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Author Topic:   How to pull more vacuum?
GRIZZLY
Dirt Newbie

Total posts: 4
posted November 12, 2001 08:14 AM
I am running a SB Chevy and have to pull 15" of vacuum. Does anyone have any tips on how to get more vacuum out of it. I barely pull 14.5 right now with my advance at 37 degrees.

BK19
Dirt Freak

Total posts: 285
posted November 13, 2001 06:51 AM UIN: 40696038
i would say use a smaller cam

avila
Dirt Full Roller

Total posts: 55
posted November 13, 2001 10:30 AM
I assume the check is performed at idle?
If you are checking your vacuum from a port in the carb, then you can adjust the carb idle *****s to pull a little more vacuum.
This adjustment usually improves off idle
acceleration too.

Wild Bunch Racing
Dirt Full Roller

Total posts: 53
posted November 13, 2001 07:58 PM
Another option is to run another vaccum can parell to the existing one.

[This message has been edited by Wild Bunch Racing (edited November 13, 2001).]

r19v
Dirt Roller

Total posts: 19
posted November 14, 2001 12:14 AM
Here are a few things you might try without changing many parts. The smoother the engine runs the better the vacuum it will pull. Make sure your ignition system is in top shape, good plug wires, new plugs, good coil etc. If the engine misses a beat every now and then, it could affect your vacuum considerably.

If possible increase your engine RPM by 100-300 RPM. Increase valve lash by .003 to .005, and increase timing advance another degree or two. Any one of these could give you another 1/2 inch and all three maybe an inch or more. Good luck.

GRIZZLY
Dirt Newbie

Total posts: 4
posted November 15, 2001 08:03 PM
r19v I appreciate the informaiton. I will definately give it a try before I start buying parts.

dirtracer17
Dirt Forum Racer

Total posts: 88
posted November 20, 2001 09:03 AM
We ran into this awhile back. I won the heat and feature only to get DQ'd because we couldn't pull vacuum! We later discovered that our adapter plate(We run 2 barrels on 4 barrel intakes)was loose! That was all it took for us to pull vacuum the next week! You might make sure that the seal under the carb is good too.

chevyscrounger
Dirt Full Roller

Total posts: 43
posted December 04, 2001 08:36 AM
I know but I ain't tellin.It's legal too.

sdhnc29
Dirt Freak

Total posts: 467
posted December 04, 2001 12:51 PM
Ok ... here is an old trick that works every time ; Set your timing at what you would normally run (36-37) . Get a vacume advance unit for your dist. Weld a stop into the vacume advance , where it will pull an extra 8-10 crank degrees . At an idle , you will now have 44-47 degrees of advance . As soon as you come off of idle , the extra advance will drop off , and you'll be back to your 36-37 degrees .
To find out how much to limit the vacume advance diaphragm , set your timing at 36 , then take a vacume reading . Next , advance your timing until your vacume peaks ! At that point , look at your timing , if it's at 46 degrees (example), then you need to limit your vacume advance diaphragm to 5 degrees , which will give you 10 degrees on the crank .
Problem solved !! Now you have plenty of vacume when checked , and you can still race with your normal advance on the distributor !! Thus ... totally cheating the tracks vacume test , and allowing you to run more cam than your competitors !!

Hope this helps !
Steve

phelps
Dirt Full Roller

Total posts: 58
posted December 04, 2001 10:25 PM
Hey chevyscrounger , that's a little stingy isn't it??

chevyscrounger
Dirt Full Roller

Total posts: 43
posted December 05, 2001 09:11 AM
Yea I guess it is but my dad did a lot of
thinking to come up with it and he'd kick my rear if I told you! Our vac went from 14.5 to
18 with one little change it surprises me how nobody else has figured it out.Sorry for the tease!!

[This message has been edited by chevyscrounger (edited December 05, 2001).]

GRIZZLY
Dirt Newbie

Total posts: 4
posted December 07, 2001 09:03 PM
Chevyscrounger, go ahead and lets here your little trick. I am sure everyone would like to here about what it is.

sdhnc29, thanks for the tip, I will deffinately look into it.

chevyscrounger
Dirt Full Roller

Total posts: 43
posted December 08, 2001 02:40 PM
Sorry, The first one to ask is the only one I told.I guess I should have told you first since this was your thread.But I don't want to tell more than one person. All you have to do is think how vaccume works and how to make it pull more vac. Some guys do it and don't even know it so if you have it already it won't do you any good any way. Put a little more thought into the everything between the carb and the valves and you'll figure it out.

wkramer
Dirt Full Roller

Total posts: 37
posted December 31, 2001 11:22 AM
ok you guys have been playing with this vac. problem for awhile our track just started a rule of 10 inches 0f vac @800 rpm it seems alot less than what your looking for if someone can tell me how to get the most cam and get the vac. it would save me alot of time and money. thanks

3xracing
Dirt Roller

Total posts: 17
posted January 23, 2002 03:01 AM
use a 2 bbl. rochester intake instead of te 4 bbl. oneif you use the 4412.

less duration on the cam.

advance the cam 2 degrees.

more carb spacer.

more compression.

turn on the dirt devil when your gettin teched! j/j

3xracing
Dirt Roller

Total posts: 17
posted January 23, 2002 03:04 AM
The only problem with 29's vacuum advance cheat is at the end of the straight when the motors vacuum comes back up and advances the distributor again, those pistons are gonna start gettin HOT!

chevyscrounger
Dirt Full Roller

Total posts: 43
posted January 23, 2002 10:20 AM
3xracing is right on track.Just be careful when you pick the parts he listed.

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