htownws6

iTrader: (1)

Well it all depends on what your DCR comes out to be really. Apparently you have a cam that compensates for that. What is your DCR?

cardo0

htown has a good idea as u could just wait it out until u catch a good price on used large camber heads. AFR, trickflow, brodix all have larger chamber LT1 reverse flow heads.

But the choice is up to u now. U need to learn how calculate your own c.r. or u will be right back were u where before with that first machinist. Taking for granted what others are telling u at work or over the internet isnt the fix for your engine - u have to confirm it yourself.

I think u hare at least $1000 or more like $2000 from a streetable engine. But a forged high compression 383 with a good cam.

Lots of options for u. Change the cam & vlv springs - cheapest.
Change the heads - more money and have to wait for correct chamber heads to go on sale.
Change the pistons - most expensive (well labor intensive) - but most reliable. U would need to find dished pistons of exact same weight and cyl wall clearance.

So what do u want to do?
cardo

htownws6

iTrader: (1)

exactly right. When its your money you need to confirm and understand everything otherwise you will likely waste your time and money.

hrcslam

That compression ratio with that cam it's perfectly fine with 91 octane. Your dcr would be lower than mine and I'm running the same gas in Tucson.

My scr is 11.76:1 vs your 11.8:1 my cranking pressure is 206 psi vs yours with that cam at 187psi (lower is better for avoiding detonation). Make sure your tune is on point and your cooling system is working good. Add a 160° thermostat and tube the fans to match and you'll be problem free.

htownws6

iTrader: (1)

The SCR on my LSx 408 is 11.28:1. I was told by my engine builder (HKE) not to go any higher than that on pump gas. Since he is considered one of the best LS1 engine builders in the country I figured he knew what he was talking about. Considering my heavy pig of a car runs mid 10's on 93 octane ...

BTW my cranking pressure is around 210 psi fwiw. Never had problems with detonation but I always pull about 6 degrees of timing out during the summer months as it gets hot in Houston, just to be safe....

95camaro21

I am going to put a CC 503 cam in it with new springs. So my set up.

383 stroker
Stock heads
Kb flat top pistons 11:8.1 compression.
CC 503 cam
Scat forged crank

hrcslam

It'll be a good performer for sure. And later when you get more funds pull your heads and send them to Lloyd or AI.

What is the part number for your pistons? I'd like to know your compression height to figure out your quench.

hrcslam

LT1s are reverse cooled and can handle higher compression than LS1s. It's more of a dcr thing though, but it's not uncommon to see LT1s running 12-13:1 scr on 93 octane. It really depends on the cam.

95camaro21

What is DCR?

hrcslam

Dynamic compression ratio. It's the compression ratio of the engine based on the valve timing events and piston position. Your bore, stroke, intake closing point, connecting rod length, and scr (static compression ratio) are all applied to find out what the dcr will be.

Along with static compression ratio and dynamic compression ratio, you also need to be aware of your quench to avoid detonation. Quench is the distance from the top of the piston to the bottom of the combustion chamber. Most builders shoot for 0.035-0.045". This is debatable, but all agree over 0.060" is bad for detonation. If your quench is over 0.060" you need to address it. If not, don't worry too much about it.

If you can get your part number for those pistons and the part number for your head gaskets we can tell you what your quench is.if it's too big you'll need thinner head gaskets which will raise your compression ratio. But with that cam and the tight quench detonation should be off no concern. However, if the quench is over 0.060" detonation is still a concern.

94FBIRD

iTrader: (5)

It is an LT1. Much different cooling system than the LS1. On the LT1 the heads are cooled first, allowing higher compression ratios than an LS1 which cools the block first sending hot water to the heads.

I ran my 383 in 110 degree Las Vegas weather last year and did not have a problem with detonation. 12.4 is my compression ratio.

cardo0

I am going to put a CC 503 cam in it with new springs. So my set up.


95/21, now who talked u into the comp cam when u already have a better cam in it (for what u need) with the LT4 HC??? Save yourself 500 bucks and leave the HC in there - just upgrade the vlv springs.

Myself im not believing the high compression claims as the PCM will pull timing (or the tuner will) and the engine runs less than optimum timing/power/torque. But please post your results as I would really like to run that high compression on my next engine at 2,500' in las vegas.

Best of luck and please post your results,
cardo

Puck

iTrader: (17)

Your new machinist doesn't know **** about LT1s. They are reversed cooled, unlike both LS1s and SBCs, and can safely run more compression because of that. 12:1 is fine for pump, 12.5:1 is OK if you trust your tuner.

Your problem won't be the compression, it will be running the crappy stock heads on a 383. Heads are more important then cubes.

