BadDog

Hmmm, looking at BPW again...

I was given equations for figuring BPW in another thread. Here they are for completeness.

New bpw=old bpw * (new press/old press).


350ci: 350 * 16.39 = 5736.5 = 5.7L

5.7l / 8 = 0.7125l/cyl

inj#/hr: / 3600) * 453.6 = gms/sec

BPC = 1461.5 * (VOL/RATE)

1461.5 * (0.7125 / gms/sec) = BPW

Much thanks to honken.


If I go up in pressure, and I don't trust the original bin BPW to just do the simple ratio of pressure conversion, plus I have no idea (though I guess I could restructure the equations to figure it out) what old pressure calc was used for the BPW, how do I figure out the #/hr change for stock Orange post 55# injectors? What pressure is 55#/hr based on? That would give me the #/hr for the current 20 psi, then I could just figure the BPW from scratch instead of assuming the original bin BPW was right. Seems like the best course and, if I'm not missing something, all I really need is that original pressure for 55#/hr.

And also, most of the posts I read are for people looking for max hp and low 1/4 times so it's not clear to me which way is better. I'm not looking for high rpm, max hp results. My main goal is rock stable with max torque from idle to around 4k+ with really snappy off-idle and mid range "tip in". So what am I looking for in ideal pressure?

I'm thinking 20 psi from my reading here. Any opinions of how this thing will idle and respond on tip-in with 20 psi? From what I've read, this seems to be about right for my purposes since it gives me more fuel over all and better atomization from higher pressures, lower duty cycle so I don't run into problems on the high end (though this is a low-mid motor and shouldn't be a problem there), and it's still well under the higher pressures run on basically the same injectors in later OEM TBs as well as lower than pressures successfully run by some here. Should also still be high enough on BPW to keep from having the idle problems some have reported due to too small pulse width I think...

Or, since I really don't need the duty cycle, would my life be easier, and perhaps the system be more robust, if I go for something more like 18 psi?

My main interest in "tip-in" comes from the use of the vehicle. Some times you need a quick inertia building "bump" to get up an obstacle and no running room, or almost over the top of an obstacle and need an instant surge to complete before you slip off, and I've even backed out of what would have been a NASTY multi roll reverse end-over that was successful only because the engine responded instantly. That was in my "hard core" truggy which is basically stock and has always had excellent and instant tip-in response. But this "built" K5 is sluggish and stumbles on tip-in at most any speed, followed by a very strong surge, which is pretty much the opposite of ideal for my use. I know there are many other characteristics in my custom chip that need addressing to reach my goals, but the BPW, and therefore pressure has to be addressed before I can even start seriously on the VE, AE, PE, and SA tables…

I include that so that you guys can point out flaws in my evolving understanding and (hopefully) offer any insights you may have to help me on my way... As always, thanks for your help and patience.

honken

I beliwe the stock press its 13 psi for inj (gm tbi has 9-13 psi in the car).

Some calculations.

http://www.rceng.com/technical.htm

Ronny

iTrader: (1)

is this helpful?

http://www.users.bigpond.net.au/ben_...ctorsizing.xls

my issue is/was not enough fuel on top (17.5 lbs FP) and almost too much at idle. last time i drove the idle was OK (850 rpms/13.8/1 A/F open loop) and the WOT was 13.30/1 on WB. a balancing act. 80 lb inj's. need to work on that in spring !

BadDog

Thanks guys.

honken:
I figured it might be at max pressure of 13 psi, but wasn't sure. Thanks for the link.

Ronny:
Handly little xls. Looks like it incorporates the formulas honken gave me. Assuming 55# is at 13 psi, my new BPW will be 125.7 and support up to 218hp. Should be more than enough hp for my needs as long as it holds up the torque down low...

Just for grins, I'll double check it by hand later.

BadDog

Well, the info looks great. 125.7 BPW for 20 psi on 55# injectors is looking like the way to go.

But, what about opinions? Do you guys think 20 psi just buying into more trouble than the return for my goals? Or are there real returns on the lower end, in particular, for throttle response? If 20 is too much, any opinions on what is a good target for that purpose? Everything I can find to read is focused on top end power and 1/4 mile times… Has nobody here ever focused on the low end before?

Fast355

iTrader: (2)

BTW, TBI 350 truck injectors are 61 lbs/hr. 305 Truck injectors are 55 lb/hr. That will give a factory BPWC on a 350 of 135.

GM injectors are rated at 12 psi. To get the new volume you take the new square root of the new pressure/old pressure multiplied by the injector size. For example my 68#s become 111 lbs/hr at 32 PSI. It is on a 312 CID engine. The BPWC is at a very low 67 which gives me fuel all the way to 6,400 rpm.

I run 32 PSI and am very concerned with low-speed torque. I have 312 CID pulling 5,600 lbs+ a trailer at times with a 700r4 and 3.73 gears. Works well for me. Helps with fuel mileage, ALOT(Once Dialed In)

BadDog

Ahh, thanks for correcting that mistake. I had read on an injector ID post that the orange pins meant 55#/hr. Likewise, the psi correction will help.

Much thanks on the corrections and opinions/experience.

JPrevost

http://www.turbocity.com/default.php?cPath=16_30
These guys know a thing or 2 about TBI even if they are pricing themselves out of the market.

RBob

iTrader: (1)

The post color isn't what to look for. When manufactured there are two paint dabs placed on the top center metal piece. After time these do wear off. The injector number that is engraved on the top can then be used for identification.

If you have the 61 #/hr injectors and bump them to 20 psi:

new flow = 78.8 #/hr = 61 * sqrt (20 / 12)

Then for supported HP at 85% duty cycle (DC):

HP = 268 = (78.8 * 2 * .85 ) / .5

2 is 2 injectors
.85 is 85% DC
.5 is BSFC (lbs/hr/HP)

So you are good to 268 HP as long as you don't take them static.

RBob.

Fast355

iTrader: (2)

The code for a stock 350 injector should be orn/blk and they are 61 lbs/hr. The grn/wht are 55 lb/hr from a 305.
268 FWHP potential is surely more than you need for a stock head, stock cam engine.

BadDog

Ah, guess I misunderstood the color ID info I read. Great info and much thanks for the updates.

Yeah, 268 hp is well beyond what I need or expect, I'm mainly looking for low torque, but want a well ballanced motor that's over all as fun to drive slow on the rocks and waterfalls as it is on the whoops, sandy washes and freeways... But it's far from a stock cam and heads. I've got decked and (very) mildly massaged SR Torquer heads along with a PowerMax 2030 (aka CompuCam 2030) Crane roller with true double roller and around 9:1 compression in a 4 bolt ZZ3/4 ballanced short block powdered rods, forged crank and a few more bits and pieces like the modified 64mm deshrouded TB with pod spacer.

Edit: The main thing I think that really doesn't fit here is the heads, which really are more hight rpm oriented. But there they were here, so I used them. Beginning to think I should have just kept the stock heads that came with the ZZ block...

Edit again: But that "stock head and cams" does certainly apply to my "rock buggy" which will be getting a similar raised pressure and custom chip treatment, (though everything else will probably remain stock) once I get around to it. It *only* needs low-mid rpm responsiveness and it already does just fine there as it, though better is always good...