Pressure and flow rates of carbs?

[CobraSix]

Okay, I read and re read and re-read the posts about carbs, flowrates, and undercarbing. I see the math behind the flow rate of carbs, but what exactly does it mean when you say flow rates of carbs are evaluated at 3.0, 1.5 INHG? What is applicable?

I"m just confused on exactly what that means in regards to our engines and how it is used.

Slade

 

[SuperMag]

It's the difference in pressures that causes fluid to flow; they go from areas of high pressure to low pressure. This business of 1.5" of HG means that when the flow was measured the difference in air pressure above and below the carb was 1.5" of mercury, or enough pressure to raise a column of mercury in a vacuum tube 1.5". More pressure difference, more flow volume.

 

[CobraSix]

Okay...I understang the how it effects flow rate, but how does it effect what I should look for in a carb for the 200?

I guess more accurately asked, do I want a carb that flows higher at 1.5 rather then 3.0?

Slade

 

[addo]

Well, really, you want a "level playing field" to begin with.

By that I mean converting all to the same pressure drop ratings. It's not a straight doubling or division. As 6 cylinder carbs are rated at 1½" Hg, it may be best to work from there.

•If the carb's too big, it won't really do its job well enough most of the time. This is the problem with just banging on a big carby, and why triple CD type carbs are long favoured on hot sixes.

The nature of the six is different to the eight in terms of what it requires for the carb, so V8 theory doesn't import well. Add to that the vanity of many eight owners and the fact that indifferent running is less noticeable on an eight; the waters become muddied.

As has been mentioned before, the best (incontestable) investments in engine performance are perhaps the most plodding and costly... From your prepped head, you need flow figures at increments of either 50 or 100 thou. Combining that and your ultimate drive ratios (rear ratio, gears, wheel/tyre sizing, and driver application, you can get a cam profile to best suit the head. The cam profile will pretty much define the carburation required for your application.

That's all much dearer and less "visual" than parts in boxes coming together wham-bam. But the basic pint is that you can't know yet what to look for in a carb for your engine. A good guess, for sure. Yet the final choice may surprise.

Xecute or one of the others will come up with the conversions; after that it's a matter of planning the motor build and maybe seeing what looks good on an analyser program like DD.

Cheers, Adam.

 

[xctasy]

I like what SuperMag says because it's BS-free!

What I find is that you should do some calculations to see what size carbs you need to run to get 200-250 feet per second of flow rate at full power.

All the cross-checks I've done with carbs has shown that these flow rates are the best for maximum power.

Eg. 1 Take a 351 Cleveland, such as a 4V HO or Boss, and you find the best street power with a single carb and passable economy is a 750 cfm Holley or Spreadbore. If the engine power peak is at 5800 rpm, and has 85% volumetric efficency, then the engine needs

((351*5800)/3456)*0.85), or 500 cfm of carby flow.

750 CFM IS WAY OVER THE LIMIT!

from line 29 on http://fordsix.com/forum/viewtopic.php?t=7015

you get the following info.

29|#4150-|750|4-bbl-|1.6875-|1.3750--|2.9700|22.7-|1.6875-|1.3750-|6.216---|~290ft/sec|229|469|


The flow is 750 cfm, but the airflow is actually 290 feet per second at wide open throttle with 1.5" Hg of pressure drop.

But the engine only needs 500 cfm to get maximum power. That pegs the actual air speed down to 193 cfm.

What happens if we place a 600 cfm Holley from a 5.0 HO 4V Mustang, what's the air speed then?

20|#4180-|600|4-bbl-|1.5625--|1.2500--|2.4540|25.0-|1.5625-|1.3125-|5.1600-|~279ft/sec|183|375|

Well, at 600 cfm, there is a 279 ft per second air speed at a 1.5 "Hg flow drop, or 600/500 less air flow. That's 232 feet per second.

See the pattern? Plot your actual air flow from the formula used for carby sizing, and then grab whatever canditate carbs or combo of carbs from my chart, and adjust the carb choice untill you get between 200 to 250 ft/sec air flow.

