Not to highjack the op but, AJ do you know why the V/P for the 340 example on the Wallace site is 269, but when you run the specs on their 340 example through their dynamic calc you get 124?
Probably you entered the wrong Ica,
you have to figure from the advertised specs, NOT the .050s..
VP of 124 is closer to the truth with the stock specs, depending on your ICL.
At sealevel;
340 w/10.5 and Ica of 66* I get 131
340 w/10/1 ..........made about
124
the early 318s .......made about
136
the 8/1 318s ... come in around
116
the 8/1 360s .. come in around
126
the 5.2M .....IIRC is also around
124
Anybody who grew up with an early 318 remembers how peppy they were, and how doggy the 360s seemed to be. Nobody ever bragged on the 340s bottom end, even tho they never came with less than 3.23s. The VPs reflect that very well.
Don't forget to enter your elevation, to get your corrected Dcr.
All my own three builds were to the corrected Dcr, and all three worked excellently.
I bet a guy could make a spreadsheet relating corrected Dcr to stall/starter gear and never make a bad call.
hang on!
You mean in the VP article right?
V/P Index Calculation
That has to be an error; If I run their numbers thru the calculator until the effective stroke and pressures match and then I get; ohboy more errors, so something is screwed up in the article.
To get an effective stroke of 2.52 the Ica has to be 66*, so they got part that right. But there is no way to fudge the pressures into line unless you supercharge it, so somebody goofed. I can get to 173.49psi using 10.7Scr, but the VP only makes 135
Here is my best fudging;
With their numbers alongside in ()
Static compression ratio of.................... 10.7:1. ...........(10/1)
Ica of 66* @ sealevel
Effective stroke is..................................... 2.52 inches.........(same)
Your dynamic compression ratio is ............. 8.37:1 ...............(7.84)
Your dynamic cranking pressure is ..............173.49 PSI..........(same)
Your effective boost compression ratio, reflecting static c.r., cam timing, altitude, and boost of PSI is 8.37 :1.
V/P (Volume to Pressure Index)...............
is 135 ................. (269)
interestingly, 135 is half of 269
Good catch on your part.
Whatever they are doing, it carries on in the article, so you can still get useful information out of it by comparing the results to eachother.
If I had to guess, I'd say they are working with absolute pressures....
Yeah that's it. Here are the numbers at 10/1;sealevel, and gauge pressure
Static compression ratio of .................... 10.:1...................(same)
Effective stroke is ......................................2.52 inches.......... (same)
Your dynamic compression ratio is .............. 7.84:1 ................ (same)
Your dynamic cranking pressure is .............. 159.28 PSI.......... (173ab)
Your effective boost compression ratio, reflecting static c.r., cam timing, altitude, and boost of PSI is 7.84 :1.
V/P (Volume to Pressure Index) ................. is
123 ..................(269)
but the VP is still goofy, so
Volume/Pressure Index
V/P = CP × VE × N × .3% (.3% or .003 is a correction factor to return a useful 2 digit number roughly proportionate to torque)
where;
CP is pressure absolute (177 in this case)
VE is effective cylinder volume (258.4 in this case)
N is number of cylinders (8)
So
177 x258.4 x8 =1097.68/ nah,still doesn't work, so
IDK what they are doing. I cannot fudge their 269 into the calculator's 123 no matter how I fudge their formula, in the thought that they mighta put a wrong operational sign in there.
I tried...........
But IMO, it really doesn't matter. If you only use the Calculator, you will always get a useful number to use for comparison.
And if you look closely at how the formula is set up, you can see that it works equally with any sized engine. So you can assume a 408 stroker with a VP of 145 is gonna act just like a 367 at 145........... up to somewhere between 3000 and close to peak torque. In terms of VP, 145 is down-build for a 408, and 145 is an up-build for a 360. But in this case up is down and down is up, the 408 will use a large cam, and the 367 will need tons of pressure and a smallish cam. So once they are performing at par, car for car, (in the range of 3000 to say 3800), the 408 will begin to run away on the 367. You get the idea.
This is also why a dragracer doesn't need to care about VP, because he is launching close to peak torque, and the rpm will never dip down that far again.
But for a streeter,
big VP means taking off like a good BB, and/or using small gears and short stalls.
BTW; a 68 440Magnum has a VP around 137, by the numbers. So I remember those, and I liked them, so 137 is always and ever will be, my minimum target. You can get a 318LA up to there but it'll cost you, and it won't be all that powerful until the Dcr goes thru the roof, past pumpgas. But it will sure be snappy down low. IMO, for a small-geared streeter, 137ish is where the fun starts.
Since you can compare VPs of any engine directly, that means you can also get an idea of what gears to run.
For instance, a VP of 137 with a starter gear of 10/1 will be daymn snappy off the line no matter what the engine-size. So that would be; 137vp x10 gear=1370 factor.
So if your combo only makes 124VP, it will need 1370/124=11/1 starter-gear to take off similarly. But if your combo makes 160, then 1370/160=8.56 starter gear. With a 4 speed and a 2.66 low gear, this is the difference from 4.72s to 4.29s to 3.67s
And
with a VP of ~153, my combo can run 1370/153= a starter of 8.95; which with a 2.33 low is 3.84s.. But I run a 3.09 low, so I could run down to 2.90s. I chose 3.55s cuz I have a .78 overdrive, giving it a final drive of 2.77 on the hiway. Cool, hey!
With a starter of 3.09 x3.55=10.97, it takes off like a 10.97/2.33=4.71s; yes she's a dump it and go deal with the factory flywheel.
But I digress; I know at least three or four guys, who are falling asleep.
zzzzzzzzzzzzzzzzzzzzzzzzzzzz
