Corto

Hi,
I would need advice from experienced people here what proper heads chamber size would be for my config?
They will be aftermarket alu heads, 185-195cc (Profiler, Promaxx, Dart,...?)

I am going to have a 383 engine in stock 1980 car, with:
- - SCAT stroker rotating assembly, 6“ rods, hyper flat top pistons with 7cc,
- - Deck clearance 0.020
- - Head gasket – still to be decided, but could be 0.026 (or 0.015)?
- - Roller cam Compcam XM276HR (276/282, 224/230@0.05, 112/110)
- - TH350 with stock converter

If I enter these data into CR calculator, for 64cc heads I got SCR 10.7:1 and DCR 8.3:1
Is this SCR too high (here in Europe we have 93 and 95.7 pump gas)?

Or it would be better to go for 70cc heads? With everything the same but the 70cc heads the results are SCR 10:1, and DCR 7.8:1

I understand that the things are not so simple and that DCR is not the only parameter that should be taken into account. So I would be very grateful if some experienced people can give me advice.
Thanks!

Dynra Rockets

I personally like the smallest chamber I can get away with for quench reasons.

HeadsU.P.

I agree with Dynra, smaller chambers. The thing about headgaskets are, you can go a little thicker to reduce your SCR but it will not affect the DCR. You want your dynamic above 8.1, I believe. So, looking at those 70cc heads comes out 7.8 DCR = too low.
And the 64 chambers 8.3 is perfect, except the 10.7 is getting close to aviation gas.

I went through this last winter, crunching numbers, how far down is the piston in the hole, different gaskets, Int valve closing events, chamber size, piston top volume, etc.
Take your time with this, do it right so there will be no regrets later. I am not too keen on thin headgaskets (shims) such as 0.015. I think you are setting yourself up for leaks because there is not much material to fill in any defect scratches in the block or head. I ended up with 0.051 thickness to reduce the too high SCR down to 10.4. But retained the DCR at 8.2. It runs on premium only, but I don't care. You put thousands of dollars into a rebuild, sure not going to squawk over 25 cents difference in gas grades.
Just for the heck of it, here's another calculator to compare / crunch number:
uempistons.com (United Engine & Machine) Calculator is on the left.

REELAV8R

With a deck clearance of .020 and a gasket of .015 and 6" rods you will be pushing the limits of piston to head clearance after rod stretch. Kind of depends on how you plan to run it, at what RPM and how long and often.
Keep in mind the piston to valve clearance as well depending on the valve lift and LSA.

That being said you can run a 10.7 CR and get away with it given good exhaust, proper timing and fuel mixture and a good quench.

I run 10.6 CR and have no issues with detonation on my 350. However I also have a .038 quench with 5.7" rods vs 6" rods. I top out at 6400 RPM and do not stay there for prolonged periods so I have no problems with piston to head clearances or using hyper pistons.
Hypers with 6" rods and a 3.75 stroke you will have to be cognizant of you piston speeds due to RPM longer stroke and longer connecting rods along with peak HP levels. Hypers are solid up to around 500 HP maybe even 540 HP or so, beyond that forged is a better option. Forged is what most folks go with for a stroker to be safe.
If your pushing detonation limits forged is going to be a better option.

6" rods dwell slightly longer at TDC which somewhat increases chances of detonation.

.026 head gasket will put you at .046", not ideal for quench for detonation prevention, but still probably adequate.

I would not use a stock stall torque converter either. Idling will probably be difficult with it and you'll be mashing the brakes hard to get it to stop.

cv67

get at least a good 2800 if you keep it too tight youll hate it and wonder where the power is

63mako

I have had no issues running the Felpro 1094. I spray it with coppercoat first. With the lift of that cam .035 valve to piston clearance is not an issue. With the tight quench and aluminum heads detonation resistance will be better than a thicker gasket and dropping compression. Quench and good cooling is the key factor. run a 165 or 180 thermostat. I have a 5 CC zero deck, .040 gasket with more lift and milled the AFR 195 75CC heads to 72. If you think about it milling the heads puts the valve closer to the piston. Mine were cut .018 and plenty of clearance left. I would also go forged pistons. https://www.summitracing.com/parts/w...06a3/overview/ This might drop it down in the hole a little too. DON'T USE KB HYPERS!! Check with this forum member for AFR head pricing, Likely less price difference than you think and You get a CNC head with correct springs out of the box. tpi421vette@comcast.net

NeverTooOld

I'd use the 70 cc heads because over time carbon will build up on the piston tops and in the combustion chambers and raise the compression ratio a bit.

resdoggie

I hear this all the time but I haven't had any issue. I did increase the ring gap per KB instructions. Maybe that's why?

