I noticed that my engine pulling about 4-5 inches of vaccum at WOT. Is this about right or do I need a bigger carb?

Thanks

You need a bigger carb. Before you do that, it would be a good idea to check that your carb is opening all the way at WOT and that you have a clean airfilter.

Mike

If the air filter was the restriction, the excess vacuum would pull fuel like crazy, right? I mean it would be horribly rich running.

That much vaccum at WFO seems like there must be something wrong.

Fuel is controlled by the flow through the venturi not the amount of air the top of the carb sees. Check your aircleaner, is the carb openining completely (throttle linkage)

I'm curious. What is the difference between restricting air flow at the entrance of the carb with a plugged filter, or restricting air flow to the entrance of the carb with a choke blade?

Both create "artificial" vacuum on the venturis, and enrich the mixture, right?

So if the 4-5 inches of vacuum is BELOW the throttle blades, the carb is too small, but should run OK other than not producing power to it's potential.

But if the 4-5 inches of vacuum is ABOVE the venturis, you're gonna suck fuel like crazy and it will run like the choke is stuck on.

Right?

In this case, the carb may be grossly undersized, the throttle may not be opening completely, or the vacuum gauge may be inaccurate. It is NOT the air cleaner, though, or the guy would be complaining of black exhaust, fouled plugs, etc.

FYI the filter is new. Car runs good though. I got this reading by T-ing into the vac. advance line. Guage is a Snap-On.

schurkey you're not taking into account where the vent for the float bowl is, gas doesn't get sucked into the engine it gets pushed in by atmoshere (spelling i know is wrong) pressure on the fuel in the bowl. so depending where the vent is controls the mixture if it has clogged filters or shut choke valve.

Based on this info, would you guys suggest a bigger carb? I'm thinking 850 vac sec (only Demon seems to have one) or a 800 DP.

You didn't get this reading by revving the engine in park or neutral did you?

Also the line was ran to a manifold vacuum source wasn't it?

I think I would prefer the reading be taken by using the line that goes to the PCV because of it's larger diameter.

Not sure how this would effect the outcome but I would feel better doing it this way.

Test done by driving the car. Idles about 15-16", moderate throttle 10", WOT 4-5". I tee'd into the vac advance line. I wonder if the vac canister would affect the guage?

**Also this reading must be taken at the top of high gear**

Kinda doubt the canister would have an effect on it, but the size of that little line...Not sure bout that.

Myself I just wouldn't feel for sure about the reading (even if someone told me otherwise) using that small line to the canister.

If a 750 carb is rated at 750 cfm @ 1.5" of drop across the carb with the throttle blades wide open, how many cfm would it take to create 4.5" of drop across the carb? About 1300 cfm, or a 396" engine running at 100% VE at 11,500 rpm.

..are you sure you have WOT?

Test done by driving the car. Idles about 15-16", moderate throttle 10", WOT 4-5". I tee'd into the vac advance line. I wonder if the vac canister would affect the guage? I think you should try it again off of regular manifold vacume rather than ported vacume. The ported vacume the distributor advance is run off of should also disapear at wot but maybe at a slower rate? What size is the engine and how many rpm are you turning? Bob

My carb is getting WOT, I had my wife floor it and I checked.

The vac advance is running manifold vaccum through the port on the base of the carb.

Test was done is 2-3rd gear between 5500-6000rpms, engine is a 454 HO.

Better put a bigger carb on if you want max performance out of it. 950HP would be good I bet.

Get your vacuum reading at the manifold.

A 750 carb should be enough for a 454 turning 5500-6000 rpms. It might have something to do with where you are taking your reading.

Originally posted by Vince g:
Test was done is 2-3rd gear between 5500-6000rpms, engine is a 454 HO. Does that mean you got the reading when the engine was at 5500+ in second going for third?

High gear at 5500-6000 is where you want to get the reading.
Also connect it to a manifold vacuum source with a large hose, not the small stuff that goes to the distributor.

I dont think I can do 6000 rpm in 4th gear w/3.31 gears around here!

it really doesnt matter which gear you get it from peak rpm is peak rpm . for a given engine. the engine doesnt care what gear your in as far as im concerned. with all the tests i have done on my own engines no matter what gear i was in at peak rpm the reading is the same.

and i very much doubt that if you have a car like mine your going to try and take a reading in 3rd gear at 6500 rpms with 2.73 gears.

even with my last test with my bbc's my 3310-2 holley 750 provides just under 1"of hg at 6500 rpms with my 414 weather im in second gear or first gear. thats good enough for me.

a healthy 454 can easly take 850 cfm without any problems. 750 is ok for a daily driver but if you want every last possable amount of hp go with the reading of 1" to 1.5" of hg.

Good info ddeennis.

How were you taking your readings?

