When is enough, just too much.
When is too much, just enough.



Lets take for instance a 502.
At what point is it too much Carb?

-beerjet-

I wouldn't have believed it but Danny Crower built a 502 (Bow-Tie short block) for a friend of his it ran for two year's with a single Dart intake and an 850 Holley. It only turned 6,200 with a Hydraulic roller cam. That would be a minimum cfm of 502 x 6200 / 3456 = 900 cfm assuming 100% ve.

 After the second year the owner wanted a T/R on it so they pulled the single carb and put a T/R on it with two 1,050's.  The engine picked up 80 hp at 6,200 with no other changes. I would have bet anything it would have been over carb'd.  I'll see if I can find the HP numbers I think it went from 660 to 740 hp (something like that)

Edit: Found my notes ; it made 75 more lbs. ft at 5,200 and 80 more HP at 5,800 for a total of 680 hp.

I still don't know the formula for Tunnel rams and dual carbs.

Sleeper CP 

Thread jack   Where ya been jet havent seen ya around these parts for awhile

Lets try this out for size
454 with 049's Dart intake Holley 850 annular carb jetted at 91 prim and 94 sec.

this was run on a 460 BBC with great results. I hope i have the same results 

Sounds to me like you MIGHT need to go up about 10 jet sizes...you might be able to get away with 9 1/2 though.  lol jk

Nordic...been hangin' out on Performance Boats a lot. Gettin' ready to start my new build.

Propless and I were talking about this very subject not too long ago. From what he told me, and it makes complete sense in my mind...

The airflow rating of most 4 barrel carbs is obtained at 1.5" of vaccuum. That being said, if you have the correct size carb that the motor wants, you should have 1.5" of vaccuum at WOT.

This makes sense because if the vaccuum reading is higher than this, that means that the carb is flowing the max that it can, but the motor is trying to pull more air through it than it can flow, causing the higher vaccuum reading (restriction = higher vaccuum). On the flipside, if your vaccuum reading is lower than this, that means that the engine cannot pull enough air through the carb to create 1.5" of vaccuum at WOT because the carb is not restrictive enough (i.e. too big).

With 1.5" of vaccuum at WOT, the carb airflow rating is maxing out right at the amount that the motor wants at WOT.

Vaccuum gauges are a very useful tool in tuning a motor. Fairly inexpensive, if you know how to read them they will tell you what the motor is doing and what the motor wants.

Remember, carburetors require a vacuum signal in order for them to work properly, so if you have a carb that is so big that at WOT it doesn't restrict airflow enough to sustain sufficient vacuuum for the carburetor to work properly, your motor will run like garbage at WOT.

I like it, but not really, the 1.5"hg is only true if the engine makes peak load, that being said, if you start to run out of camshaft (volumetric efficiancy drops off) your going to get faulty info on your vacuum gauge and read it like it is overcarbed, when in reality it is turning too damn fast (too much RPM for the engine to use).

The debate would be, is the carb matched to the engine? AND is the engine matched to the load?. Let's make an example of a stock 454 with a super mild cam and put a 800 CFM carb on top of it, now run the engine with a "C" impeller and run WOT which should bring the engine RPM to 5,500 RPM or so, you will have less than 1.5"hg under the carb but the engine would be completely out of it's comfortable VE and not gain any HP, would probably lose HP...Now there could be two possible "fixes" (probably more, but lets keep it simple) 1. Valvetrain/camshaft change to get the motor to use the higher RPM. 2. taller or bigger impeller to lower the RPM at WOT, either should get you closer, but a carb would not fix anything...

Now take the same 454 with baseball sized ports and rediculous valves and the gnarliest cam money could buy, same 800 CFM carb. this time use a "AA" impeller (not bringing up an "HH" because most of us won't see one for a while ). At WOT the motor may make 5,500 RPM again but will be under a tremendous load causing a false vacuum reading again (call it less than 1.5"hg), so is it over carbed or overloaded?

There is a lot to selecting the proper size and type of carb/carbs. and a lot of info. needs to be taken into consideration, I like the trial and error method myself, a vacuum gauge, AF ratio meter and cylinder pyrometer or if headers an infra red thermometer are very useful tools to get you there. Which is why on an everyday non race mill I like a spread bore vacuum secondary, they will literally only pull in what it can use (providing it's set up right..).

