holley carb. tuning question

[eggman918]

I am running a 4160 750 on my 390. now I need to readjust it for towing,jets seem to be correct.my issue is with secondary opening timing,the book says use a lighter diaphragm spring for increased vehicle weight.that sounds reasonable but how light for 10'000# total?

[BobbyFord]

I am running a 4160 750 on my 390. now I need to readjust it for towing,jets seem to be correct.my issue is with secondary opening timing,the book says use a lighter diaphragm spring for increased vehicle weight.that sounds reasonable but how light for 10'000# total?

Not sure if your book has this information;

http://www.holley.com/data/Products/Tec ... 8219-2.pdf

[eggman918]

thanks again .....You must have 1 hell of a library nice of you to share

[mylifeswork]

Eggman, Since you have a 750 CFM and you are trying to tow you need more low end grunt. That would mean to maintain volumetric efficiency the highest possible for the rpm range you are using the truck for. Most of the time that will be 2000 to 3500 RPM's. Assuming your primary's flow 375 CFM and the secondaries flow 375 CFM, giving you 750 CFM total the rpm range of 2000 to 3500 only needs the primarys to operate properly. If I use the formula to find you CFM requirement 390x3500/3456=395CFM. That is at 100% Volumetric Efficiency, VE. Our trucks are not 100% VE so if we figured at best the VE is at 85% then the CFM requirement is 335 CFM. Below the rated flow of the primarys. It's all about air velocity when it comes to carburetion. When need fast flowing air through the carburetor to allow the carburetor to mix air and fuel correctly and to fill the cylinders with as much as possible. That faster the air flow the better our engines run. In my opinion I would be putting in a heaveir or stiffer spring for a heavy vehicle, not lighter. I would want to stay on the primary's as long as possible. When running a heavy vehicle it is recommend to run a smaller carburetor for the simple fact of increased air speed. If your secondaries come on sooner it will slow down air flow. The smaller primarys with a smaller carburetor provide for high air velocity as slow speeds. Another option available is a spread bore style which provides for much smaller primaries and larger secondaries. This is for increased air flow during low speed cruise situations.

Now to determine which spring to use I would try different ones and drive it under loaded situations to see which performs best. Most Holley's come with the plain spring in the secondary diaphram. Purple and two versions of yellow springs are lighter then Brown and Black make the heavier springs from stock. If you have a selection change them and test drive to determine the performance difference. According to my information a plain spring will not allow full opening of the secondaries until 7130 rpm. That is for a 402 engine. Slightly larger than our 390's. Meaning the engine never uses the full potential of the carburetor CFM. Mainly because the engine cannot use that much air flow. The lightest spring, which is yellow short spring, allows the secondaries to open fully at 4960. But at that rpm our engine only reguire 560 CFM at 100% VE. Remember our engines are not 100% VE. So why would we fully open the secondaries when the engine cannot use the air flow. This causes the air flow to slow down and our trucks lose power due to the decreases in airflow. In my opinion stay on the primaries much as possible, better is change the carburetor to a smaller version if possible. Granted I know this means spending money.

[mylifeswork]

I'm sorry, to clarify the airflow in the primaries slow down since the secondaries open up to soon. Since the volume is more with 4 barrels open the air speed or velocity slows down. It would be better to keep only two barrels open and have faster air speed instead. To help understand how the secondaries work. The secondaries open due to the difference in airflow thorough the primaries and the spring tension in the secondary diaphram. The faster the airflow in the primary the stronger the venturi vacuum signal which will overcome spring tension and open up the secondaries. If we lighten up the spring we open the secondaries sooner. If trying to maintain very fast ariflow through the primaries it's best to hold off secondary opening to maintain good airflow, especially for heavy vehicle or towing.

[eggman918]

spring kit is on the way.my last 390 ran 600 vac holley, motor was not happy with it intake stream velocity was my concern i can feel it slow down .if I keep mph at 65 or more it is fine.Staying on primary's makes sense.the trailer is new so I expected tuning issues

[mylifeswork]

Hopefully you have a quick change cover so changing springs are very simple. If not you may have to take the diaphram off entirely to make the changes.