Vince,

IF your test is accurate, a bigger carburetor would be in order. I'd suggest checking it from direct intake manifold vacuum through a dedicated hose first. If you’ve already verified that the car goes wide-open throttle that should be OK. Normally you’ll get better performance using a mechanical secondary carburetor on a vehicle with a manual transmission. Not knowing your cam specs it’s hard to say specifically which carburetor to use, but an 850 with annular boosters should be a good choice for your 69.

Thanks Tech@BG.

I kinda figured that an 850 might work b/c GM recommends it for the ZZ454 which is close to the 454HO.

Wonder why they recommend the 750?

Let it be noted that the Annular 850DP that the Tech mentioned is not the same animal as a regular 850DP.

Those Annular Boosters make a BIG DIFFERENCE!
Throttle responce is greatly improved.
Don't slap a regular 850DP on their and wonder what is wrong.

Vacuum is vacuum, doesn't matter how big the hose you're testing off is, it will read the same.

You say your wife held WOT and you verified it. How? It is a vacuum secondary carb, and they will not fully open 'til under load. Not to be a smarta$$, but were you riding under the hood?

You cannot verify the opening of the secondaries on a VS carb by revving it in neutral, at any rpm. If you got this reading while driving, it does not mean the secondaries were open, and chances are they weren't. Change to a lighter spring in the diaphragm, assuming it's working properly, and I'll guarantee a lower , more appropriate vacuum reading. Your 454 should not require more than that 750 carb.

Steve

Sorry, I didn't see the DP in your profile. The same thought goes, shouldn't need more than a 750. Certainly shouldn't pull more than 1-1/2 to 2". If it is real, you've got one hungry beast!!

Is this the carb from GM? As in, are you sure it's a double pumper?

Steve

Yes Steve its a Douple Pumper.

when is Jason gonna chime in here?

my little 414 ci bbc is pretty much using every last drop of that 3310-2 i have. at 6500 rpms...........

so just imagine what that carb would be like on a 454 thats just as healthy.........

and for those who are wondering how i know my secondary's are fully open.......thats easy i run a paper clip on the vacuum secondary rod (and this shows the location of the butterflys at max rpm.....) and wether i have a purple, or yellow or plain spring in there they open fully..they just start coming on at different rpms.........

infact i tweaked the secondarys to open to full tilt straight up.....this is one of the first checks i do when i buy a carb to make sure all opens to expected position


and i run my test off full manifold vacuum using a big 3 inch gauge taped to the windshield.........and i record readings from all kinds of rpm ranges ...this also helps me to pick my P.V......for cruising......i take the readings at peak rpm.....just to make sure carb isnt to big or to small.......

Hooked up the 800 DP and noted about 2.5 -3 inches of vaccum @ wot. Just a refresher I have the guage taped to the window and t'd into the vac advance line (manifold vac). test was done in 2 and 3rd gears.

Idles good and no hestations (even w/the 50cc pump on the rear). Am I leaving any performance on the table w/this high a vaccum reading?

BTW - the 50cc pump cleared the manifold (no need for spacer).

Vince

Vince, my guess is your are still seeing vacuum because of the real short cam timing that comes in the HO motors. That particular cam has .050 closed measurement at only .25" up from ABDC and has the intake opening at 3.5* ATDC @ .050. The lack of overlap is creating a very strong signal. There is an advantage to this and you are seeing it, the carb will have a strong signal through the carb and you get good fuel atomization. I wouldn't worry about running any more carb, but instead pay attention to throttle response and fuel mix. For comparison, I intend to run the 870 Vac Sec Holley I have here on the 502 motor. Same principle, I want a strong signal for good fuel atomization and overall throttle response.

Joe,

I understand your point. Maybe your theory explains why this motor should be happy w/a 750. But given the reduction in vaccum that the 800DP gave you would think a bigger carb would be needed. My vaccum at idle went up too - now 18inches. I bet you'll see even more vaccum w/the 502. I'll just stick w/this I guess. Its been mentioned here that 850's (pumpers anyway) don't meter very nicely as the smaller units.

I was thinking about one of the 502 vac sec carbs I see floating around on Ebay.

Vince

Vince,

I have a brand new 870 Vac Sec here that you are welcome to try out. New in the box, I originally had picked it up to try on the 402 motor but it is destined for the 502 now. Give me a yell, I will be home the next 4 days, redoing the landscaping out back.

Have you ever tested your vacuum gage for accuracy??? Been there, done that!.......mine was out 2".

The other question, on your first carb, did it have a choke and if so is there any possibility that it could be closing up at WOT?? You could wire it open or remove it temporarily and try it.

BoldM,

The 1st carb had an electric choke (that worked too) so I don't think that was a problem. If it was causing a problem I think it would go to a fast idle. As far as the guage goes, who knows. Its reading 17-18 inches of vaccum which I think is about right for this motor.

BTW...this is the 1st performance motor I've owned that actually put the vaccum gauge needle in the "normal" part of the scale!!