Manifold vacuum ?

[harleytoprock]

I hoping you can shed some light and educated me on this. I've read that 30 in-hg is considered WOT and that 16 in-hg is deccel. Since this is the manifold vacuum extremes of an engine, how does a VOES operation at 5 or 6 inch fit whithin this range? Is there a different vacuum rating when talking VOES? DUH, I don't get it. Thanks!

[Don D]

This is simple but gets complex when the units and sampling method changes
You speak of in Hg and that is common in automotive and so is in H2o. The EFI bikes work in Kpa

[mayor]

I'm not well versed, but I think that a lower in-hg number would imply WOT.  I'm thinking the 30 in-hg is probably closer to deccel.

[Don D]

Depends
If you talk Kpa manifold "pressure" higher number is WOT
If you talk manifold vacuum carbureted in in Hg lower number is WOT and the VOES kicks in at high load / throttle

[harleytoprock]

When I look at DTT's Advance slope graphs, the high number 30 in-hg is labeled WOT and shows the retarded slope. Am I misunderstanding something? or is the 30 in-hg labeled WOT an error?

[Sonny S.]

I hoping you can shed some light and educated me on this. I've read that 30 in-hg is considered WOT and that 16 in-hg is deccel. Since this is the manifold vacuum extremes of an engine, how does a VOES operation at 5 or 6 inch fit whithin this range? Is there a different vacuum rating when talking VOES? DUH, I don't get it. Thanks!

30 in-hg @ WOT is MAP    " manifold absolute pressure "
0-2 in hg @ WOT is vacuum using vacuum as a scale

Think about manifold vacuum this way. As the intake valve opens, and the piston is on the down stroke, there is a suction created. Now, with the throttle plate mostly closed there is a huge suction ( vacuum ) in the manifold because air can't enter. Now as you open the throttle plate the vacuum decreases.
Sorta like when the shop vac gets clogged then unclogged...lol

Now MAP.
Same scenario. As the throttle plate if fully opened, 14.7psi of atmospheric pressure or 30 in-hg  equalizes in the manifold. So using MAP as a scale, 30 in-hg is WOT.  Actually about 29. something

[mayor]

hey Sonny, show off.   

[Don D]

Yep
and 1 bar is 30 in Hg is 100Kpa is 1 atm and on and on
Leave it to DTT to use that unit of measure. Confuses many when trying to compare SERT timing maps

[harleytoprock]

So, Map is different from manifold vacuum? If so, why. Thanks for your patience.

[Don D]

MAP is Manifold Absolute Pressure
The number is a relative to atmospheric pressure
Vacuum is an actual pressure reading and MAP is a relationship between atmospheric and actual

[harleytoprock]

Thanks to all of you, I think I'm starting to get it. Is there a way to equate, lets say, 5 in-hg manifold vacuum to MAP?

[Don D]

Yep
5/30 = X/100 where X is the missing MAP number

[HD/Wrench]

Dont forget that based on what elevation you  are checking it at , hugh elevation you will not see the max number on either side of the scale. Like us here at 6000 ft.

[Sonny S.]

Dont forget that based on what elevation you  are checking it at , hugh elevation you will not see the max number on either side of the scale. Like us here at 6000 ft.

you guys up there only need half a tune 

[HD/Wrench]

YEP half a tune   

[Don D]

Yep
I know all about that Steve, used to live up ther near you in Pinetop. Our diesel naturally aspired equipment and gensets were derated by 30% which is directly translated to the lack of air pressure.

[barny7655]

There are different VOES out there that have a wider adjustment than the standard bikes,modified bikes needs the VOES varried to compensate for the cams and pistons C/R ,some times the VOES that comes with the bike cant be altered enough to suit,so it comes on around 9 instead of 5 or 6 , a vaccumme gauge you can hear it click when operating it then adjust to suit, on cruise mine runs around 18 , cheers Barny