turbo 2.8l jap car is going to be ~7.4:1, it should love 25 psi.

 

Thats interesting, i was told around 650.

I'd love to do a piston swap but i can only find JE's to give me 8.7 which i thought was abit low for my goals but you know what, i'll have 550-600whp and not be happy so that leaves room for 700+

Maybe i will consider new pistons and keep my heads

What i was gonna do was see if i could seel these heads off for 1000 bucks and purchase new set for 1500. thats 500 bucks spent, making pistons more expensive at 700-800 for the JE's

 

yeah, honestly, with a lower compression and forced induction, more HP is just a click of the boost dial away anyway. You will have to run a smaller cam though, so your dynamic compression doesn't dive into the toilet.....

since you have good heads though, you are already well ahead of the game. I'd keep the lift high, and the duration on the smaller side.

I think you'd be fine either way....just depends what you want to build when, and how much you want to spend.

there are alot of GM blocks running in the 700HP range with just some minor work. above that, most guys will fill to the bottom of the freeze plugs. forced induction really doesn't hit the block as hard as nitrous, so they tend to generate more HP than you'd think possible.....

 

i just wanted something to spool rather quickly and make 600whp for now. If the block will handle that thats great. I do plan on a custom grind turbo cam and expect it to be small which is fine with me. car may not sound great at idle as it does now with the lope but oh well, power is all that really matters and if the smaller cam and turbo quiet it down, the neighbors will be happy.

i wanted peak power by 6000 rpms shift at 6300. I'm at 6300 shift by 6500 now n/a. So i expect the cam to be a good deal smaller than the 230 intake lobe. I'm thinking 218-224 on the intake.

 

Reasons you don't have to worry about that: 1) 383 2) stall converter

You could bolt an mpt70 on and make 600rwhp and have boost at something retarded like 2500rpms, you'd have to shift at like 5500 though. With a decent converter you will never be below the stall point at WOT, say you get full boost at 3300rpms and your 3000 stall converter flashes to 3500, you get tire smoke.

 

i didnt think 600 would be that easy to make. mp t70 is small from what i hear. i wanted either a T76 or am thinking S300 turbo.

T76 should work awesome at 15psi for a later build

i have 3600 stall now but if i do this turbo build i may swap it out for a slightly smaller stall speed. I want peak power by 6000 and shift by 6250-6400

 

That should be good for roughly 675 maxed out(about 10hp per lb/min), people have been in the 9's on one. But I agree bigger would be better. Maybe consider an s400.

600rwhp should be relatively easy with a good breathing 383, I'm looking to make 350-400rwhp with a poorly breathing 2.8L nissan with full boost at 3200(shift at 6k) and 91 octane.

Peak power by 6000 is very obtainable, no reason to spin the engine very high when you have boost. You could probably do it at 5500 if you were so inclined.

 

either way would be cool. S400 is massive tho, i'm not sure i want that one. I"ll have to start looking at the maps and size something out. I'm just not ready for it yet