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 ideal vacuum of a motor crossing the stripe 
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Joined: Sat Jul 25, 2015 2:31 pm
Posts: 75
Can someone explain to me what is the ideal vacuum you should have when you cross the 1/4 mile finish line?

If it is higher or lower than ideal, why is that?, Carb too big or too small? I have read I don't know how many posts with differing opinions on what is best or what its telling you. I trust you guys to tell me how it is and please explain so a layman will understand.

FWIW, you guys helped me with 2-660's and as best I can tell when it goes across the line , its at about 1.5 or less of inches of vacuum.

I'm not looking for every HP out of the engine, really just trying to understand what I read.

Thanks Bill


Mon Nov 02, 2015 6:23 pm
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Joined: Mon Dec 29, 2014 1:26 am
Posts: 433
Only thing that matters is if there's enough booster signal to move the fuel out of the bowl and into the airstream. If the booster is efficient enough (or, generates a low enough pressure (enough suction applied to the main jet)), with zero pressure differential between above venturi and below venturi, then it will work fine. If the booster is inefficient, it may need assistance from a more restrictive venturi - essentially, using manifold vacuum at WOT to help suck the fuel out of the bowls, instead of the booster doing it with airspeed alone.

This is why an annular booster will allow a larger venturi on any given engine - more efficient booster works with less WOT vacuum. At the other extreme, the 'straight' booster with the bar across the top doesn't do much boosting (there's very little airflow through the center for it to work with, because of that bar) and so depends more heavily on just plain vacuum under the carb to move the fuel. Downleg boosters are somewhere in the middle.

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Mon Nov 02, 2015 7:17 pm
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Joined: Fri Feb 08, 2013 5:50 pm
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It also depends on how well the intake distributes fuel. If distribution is poor then a more restrictive carb is needed, more WOT vacuum will improve vaporization and distribution, but at the expense of pumping losses. You have to find that line between the two. As an example my 23˚ head 427 SBC runs best with a 1050, an 1150 is within .02, and a 1250 loses .03-.05. On my 461 SB2 I have not found too big yet...


Mon Nov 02, 2015 10:45 pm
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Joined: Mon Feb 18, 2013 5:52 pm
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Jmark, do you happen to have any vacuum numbers between the carbs. I think this is what has happened to me. I have done stuff to improve the flow of my carb, which dropped the manifold vacuum to a point that it is now hurting distribution and vaporization. I just ordered some .100 skirted boosters to try and bring the vacuum back up.


Tue Mar 22, 2016 5:01 pm
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You remarked you noticed the distribution changed when you changed the throttle shafts. Either the shape of the shaft or the angle of the butterflies is different and is the cause of this, so perhaps a different WOT throttle angle slightly less than 90 degrees is the solution.

I would figure out which are the lean cylinders and steer fuel to them by pointing tie-wraps at them before using more carb restriction to vaporize the methanol in the intake manifold. If you figure out a distribution scheme with tie-wraps you can use a skirted booster to make a spivy by removing the skirt except the part you want to point the fuel to were you want it.

One of the power advantages gained from using methanol is the work of the compression stroke is reduced by the large heat absorption in the change of state from liquid to vapor during the compression, the result is the temperature at the end of the compression stroke is lower than with gasoline. Lower temperature results in lower pressure so the compression stroke requires less crankshaft power to accomplish and less internal loss results in that power being available in the engine output.

You don't want to evaporate the methanol in the intake manifold because the large amount of fuel vapor displaces air and so reduces the volumetric efficiency. This is why sprint car engines evolved to have"down nozzles" in the spark plug side of the head to inject the methanol right at the valve, the purpose is to get as much of the fuel into the cylinder in liquid form as possible.

Nozzles above the sparkplugs in this pic...
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Tue Mar 22, 2016 6:11 pm
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Joined: Mon Feb 18, 2013 5:52 pm
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I understand what you are saying, but if I am having an issue getting the methanol to the cylinder consistently, wouldn't a little vaporization help?

The new throttle shafts where half shafts where the original were full. So I gained some flow from there. I am not talking about adding alot, maybe .2-.3 to get it back around what it was before.

I got 6 cylinders(3-8) which I consider to bet hot egts, and 1 and 2 seem to be in the range in which I would expect them to be in. It has wanted more jet, and they still seem hot.


Tue Mar 22, 2016 9:07 pm
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Hot EGT doesn't always mean lean...And it only takes a small change in butterfly position to shift distribution.


Tue Mar 22, 2016 9:29 pm
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I will reverify it, but I checked it before, and made sure that they were opening all the way. Actually found that the rears were over rotating which was causing the throttle to stick.

I think mixture quality isn't there.

I think the rear cylinders are rich and fuel is burning in the pipes showing the hot egt.

I can also say this, before I made the changes, the afr trace swas alot smoother which what I was taught means I have a better quality mixture. They have not been happy. I actually though I had bad o2 sensors.


Wed Mar 23, 2016 1:01 pm
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I forget what allyou changed and where I read it, throttle shafts and what else ?? Why don't you configure the carb back the way it was? I think the issue is the distribution of liquid fuel.

I have run methanol so rich it makes a trail in the exhaust port of the excess liquid passing through and that definitely didn't raise the exhaust temp. The way we jetted methanol carbs for circle track was to make it so rich it leaves evidence of a liquid trail in the exhaust port and then lean it in steps until the trail just disappears. The important thing to be sure of is that it is that rich at WOT but not at part throttle. Some of the store bought methanol carbs (like BG's modified Holleys and others who apparently copied them) would drip liquid from the exhaust at idle and driving around the pits but were so lean at WOT the engine would overheat.


Wed Mar 23, 2016 1:45 pm
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