REELAV8R

I am using KB hypers and have had no issues.

His comment may have been more to hypers vs forged perhaps.

.035 is piston to head clearance NOT piston to valve clearance. With rod stretch a .035 on a 3.75 stroke with 6" con rods could be an issue depending on use of the engine and the peak RPM.

C3 Stroker

With my 383, I used AFR 195 heads, comp. was 10.9 to one. Comp Cams 286HR 230/230 @ .050 hydraulic roller. 93 octane gas with no issues.

63mako

IIf you rarely hit 6000 rpm with a hydraulic roller .035 is fine. You will float the valves before you get .035 rod stretch. I had a 302 solid flat tappet that ran 7500 with pink rods and .035, no sweat. I do realize The piston speed is slower on The 302 but with good modern rods I wouldn't worry. The piston link I posted is .005 down in the hole compared to the KB Hypers for $100 more so if he goes with them will likely end up .040. Those are a low expansion forged.

Corto

Thanks guys,
this should be a street car, will never see 6000rpms.
Long tube headers already mounted.
Recently rebuilt TH350 with stock converted, which I would like to keep, and the cam has been chosen/proposed to fit (thanks mako!).

Nothing else has been bought, I can still decide to go for forged pistons, or 5.7" rods instead of 6",.....
So the most of the config is still open, I would like to get max of it but definitely I don't want to push to the limits.

JoeMinnesota

I've run KB hypers in a 383 in the past with no issues, but was not pushing the envelope on compression. The argument about KB is that the top ring is located up higher on the piston, so thinner top lip between the top ring and face of the piston. My machinist recommended I go sealed power and not KB because of this. On my recent 355 build I went higher compression and sealed power hypers, which place the top ring further down.

My brother actually just broke 2 KB hypers after roughly 4k miles on his 462 Pontiac motor. We assumed possible ring bind, but the engine shop said they appeared to be classic case of detonation. He did have a bad curve in his disti early after his build and rattled the motor some. I never had an issue with KB personally but would say be careful about detonation because the hypers won't tolerate it like a forged piston.

v2racing

I run a Scat forged crank with Scat Pro Comp "I" beam rods with 7/16" cap screws. My squish is set at .034". I've spun this engine to 7500 plus many times without issue. I do have very light pistons and wrist pins, Ross Ultralight forged and tapered wrist pins.

Mike

Corto

Thanks guys, based on your recomendations I have checked another Scat kit with forged pistons (Icon) and they bring in some changes:

- - SCAT stroker rotating assembly, 6“ rods, forged flat top pistons with 5cc (or 3.7 still to be confirmed!)
- - Deck clearance 0.025
- - Head gasket – 0.015 to have 0.040 quench?
- - Roller cam Compcam XM276HR (276/282, 224/230@0.05, 112/110)
- - TH350 with stock converter
(long tube headers, street car max 5500rpm, premium 93 and 95 gas)

With bigger deck clearance (0.025) I can use 0.015 gasket to have 0.040 quench, is it ok?

Smaller piston head volume (5cc) is asking for bigger heads (70cc).

The CR calculator is now showing SCR 10.4:1 and DCR 8.1:1.
Are we now on the safe side, with some room for playing with cam phasing for few deg?
Thanks

Street Rat

I like this thread.

63mako

nice setup. I don't push the 8.5 even if everything is right. my 10.4 with XR 288 HR 114 lsa, 6 speed, 411 will run on regular with no ping. Likes to run on when I shut it off though so I use premium (optimized timing). it makes 535 hp and very streetable so I did not give up much. Also got 20.1 MPG on a road trip with 3 deuces running 80, no fuel injection.

c3_dk

What ?

resdoggie

That's like saying drop a diesel in so you won't have to worry about fouled plugs.

cv67

Bet so, probably properly tuned also, with the cheap 02 sensor kits available its a wise investment long term

GM has killer spark and fuel management, probably the reason theirs live so long behind high hp motors (guessing they have a better quality piston too)