Inexpensive 350 Build

[vortecpro]

Great job and keep up the good work. I love seeing somebody who doesn’t just build an engine and think they have a race car. You are going to upset a lot of hobby types who do not understand average power and making a car work. Love following along with this and look forward to you continuing to update us.

Update:

Preliminary carb tuning has delivered a new best 1.43 60 foot, in 1700 DA on a fair track. This was accomplished with a 750 DP in place of the original 850 DP, the 750 did lose MPH over the 850, but the 750 ran harder to the 330 foot slowing down from 330 foot on. So working with the 850 to get rid of the slight hesitation is the answer, we are working with Vintage Muscle car on this carb issue. It would seem the 750 main body would be sufficient for such a mild low HP 350, but it isn't. Interesting enough dyno testing also showed the 750 main body (950 HP) down 10 HP through out the test, mirroring what we are seeing at the track. Now the question is: if the 850 was worth MPH over the 750, is the the 1000 worth MPH over the 850? We believe the carb is the way to a better 60 foot which will hopefully get us into the high 10.70s. Next the rear suspension will have to be addressed.

I have found on my 383 engines the 950 hp carb 60 ft and ET the best as well.
The 950hp carb responds more as well with mufflers on/ off as in fuel curve and needing more jet with open headers.
At WOT it pulls 0.75" vacuum across finish line so that could be a restriction.
With my 4150 QFT 1050 carb with annular boosters manifold vacuum is 0.20" at WOT .
60 ft is always down slightly doing back to back at track testing. 1.43 (950hp) - 1.46 (1050) carbs.
ET is near on the same but around 1/2 mph at the long end in difference.
Mark are you playing with spacers on top of the airgap intake? Or are you in stocker trim.
Appreciate you posting your results, always plenty to learn.

I have a 1 inch open spacer on there.

[vortecpro]

Update:

Preliminary carb tuning has delivered a new best 1.43 60 foot, in 1700 DA on a fair track. This was accomplished with a 750 DP in place of the original 850 DP, the 750 did lose MPH over the 850, but the 750 ran harder to the 330 foot slowing down from 330 foot on. So working with the 850 to get rid of the slight hesitation is the answer, we are working with Vintage Muscle car on this carb issue. It would seem the 750 main body would be sufficient for such a mild low HP 350, but it isn't. Interesting enough dyno testing also showed the 750 main body (950 HP) down 10 HP through out the test, mirroring what we are seeing at the track. Now the question is: if the 850 was worth MPH over the 750, is the the 1000 worth MPH over the 850? We believe the carb is the way to a better 60 foot which will hopefully get us into the high 10.70s. Next the rear suspension will have to be addressed.

It may seem a naïve question however I'd like to know what you attribute the differences in performance between the carburetors tested?
Is it simply a fuel curve thing? Or signal and responsiveness? CFM?
If I recall correctly you don't do any datalogging. So is it then a matter of swapping carbs and looking at the results?

Following along with your post above, I'm also very interested to see what you do to the 3rd F-body rear suspension.

I believe its CFM.

[Tom68]

It would seem the 750 main body would be sufficient for such a mild low HP 350, but it isn't.

So many engines under carb'd, good for drivability and ease of tune.

[tuffxf]

1000 cfm and an old 850 are identical sizes
Cheers

[HQM383]

Update:

Preliminary carb tuning has delivered a new best 1.43 60 foot, in 1700 DA on a fair track. This was accomplished with a 750 DP in place of the original 850 DP, the 750 did lose MPH over the 850, but the 750 ran harder to the 330 foot slowing down from 330 foot on. So working with the 850 to get rid of the slight hesitation is the answer, we are working with Vintage Muscle car on this carb issue. It would seem the 750 main body would be sufficient for such a mild low HP 350, but it isn't. Interesting enough dyno testing also showed the 750 main body (950 HP) down 10 HP through out the test, mirroring what we are seeing at the track. Now the question is: if the 850 was worth MPH over the 750, is the the 1000 worth MPH over the 850? We believe the carb is the way to a better 60 foot which will hopefully get us into the high 10.70s. Next the rear suspension will have to be addressed.

It may seem a naïve question however I'd like to know what you attribute the differences in performance between the carburetors tested?
Is it simply a fuel curve thing? Or signal and responsiveness? CFM?
If I recall correctly you don't do any datalogging. So is it then a matter of swapping carbs and looking at the results?

Following along with your post above, I'm also very interested to see what you do to the 3rd F-body rear suspension.

I believe its CFM.

Do you know the make up of the 850cfm carb spec wise?

Without that info its like declaring the intake cc's of a cylinder head is too big without considering valve size, mcsa, average csa, throat size, valve job etc.

Max cylinder density fill and shift recovery are two demands of carburetors for 1/4 mile. The 850cfm is doing one of those and maybe with some adjustments can do the other.

[vortecpro]

It may seem a naïve question however I'd like to know what you attribute the differences in performance between the carburetors tested?
Is it simply a fuel curve thing? Or signal and responsiveness? CFM?
If I recall correctly you don't do any datalogging. So is it then a matter of swapping carbs and looking at the results?

Following along with your post above, I'm also very interested to see what you do to the 3rd F-body rear suspension.