E.g 2:- Take two Holley Weber #5200 carbs, rated at 230 or 280 cfm depending who you talk to, at 2" or 3" Hg. Try this listing:-

04|#5200-|280-|2-bbl|1.2800-|1.0400|0.849â€￾---|23.1-----|1.4375-|1.0625----|1.736“-----|387FT/sec|061bhp|124bhp|

On your six, reving to 5000 rpm where maximum power whould hopefuly come in, then it would need 200*5000 rpm all divided by 3456, then multiplied by, say .85 ve. That's 246 cfm.

But we've got two carbs, each running at 387 feet per second and 280 cfm for a grand total of 560 cfm at 3.0"Hg.

The peak air speed is now lower than this, by 560 cfm/246 cfm, or 2.276 lower.

387 ft/sec> ends up at 170 cfm. Not ideal, but if your engine is having to flow 340 ft/sec with a single 2-bbl Holley Weber, then this is a very good compromise.

Eg.3:- Best one I can think of is a 2V 200, running to 500 rpm, needing 246 cfm of air, and getting a 390 cfm Holley. Air speed at this flow rate is 264 ft/sec at 390 cfm, or 167 cfm at 246 cfm.

11|#4150/60|390|4-bbl-|1.4375-|1.0625-|1.7730-|43.7-|1.4375|1.06250-|3.547-|~264ft/sec|122|244|

The twin Holley Webers would be a close match, and have better fuel distribution if you could create a custom intake.

Eg. 4:- A single #2300 500 Holley, rated at 404 ft/sec at 500 cfm, would have 199 ft/sec at 246 cfm, which would be ideal, but there would be the issue of starvation at cylinders one and two, and oversupply at 3 and 4.

A single 350 cfm#2300 would be right on the limit. The air flow would be 266 ft/second at 246 cfm, since the flow speed is 379 ft/sec at 350 cfm.

Eg. 5:- Dual 350 cfm Holleys for a screamer race 200 cross-flow with revs to 7500 rpm, and 100% ve by a full-race cam, could suck 434 cfm of air. The flow speed would drop from 700 cfm/432 cfm, or from 404 ft/sec to 250 cfm, almost perfect for performance. There could be 300+ hp hidding there!

The key is good air flow first, then good distribution, and then trying to make it progressive.

Just in case you think the 390 or Twin Holley/Webers are silly because of low air speed, just remember...they may never open up to the fullest if the engine doesn't need it. Actual air speed at wide open throttle is likely to be more than tose figures.

 

[SuperMag]

I like what SuperMag says because it's BS-free!

Thanks buddy...

Apples, oranges, and apricots...

4V carbs are rated at 1.5" HG
2V carbs are rated at 3.0" HG
1V carbs are rated at 6.0" HG

Don't ask me why, they just are. However, you can convert from one rating to the other so you can compare apples to apples by using this nifty little formula Whittey dug up...

Where Q= flow rates (cfm), and P= the pressure difference (inches HG)

So how big?
You will find two schools of thought on this here at FS.com. School one will tell you to size the carb based on engine size, redline RPM and Volumetric Efficiency (VE). I'm not a big fan of this approach for street motors, because this formula only works well when you know what your VE is (do you?) and you drive at or near your redline most of the time. It's good for squeezing every last 1/4 HP from your motor, but economy and low speed tractability are compromised more than they have to be.

The other school (me) prefers to size the carbureter based on the planned HP output of the engine. By picking a realistic horsepower number to build to, you can pick all your engine components based on a standard of performance so that they work well together. The carburetor should be picked with the idea that it will flow just enough air to support the level of HP of the engine. This way, you meet your HP goal and you don't compromise economy and low speed driveability any more than you have to.

Sooooo... How much HP do you wanna make?

 

[Anonymous]

Okay...I understang the how it effects flow rate, but how does it effect what I should look for in a carb for the 200?

Slade

Slade,

For a mild 200 use a 350 cfm Holley and for a full tilt 200 use a 500 cfm Holley.

