Done with six pack

[BSB67]

Sure.  A lot of numbers fly around in this hobby without proper context.  Dyno numbers and head flow are a couple more beauties.

[c00nhunterjoe]

I purposely stayed away from this last time.  But I'll give it a shot.

It seems that everyone talks about "cfm" as some ultimate measurement, where in reality, it is a number that has no meaning by itself.  For a 4 bbl, it is actually XXX cfm @ 1.5" hg.  So a carb can, and will flow more or less than the advertised number depending on.....wait for it.....the pressure drop.  Guess what, our type of engines (street/drag) usually do not pull 1.5" hg at WOT.  At WOT, the carb will create a pressure drop across the venturi, which in effect restricts air flow (but provides the necessary pressure drop to meter the fuel - can't have your cake and eat it to).  It is absolutely incorrect to say that an engine only wants XXX cfm.  What is accurate to say is that an engine will only TAKE xxx cfm @ x.x" hg.  This part is important....if you reduce the pressure drop, the engine will TAKE more cfm.  Of course there is the point of diminishing returns...i.e. the carb is too big.

So let me give an example:  We run an engine on a dyno and it makes 550 hp.  The carb is a 950.  The dyno instrument tells us that the engine actually had an air flow of 800 cfm during the pull.  Wow.  I guess the motor only wants 800 cfm.  So we put a 800 cfm carb on and do another dyno pull.  Hmmm.... it only made 535 hp, and now the air flow is only 725 cfm.  This happens because the engine is pulling less than 1.5" hg.  So for the first pull the engine was pulling 1.1" hg, and on the second pull, the engine was pulling 1.25" hg, caused by the more restrictive smaller carb.  Of course the engine made less power because it pulled less air.

Thank you.   

[c00nhunterjoe]

In other words, advertised cfm is much like advertised duration with cam specs.

Comparing the 2 barrel vs a 4barrel cfm numbers would be the same as comparing hydraulic cam to mechanical cam specs. 

Advertised cam specs vary from manufacture to manufacture based on where they are measured at.

[firefighter3931]

Here's some data to chew on....several builds coming off the same dyno, with the same dyno operator (Dwayne Porter) using the same correction factors. Translation ; the results are DEAD NUTS reliable....no overinflated magazine bullshit ! 

I've yet to see one of these magazine beauties make it into a car to back up the horsepower claims with a timeslip. I guess that makes me a skeptic....oh well it is what it is 

From another post i made in Cooters thread a week or so ago :


Here are some engine combos with dyno numbers/CFM @ peak HP to illustrate ;

(1) 446 E-head > 535hp/540tq with 830cfm dp'er (Proform 750DP) = 760cfm
(2) 493 E-head > 563hp/592tq with 850cfm dp'er (Holley 4781 850DP) = 785cfm
(3) 572 Indy head > 715hp/720tq with Quickfuel1050 (4150) dp'er = 930cfm
(4) 572 Indy head > 722hp/725tq with Holley 1150 Dominator = 960cfm

The comparison between number 3 & 4 are interesting ; this is my 572 and we tried to see if the engine really wanted more carb. Despite a 100cfm increase in carb sizing, going from a 1050 (4150) to an 1150 Dominator it only made an additional 7hp even though the engine was ingesting 30cfm more air. What I learned from this was that throwing more carb at it doesn't allways translate into bigger numbers....and lots of air equals a small amount of power if the engine doesn't need it. In most cases if the carb is sized appropriately going bigger will just soften up the throttle response and make the driving experience less enjoyable. 

I can say that the smaller 4150 Quickfuel 1050 was waaaay more responsive than the Dominator. Smaller venturies mean increased airspeed and sharper throttle response.....allways has, allways will ! 

I'm not even sure that 7hp would even show up on the timeslip at 4100lb raceweight and there was no way i was going to trade off a razor sharp throttle to find out.   


**So if after reading all that you're still convinced that your stock or mildly modified 425-450hp Big Block still needs an 850 carb then i guess my response would be..... good luck with that. 



Ron

[Cooter]

Well, I think I'm gonna try all three mentioned (700 DP, 800 DP, and 850 DP) on the dyno just to see what is what for this(my) 440. If it likes more carb, ok, fine, but if it doesn't, ok fine...

I was just thinking a modded 650 DP might be a little small for a modded 440. I guess we's bout to find out.

[ACUDANUT]

Yea, a 650 is too small for a 440.

[c00nhunterjoe]

I won't argue with your numbers, but on the same note, I know what I have seen with my own eyes in this area. I know what my local cars have tried and run.

I also know what worked on my car. I ran a 650dp for a long time because that is what everyone said was the biggest I could go. My cousin (who builds race engines) talked me into a run of the mill 4779 750dp. The power difference was enough I could feel. I have never dynoed my car so I can't quote you numbers.  After pulling the properly tuned 650 for the 750, the only things changed were the jets and the primary squirter was changed from a 24 to a 31. The response it quick and crisp, I lost nothing in the upsizing.

On the same note, for the same reasons quoted above about numbers being tossed around on the internet, I have not bought a holley 850 to try for several reasons. I know its a lot of carb, I know what I have read though. My 750 runs great so I am skeptical to buy another carb.

[mauve66]

my 6pak on my "little" 383 worked great, never had to change anything out of the box other than a power valve and accel pump and that was after moving to 2 different states and 2 years of running and upgrading the cam, maybe it could of worked alot better, maybe it was too much CFM for the motor but it ran a LOT better than the stock carb that came on it in 66, yea the car was completely bone stock when i got it.

[XH29N0G]

Just a thank you to those who have written the detailed explanations for whys and hows of dynos, carburators and CFM.  This teaching is great for someone like me who is reading this thread and learning.