Not trying to start any shiat, just joining in the discussion....(I think the term "grain of salt" should apply  

GT

Propless and I were talking about this very subject not too long ago. From what he told me, and it makes complete sense in my mind...

The airflow rating of most 4 barrel carbs is obtained at 1.5" of vaccuum. That being said, if you have the correct size carb that the motor wants, you should have 1.5" of vaccuum at WOT.

This makes sense because if the vaccuum reading is higher than this, that means that the carb is flowing the max that it can, but the motor is trying to pull more air through it than it can flow, causing the higher vaccuum reading (restriction = higher vaccuum). On the flipside, if your vaccuum reading is lower than this, that means that the engine cannot pull enough air through the carb to create 1.5" of vaccuum at WOT because the carb is not restrictive enough (i.e. too big).

With 1.5" of vaccuum at WOT, the carb airflow rating is maxing out right at the amount that the motor wants at WOT.

Vaccuum gauges are a very useful tool in tuning a motor. Fairly inexpensive, if you know how to read them they will tell you what the motor is doing and what the motor wants.

Remember, carburetors require a vacuum signal in order for them to work properly, so if you have a carb that is so big that at WOT it doesn't restrict airflow enough to sustain sufficient vacuuum for the carburetor to work properly, your motor will run like garbage at WOT.

Hummm I would only disagree with about 99% of what you just wrote in-regards to making power. When my engine dyno'd at 935 hp at 7,000 it was pulling 1.4" of vac. For max power you want to pull less than 1" ,anything above 1" of vac is showing a restriction as where my carb was pulling 1" at 6,000. The 1,050 I was running will be replaced with a new 1,200 this year. It will pull less vac and may make up to 25 more hp. Patrick and John at C&J carbs both asked why I was making my engine suck through a straw.

Patrick James from Pro-System set up my new carb... he is one of the best known names in the country when it comes to carb tuning:


The 1.5" of vac is important in a car where you are at part throttle driving around town, pulling away from stop lights and where you don't stand on it like in a boat.  In a street car where you need driveability a smaller carb is needed, but if you take that same engine to a drag strip you could pull a 700 cfm carb off an 850 or even a dominator will out-perform it on the strip. IE cruise at 3,000 and a steady part throttle and then wide-open.

Oh yeah, Pro Stock 500" 9,500 rpm 1,200+ hp engines pull less than .75" vac some closer to .5 " vac

Sleeper CP 

Big Inch Ford Lover

I like it, but not really, the 1.5"hg is only true if the engine makes peak load, that being said, if you start to run out of camshaft (volumetric efficiancy drops off) your going to get faulty info on your vacuum gauge and read it like it is overcarbed, when in reality it is turning too damn fast (too much RPM for the engine to use).

The debate would be, is the carb matched to the engine? AND is the engine matched to the load?. Let's make an example of a stock 454 with a super mild cam and put a 800 CFM carb on top of it, now run the engine with a "C" impeller and run WOT which should bring the engine RPM to 5,500 RPM or so, you will have less than 1.5"hg under the carb but the engine would be completely out of it's comfortable VE and not gain any HP, would probably lose HP...Now there could be two possible "fixes" (probably more, but lets keep it simple) 1. Valvetrain/camshaft change to get the motor to use the higher RPM. 2. taller or bigger impeller to lower the RPM at WOT, either should get you closer, but a carb would not fix anything...

Now take the same 454 with baseball sized ports and rediculous valves and the gnarliest cam money could buy, same 800 CFM carb. this time use a "AA" impeller (not bringing up an "HH" because most of us won't see one for a while ). At WOT the motor may make 5,500 RPM again but will be under a tremendous load causing a false vacuum reading again (call it less than 1.5"hg), so is it over carbed or overloaded?

There is a lot to selecting the proper size and type of carb/carbs. and a lot of info. needs to be taken into consideration, I like the trial and error method myself, a vacuum gauge, AF ratio meter and cylinder pyrometer or if headers an infra red thermometer are very useful tools to get you there. Which is why on an everyday non race mill I like a spread bore vacuum secondary, they will literally only pull in what it can use (providing it's set up right..).

Not trying to start any shiat, just joining in the discussion....(I think the term "grain of salt" should apply  

GT

 OK GT,  "grain of salt" taken.

 Now lets assume were not stupid enough to run our motors that far out of there intended operating ranges (well most of us  ). Obviously we need to swap those impellers around and try again. 

