F.A.S.T Crank / Afterstart help
- Thread starter TEST MY 6
- Start date
Roland Black
New Member
- Joined
- Dec 20, 2001
A day late and a nickel short, but I'd appreciate seeing this also. Thanks in advance,
KLHAMMETT said:
Any chance to take a look at this. Having the same problem on my NA SBC 396. Otherwise my program is very good, but the cold starts are an issue.
redcorvette@brightdsl.net
Thanks,
Ron
I spent a little time pondering the cranking fuel issue earlier this year, trying to help out a guy who was having cold start problems (among other things). I was thinking that if we started with a car that has a known good cranking fuel table, then we could use that on any car with the following relation:
new cranking pw = old cranking pw x (old inj size/new inj size) x (new cyl size/old cyl size)
For example, the demo program I seem to remember coming with the FAST box was for a 265 cid V-6 with 83# injectors. At the coldest temperature the commanded cranking fuel was 11.5 ms.
Now, big assumption, lets assume that is a good, well tuned value. We now want to figure up the value for a 396 cid SBC with 55# injectors. The V-6 has 265/6= 44.16 cid per cylinder (or per injector if you want to think about it that way). The V-8 has 396/8=49.5 cid per cylinder. So that first cranking pw point would be = 11.5 x (83/55) x (49.5/44.16) = 19.45 ms.
So we'd put 19.5ms into the first point on the cranking fuel table, and then go to point #2 and do the same thing. To my way of thinking this should give about the same volume of fuel per cubic inch of displacement during cranking for both engines. And so if the base engine was tuned well, then the new engine ought to be dang close.
I don't think the guy I was helping ever tried this out though, so don't know how well it worked. I'd like to take the cranking pws from a stock GN and do this exercise, see how that compares to the values people end up with that work well, but I never got around to it. Be interesting to see though.
Maybe if this works, someone could post a table of their (cranking pw x inj size / cyl volume), and then anyone could take that table and use their own cyl vol and inj size and create their own cranking table easily.
If anyone gives this shot, let us know how it works.
John
new cranking pw = old cranking pw x (old inj size/new inj size) x (new cyl size/old cyl size)
For example, the demo program I seem to remember coming with the FAST box was for a 265 cid V-6 with 83# injectors. At the coldest temperature the commanded cranking fuel was 11.5 ms.
Now, big assumption, lets assume that is a good, well tuned value. We now want to figure up the value for a 396 cid SBC with 55# injectors. The V-6 has 265/6= 44.16 cid per cylinder (or per injector if you want to think about it that way). The V-8 has 396/8=49.5 cid per cylinder. So that first cranking pw point would be = 11.5 x (83/55) x (49.5/44.16) = 19.45 ms.
So we'd put 19.5ms into the first point on the cranking fuel table, and then go to point #2 and do the same thing. To my way of thinking this should give about the same volume of fuel per cubic inch of displacement during cranking for both engines. And so if the base engine was tuned well, then the new engine ought to be dang close.
I don't think the guy I was helping ever tried this out though, so don't know how well it worked. I'd like to take the cranking pws from a stock GN and do this exercise, see how that compares to the values people end up with that work well, but I never got around to it. Be interesting to see though.
Maybe if this works, someone could post a table of their (cranking pw x inj size / cyl volume), and then anyone could take that table and use their own cyl vol and inj size and create their own cranking table easily.
If anyone gives this shot, let us know how it works.
John
Yes IME that is the best 1st approximation (to ratio ecu values that are in terms of raw inj pw by the ratio of the relative inj sizes and/or displacement). But generally requires a bit more tweaking after that. Even on the same engine when say, swapping to different size injectors. My feeling is that different engines respond to heating conditions (cold start, warm start, hot start) enough that it probably prevents this from being a pure plug and play. But again, is the most obvious and best approximation to start with, and what I usually do myself for starters.
TurboTR
TurboTR
JDEstill said:
Well, figured I'd post something that might help someone someday. Here's a set of points that I came up with (these are theoretical and untested):
deg F / unitless pw
5 / 29.0
16 / 24.21
27 / 20.39
39 / 17.17
50 / 14.93
61 / 13.24
72 / 11.94
84 / 10.83
95 / 9.93
110 / 8.77
125 / 8.56
140 / 8.35
155 / 8.15
181 / 7.84
218 / 7.43
254 / 7.08
So, if you don't have anything better for the cranking fuel table, take the table values above, divide them by your injector size, multiply by your cylinder volume, and enter the result in the cranking fuel table.
For example: lets say you've got a 455 V-8 with 42 lb/hr injectors. The cylinder volume is 455/8=56.88 cid. At 110F, your cranking pw would be 8.77 /42 x 56.88 = 11.9 ms
If instead you have a 270 cid V-6 with 96 lb/hr injectors, at the same 110F you'd have 8.77 / 96 x (270/6) = 4.1 ms.
These table values are pretty rich at the colder temperatures and lean out gradually up through a coolant temp of about 110F, that's why the curve is so steep through those points but then flattens out after that.
So, how good are these numbers? Well, can't say for sure, I'd like some input from someone that has a car with good cold start manners, but I'd hazard a guess that this table should be good to plus or minus 100%. In other words, if this table says you should have a value of 10 ms at some temperature, I'd guess that you'll find in the end that something in the 5 to 20 ms range works best. Not great, but better than nothing.
Oh yeah, I went back and double checked these values against the demo file Demo.gct, and they were pretty close. FWIW.
John
deg F / unitless pw
5 / 29.0
16 / 24.21
27 / 20.39
39 / 17.17
50 / 14.93
61 / 13.24
72 / 11.94
84 / 10.83
95 / 9.93
110 / 8.77
125 / 8.56
140 / 8.35
155 / 8.15
181 / 7.84
218 / 7.43
254 / 7.08
So, if you don't have anything better for the cranking fuel table, take the table values above, divide them by your injector size, multiply by your cylinder volume, and enter the result in the cranking fuel table.
For example: lets say you've got a 455 V-8 with 42 lb/hr injectors. The cylinder volume is 455/8=56.88 cid. At 110F, your cranking pw would be 8.77 /42 x 56.88 = 11.9 ms
If instead you have a 270 cid V-6 with 96 lb/hr injectors, at the same 110F you'd have 8.77 / 96 x (270/6) = 4.1 ms.
These table values are pretty rich at the colder temperatures and lean out gradually up through a coolant temp of about 110F, that's why the curve is so steep through those points but then flattens out after that.
So, how good are these numbers? Well, can't say for sure, I'd like some input from someone that has a car with good cold start manners, but I'd hazard a guess that this table should be good to plus or minus 100%. In other words, if this table says you should have a value of 10 ms at some temperature, I'd guess that you'll find in the end that something in the 5 to 20 ms range works best. Not great, but better than nothing.
Oh yeah, I went back and double checked these values against the demo file Demo.gct, and they were pretty close. FWIW.
John
