Hey guys,

I'm working my way through a chip currently for a fairly hot TBI 4.3L V6 on a 7747. I've got the part throttle fueling mapped out, and have started working through the WOT fueling when I noticed that when I get above about 4 grand, the MAP starts to drop off to about 92-95. I'm interpreting this as a slight vacuum in the intake, but with the 670cfm TB on this 4.3L, there should be no way this motor is drawing that much air between 4 and 5 grand. Am I just crazy, or is this normal for TBIs in general?

Thanks,
Teeleton

 

No, you should not be seeing that much of a drop. There are a couple of things that will do this and need to be checked. I would first check to see that the throttle fully opens the butterflys. May need another person to do this. I usually stand outside of the vehicle and wack the pedal down as I look at the TB linkage. If it hits the TB stop, then it is open.

The aircleaner element (filter) may be constricted, or the intake air ducting can be too small.

I have also seen (and experienced) intake duct work that collapses. Does that ever slow one down. . .

Just to be sure we are on the same page, that 670 is a 2bbl with 48mm bores?

RBob.

 

Yep, it's the oversize Holley 2-barrel with upgraded injectors. Here's a pic (this is actually the older model, the injector pod is different on mine):


And there's no way the intake is collapsing, because it's a fairly large open element filter. I'll likely replace it with a K&N eventually, but if the paper filter is my problem, that may be my next purchase. Here's a shot of the intake that was taken at my last car club gathering (it was the truck's debut back from the garage).



I'll have to have my wife mash the gas so I can check if the butterflies are fully open. The TPS reads 99.7% at full pedal travel.

Teeleton

 

Butterflies are all the way open. Guess I just need a better air filter.

Teeleton

 

The air filter is probably fine if that is the one in the picture. It is possible that the 4.3 is drawing so much air at WOT and high rpm that you are getting a vacuum. I get almost 3 in/hg under my Q-jet on my 355 at 5,000 rpm.

 

One item I forgot about is barometric pressure. Measure this at key on engine off, it will be the MAP reading. If that is say 97 KPa, then a MAP of 92 (> 4,000RPM) is only a 5 KPa drop. Which isn't too bad.

If baro is 102 KPa and the MAP drops to 92 KPa at > 4,000 RPM then that is almost 3" of vacuum. here is a handy chart to convert the two:

How hot is a 'fairly hot TBI 4.3L V6?' Heads, cam. . .

RBob.

 

Key on with the engine off, the MAP reads 102.6 in that log.

Using a calculator over at http://www.smokemup.com, my motor would have to be operating at 150% VE in order to need 634 CFM at 5500.

The V6 is pretty much a full-on rebuild.

balanced forged pistons & rods @ 10:1 compression
ported & polished 96+ vortec heads (iron)
shorty headers
2.5" to 3" y-pipe, 3" cat, 3" muffler
holley intake & oversize tbi w/ 85lb injectors
intake gasket matched to the heads
full roller valvetrain
cam is 222/230@.050, 112 LSA, and .515/.531 lift at the valve
MSD distributor/coil/wires/6A

I get about 15in. (~50kpa) of vacuum @ idle

At this point, rather than try buying a K&N, I may just take the hood & air cleaner off and make a few runs like that to remove all doubt.

Teeleton