Maybe I've just been fortunate for never having a problem with this, but what throws off cranking VE so bad that it needs to be tuned? And what do you log for to tune it? Is there a cranking fuel trim that I've over looked somewhere?

I was browsing through a couple other threads for some other info and ran across this being mentioned a few times, but without much detail.

Any opinions, questions, or comments are welcome.

 

I would like to know this as well. TTT for ya.

 

No one seems to know....

 

Is there any correlation between the stock cranking VE and normal VE on a stock car?

 

I wouldn't think so, except that VE varies according to RPM which would result in cranking VE being far different. I'm not knowledgeable though, just looking for info from the rest of you.

 

Looks like @
400: 124% -> 103% greater
800: 112% -> 104% greater
1200: 109% -> 105% greater

 

I don't have my laptop with me, would you mine elaborating on what those values mean Phil?

I may play around with this a little tonight to see what effects it has on the car. I suspect lowered compression or a larger camshaft would make you want/need to decrease the cranking VE some.

 

The cranking VE is 124% bigger than your normal ve....
If your VE is 100 than cranking ve is 124

 

When the engine is cranking at only 200 - 300 RPM or so it is very hard to keep enough fuel in suspension to get complete burning, and especially since the chamber, valves, and plugs are relatively cold during a start.
Increasing fuel delivery during cranking eases the process.

Things that would significantly affect this value would be a large displacement change, an injector change with the IFR out of perfect adjustment, or anything else that would change the amount of cylinder filling during cranking. IAC adjustment being off, a big change in minimum-air-rate, or a change in TB size could do it.

 

Thanks for the clarification. I may just multiply my normal VE table by 124% and place those values in cranking VE to start with just to see what happens.

White2001s10, you seem like you have a pretty good grasp on what's going on with the cranking VE stuff. How do you go about tweaking it?

 

Well considering carburetors get by with no extra cranking fuel unless you press the gas pedal, the object would be to use as little as possible.

There's also a fuel prime that can be adjusted for cranking.
Between the two fueling methods, you whittle them down to the minimum that still gives smooth starting.

Adjusting other variables such as cranking spark will change the fueling requirement. Generally more advance will want a little more fuel.
A cold engine will like more cranking advance and more cranking fuel, where a warm engine will want much less advance and very little cranking fuel.
For example your engine may like 16* crank advance at 0*F
but only maybe 5* crank advance at 200*F.

The amount of air the engine gets (due to wide open IAC or air getting through or around the throttle blade) will change the amount of fuel needed.
Say you have your IAC opened pretty far for cranking, and you also have a good size idle hole drilled in your throttle blade, then you'll need more cranking fuel.

Very large injectors may flow so much at their minimum PW that you simply get too much cranking fuel.

Anything in particular you were wondering?

 

Not really, I'm basically just trying to get a broad understanding of how it works. I've seen it mentioned by a few people, a few times, and I never understood why there was a need to modify it in the first place. Every time I've nailed down the big 3 tables on a car, it starts without a problem. Idle tuning takes a little tweaking of course, but the actual start-up itself is always fine, so I never understood why people modified it. From what I'm gathering from your post, the only real reason to modify it is to make the fueling at start up just that much more efficient?

Thanks for the info on the basics though, that should at least give me some working knowledge when I get some free time to start messing with it.

 

tttt....

 

If your VE table (Low RPM) is significantly LESS than your stock VE, then your STOCK Cranking VE may be excessive - resulting in difficult hot starts (i.e. too much cranking fuel).

Bigger cams with a narrow LSA (lot of overlap) have lousy VE %s below 2000 rpm. Stock 67.834122 %....Cammed 24.491218 %. I get a kick out all those decimal places...as if I'm that accurate!

FWIW.

 

I tuned my cranking VE down by logic after reading Bink's post (thanks buddy ), and my car has a much more mellow time firing up. Instead of the usual (borderline) violent start up, it starts smoothly now, just as its "supposed" to. I didn't really realize I had any issues with it before messing with it, but it also seems to have effected my throttle cracker issues a little in a good way. I still need to iron that out so it doesn't try idling a little below desired idle when I first start rolling, but thats certainly tolerable.

When I tuned it, I based it off of how much I had changed the primary VE table from stock to my current. I used the difference as my values for how much to change the cranking VE table.

 

So your saying that if your new VE table is 30-50% of what it orginally was you multiplied the VE table by 30-50% to correspond? If so that kinda makes sense and follows what I said up top.

 

If your idle drops a little when the car first starts to move, at an MPH below the Throttle Cracker enable value, then try adjusting your RAF/Desired table at that/those ECTs. Add .05-.10 g/sec at a time and see how the idle does.
If this makes it worse then maybe look at base spark/timing values (too low?) and spark overspeed/underspeed values.

 

Yep, you got it.

I'm pretty sure my Throttle Cracker enable is 1-2mph, so I'm 80% sure that's my issue, it just hasn't been enough of an issue to put priority on. Spark overspeed/underspeed would, as you said, be my next likely culprit. My RAF is pretty much dead nuts on. Thanks for the tips though, much appreciated.