I need a VE factor for my combo

[tminer]

Stock stroke, .020 over bore, 218/218 Comp Roller (3313 lobe) 113* & +2, GN1 heads with 1.60 rockers and mild hand porting, 70mm TB/UP, ported intake, Poston headers, 3" THDP. Compression ratio is undetermined but shooting for 9:1, maybe a little less.

Looking for a reasonable VE factor to use in various calculations. Also, will it change significantly with RPM (3500-6500 range)?

Thanks,
Tom

 

[tminer]

Sorry that this got in this section, not my call. But maybe you are the only ones that understand what I want.

I am not looking for any tuning data, just the general VE. Then I can better calculate air, fuel and turbo requirements at various rpms and operating conditions.

Thanks,
Tom

 

[bobc455]

You can use the following approximation:

VE (at WOT) = (0.85 * absolute manifold pressure)/14.7

If you run 15 PSI of boost, your absolute pressure would be 15 + 14.7 = 29.7.

(0.85 * 29.7)/14.7 = 172%.

NOTE: This is a TOTALLY USELESS BS calculation and is completely WRONG. Use at your own risk. There, I said it.

-Bob Cunningham

 

[tminer]

Actually, I'm looking for a less generic value for the .85 in your formula.

Tom

 

[Nick Micale]

Originally posted by tminer
Sorry that this got in this section, not my call. But maybe you are the only ones that understand what I want.................
Tom

That is why I moved it here Tom!

 

[Jackson]

Originally posted by bobc455
You can use the following approximation:

VE (at WOT) = (0.85 * absolute manifold pressure)/14.7

If you run 15 PSI of boost, your absolute pressure would be 15 + 14.7 = 29.7.

(0.85 * 29.7)/14.7 = 172%.

NOTE: This is a TOTALLY USELESS BS calculation and is completely WRONG. Use at your own risk. There, I said it.

-Bob Cunningham

I'm learning more EFI principles as I go but WTF did this come from?

 

[tminer]

People look at VE different ways. That formula converts the "normal" VE into a boost relative VE. Besides the fact that it is assuming a value for the normal VE, it does not take temperature change into account. That may be OK for tuning purposes where temperature compensation is handled separately, but not for the calcs that I am attempting.

Tom

 

[JDEstill]

tough one... The best thing would be if someone has a setup similar to yours and is running a FAST then they could give you their VE table and you'd have a pretty good idea from that.

Barring that, if you know what rpm your peak torque is at then that is also where your peak VE should be. For the calcs you are doing, turbo and injector sizing and such, I'd prolly use something like, I dunno, 90% maybe? It'll drop off some on both sides of the torque peak, and points that have the same rpm and different manifold pressures will give you different VEs.

If you are picking out new parts the 90% should be conservative enough. If you are trying to evaluate existing parts, then something like 85% or even 80% might be a more realistic value.

And if you really really need a good number, Extender chip DS readings + a data log of MAP, MAT and rpm values would let you calculate your own VEs.

John

 

[bobc455]

Originally posted by tminer
That may be OK for tuning purposes where temperature compensation is handled separately, but not for the calcs that I am attempting.

Tom

Tom,

Maybe you could share exactly what you are trying to calculate?

VE does drop off a bit as you go very high in RPMs, but of course that depends on all of the charachteristics of your engine. Actually, the high temperature increases your VE, but decreases your Mass Efficiency (or density efficiency or whatever), which is actually what makes HP.

Perhaps some cam design software, or engine simulation software might help you. There are obviously a lot of numbers that affect your VE, and you seem to need a dead-nuts estimate for whatever reason.

-Bob Cunningham

 

[tminer]

I don't need it pinpointed, I just have no idea what to use and thought that the combo would be an important factor. I've seen numbers from .6 to .9 mentioned over the years. I'm sure that some were used to represent worst case scenarios.

I'm picking new parts and also have two goals (short and long term) so evaluating things a little closer than normal to see if they can work for both.

Looks like .85 (85%) will be close enough.

Thanks,
Tom

 

[S2V6RACER]

I would start at around 75-80 VE in your boost range. Dial about 11.1:1 and get it dialed in, then lean it out to 11.6 to 11.8:1 with 26 degrees or less timing on race gas.

85 or 90 VE will be too high IMHO. Take short blasts, don't hold into it for long at first.