 Now the monster 454 has the "C" impeller and is turning 6500rpm (its intended rpm range). it still has the same 800 CFM carb and shows, lets say 2"hg on the vacuum gauge. Sense the motor is in its intended operating rpm range the high vacuum reading would mean it wants more carb,  resulting in more power.

 And now the stocker has the "AA" impeller and is running at 4500rpm (its intended rpm range) and shows 1"hg on the gauge.  I would think it might benefit from a bit smaller carb, that might have better velocity resulting in better fuel atomization. 

At least that's how its been explained to me.

OK GT,  "grain of salt" taken.

 Now lets assume were not stupid enough to run our motors that far out of there intended operating ranges (well most of us  ). Obviously we need to swap those impellers around and try again. 

 Now the monster 454 has the "C" impeller and is turning 6500rpm (its intended rpm range). it still has the same 800 CFM carb and shows, lets say 2"hg on the vacuum gauge. Sense the motor is in its intended operating rpm range the high vacuum reading would mean it wants more carb,  resulting in more power.

 And now the stocker has the "AA" impeller and is running at 4500rpm (its intended rpm range) and shows 1"hg on the gauge.  I would think it might benefit from a bit smaller carb, that might have better velocity resulting in better fuel atomization. 

At least that's how its been explained to me.

Absolutely, although methinks (that's a Rayizm  ) an 800 CFM carb is pretty damn close for a stock 454  , but more importantly the point I was making is there are more things than just carb size that effects vacuum readings, for example; exhaust size and efficeincy, ignition timing, cam choice, cam timing, flame arrestor, intake manifold hieght and type (dual plane vs. single), among others I'm certain, I am just trying to keep things above the table and showing that there are ways to get the "wrong" carb to work. I have been around engines and racing and performance and more importantly boats all of my life and can say without question that there are no guaranteed methods of getting a setup dialed in, the variables are virtually endless, some of the stuff I have seen done defies belief, but still works awsome, shouldn't, but does...

I do know that you will learn as you go and that sometimes the same thing won't work twice. I really wish I understood that........

Like I said origionally, grain of salt  

Hey BTW, just food for thought, be careful where you take the vacuum readings, I have seen it once before where the vacuum sample was taken at the base of a carb adapter and was actually reading velocity pressure because of the way the hole was drilled and tapped, it read as close to "0" as you could get with what we thought was going to be too small of carb, the actual vacuum was over 3" hg of manifold vacuum......

GT

Define. ? ...... what does  beerjet want to make more hp for, or me for that matter ? A Cup Car on  a Super Speedway  would run at sustained WOT , other than that I can't think of many others. Can you ?  Road racing and drag racing is an engine accellerating at least I think they are.

S CP  

Sorry Sleeper, I am from the Whitewater world, the only time we get out of the gas is to keep from taking flight.. , most of my high performance tuning took place in an alloy boat with a rediculous amount of time put towards reliability and fuel mileage...I am sure that for the most part all of the information on this thread has value, but probably for different applications, which is again why it's important tom look at everything in the setup...

Hope you didn't take any offense, there was none intended...

GT

None taken what so ever. Flat footin it in a river racer is sustainded WOT but your typical river racer drag boat guy is something else.

S CP 

You can say that again LOL  So here goes the stupid question of the day, why not a big ass pilot operated vacuum secondary, I would think it be awsome!  Never really understod the mechanical secondary deal all the way, I would think that if you electronically controlled the secondaries based on throttle position and manifold vacuum you would have the best of both worlds...Just thinking out loud...

GT

Not to thread jack, if I am, but what ever happened to the Predator Variable venturi carbs? They were all the rage and everywhere in the 80's? Seems to me a design like that would solve everything, no?

Ray

Update, just checked their website. They're still around. Not very technologically advanced, judging by the website, but might be interesting to experiment with this, lol. Bid on a used one on Ebay. Just to see one, really, might find a use for it if I get it.

Apparently, very limited production. Kinda like the Seecamp .32 automatics everyone wants.

 I seriously thought about a pair of those for my motor, but was told they don't like manifolds with dividers in the plenum unless you mount them sideways.  Something about the way the fuel bar works  .  Oh well, I'm sure my new (to me) 660s will do the job. 

I do know that you will learn as you go and that sometimes the same thing won't work twice. I really wish I understood that........

Like I said origionally, grain of salt  

GT

 All I know is, if we ever stop learning new things its time to do something different.  And every time I start that stupid OB the fumes make me dumber.