whyrun

I'll be hopefully be putting my 383 together soon, would it be better option to drive the static compression up high also bringing up the DCR with a really tight quench and let the cam grinder tame down the DCR with the IVC to were you need it, that way you can keep a higher static comp and tighter quench without risking to high a DCR. Is my thinking correct here if not please explain thanks...

rskrause

There is no advantage for a super tight quench and it requires meticulous assembly to avoid disaster. You pick the cam first and then figure the CR, not the other way around. You pick the cam based primarily on the desired rev range, keeping in mind the need for the other components need for that rpm range. Unless the cam is going to be very small or very large (a torquer or a screamer) figure ~230 degrees intake duration and making peak hp in the low 6,000rpm range with a CR in the 10.5:1 range.

Rich

whyrun

Speaking for a LT1 what would be the advantage in selecting such a low compression and not take advantage of the higher compression's available to the reverse flow heads this being a N/A engine with nitrous..

SS MPSTR

Agree on the "meticulous assembly" comment, not so much on the "no advantage" part. I guess we need to define "super tight". If someone is suggesting something in the 0.030" to 0.039" range without putting forth the effort for attentive detail, then I agree. Mine's in the 0.032X" range and the benefit I see is resistance to detonation and a cleaner burn.

Perhaps my information is completely off base?

rskrause

You have to consider the whole combo. You really need to list all the details, so perhaps my being specific was premature. Things like:
intended use
fuel used
priorities between things like peak power, low end torque, etc.
gear and tranny
HR v. SR lifter
willingness to spend $$$ for high dollar supporting parts (springs, pushrods, etc.)
and so on.

Rich

ByByC5

Whyrun,
Agreed about needing more specifics in your build up, fuel required...etc..all important!

But I think I know where your headed, as I had very similar thoughts in my 383 as I was laying out what I wanted. A reliable street car that performed well at the occasional trip to the track.

I zero'd my deck, and used the Felpro 1074 gasket .(039). Looking for the "safe" tight quench.
I don't have my notes in front of me, but I ended up with about 12.2 static, and 8.9 DCR.
Very streetable on 93 octane, and very N2O friendly!

Reason being a better burn, reduced detonation. The added compression does help to smooth out a lumpy cam, not to mention that a bump in compression is added power across the complete rpm range. Maybe not a whole lot...but I want all I can safely get!

I can say that attention to detail like this helped smooth out my cam.
HTH
Keith

whyrun

well let me list it out I was just looking for a general answer but Lets see for sure if I'm heading right..

383 crank 3.750 Rod 6.0 or maybe a 5.85 not for sure yet fully forged bottom end
pistons forged -5cc trying to hit around 12.5:1 to 13.0:1 93 pump gas and stand alone with 110 for the N20,rings gapped for heavy n2o 200min 300hp max
200cc ported stock casing 53cc 2.055/1.600 valves .39 HG,pro mag RR's ,pac springs
ported intake direct inject N2O
58mm TB 42lb injectors LS1 Lid, Long Tubes Going with a custom cam so no specs on that but since its going to be a limited street car its probaly going to be fairly radical but I want to keep it HR I dont care for SR.. WIth around 4K stall A4 and 3.73 gears depending on how it runs with the hose may go to 4.10..

I probaly left out alot of stuff but I think this should get it in the ballpark. And its kinda hard to figure DCR without knowing cam IVC... It seems like it would be really diffacult to design a cam around a engine..

rskrause

That CR is way too high for pump gas regardless of the cam chosen.

Rich

blue 79 Z/28

advanced induction told me they have successfully ran SCR to 13.0:1 on pump gas with zero detonation, but an ace tune on their top end and custom ground street HR cam. all of their street setups in 383's run SCR in the 12.4:1 area on pump gas and street cams. in fact thats the setup they told me to run as i bought a setup from them. one thing they stressed though is a dead accurate buildup with quality parts and a REALLY tight quench.

Zaknafein

I guess I'm missing something.

Why/how can anyone recommend pushing it to such a limit when the customer base spans across the country, lives at different altitudes and climates and sees different variations in pump fuel octane? And all for what... a few hp?

Not trying to come across like an *** but I know a few chemical engineers and one in particular that has worked in the petroleum industry. I know enough not to trust pump gas octane and understand that there is a seasonal variance, even within a single source, ie., you fill up at the same station all the time.

