I think it had 5 screws in the bottom, But anyway Im going to buy the new middle and 750 base plate after the holidays and go that route, so it will be a whole new ball game !

 

You don't necessarily want the carb to *max out* on airflow at peak RPM. The airspeed gets pretty fast at that point. Carb spacers work partly because they slow down the airspeed somewhat and make it easier to make the turn into the runners.

They rate 4bbl carbs at 1.5" vacuum usually..and I can tell you that many carbs pull much more than that at WOT. If you have a dual plane intake...a larger carb is usually preferred since each cylinder can only *see* 2 throats rather than 4 on a single plane. Ck Edelbrock's website..they have it pretty close. This is why you see 780 Holley's on 302's and 750 cfm Q-jets on mild smallblocks. It just works. A 600 DP on a single plane works pretty fair...but a 750 will do better.

I've used the same 750DP on a stock 5.0 Mustang, a solid lifter 454, solid lifter 427, hyd cammed 440 Mopar, 450Hp 327...etc etc . The thing just flat works.


JIM

 

My favorite carb is a 750 Holley whether vacuum or double pumper.
My view is I would use a double pumper on an auto or 4 speed but wouldn't use a vacuum on a 4 speed.
I have used a 850 dp and it is a really over adder for most street cars.
My 72 355 nova auto had a 650 dp but also a 3500 stall and 373 gears to be able to raise the RPMs to use the carbs gas feed when I stomped the gas!

 

I've also had a moderate 427 with 1050 Dom on the street. Carb was progressive secondaries model; intake was an Edelbrock C454 (still have). Very stout runner and still had decent manners, MPG aside. Dominators are awesome. That said, such relatively large carbs are certainly not going to work on just any build.

TSW

 

Right now Jegs has the proform mid section on sale for $99.
It comes with gaskets, screw in air bleeds and jets, thats $25 off.
now 1 thing I decided against was the anodized base plate for $162 and just bought a Holley replacement base for $123.
Both pieces will be here at weeks end.
I'll use my bowls and metering plates and turn the 600 into a 750 for $240 verses $500 for a new carb.

 

Tim,

If there is a Dyno shop close to you that has a Dyno mule with a O2 sensor, do your self a favor and see what they will charge you to dyno your new build, if you have the opportunity you wont be sorry you did.

Just a word to the wise; if they tell you they can do it with out an O2 sensor by crunching the numbers keep your money and try to enjoy the ride home...

Most Dyno shops have a killer carburetor they only run on the Dyno mule as a benchmark for all of their work you can use as a comparison.
It is a pretty eye opening experience if you have never done it.

I have seen carburetors run fine and never exhibit any ailments and be down as much as 30 hp from a good carburetor.


FWIW , Jim

 

If you did have access to an AFR gauge, what values are you looking for at cruise and WOT?

 

Depends on how much ethanol is in your gas.

Pure gasoline is ~14.7 cruise and WOT ~12.5-12.8. (WOT is a moving target based on engine/timing)

E10 can knock those numbers down quite a bit, as ethanol wants a lot more fuel in the mixture for complete combustion.

A lot also depends on what your motor will accept.

 

As Strictly Ball Park numbers

all engines are unique

idle = 12.5 -12.7


WOT = 13.1-13.3 (This is RPM Dependent) or leaner


Cruise wll be somewhere in between the two


but for economy its a whole other can of worms, that has other dependencies such as ignition quality, a leaner mixture is hard to light...

Trying to get the best gas mileage has never been high on my priority list and is as you know another science all together.


FWIW, Jim


I guess I should have qualified my statement:

I am talking strictly gas...

 

This is true. I do quite a bit of dyno flogging and have seen some wild stuff. For example:

A BG 850 Demon has the same sized venturi's and throttle bore sizes (within .001") as a Holley HP 1000. The Demon ran well...looked good on A/F ratio and BSFC numbers...very responsive. Dropped the Holley on out of the box and picked up 20 HP across the board on a 555" BBC. The Holley stayed!

A well proven homebuilt old style 1050 Dominator against a much larger high $$ Braswell race carb on another 555" street motor. This Braswell carb has always kicked butt and makes killer top end and mid range power. In this case the old Holley made 2-3 more peak HP...but the Braswell made better overall averages with about 10-15 lb ft more throughout the midrange. Not sure if driveabilty was as good as the street tuned 1050..but it made great power.

Same 555"...dropped on a 1090 King Demon that always drove well..but we knew was down on power from the Braswell. Just never knew how much. We honestly thought we had hurt the motor because it lost 60 HP! (mid 800's HP at this stage). We played with jets, yada yada yada....nothing. Dropped Braswell back on and power was right back where it began.

Not sure about the development of the fuel curve in the Proform body. Who knows who they copied? Hopefully Holley. There's a LOT to getting a carb to work right and mixing and matching stuff can get pretty wierd without some good data to work from.

Out of the box it's hard to beat any of the Holley's. They still know how to calibrate a carb. There was a recent shootout with all the big name carb builders testing their best on a race small block and a big block. The test was acceleration rates on a specifically set up dyno. There were some tuning parameters..but overall they could bring what they wanted. Holley brought shelf carbs and they made the top 10 against the customs ( I think they were #8) and all the top ones were just a few HP apart.

