tee-boy

iTrader: (8)

"LSA - Vizard's LSA to CI/Inch of valve diameter chart puts my 21.4 at 108. This chart is supposed to be for accurate for 9.0 to 11.0 compression."

I pulled this statement from a recent post. Regarding Vizard's chart, do you think it is applicable to Gen III SBC heads? I read the article and am a bit weary to apply his principles to Gen III, when it seems he is referring to Gen I heads.

IFRYRCE

iTrader: (3)

I read the article and thought the same thing. I rarely see LS1s anywhere near 108LSA. I just assumed he meant Gen I heads and that it wasn't applicable to LS1s.

Could someone explain why it is/isn't applicable?

1989GTA

I was wondering if the chart leaned more to carb applications versus EFI. It's my understanding that the narrower the LSA the harder it is to tune the ECM.

tee-boy

iTrader: (8)

Well one things for sure... Our intakes are totally different from single and double plane intakes of gen 1's. Therefore, I assume reversion characteristics relative to LCA and overlap would not be highly correlated b/w gen 1 and gen 3.

tee-boy

iTrader: (8)

I'm not a tuner, and I have heard statements like that before; however, I am under "assumption" that anything a carb can do, a computer can do better. It just depends on the quality of the individual who is doing the tune.

edcmat-l1

iTrader: (10)

Thats exactly it.

LS1curious

I think that chart is the biggest bunch of bullshit I have ever seen. Overlap amount is going to change with port efficnecy, cubic inchs rpm manifolding headers etc etc etc. Valve size doesn;t have **** to do with overlap.

edcmat-l1

iTrader: (10)

Try gettin the surge out of a cam like that in an EFI car. You wont have near the problems with a carb.

Asmodeus

That was me that used his chart and refered to it in another thread.

I got the info from reading this article:

http://www.compcams.com/Community/Ar...ID=-2026144213

I learned a lot from it too.. it's a great read.

stevieturbo

And efi car will tame a wild cam far far easier than a carb ever will.....


And the same rules probably dont apply. Tight LSA cams tend do be favoured on engines with crap heads, as they have to resort to such measures to try and make some power.

LSx heads flow very well...so these tight LSA's arent needed, or wanted.

edcmat-l1

iTrader: (10)

SAY WHAT? Who's buildin your carbs? I run/build carbs for cams I would never even think about running in an efi car. 250/260+ duration on a 108. Silky smooth with a carb.
There are some very specific carb mods that need to be done to make them work right.

stevieturbo

I havent used a carb in about 5 years....I have moved on with technology.

All thats required with efi...is a laptop and suitable software/computer.

Fuel and ignition control at your fingertips. No very specific mods at all. Just simple tuning.

its great.

edcmat-l1

iTrader: (10)

Yeah. Preachin to the choir. I do a bunch of LT, LS stuff, not to mention the stand alones.
But fact is a carb doesnt have the same issues as a computer with high overlap cams. I'm talkin cams that are pretty much race car ****. Over .700 lift, over 260 degrees duration, 108, 106 LSAs.
A properly tuned carb will make just as much HP as an EFI system. And just as good of driveability. The only trade off, is cold start, because most perf carbs dont use a choke.

99Fbody99

I agree and have found the same, Carbs as archaic as they are, do allow you to get away with stuff EFI will not in general. But you have to get dirty tuning them

Dave

tee-boy

iTrader: (8)

I don't think overlap has anything to do w/ port efficiency, ci, or headers. You are correct however, that valve size doesn't have anything to do w/ it either. I'm willing to bet that all those variables do affect reversion-including valve size.

edcmat-l1

iTrader: (10)

Overlap, or the amount of it, affects scavenging. Scavenging aids in cylinder filling. This can either help or hurt the output. Depending on the flow characteristics of the entire motor.
So, how does it not have anything to do with any of those parts? They all have an effect on the engines ability to breathe. Well flowing intake runners combined with well flowing heads and headers would seem to not require as much overlap. So, running a bunch of overlap would tend to hurt the power in the way of wasted energy right thru the motor.

tee-boy

iTrader: (8)

The point is... Overlap isn't a funcion of port efficiency. Engine's ability to breathe is a function of port efficiency. My statement is only true if overlap is defined as degrees of rotation over which both valves(assuming 2 valve engine) are open.

71CamaroLS1

To the point of the original post: I'm not so sure that Vizard's handy graph applies particularly well to most LSx applications. In particular, LSx motors seem to like later intake valve open and close points (and hence, a larger LSA) than older generation small and big blocks of similar displacement.

I suspect that this difference is due primarily to the very different characteristics of the plastic "candy cane" style intakes, which tend to have runners that are longer and less tapered than those of performance 4-barrel style intakes. I would bet that a 4-barrel single plane LSx intake would require very similar cam timing as a first gen SBC with similar intake.

edcmat-l1

iTrader: (10)

Well, duh. Never said it was. Overlap is nothing more than how long both valves are off their repective seats.
Yeah. And the ability to breathe can be influenced, both positively, and negatively by valve events/overlap.

Not really sure what you mean there. Overlap is ONLY defined as degrees of rotation over which both valves are open. So, thats another DUH statement.
Virtually all of the components and specs you've regarded as meaningless, all have an effect on what cam specs will work best.

tee-boy

iTrader: (8)

Ed,

Thanks for the clarification. So going back to your original post... I assume your answer to the original question is that since an EFI application is harder to tune on a narrow LSA when compared to a carb application, then Vizard's Cubic inch:valve diameter ratio chart is useless... Now it's clear.