If not 42's then ?

[Hooligan]

Need a little help deciding.
I'm in the process of building a low comp 347 that will see around 10-12 psi to start.I don't have the whole combo together but I will list what I beleive at this point will be the end result.
Dart sportsman 347
Custom camshaft
Vic Jr's
Holley systemax II intake

Now heres where I need a little coaxing..
I have 42# injectors with a SCT big air Maf that once everything is done I will get tuned with a SCT chip.
Do I have enough injector? I have done the formulas that are readily available to determine what size injector I would need but I'm not satisfied that the calculations are accurate.
According to this formula...
Injector Size = (FWHP x BSFC)/(# of Injectors x duty cycle)
My calculation
injector size =(600 x .6)/(8 x .8) = 56.25 lb/hr
Rounding up thats a 60 pound injector!!
Now I'm assuming thats at 43.5 psi but am I being too aggresive using .60 as a bsfc and 80% duty cycle are those realistic figures?

[93gt]

Im running something similair with 42s. They are maxed out completely with the chip and the FMU. 60s are the way to go according to my dyno tune shop. I will be going this route next year for sure. The whole sytem will be different, and the 60s will be at the end of it all, so youve got your math correct!

[fanglemeister]

I agree with 93gt. If anything you are being conservative, with .6 bsfc and 80% dc.

suggestion: if you are going to go with a Dart block, don't limit yourself to the 4.00" bore version, unless a class you want to compete in requires that bore size. Consider the big bore version of the Dart block instead, you can take it out to a 4.125 bore and get 363ci from a 3.4" stroke. Or you can run a 3.25" stroke crank, which with a 4.125" bore would get you 347ci, but the bigger bore unshrouds those big 2.08 V-Jr valves very nicely, will make enough extra "free" power to drive around the 4.03" bore 347 in the next lane, while the 3.25" stroke will make the engine run longer, and rev higher.

[Hooligan]

I agree with 93gt. If anything you are being conservative, with .6 bsfc and 80% dc.

suggestion: if you are going to go with a Dart block, don't limit yourself to the 4.00" bore version, unless a class you want to compete in requires that bore size. Consider the big bore version of the Dart block instead, you can take it out to a 4.125 bore and get 363ci from a 3.4" stroke. Or you can run a 3.25" stroke crank, which with a 4.125" bore would get you 347ci, but the bigger bore unshrouds those big 2.08 V-Jr valves very nicely, will make enough extra "free" power to drive around the 4.03" bore 347 in the next lane, while the 3.25" stroke will make the engine run longer, and rev higher.

Already have the block but thats good info for others to digest thanks.

[86merc]

Two years ago everyone told me 42's would be good for 550rwhp. Every car is different but I made 513rwhp @14#. We stopped tuning due to the stock block and a DC of high 90's with 55# injectors. That is with a base fuel pressure of 50psi and A/F ratio of 11.8:1. I would say it is better error on the high side becuase if you ever add more boost/power you might max out the 42's. Then you and the need to buy new injectors and recal the MAF. Better to spend the money once than buy parts over and over again. I did it three times with my injectors. PITA.

I would say buy at least a 55# injector. Look around you can find good deals on used 72's.

[Chuck]

Told ya Jimmy!!

[Hooligan]

Yep you did I think I already knew the truth I just needed a little more reassurance. :teeth:

[fanglemeister]

Already have the block but thats good info for others to digest thanks.

You'll still be making very good power with the standard bore block in any case, so no worries. :happy: In fact I would be very tempted to run your listed combo naturally aspirated on the 42# injectors for a little while, just to try it out while everything gets broken in and seated properly - cylinder walls less likely to get washed down with smaller injectors. I bet it would be some good fun on street tires even without any boost, particularly if you turn some high revs with it using an Anderson hi-rev cam or similar.

[Hooligan]

That's definetly been a consideration and makes good sense.I have been pondering driving n/a to break it in then adding a T-trim to the mix, but at this point I'm just trying to figure out a fuel system that will be capable of taking whatever I throw at it.

Anyone have thoughts on the Siemens/Mototron 60# high imp injectors?
I'm just looking for some real world testimony good or bad.

[93gt]

My set-up made some killer #s with the 42s, but I would get the 60s if youre buying injectors anyway. Its just safer. As far as cylinder wash- if its tuned correctly, this shouldnt be an issue.

[Hooligan]

I bought brand new 42's awhile ago haven't even used them!!
I was going to build something totally different than what I'm going to end up with here eventually.Isn't that always the way ?

[RSNovi]

I used to run the Siemens Deka 55# high impedance injectors. They seemed to work pretty well. I made around 550 rwhp with them.

[Hooligan]

What made you change?

[RSNovi]

I found a good deal on some Siemen's Deka 83's and a low impedance driver. I just figured that it would be a good purchase and allow further room to grow. If I would have had the 60's, I probably would of just stuck with them.