Then again... what is the air density like in your neck of the woods at the peak of summer? Of course it's going to vary from area to area so how do we arrive at a blanket value for everyone? Combustion temps are always going to be the problem and once you exceed the auto-ignition temperature of the fuel...

Personally, I'd err on the safe side. I'm sure you could stay on top of fuel/spark tables and quench it that way but that sure seems like a lot of headache for a daily driver.

blue 79 Z/28

while id agree with everything you said, and i voiced those concerns also. they assured me that it is far enough from the limit on their setup. while im lucky i live at sea level on the west coast and our gas in canada is 94 octane for premium plus. i dont know the valve events on their cams because its a secret but im sure they must have worked it out to make the DCR within range for pump gas otherwise they wouldnt have made it seem so "nothing special" they told me to think outside the box and forget what i went by for sb1 and things of the past. something must be right because im sure not the first to make a 383 with their gear, and not the last.

whyrun

Mine Is Not a DD And why would 12.5:1 be to high for pump gas isn't stock around 11.0:1
. Of course In all actuallity you should be able to lower the DCR quite a bit with the cam should be able to get it back down around 8.2 with cam timing.. I'm going to talk with my builder some more I've always been told a tight quench not insanely tight just tight will wardoff detenation better than anything plus keeping it cool...

rskrause

It will detonate. The DCR concept works for low speed/high load operation. Of course, a high performance car shouldn't be driven in that regime much, if any, in any case. At higher rpm, especially with a big cam and good heads the VE will increase to the point that cylinder pressures are even higher than low speed/high load. Peak torque at WT is where cylinder pressures reach their peak. Detonation also occurs under those conditions and it's even more of a problem than low speed. That's because you can't simply avoid running at WOT/peak torque (that's why you have the car!) and you will probably not hear the detonation. But it will kill power and damage parts.

IOW, 12.5:1 is just plain too high for a SBC on pump gas, IMHO. Stock is 10.4 and at that CR detonation often occurs. That's why GM included a KS and knock retard. If you have a big cam you can bump the CR up a bit, but again, 12.5:1 is just too high. Look at some of the PHR engine builds and consider that these are dyno queens.

Don't try to reinvent the wheel. Use a proven combo unless you have the time, money, and knowledge to experiment.

Rich

blue 79 Z/28

static compression means little, its just a number and has little to do with what the true cyl pressure is. dynamic compression and cranking pressure will tell you what you can do, combined with the LT1's awesome chambers, reverse cooling, and a tight quench motor with flat tops, you can get away with alot more then a std sbc. camshaft events will determine what those will be. no motor is the same and its all in the cam. one motor may rattle at 11:1 with a certain cam yet another can take alot more. over on yellow bullet, theres a guy that has a big block mopar and run in the hot rod pump gas drags, he runs damn near 13:1 on 93 and runs 8.50's. 200 cranking compression is about where you want to be to run pump gas.


http://www.yellowbullet.com/forum/sh...ad.php?t=81880

ByByC5

I disagree about 12-ish:1 static compression ratio detonating in a "performance built" LT1 engine, (NOT a STOCK mass produced engine).
To many people doing it, myself included. It is about which cam AND attention to detail.

In a stock LT1 with 10.5 cr, and the LT4 10.7 cr and the generic tune GM gives these cars, 180* thermostats, fan temps pushing melt down, poor differential gear ratios and people not always running the best of fuels, different driver types...etc, all the variables GM can try to factor in IS the reason for the knock sensor circuit.

GM and performance vehicles are not the only applications using knock detection. Many non performance applications do for the above reasons.

Every car/engine combo is different for sure, and true it is all about cylinder pressures.

Other items to consider, rod length, air inlet temps, ICL that the cam is degreed at (relative to IVC) the LT1 reverse flow cooling system...alot to consider.

My set up does not detonate, street driven, at sea level in S.W.Florida...doesn't get much worse for an engine to detonate as far as abient conditions (heat, humidity, sea level). Running close to stock timing tables. But my cooling system is not stock, nor my gearing...weight reduction..etc...

It's all about attention to details! Running a tight quench is a detail that requires it to be done right for sure. An LT1 cylinder head and a flat top piston would benefit from a closer quench IMO.I did it for that reason.

I guess opinions vary

I dug out my notes from when I built my engine in sig, SCR 12.1740 and DCR 8.56291

HTH

Keith