holley carb. tuning question

[eggman918]

If the 600 is what I need,and it appears it is.................what do I tell the WIFE

[mylifeswork]

Didn't you have it on the shelf already?

[eggman918]

Didn't you have it on the shelf already?

Yes but I recently purchased the 750 and she NEVER forgets

[70_F100]

Just don't say anything about it, and sell that one here, on Craigslist or on ebay...

[mylifeswork]

Look at that, who's got your back . Besides, if she saw one carburetor on the shelf you could just put the 750 on the shelf it all looks the same

[eggman918]

It's good to have friends

[DuckRyder]

Besides, if she saw one carburetor on the shelf you could just put the 750 on the shelf it all looks the same

I hope they look essentially the same, women are pretty observant... I can hear it now...

"I thought the old one didn't have that thingamajig on the side...."

My advice, fess up...

[OldRedFord]

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.

Where did you get the 3456 in that formula?

[mylifeswork]

Most all formula's have what is called a constant. The 3456 is used so we can make the formula work. CFM, Cubic Feet per Minute, RPM, Revolutions per minute, and CID, Cubic Inch Displacement, are all different factors. The 3456 allows all factors to be calculated. There is a more detailed explanation but would require more research. It's like pie, 3.146. It's used as the constant to find the volume of a circle. Piexradius squared=volume. If I remember my algebra correctly. You use pie as a constant to find the area of any circle.

[eggman918]

first She knows I dont walk on water,but I dont lie so yes fess up
second the math makes sense,going up the hill i could hear/feel the intake velocity drop.As I have the carb and jet kit and springs /quick change housing are on order I will go back to 600 and tune it.The grade is only 15 miles away so it should a relativity easy fix at this point Thanks again

[hotrodfeguy]

Good way to fess up is to put a smaller 64 jet in the 600 and say your trying to get milage out of it with the 600 How can she fault you for trying to save money

[eggman918]

That,s how I killed the last motor

[Dragon]

a 390/2*3000rpm/1728 needs 287 cfm at 85 % VE. As for the 750 the 80508S model has a 72 jets on the primary. At this altitude I have 70s as the primaries.

So your towing application only needs a 600 cfm carb for everything you do to it. And then it will only see the primaries for towing, now as the rpms rise it needs more CFM. My 71 has been to 120 mph with a 600 but only reached the same 120 with my 750 very slightly faster. Now at my max of 4000 I ever drive the truck I only need 406 cfm at 90%VE. I have used my truck to tow my 69 F350 on a tow dolly. 6920 lbs and the 750 opened the secondaries in 2nd and 3rd smoothly at this altitude as I pulled out onto the highway. Steady state 4th ear and I barely had the throttle down to cruise in the tow at 65, barely more than the unloaded throttle position. So did the 750 help me tow any better than the 600? Nope.

[eggman918]

a 390/2*3000rpm/1728 needs 287 cfm at 85 % VE. As for the 750 the 80508S model has a 72 jets on the primary. At this altitude I have 70s as the primaries.

So your towing application only needs a 600 cfm carb for everything you do to it. And then it will only see the primaries for towing, now as the rpms rise it needs more CFM. My 71 has been to 120 mph with a 600 but only reached the same 120 with my 750 very slightly faster. Now at my max of 4000 I ever drive the truck I only need 406 cfm at 90%VE. I have used my truck to tow my 69 F350 on a tow dolly. 6920 lbs and the 750 opened the secondaries in 2nd and 3rd smoothly at this altitude as I pulled out onto the highway. Steady state 4th ear and I barely had the throttle down to cruise in the tow at 65, barely more than the unloaded throttle position. So did the 750 help me tow any better than the 600? Nope.

COOL 600 it is.Already broke it to the wife so all is well
Thanks to all for the help/support