96capricemgr

iTrader: (1)

The LT1 likes lots of compression and fairly low timing compared to old school SBC engines.

The 87 octane tuned iron head LT1 was 10:1 and cranks in the 190s reliably even after high miles, the aluminum head LT1 tuned for 91 octane is 10.4 and the LT1 was 10.8 again on a 91 octane tune. Those are all mile wide quench, available in 96 with 100K mile tuneup interval, conservative enough to be warrantyable.

When someone looks at 12:1 compression in that context it should help them understand this is not the same as taking an old 8.5:1 smog SBC running 40degrees of WOT timing and jacking the compression to 12:1. While most LT1 parts are very much the same as SBC the cooling and good chamber allow for lots of compression and don't need a lot of timing.

My car has run best with WOT timing programmed at 29degrees the pcm does add a few beyond that but it is not the high 30s old school smallblocks often ran.

cardo0

Your looking at "old school" iron head 350" @ 8.5:1 out of context - nothing like alum head sbc. '92 only had a iron headed sbc that had more like 9.3 c.r. in the iron headed LB9 (305).

Yea the "old school" 3rd gen L98 alum head camaro had 9.7 c.r. in '92. Then in '93 the LT1 reversed cooling goes to a wopping 10.4 with similar cam (maybe even a little lower DCR) while the LT4 went to 10.8 with the larger cam/lower DCR. U cant say GM saw more than 0.7 c.r. increase and failed to use it. Your stretching the logic too much as 12:1 is way out of context. U continually stretch the logic. Real alum head sbc goes from 9.7 c.r. to rev cool 10.4 LT1 and u want to claim 8.5 c.r. iron head sbc vs a 12:1 rev cool. Not even close! Another one of your "stories".

And the older flat tappet cams didnt spin to rpms the newer rollers do as most engines use less adv timing at 6,000 rpm than at 5,000 rpm - mine does. So your stretching the logic again with the timing adv.

And as u do the same stretch the logic with advance. Who knows how much timing a rev cool hi comp LT1 should have without a dyno to prove it. GM may have that dyno info. TPIS or Lingenfelter or Golen Engines may have that dyno info. I dont have that dyno time. And i definitely dont take your word for it. U definitely exaggerate too much.

hrcslam

My Dyno run proved 26.5 degrees of advanced timing at WOT at 3200rpms peak. 25.5 at 4000rpm+ at WOT. 11.76:1 Compression with 206psi cranking pressure. 414RWHP. My Combo isn't even extreme for an LT1, but for any other SBC it would be on the cautionary side of high.

His point was that the LT1 isn't in the same boat as the GenI SBC in terms of rough SCR vs detonation guestimations or rules of thumb. LT1's routinely run higher SCR and DCR than other SBC because of it's reverse cooling. There's no stretching it. His point is correct.

96capricemgr

iTrader: (1)

I remember when I first became interested in cars/engines, all the magazines would say things like 180psi was max cranking compression on pump gas, realistically this was just about the time the LT1 was being produced and as we have covered the iron head LT1 runs in to 190s cranking compression on 87 octane. Some folks are stuck in decades past though and think magazines/books are the only measure of what works. To hell with all of us who have EXPERIENCE.

My two heads/cam setups over the years have cranked in the 220s and in the 240s. I wouldn't try that with a LS1 or SBC but my EXPERIENCE in a mid 11 second 4200lbs car says it is working and I am not dialing back timing to make the compression work. It wouldn't perform that well if the timing were being held back to avoid detonation.

hrcslam

If I remember right the LT1, when set up right with a tight quench and high SCR/DCR and a good cooling system will produce MORE power with less advance than a typical Gen I SBC. The LT1 has a faster burn and tightening the quench speeds it up further.

All this combined allows us to lean the PE mode a bit (13.5:1 for some) and retard timing and GAIN power, not richen and pull timing to avoid knock like most other engines and lose power.

cardo0

BTW the OP has a stock quench height as far as is known.

25.5* adv @ 4000rpm is extremely low while 206psi is nothing to be a scared of. 415rwhp reasonable too for hi comp 383.

The 25.5* just dosnt fit as my stock engined car is 30* near 6000rpm.

Improvement with rev cool - yes. That much? Im not buying it at all. GM would have left way too much pwr on the table then at 10.4 c.r..

hrcslam

25.5* it is. 414 rwhp at 6100 (not peaked yet) on a MD SAE corrected on a 355. My peak timing is at 40* advanced but that lower in the rpms at less throttle.

Quench needs to be addressed with the OP's 383 build. Piston compression height and head gasket compressed thickness is needed to know where that's at. But right now it should be assumed that the quench is the stock and very bad .071".