I believe its CFM.

Do you know the make up of the 850cfm carb spec wise?

Without that info its like declaring the intake cc's of a cylinder head is too big without considering valve size, mcsa, average csa, throat size, valve job etc.

Max cylinder density fill and shift recovery are two demands of carburetors for 1/4 mile. The 850cfm is doing one of those and maybe with some adjustments can do the other.

4781 plugged power valves. Feel free to educated me.

[HQM383]

I believe its CFM.

Do you know the make up of the 850cfm carb spec wise?

Without that info its like declaring the intake cc's of a cylinder head is too big without considering valve size, mcsa, average csa, throat size, valve job etc.

Max cylinder density fill and shift recovery are two demands of carburetors for 1/4 mile. The 850cfm is doing one of those and maybe with some adjustments can do the other.

4781 plugged power valves. Feel free to educated me.

Those old school calibrations of the 4781 make for a great all round carburetor. However the small amount of air introduced into the mains via the small mab and emulsion that provide good stable AFR over a wide rpm range can cause fuel delivery response from the mains with rapid throttle angle changes to be slower than needed in a race application. No doubt people have got 4781’s to work for racing applications but probably on an engine larger than a 350ci. Smaller cfm would have signal strength to help with mains response but as evidenced by your testing smaller cfm is not quite giving the engine what it wants at the top end.
Think about rpm/sec engine acceleration rate in 1st and 2nd gear on the track v dyno rpm/sec rate. Adding more air to the main well with mab and emulsion will lower the density of the fuel. Easier to make a less dense, lighter fuel change velocity faster. It doesn’t have to be drastic increases in mab and emulsion sizes, just enough until performance peaks. Another benefit is the extra atomization when using downleg booster. Take a look at the calibrations of racing carburetors. If they didn’t work for the purpose, rpm window and engines they were designed for they would be nothing but a memory now as racing doesn’t tolerate what doesn’t work. They get labeled as poor fuel metering devices through their overuse on the street.

So with the reduced signal at the booster with the bigger venturi carb, make the fuel less dense so it’s easier to get moving with less signal.

As tuffxf said the 4781 850cfm and a 1000cfm are both 1.56” venturi and 1.75” throttle bore. You probably don’t want to start drilling and tapping on the nice 4781 just to trial some changes to see if it improves 60’ but I assume the 1000cfm is a performance oriented carb with some threaded adjustments?

[rgalajda]

As a lot of things, it comes down to the details. The Holley List # is important when comparing carbs.

In regards to 4150 Holley Carbs

Venturi Throttle Bore

750 Holley 1- 3/8 1-11/16

4781-850 Holley 1-9/16 1-3/4

1000 Holley 1-9/16 1-3/4


750 Holley carbs were built over the years in various configurations such as with a choke horn, or not.

4781- 850 Holley carbs had a choke horn that was shorter than a regular Holley ( flow reasons )

1000 Holley carbs had no choke horn with the entrance on the venturi smoothed out for better flow.
These carbs flowed better than the 850 but not 1000 cfm as rated. These were built with down leg or annular boosters.

[rgalajda]

For larger Venturi carbs such as the 850 cfm Holley I start with the Booster to improve signal . I don't believe the the 4781 - 850 had stepped boosters.
Look at the pictures , turn your carb over , open the throttle and look at yours.
First thing to do is machine a step in the booster. Next , I always machine the top of the booster to have a sharp entrance point. I do these things before changing air bleeds/emulsion.
The 1000 cfm Holley may have stepped boosters already, depending on the list #.

[vortecpro]

Thanks guys, now we're getting some where! Is the BLP booster the same as your modified booster? We were going that direction.

[rgalajda]

I suspect the BLP boosters are, but I have never used their' booster. I would suspect they could use a little dressing on the top of the booster.

The modifications I make do not require the booster to be removed from the carb. It is quicker and less expensive. Another note is to level the booster in the Venturi. This is also a rather simple operation.

For a street application I prefer to install an Annular Booster.

[tuffxf]

The braswell stepped downlegs are a nice piece

[HQM383]

The braswell stepped downlegs are a nice piece

They are a good thing with the larger diameter than std banjo. Had them in the first carb on the dyno a few weeks ago. Felt they were pulling a bit early for the carb they were in on the dyno so I swapped them out (threaded) for regular Holley downleg. They will however be getting another go when testing in car.

[HQM383]

I suspect the BLP boosters are, but I have never used their' booster. I would suspect they could use a little dressing on the top of the booster.

The modifications I make do not require the booster to be removed from the carb. It is quicker and less expensive. Another note is to level the booster in the Venturi. This is also a rather simple operation.

For a street application I prefer to install an Annular Booster.

I like your in-carb modifications to the booster. Neat job.

BLP sell a 'machined' downleg booster that tidies up any casting flaws to do with consistency. I have a set in an 825cfm Race Demon and they work great.

[1980RS]

I know my recently acquired 1000hp has stepped booster from Holley. I have never had any luck with any 4781's I've ever used out of the box and my 4779 was quicker and faster then the last 4781 I tried. Even my 4780 800DP was better then the 4781, and the HP carburetors just seem to work better for me.