[eggman918]

Hopefully you have a quick change cover so changing springs are very simple. If not you may have to take the diaphram off entirely to make the changes.

stock cover It's never as easy as we would like,but if I wasent doing this I would be doing something else to it

[BobbyFord]

Quick change cover part #20-59 is only about $17.00

[eggman918]

I'm sorry, to clarify the airflow in the primaries slow down since the secondaries open up to soon. Since the volume is more with 4 barrels open the air speed or velocity slows down. It would be better to keep only two barrels open and have faster air speed instead. To help understand how the secondaries work. The secondaries open due to the difference in airflow thorough the primaries and the spring tension in the secondary diaphram. The faster the airflow in the primary the stronger the venturi vacuum signal which will overcome spring tension and open up the secondaries. If we lighten up the spring we open the secondaries sooner. If trying to maintain very fast ariflow through the primaries it's best to hold off secondary opening to maintain good airflow, especially for heavy vehicle or towing.

Carb is off an it appears that the secondary's are not opening,no sing of fuel on ventures compared to primary's.....new clue?

[hotrodfeguy]

A slower opening secondary is what your after, with a 750 and towing. On towing a load I would venture to say the 600CFM would work better down around towing RPM. Towing is all about the 1500-3000 RPM lug. Thats what makes the 300-6 such a good engine. It has the same stroke as the 410/428 FE !!!

[eggman918]

A slower opening secondary is what your after, with a 750 and towing. On towing a load I would venture to say the 600CFM would work better down around towing RPM. Towing is all about the 1500-3000 RPM lug. Thats what makes the 300-6 such a good engine. It has the same stroke as the 410/428 FE !!!

My thoughts also.started with 600 vac.motor was not happy my motor builder said I was better off with 750,and until i got trailer it was OK.so now i will see if it will tune for trailer,it will pull 6% grade at 65mph/2600rpm.I want 58mph/2200rpm.....we will see

[mylifeswork]

I agree with hotrodfeguy. Those speeds and rpm figures you provided are much better suited for the 600 CFM size of carburetor. A 750 CFM is suited to hot street running which means less vehicle weight and higher rpm's. Your after low end grunt which lends itself to the smaller carburetor. The smaller carburetor will fill the cylinders with more air/fuel mixture in those rpm's.

[eggman918]

I agree with hotrodfeguy. Those speeds and rpm figures you provided are much better suited for the 600 CFM size of carburetor. A 750 CFM is suited to hot street running which means less vehicle weight and higher rpm's. Your after low end grunt which lends itself to the smaller carburetor. The smaller carburetor will fill the cylinders with more air/fuel mixture in those rpm's.

The 600 is still on the shelf, I am running #83 jets in 750 primary's where should i start in the 600?
I never spin the 390 beyond about 4200 so I see your point

[mylifeswork]

Eggman, you'll most likely have the correct size in the carburetor already, as long as nobody has messed with. They should be from 69 to 72 jet size. 83's, wow those are pretty large. I ask my students if I run a larger carburetor on a vehicle will it runner richer or leaner. They will always say richer. What they don't understand is when you install a larger carburetor you are providing more air, not fuel. The driver is controlling air only. The physics of pressure differential effect airfow. Carburetors need good air velocity to work properly. Knowing the physics of pressure differential allows us to understand what we need to do for improvement of air velocity. Atomospheric pressure along with manifold vacuum create pressure differential. When we install larger carburetors we drop manifold vacuum and slow down airflow due to more volume. Air velocity along with, float level, jet size, and air bleed size determines if the fuel mixture is rich or lean. When carubretors are bigger we need bigger jets to offset loss of air velocity. When smaller we can run smaller jets. 500 CFM carburetors come with 58 size as a rule. The faster the air velocity through the venturi of the carburetor the better you pull fuel through the jets. Most important is cylinder filling at lower rpm's. Helps with power down low.