carb CFM

[Falcon67]

>Shame on the crab manufactures
God made the crabs, Holley makes the carbs    OK, slap me.

FWIW  - Somebody on one of these lists (some Ford related forum somewhere) in the way back put a ProForm "750DP" bodied 4779 Holley on a Superflow and ran it at the 1.5 in/hg carb rating, netting as I recall 840 CFM. 

[manofmerc]

On my 452 comet with Edelbrock heads c6 237@.050 569 lift lunati cam I had a 750 .It is a proform main body quick fuel metering blocks .It is really responsive idles good and whenever you kick it naturally it hauls ass!I had a left over 950 proform main body with annular boosters .I had a pair of metering blocks from an 830 bought a new base plate from quick fuel a rebuild kit and some more parts I needed and assembled my carb.It ran circles around the 750 .It idles better going down the highway it just runs better .And when I first kicked it it was like waking the dead .So much more power .And from just a driving perspective it doesn't use any more gas that the 750.I would say bigger is better if you have a larger than normal engine that can flow more air .One thing about quick fuel base plates  on their billet aluminum the throttle shafts are milled to increase flow and they use button head screws as well every little bit helps .On the 1000 cfm carbs.do they really flow 1000 cfm? Seems to me a 1050 dominator is a lot bigger than a 4150 series that is supposed to flow 1000cfm .Doesn't a 1050 dominator have 2" throttle plates ? How does that compare to 13/4 throttle plates for the 4150 carbs. I haven't figured that one out yet.Merry Christmas Doug

[Barry_R]

Its been a while - but I did a test with a 750 & 850 swap on a 445 and got something like 17 HP in 10 minutes.  Also did one with four different carbs and saw significant differences between a 750, a modified HP900, 850 vacuum, and a 950 3 barrel.  The 3 barrel "won" that contest - it was a simple matter of airflow at WOT.  Bigger is better.  You just have to tune around it.

And if you can work around the intake and air cleaner stuff a Dominator is hard to beat.

[ScotiaFE]

Its been a while - but I did a test with a 750 & 850 swap on a 445 and got something like 17 HP in 10 minutes.  Also did one with four different carbs and saw significant differences between a 750, a modified HP900, 850 vacuum, and a 950 3 barrel.  The 3 barrel "won" that contest - it was a simple matter of airflow at WOT.  Bigger is better.  You just have to tune around it.

And if you can work around the intake and air cleaner stuff a Dominator is hard to beat.

Or just put two carbs on. Looks stock also.

[gordonr390]

I have a belief that most all these issues stem from a plenum being under sized. Plenums should be the same size in cubic inches as the engine it sits on or even larger. There has to be a reserve capacity sitting in the plenum to fill the requirements of a given engine. So in the end there is only one cylinder filling and firing at a time so having to mount large carbs to gain more power makes me suspect. 

[Autoholic]

The opposite could be true about that though. Too large of a plenum and you will hurt the performance of the engine, and waste fuel. If the carb can easily handle the demand of the engine, chances are the plenum volume won't be the restricting variable. If the carb is too small, a larger plenum will only delay starving the engine of air a moment longer. You can hurt an engine by having too large of a plenum, you can't hurt the engine (within reason, paying attention to AFR) from having too large of a carb.

Eliminating the plenum all together is actually the best solution. Once fuel has left the carb, it is no longer being controlled. A plenum is a large area where air can slow down. If the plenum is too big, the air will slow down just enough to cause fuel separation. This is why individual carbs like Webers develop better power, there isn't a plenum for the air to slow down at any point in the RPM band. So the best intakes would be ones that decrease the plenum size and opt in favor of individual runners that go almost from the head to the carb without sharing fuel. An example of this would be the 2x4 sidewinder intake for the 427 SOHC.

[gordonr390]

Never put to much thought into fuel separation. I could see cold low speed issues until an engine warmed upped. But shouldn't goal of the intake manifold be that the ports draw off a semi calm clean air supply though? Yea I get that the IR is the ultimate way to go but I also believe that the inlet of the IR has a radius that air travels across all 360* but in an shared intake manifold it physically can't. So many diff port entries in there causing turbulence or dirty air causing the lack of good flow to the combustion chambers.

