bence13_33

I'm heading to the track tomorrow and am hoping for the 11s (I'm pretty confident it's going to happen tomorrow). I ran my best 1/4 mile pass ever with this jetting configuration:

Primary: 6.5" Power Valve, 78 Jets
Secondary: Power Valve Blocker, 87 Jets

It ran five passes in 80 degree weather and the mile per hours were: 117.55, 116.94, 116.61, 116.80, 117.72. That was the last time I raced with that carb setup. About a month later it was back to fouling plugs so I jetted it down...only to find out my secondary throttle adjustment screw had loosened exposing more of the transfer slot more than it should have been exposed.

I have since made a few more passes down the 1/4 mile with different jetting configurations...but the MPH dissappeared down to low 115MPH- low 116MPH. I just switched my jetting back to 78s in the primary in hopes of bringing back the MPH. I pulled my plugs after cruising along the highway and they are white (which is how I like them).

I guess what I am wondering is if anyone has any jetting ideas for me to try at the track? Do my secondaries seem jetted a bit low? I'm heading down tomorrow in hopes of 11s.

GrandSportC3

I think that - in order to get the perfect jetting setup - you should take the car on a dyno and dyno it with A/F Ratio.. That should give you an idea where you are on the A/F scale... in order to know if you need to up your jets or lower them... I wasn't jetting big enough and I was running way too lean on the dyno.. I increased my jet size by 4 numbers because of the dyno results...

chewy

Way to many variables to even give a guess. What size carb, what intake, I always ran power valves in both ends if it came that way even on extremely radical race only motors.

norvalwilhelm

I definitely think that 74's in a 850 are too small. I run 80's square in all 8 jets in my 2x4 setup. Other things are done to lean out the idle and low speed circuit.
Shane I still owe you and Robyne a lunch. Don't screw it up and turn 11.??.
Good luck Shane
Norval

bence13_33

Chewy:
It's a Holley 850cfm Double Pumper on top of a Holley Strip Dominator Intake. I'm pretty sure I have the jetting close to where it needs to be with the 78s in the primary.

Norval:
Tonight's the night so I guess we'll see :) . I'm feeling pretty confident about tonight and am really excited. By the way my Pro Topline 360cc heads came in today for the blower motor :D . All I can say is :eek: those are some huge intake runners. I really had no idea how big 360cc was until I saw them first hand. The 2.30 intake valve size is also impressive. I'll be posting some pictures of the heads when I get a spare moment. I'll be in touch Norval.

gkull

If your trailoring your car to and from the track you need to only wory about WOT.

You need to run your jetting square. I'll sit down and add up the flow of 78's and 87. But off hand I would guess that 81 or 82 all around flow the same. Blower motors like Norvals are a different animal. They operate at a higher vacuum level so you can't judge how jetting will go

Matt Gruber

back in the old days before A/F meters guys would change jets at the track and see how it runs. If it is way off big gains can be had, otherwise it could be a waste of time.
Looking at the holley jet chart 87=.103
94=.108 a 10% increase in flow area.
95=.118, a whopping 31% increase in flow area.
My GUESS is:
do 1 pass with what you have now.
Change the rear to a 94. if it slows down, try an 85 (6% leaner than 87) But my guess is the 94 will speed up, you will try the 95 and it will............(continued next week, stay tuned for the next exciting episode)
OK, i think the 95 will be too big,so don't skip the 94 :steering:

gkull

Shane - It took me a little bit to refresh myself on the math involved in flow area of a give jet orifice.

What I came up with for square jetting your four main jets was. You stated that the A/F ratio was right with 78/87. The equivolent flow would be 4 Holley 82's which is near right on. or a tad richer would be 4 Max-jet .091's

There are many benefits to square or near square jetting.

Matt Gruber

george
i am closer with 78/94 to square.
you forgot shane has a plugged PV in the rear.
i don't know his PVRC on the front, but i am way way closer :cheers:
.
Good idea to use max jets, if he has them available, i would go for +10%, then +20%, if it goes faster.
Keep in mind +20% on the rear is only +10% on the whole carb; could easily be 10% lean and still run 116mph. if the plugs are white WOT it could be 40% too lean :eek: but i doubt that!


[Modified by Matt Gruber, 8:27 AM 7/2/2003]

Jvette73

Air temp and barometric pressure at the time of the run can effect what jets will work best. Its so hard to just guess. A good way is to start with what the carb came with, then increase 2 sizes each run and look at the mph in the traps. When you see the decrease in trap speed, youve gone to big. How about yur pump squirters? Have you tried different ones of those. They might get you a better launch which would in turn help you break into the 11's.

The Dude

I've got an 800 CFM Holley on my 406. It's jetted 78P/82S.

I've noticed--and graphed--that the car runs better and better the hotter it gets. It's slower in the morning and evening when there's "good" air (12.20s at 112 or so), but in the heat of the day is when it really hauls the freight, relatively speaking (12.0Xs at 114 and change).

That would seem to indicate that I might be a little too lean when it's cool and that the weather "catches up" with the tuneup at mid-day.

I'll play with it some more when I can get back to being a test 'n tune w*horse...

Matt Gruber

saw your parts pics, nice heads!
did not see any jets :nonod:

Matt Gruber

11's could be right around the corner with a simple jet change!