[firefighter3931]

Well, I think I'm gonna try all three mentioned (700 DP, 800 DP, and 850 DP) on the dyno just to see what is what for this(my) 440. If it likes more carb, ok, fine, but if it doesn't, ok fine...

I was just thinking a modded 650 DP might be a little small for a modded 440. I guess we's bout to find out.

It never hurts to test while you have the engine strapped to the dyno....that way you know for sure what the engine wants/needs for best performance. 

I'll be interested to see what the results show. 

Ask the dyno operator to supply cfm numbers when he prints out your dyno sheets...that is valuable data 



Ron

[JB400]

I don't see what all the hype is about the six pack setup.  Back then, Chrysler wanted to win everywhere at any cost.  Win on Sunday, sell on Monday.  NHRA rules were like NASCAR and SCCA.  Run what you sell on the street.  It might not be practical now, but it was fun back then.  Still fun now.

[BSB67]

Well, I think I'm gonna try all three mentioned (700 DP, 800 DP, and 850 DP) on the dyno just to see what is what for this(my) 440. If it likes more carb, ok, fine, but if it doesn't, ok fine...

I was just thinking a modded 650 DP might be a little small for a modded 440. I guess we's bout to find out.

It never hurts to test while you have the engine strapped to the dyno....that way you know for sure what the engine wants/needs for best performance. 

I'll be interested to see what the results show. 

Ask the dyno operator to supply cfm numbers when he prints out your dyno sheets...that is valuable data 



Ron

A couple of data geeks here.  And I'll gladly pile on if we're spending Cooter's money.  Seriously, maybe we should start a collection to help finance Cooter's  data collection?  Dyno time is not cheap.

If my 750 DP wasn't on a car, I'd send that to you to try as well.  I have a 1" phenolic spacer that I'll send you if you're interested.  They always seem to affect the results.

The operator may have a standard data package that he gives with his service, or he might ask you what you want.  With the air flow, get the DP, mass fuel flow, VE and BSFC.  In the heat of the pulls, you won't look at this stuff much, but when you look back, you will want it.  Also, they usually only give the corrected results, I like to get the actual results and the corresponding atmospheric data.  He should be able to give you average power and torque for each pull real time, and that is the data you need for dyno engine tuning and evaluating your changes, FWIW.

[bobs66440]

Six Pack and twin four set ups are like Trophy wives...The tend to impress, but require high maintenance.

I don't know. I have a Eddy dual quad setup and never had to do anything to it. Works perfect. Though I have to agree, a good single carb setup is hard to beat. 

[Cooter]

I just hope my builder/machinist has an up-to-date Dyno that records and prints out all the info required here. Not real sure if his 1200 HP Engine dyno will give that, but we will see.

Nah, I hoping for a deal for referring about 50 people in the last year to him, so hopefully it won't cost me much. I'm doing it for more of a curiosity/braggin rights thing than anything else.

[charger1972]

Got my proform carb the other day in the mail.Came home from work and went straight to garage to install it.About 11:30 at night i took off for a test drive(my neighbors must love me)and what a difference!That's a great carb ,very responsive.Was a used one off eBay,but only a month old.Took the bowls off before i put it on to see what jets were in there...74 primary 84 secondary.Does that seem a little to much?I'm not sure ,it definitely runs strong.Thanks for all the advice and for any further advice,starting to cool down here in NY so not sure how much more tweeking i can get in before the snow comes.   

[JB400]

Glad to hear it works for you. Are you going to do anything about the metering blocks and the carb base?  Kinda tacky on an orange car in my opinion.

[Cooter]

750 CFM?
Why was Ebay seller selling? Sorry, but I'm always kinda suspect when buying used carbs.

[BSB67]

If that carb has 1 3/8" venturi then that jetting is similar to what I have found to work on other 1 3/8" venturi carbs of the same style.  Maybe a little fat on the secondaries.

[charger1972]

750 CFM?
Why was EBay seller selling? Sorry, but I'm always kinda suspect when buying used carbs.

He said he sold the car and kept the carb,didn't need it anymore.Normally id be a little hesitant to but he had 100% feed back and it looked good in the pictures so,i gambled it was OK.

[charger1972]

Glad to hear it works for you. Are you going to do anything about the metering blocks and the carb base?  Kinda tacky on an orange car in my opinion.

Like what?All iv seen is purple and pink or whatever color mine is.Black would look cool but from what i have seen only the street carbs have black bases and plates.Maybe polish them?

[ACUDANUT]

How old does that e-bay carb look.? Price ??

[charger1972]

How old does that e-bay carb look.? Price ??

He said a month old,looks like new.He had a buy it now for 375 so i jumped on it.

[ACUDANUT]

375.00 IMO is waay too much.  Is it a DP or what ?

[firefighter3931]

375.00 IMO is waay too much.  Is it a DP or what ?

Not too much at all for a 1 month old carb that sells for $550.00 

I'd call that a score 


Ron

[firefighter3931]

Got my proform carb the other day in the mail.Came home from work and went straight to garage to install it.About 11:30 at night i took off for a test drive(my neighbors must love me)and what a difference!That's a great carb ,very responsive.Was a used one off eBay,but only a month old.Took the bowls off before i put it on to see what jets were in there...74 primary 84 secondary.Does that seem a little to much?I'm not sure ,it definitely runs strong.Thanks for all the advice and for any further advice,starting to cool down here in NY so not sure how much more tweeking i can get in before the snow comes.   

You should install the holley throttle adapter (pn 20-7) to get the right throttle ratio....it'll run even better than it does now ! 

I knew you'd like that carb ! 

Have  a look at the plugs to see if it's running rich 


Ron