[plovett]

I think in most, maybe all intake manifolds with a central plenum, the flow is much more chaotic than one would expect.  It's not like a linear "here to there" type deal.   Pressures are constantly changing from place to place, flow changing directions, etc.  I could be wrong, but that's my belief.

For instance, I have seen in oil "communicated" from a leak on one side of an intake manifold to the opposite side.   

JMO,

paulie

[CaptCobrajet]

Phooey!!  Too much carb can look great on a dyno and not accelerate near as well in a car.........especially a heavy car.  Carb spacers are the same way.  Sometimes you can keep stacking spacer on one and keep seeing dyno power go up.....only to hurt the ET slip later.  Rather than say the biggest carb is always better, I would say that SOMETIMES a bigger carb is better, depending on the carbs in question.  I seem to remember a certain 4150 carb that tested better than a Dominator on Jay's test engine...........but got excluded from the final decision, LOL.

[Autoholic]

I think when it comes to an intake manifold plenum, a lot of thought needs to be given to fuel separation and pooling. If you really wanted to get an engine dialed in, I'd run a wet flow simulation on a completed engine and then remove the intake and heads. Look at the dye traces to see how the engine is performing at your critical / desired RPMs. This is an extreme analysis though, that would require a hell of a setup. It would be the most realistic though, short of actually running the engine on a dyno and making various changes.

An intake plenum is very chaotic, and chaos is probably better than calm when you need that air to keep fuel in suspension. If you go for an EFI conversion where the fuel is injected into each individual port, then slowing down the air is better. Laminar flow is the best for performance but if that carries the risk of fuel separation, then it's better to have turbulent air. This is why I say it's better to not rely in the volume of the plenum in the first place. A small plenum and a properly sized carb that has no issues on feeding the engine what it needs. If you can keep a steady and fast supply of air/fuel to the plenum, you can keep it pressurized that way and not risk fuel separation. So the air/fuel is delivered more on a just-in-time basis rather than relying on a reserve capacity.

Remember, I did say paying attention to the AFR. Too much of anything can hurt an engine. A bigger carb will be better if it's dialed in to control the AFR better. That's my point in my previous post. It's better to deliver the air/fuel when it's needed rather than have it waiting around. Also, when an intake manifold is created, it needs to be able to manage the needs of a wide range of displacements reasonably well. So unless you are fabricating your own intake manifold, chances are the intake you are using does not have a plenum volume that suits your engine the best. If you really want to use an off the shelf intake and make sure it has the right volume for your engine, you would need to find out the volume of the intake manifold and then use this volume to dictate displacement.

[plovett]

Phooey!!  Too much carb can look great on a dyno and not accelerate near as well in a car.........especially a heavy car.  Carb spacers are the same way.  Sometimes you can keep stacking spacer on one and keep seeing dyno power go up.....only to hurt the ET slip later.  Rather than say the biggest carb is always better, I would say that SOMETIMES a bigger carb is better, depending on the carbs in question.  I seem to remember a certain 4150 carb that tested better than a Dominator on Jay's test engine...........but got excluded from the final decision, LOL.

Is it possible to vary the load on a dyno during a run, to better simulate the actual conditions in a car doing a quarter mile.  Instead of a certain amount of rpm per second, could you vary the load based on rpm and calculated speed?  Maybe with some "swoopy" electronic controls?

I'm not a big fan of chassis dynos in general, but it might be easier to do there.

paulie

[jayb]

  I seem to remember a certain 4150 carb that tested better than a Dominator on Jay's test engine...........but got excluded from the final decision, LOL.

I dont' remember it that way Blair, I thought that your 4150 carb and my box stock 1150 Dominator were pretty much neck and neck.  I'll see if I can find that data...

[CaptCobrajet]

I gotta have a little fun Mr. Jay.

[cjshaker]

It's all bench racing until it's proven on the track or street. We've all seen plenty of "That won't work as good" examples, only to be proven wrong. In the end, the engine always tells you what it wants, it just takes lots of arguing with it before you listen. It's almost like having a wife

[jayb]

It's all bench racing until it's proven on the track or street. We've all seen plenty of "That won't work as good" examples, only to be proven wrong. In the end, the engine always tells you what it wants, it just takes lots of arguing with it before you listen. It's almost like having a wife

No wonder I have so many FEs