I dont understand all this other talk either, I just know what works

Later,

Doug

 

[SuperMag]

If you peel away the fruit you get down to the nut. What Doug said...

 

[Anonymous]

 

[addo]

The other school (me) prefers to size the carbureter based on the planned HP output of the engine. By picking a realistic horsepower number to build to, you can pick all your engine components based on a standard of performance so that they work well together. The carburetor should be picked with the idea that it will flow just enough air to support the level of HP of the engine. This way, you meet your HP goal and you don't compromise economy and low speed driveability any more than you have to.

pretty much what I advocated, but with the addition of "benchmarking" - logging flow rates on the prepped head in increments of valve opening. Note the last word:

But the basic pint is that you can't know yet what to look for in a carb for your engine. A good guess, for sure. Yet the final choice may surprise.

Knowing more about the motor is a good thing, no?

Cheers, Adam.

 

[Anonymous]

I like the HP ratings that Execute gave for the various carb options. For the techno junkies it appears from the above to be an almost exact science (Maybe, Im not arguing with this). For me I use as a rule of thumb Executes HP figures for example if a 500cfm Holley (350 cfm at 3HG) is good for 220 hp tops.
When I took my car out for a time trial the other day Execute posted that I would have need at least 155 hp at the fly to do those times. If I added say another 5 to 10 hp for gear change time. Maybe Ive got 160 165 HP max. My 500 Holley is going to be ample big enough but not too big. If with a few mods (with new cam ect) I were to get near 200hp (500Holley 220 hp tops) I would go to the next carb up (say a 390 four barrel) and I would expect this to make a significant difference to the HP.
There must be a near percentage, close to the peak HP rating of the carb when the next size up should be seriously be considered.
I dont consider my 500 is too big because a 350 Holley (247cfm 3hg) is good for 175 HP (a bit to close too 165 HP for my liking)
This is how I do it. Maybe its not very technical but I like it.

Oh, just as a side note. I plan to put another cam in my 250 2V to get it to peak at 5000 rpm instead of 4000rpm with more lift than it has at present and maybe bigger valves as well. I dont think this will put it over 200, so Ill leave the 500 holley on (as I am still worried about fuel dispersion with 4 bbl anyway). I think the 500 will be fine.
Alternatively if I were doing exactly the same mods to an Alloy crossflow allowing for the fact that performance cams for a crossflow tend toward higher lift and that these heads gain well from headers (2V already had headers as standard) and the overall design is better. I would definely go to a bigger carb at least a 390 cfm. (I wonder how Dick Johnsons 147 KW crossflow would have gone with a 4 bbl). I would expect there to be some gain in doing so.
Im open to any comments, or scrutiny.

 

[Anonymous]

This is an old post from A7Ms for crossflow re-build. 165 hp sounds accurate. According to a previous post the EFI cam I a close to .400 lift about 25/65 270 duration (peaks at 4000). The cam Im running in my 250 2V is a 25/65 270 dur .417 lift (4000).

Try these.
Use the later EFI grind cam with a set of 351 4v springs, change to non sintered spring retainers (MUST) suggets CROW. Dont waste money on roller gear, not necessary. This will allow an extra 500rpm.
Dont bother with head porting, just freshen up head.
Go to roller timing chain.
Extractors are not needed for this level, stock exhaust manifold was used on EFI anyway, just get as big diameter system as you can get on the mainfold, 2.25 should be about right. use low restriction muffler.
Use the stock intake, but try to improve size of passages.
Find a 2 barrel weber that non emission that will drop on, 2 litre Cortina V6 capri?
Get a cold air induction to the air cleaner.
Or use EFI manifold and go straight LPG.
Check dizzy advance curve. You should have electronic ignition.
Stock rebuild is all that necessary, fancy pistons oil pumps etc not needed dont go above 8.5 :1 for unleaded, or 9.5 for LPG, get COME LPG cam for straight gas.
Chuck your cooling fan, go electric, run an electric fuel pump.
Otherwise its easy, this setup will give around 165 honest BHP and 5000rpm, whith smooth idle and reasonable fuel consumption.
A7M