Pitot size in Regards to jetting

[dirk_03]

Ok heres the deal. Last year I ran my vector with a custom mid mount turbo system with a stainless charge tube with a different style pitot pickup. It was a 3/8 straight tube through the charge tube with a .125 hole facing towards the turbo. I ran a mikuni jet in the one end of it and the other end was referenced to the carbs. Jetting was pretty predictable.

Fast forward a year.

This year I have the motor in a rev chassis with a different style intercooler and a different charge tube and different pitot style. The charge tube is 2 inch id. The pitot tube is a 3/8 tube with a .250 ID with a 90 degree bend on it facing back towards the turbo. And it seems like I need to jet it way down from last year, from a 140 to a 132 or 135?

So what does anyones thoughts on this? do I just have to large of a pressure reference? How will this pressure reference affect my jetting other than my main jet. I am debating going back to the old "bender" style pitot jet bleeder system, To make fine tuning easier?

What do you guys think?

 

[dirk_03]

Thought

One thought, I don't really know but I know that powderlites has the pitot tube pickup referenced to the carb side of the intercooler as well. Can anyone tell me what this is? Why they do this? What size it is? I guess I have heard that thier system just worked so what can I do to make mine work as well as the PL kit worked.

 

[Bamboozler]

What does your wide band say? Are you a running satisfactory A/F at idle, mid throttle, and wide open throttle?

I have the same pitot setup on my turbo Honda CB750 (3/8" tube 90 degree facing into the air charge) and adjusted jets until the WOT A/F was happy. If it enriches a bit too much in the low/mid throttle from the extra dynamic pressure in the fuel bowls lower the needle until happy. Shouldn't affect much for idle. I don't think you're hurting anything if you can get the right tune down. You could change the pitot setup to what you had last time and adjust the correct dynamic pressure to the carbs instead of adjusting the carbs to the fixed dynamic pitot pressure, all a matter of what you find easier (refabbing your pitot for variable pitot pressure or changing jets/needles to match a fixed pitot pressure).

My friend had a powerlite kit and noted the line that "T"s off the pitot line and plumbs into the intake manifold. I don't have this guy plumbed on my bike and it runs a pretty flat air/fuel line WOT aside from a slight tip in throttle rich state. I have been wondering if this may be due to the accelerator pump built into my rack of Keihin carbs or maybe this tube not plumbed to the intake manifold?.

Best I can figure without trying it first hand is that the pitot tube that is "T"ing into the intake manifold may help reduce the pressure in the pitot line for a brief moment when the throttle is cracked open. Would this prevent a slight enrichment on the onset of opening the throttle up?

Maybe I'm thinking this incorrectly, but during the short brief moment of cracking the throttle the air flow/verturi effect over the main jet/needle stalls a bit transitioning from vacuum to a positive pressure (boost). However, the dynamic pitot pressure may still be unchanged because the intake manifold's feed pipe (charge pipe/intercooler pipe) keeps dumping at the same rate (for a brief moment, think turbo lag). This would put a greater pressure (dynamic from pitots) on the fuel bowl verses the main/needle jets from the lack of venturi effect (for a brief moment. This would make your A/F rich for a second until your turbo/engine responds to the WOT and establishes the equal pressure above jets and below in the bowls via the pitot tubes.

Does that make any sense? Someone correct me if I'm missing something or failing to bring something else into that thought process.

 

[dirk_03]

makes sense

I do think that this makes sense.

I am going to put a nipple on the carb side of the intercooler and see what that does. I imagine that it is something to do with smoothing out the reference from maybe a restrictive intercooler?

I would really like to figure it out like mcx had on the turbo vector kit so the reference is good at all elevations. As jetting is a pain to change.

 

[zx670]

you have more than doubled your opening area of you pitot tube from last year, area has a huge effect on pressure related to velocity, larger pitot area=more pressure. more pressure in the float bowls means more fuel, you will need either a much larger bleed jet size, or much smaller mains, from my experiance going to large on the bleed off jet creates a lean mid range spot thats hard to tune out, so I recommend the small mains to solve your issue, I think I ended up down at 115 mains on mine with a similar setup

 

[Ski-Dooin' it]

yep, your gonna have to jet stupid low. excessive bleeders dont work as the midrange gets so lean you cant rev past it. and the sled will stall if you cruise in that range.

once you get your jetting in dialed in for your reference, you shouldnt have to change it much still. I had mine down to 115's and it was still a hair rich, but that could be because of the non intercooled setup. either way, it ran good from 3-10k with only bleeder adjustments. there was definately more on the table, but it worked damn good, but mine got an insane amount of dynamic pressure from the reference alsmot being in the mouth of the turbo..

and a 1/4" id line facing straight into the airstream the way you have it setup is going to get an insanely strong amount of signal, that is literealy as MUCH pressure as you can get right there.

 

[dirk_03]

you have more than doubled your opening area of you pitot tube from last year, area has a huge effect on pressure related to velocity, larger pitot area=more pressure. more pressure in the float bowls means more fuel, you will need either a much larger bleed jet size, or much smaller mains, from my experiance going to large on the bleed off jet creates a lean mid range spot thats hard to tune out, so I recommend the small mains to solve your issue, I think I ended up down at 115 mains on mine with a similar setup

Hmmm this all makes good sense. I was so alarmed at why i need to jet it so excessively different than last year.