I just confused myself (again)
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Joined: Mar 2004
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I just confused myself (again)
From the responses to my last post seems to indicate that I need to get my VE tables dialed in closer before working on my O/L woes, I think I'm going to start over (again!).
I went to the diy-efi site and returned with a stock AMUR bin. This is what they say is for an 88 C/K truck, TBI EFI system.
Next I used the search engine to come up with the formulia for setting the BPW.
Now here's where the confusion sets in. According to the above formulia, the stock 55lb injectors *should* have a BPW setting of 151.2; and the Dilphi's on the Holley 670, 60lbs @ 12 PSI (lowered the psi because of the age of the pump) *should* have a BPW setting of 138.6, right?
If I am correct so far, why is the stock BPW set at 135.0? It seems like at that setting, everything is going to be slightly lean. Also, wouldn't this setting cause that injectors to go static earlier? Speaking of the injectors going static, how can you tell when the reach the static stage?
If this is all correct. stand by for a couple of VE tuning questions if I can't find them by searching.
As always, TIA!
I went to the diy-efi site and returned with a stock AMUR bin. This is what they say is for an 88 C/K truck, TBI EFI system.
Next I used the search engine to come up with the formulia for setting the BPW.
Ok our BPC (base pulse constant) is basically an injector constant and engine size all rolled up into one. So to calculate it we need to know a couple of things and get them in the right units and then plug it into the formula below.
BPC = 1461.5 * (cyl size / gm/s of fuel)
For cyl size you find your engine size in Liters. 350 * 16.39 = 5736.5 cc or 5.74 liters Then divide this by 8 to get your cylinder size 5.74/8 = .717L
For say a 65 lb/hr injectors : 65 / 3600 * 453.6 = 8.19 gm/s
So our formula works out to be:
BPC = 1461.5 * (0.717 / 8.19) = 127.95
BPC = 1461.5 * (cyl size / gm/s of fuel)
For cyl size you find your engine size in Liters. 350 * 16.39 = 5736.5 cc or 5.74 liters Then divide this by 8 to get your cylinder size 5.74/8 = .717L
For say a 65 lb/hr injectors : 65 / 3600 * 453.6 = 8.19 gm/s
So our formula works out to be:
BPC = 1461.5 * (0.717 / 8.19) = 127.95
If I am correct so far, why is the stock BPW set at 135.0? It seems like at that setting, everything is going to be slightly lean. Also, wouldn't this setting cause that injectors to go static earlier? Speaking of the injectors going static, how can you tell when the reach the static stage?
If this is all correct. stand by for a couple of VE tuning questions if I can't find them by searching.
As always, TIA!
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From: Chasing Electrons
Car: check
Engine: check
Transmission: check
305's ran 55#/hr injectors, 350's ran either a 61#/hr or a 68#/hr injector.
The higher a correct BPC value the lower the RPM the injectors go static. Can output the PW in the ALDL stream and do the math for DC%.
RBob.
The higher a correct BPC value the lower the RPM the injectors go static. Can output the PW in the ALDL stream and do the math for DC%.
RBob.
Joined: Jan 2005
Posts: 10,450
Likes: 509
From: Hurst, Texas
Car: 1983 G20 Chevy
Engine: 305 TPI
Transmission: 4L60
Axle/Gears: 14 bolt with 3.07 gears
Originally Posted by TierAngst
135 if calculated correctly is almost exactly 61pph for a 350. The injectors go static when the DC = 100%
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Joined: Jun 2004
Posts: 386
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From: San Antonio
Car: 78 Caprice Coupe
Engine: 355
Transmission: 4L60
Axle/Gears: 3.42
Originally Posted by Fast355
Correct on the injector sizing, BUT the because of the nature of the injectors, they start having noticeable trouble around 85-90% DC. By trouble I mean the injectors tend to hang open and give a very rich mixture.
I really don't know what's up with my fuel pressure since stock BPC is 135 and right now I have to set mine at 126 to get the stock VE tables to work and come close to 128 BLM's but it was actually very close right there and my car is running better than ever... er engine wise anyways cause like I said the trans is going soon.
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