negative vacuum readings at WOT

[mmigacz]

I was datalogging a hole shot and noticed that the vacuum dropped to -3 HG (yes negative) after the throttle was fully opened up. I think its impossible to get pressure rather than vacuum at WOT. Has anyone seen this when datalogging using the $42-1227747-V5.2 acquisition file?

Attachment 8966

[jay72]

Maybe you could upload the adx so people who don't have it don't have to look for it???

I don't understand the question. Inches of Mercury is negative. If you connect a typical vacuum gauge to an engine and it reads 20 at idle, thats -20 inches of mercury below atmospheric pressure. Shut the engine off it reads 0 and positive pressure is going to read pounds per square inch.

So it's impossible to read negative inches of mercury. Must be a problem with the adx.

[mmigacz]

Attachment 8969

Look at the datalog. Tunerpro is displaying 11 inches of Hg (positive value) at idle. When I open the throttle up (WOT) it displays -3 inches of HG (negative). Tunerpro must be displaying vacuum (negative pressure) with positive values (right or wrong, that's what its doing).

So does it seem strange that at WOT the vacuum values become negative?

[steveo]

i think that adx is broken.

it takes a linked value from MAP KPA, then applies this formula: 29.9-(X*.349)

that isn't a good conversion afaik. think about it for a sec, 100kpa (which should be atmospheric pressure, WOT, no vacuum, should be 0 inches of mercury)

that'd be 29.9-(100*.349) = 29.9-34.9 = -5

derp?

kpa to in/hg should be 0.295

i do suck at math but i think you should go with 30-(X*0.295)

[steveo]

also you should get used to using MAP KPA to evaluate everything, thats what every scan tool, tuning program, etc. displays. i wouldn't even have inches in my adx.

[mmigacz]

Here's the whole story:

I go on a web site called correctcraftfan.com (inboard ski boat forum). The guys on the forum go back and forth about what power valve to use (2.5 or 6.5) on their holleys. With my capability to datalog (TBI conversion), I wanted to show them when the power valve actually opens based on engine vacuum (the power valve number is based on vacuum in inches of mercury).

[mmigacz]

i think that adx is broken.

kpa to in/hg should be 0.295

i do suck at math but i think you should go with 30-(X*0.295)

Thanks Steveo!

that definitely makes more sense.

[96lt4c4]

Usually if you still have vacuum at WOT then you have a restriction on the intake, TBI, air cleaner...etc