Manifold vacuum at WOT

[splicer]

I'm having an ongoing argument with a good friend over the amount of manifold vacuum present during a 1/4 mile drag run.

At the drag strip, I leave my vacuum advance hooked up because I believe the vacuum drops to near zero during WOT for the entire run.....

He, on the other hand, is trying to tell me that the vacuum is coming back to as high as 8" near the end of the run, activating the advance unit and reducing power.

His approach doesn't make any sense to me but I haven't had a chance to make a run and record vacuum gauge readings to prove him wrong...

Have any of you tested vacuum during a drag run ? Or maybe IQ52 can tell us what he normally gets for vacuum at WOT on the dyno...

Thanks
Splicer

 

[Photon440]

There is one way for the vacuum to come back as high as he said - having a carb that is far too small for the engine. How does your friend think that power will be reduced by activating the advance unit?

 

[splicer]

I believe he thinks that too much advance will rob power....that's contrary to the principles behind vacuum advance....

FYI.......engine is s 505 stroker, carb is a 950 street hp and shift point is at 6200, so I don't think carb is way too small...

Splicer

 

[IQ52]

If I see 1.5" of vacuum at the end of a pull I figure my carburetor is a little too small. The 505 with the Q950 was pulling 1.0" and the 750 Holley was pulling 1.3".

 

[nm9stheham]

The 8" number is silly...unless the throttle is not any where near fully opened. Did your friend say where he got/measured this?

 

[383man]

As long as your at wide open throttle for the complete run your eng vacum should be right near zero but as was stated too small a crab and sometimes the air velocity through the carb can create a small amout of vacum but I have never seen over 2 inches of manifold vacum at wide open throttle. His point is that alot of racers actually retard the timing a tad in high gear at higher rpm to help top end power because they have so much advance for good low end. So thats why sometimes it helps to retard the timing a bit in high gear if you run alot of advance in the lower gears. Many years ago Bill Jenkins would retard his timing in high gear by cutting out one set of points on his dual point dist. Most street cars dont use enough advance to worry about the high gear retard but some of the aftermarket boxes can be set up that way. I really dont think you will have much if any vacum at wide open throttle to worry about it being enough to move the vacum advance. In theory with the throttle blades wide open you should have 0 vacum but most engines will show a tiny bit of 1 to 2 inches of vacum because of eng and carb design. Ron

 

[rumblefish360]

As I have read, anything more than .5 is to much, go to a bigger carb. This is for racing. Street? Not so worried. Though lower the better. You just enter a catch 22 where the carb maybe bigger than you like for your street car.

It's a tasters choice. (What you like)