nize

it's because the stock injectors can't flow enough fuel at higher boost.

billindenver

I would have to respectfully disagree. Last year I finally replaced my stock injectors. Not because they failed in any way, but because they were pushing 20 years old. I was running 50psi of fuel pressure, but they never let me down or failed to keep me plenty rich at 22psi.

Since I was replacing them anyway I went with 55's and dropped my fuel pressure into the forties but it runs exactly the way it did on stockers with 50psi and ARC2 controlling them.

YMMV, only relating my experience.

toddk911

Why didn't you post when I was asking what fuel pressures everyone was running???

billindenver

Because I don't like Orlando people?

Oh...uh...46psi. That help?

toddk911

LOL.

So is it 50psi or 46????

toddk911

So I assume you are still running stck fuel pump also?

billindenver

Yes, stock fuel pump. 46psi with 55# injectors. 50psi with stock..IIRC At the time I was tuning based on what the A/F meter told me...and I turned up the pressure until it would remain rich enough wide open. When I put the 55lb's in I put the fuel pressure guage in as well. Pretty sure it was 50 psi on the stockers.

Geneqco

Have you done any head work? I've been told that at around 17 to 18 psi you can start to get valve float at high rpm with the stock valve springs. I recall a thread where I think ST confirmed he had experienced this.

Is this a common experience or is it just that some springs may be a little fatigued with age?

billindenver

Stock springs on mine. The springs in a 951 are pretty stout, akin to racing springs in american iron.

nize

do you have any way to monitor the percentage of duty cycle your injectors were (or are) running?

i ask this because i do have that ability, and i have seen how stock injectors hit 100% duty cycle at 18-19psi of boost.

ehall

actually If I recall correctly, ST actually had the whole fricken head float...and he had it torqued all to hell. He changed to very different head when he hit his 427hp number. The one he's usiong now, is not stock.
I'm getting old so my memory could be off a bit.

nize

head float is probably due to knock, not boost psi.

can you guess how much psi is generated from the cylinder combustion?

Geneqco

I do recall reading about the head float but that was at substantially higher boost. The valve float at around 17+ psi was an independant observation IIRC. It could have just been that the springs in his particular head were a little fatigued if this is not a more general problem.

I also know of some who have turned their boost up on occasion to mid 20's without issue.

nize

just because you can run your injectors at 100% duty cycle doesn't mean it's safe to do so on a regular basis.

ehall

lol...not in this case it wasn't.