JIM

 

Often wondered about that

Have a 10 yr old Holley 750DP platinum series
Never quite had that razor sharp response Ive wanted.
Done everything I can to it still not happy

Thought about sending it to (forget his name)XX carbs in Tx..hesitant to spend another 200 on this thing yet an HP is more than I can afford.

 

Jim,

I am glad some of you racerly types are crawling out of the woodwork...

I am very reserved about what I say on here, seems like everytime I post
something I gotta hear a load of crap from somebody

There is something to be said for reading between the lines

As you stated, when it comes to carbs things are not always as they seem...


As far as builders go I have respect for any one who has the patience to deal with the general public when it comes to perfromance carburetors.
The general attitude is you can bolt it on thats all you have to do...

Performance and going fast is a different game, you will never catch a racer selling a good carburetor even if he sells his whole rig. That one will just go on the shelf in a bag for another time.

For after market, Holleys as you stated are hard to beat as far as value for money. (Gas or Alcohol)

Carburetor calibration is the name of the game!!! Each circuit should be calibrated to work as it was designed to function and then be made to function as whole with the rest of the circuits in the carburetor. This is much harder than one might think. Then factor in drivability its almost easier to believe in the Tooth Fairy

The next contributing factor is Airflow, Ruh Roh...

As you described about the main bodies... they make look the same but are they really???

NOT!!!

Then move on to spacers

This is where all of the after market shister's make there money.


Before I get a load of crap for that statement!!!


There are spacers that work but there are ten times that many more that don't

and

we haven't even covered air cleaners or base plate gaskets yet...

or air cleaner element heighth or compositon...

Then move on to exhaust tuning...

I digress

Bottom line unless you can measure it you cannot improve it...

I would advise anyone serious about performance to go to a competent tuner with a dyno and spend a little money.


Best Regards, Jim

 

On the money.

My old funky home brewed 9375 Dominator isn't going anywhere. I can't tell you how many dyno operators and street folks have tried to pry it from my hands!

The common comment is " It just flat works!".

Getting one to drive right on the street takes a little time. But the trick is to NOT move too far from the baseline setup. I've fixed many a carb that was worked on by various modifiers by taking them back to the baseline and then slightly adjusting what may need to be changed.

*Universal* carbs are in the ballpark..and a little work will make them dead on.


JIM

 

Great post Jim! Thanks

I guess this means the "formulas" only provide a rough starting place, and it almost seems like different brand carbs need different formula values (constants).

 

It is a complicated topic...

The above sounds to me like a calibration issue but could be other things too...

Provided you have a healthy ignition. (Mechanical advance operates properly, if you have not welded it up)?

I would suggest if the carburetor is at the baseline it was setup at from the factory, taking it to a carb shop with a Dyno and an o2 sensor. After one pull they can tell you if you are in the ballpark and the cost is usually minimal ( just request to be present when they do it)

If it is not put it back to the base line (air bleeds, squirters, Jets, etc.) and try it?

The level of detail and tools required for ringing the most Horsepower out of a carburetor is normally beyond the average guys tool box. But sometimes you just get lucky and get it right.

Things considered are Fuel Delivery System ( all encompassing) everything that touches the fuel delivery system.

Booster design

Booster placement

Booster signal tuning

Booster Positioning

and over all carburetor calibration

all play a part in the above converstation and all can either have a negative or positve impact on overall carburetor/engine performance...



FWIW, Jim

 

Thats what I did last time around.
Found if I tuned by LM1 to its #s car ran worse.
Perhaps because it had a lot of overlap and the probe was in the tip not a "Coming from the collector" readout?
New cam/motor has 80 deg overlap
THink for the new motor it should be close but if its just way far off I give up on it. Set things bout as close as I can as far as what it should want (squirters jets cams etc)

Every carb shop its been to has just drained me of another 200 with no change in how it runs. I can screw it up myself for free.

 

Have any of the carb shops given you the read out on the Air/Fuel curve of your carb?

I would be interested to see what the A/F ratio looked like on the pulls they made? If they did not give you one I may would have to call BS on their work

Is your LM sensor heated?

I would check with the LM Manufacturer for placement if you haven't already. I would also check for fuel compatibility? I do not have alot of experience with the hand held jobs, just had to ask the obvious.

Hate to ask on the forum because you will get a hundred diffrent opinions on this.

But

How do the plugs look?

At the end of the day you can tune like this but unless you are not faint of heart will never get lean enough reading plugs to make max power.

FWIW, Jim

 

I'm used to figuring overlap on LS roller cams. It must be figured differently on the Gen 1 stuff? Gen III/IV stuff is figured at .050 lift ....(intake duration + exhaust duration /2) - (lsa X 2) = cam overlap. A cam with 80 degrees of overlap figured that way would have a real tough time even running. What's different?

 

The probe needs to be way in the tip to work to prevent reading ambient air, and if you have catalytic converters the reading will be very skewed - you'll need to measure from before the cat.

 

http://www.wallaceracing.com/overlap-calc.php

This calc says 82 deg cam card says 80. Oh well.