Will my Carb work good with my new engine???
Thread Starter
Team Owner
Joined: Jul 2001
Posts: 182,997
Likes: 88
From: Lakeland, FL
Cruise-In IV-V-VI-VII-VIII Veteran
St. Jude Donor '03, '06, '17
Will my Carb work good with my new engine???
I have a Holley 750 Vacuum Secondaries Carb - Part # 80508.
I just ordered a new main body to get better performance out of the carb and more clearance (because the new main body doesn't have a choke tower). However - I start to doubt that this was a good choice. I think that with my new engine I should use a mechanical secondaries carb. The new engine has a solid cam with 238/248 .050 duration. This cam will very likely produce less vacuum than my current engine. I am worried that this will affect the function of my carb.... My technical knowledge about carbs is very limited and therefore I really don't know what I can expect running a regular vacuum secondaries carb with a relatively radical cam....
I really need some advice here. Please help!!
Thanks in advance
Olivier
I just ordered a new main body to get better performance out of the carb and more clearance (because the new main body doesn't have a choke tower). However - I start to doubt that this was a good choice. I think that with my new engine I should use a mechanical secondaries carb. The new engine has a solid cam with 238/248 .050 duration. This cam will very likely produce less vacuum than my current engine. I am worried that this will affect the function of my carb.... My technical knowledge about carbs is very limited and therefore I really don't know what I can expect running a regular vacuum secondaries carb with a relatively radical cam....
I really need some advice here. Please help!!
Thanks in advance
Olivier
Tech Contributor
Joined: Aug 1999
Posts: 14,384
Likes: 6,408
From: At my Bar drinking and wrenching in Lafayette Colorado
Re: Will my Carb work good with my new engine??? (GrandSportC3)
The vac sec carb will work fine. The secondaries are not pulled open by manifold vacuum - they are pulled open by venturi vacuum. Venturi vacuum increases as airflow through the primary side of the carb increases, and is completely independent of manifold vacuum (even on a mild-cammed car, there is little or no manifold vacuum at wide open throttle, yet the vacuum secondaries open just fine, right?). If you're planning on driving the car on the street, the vac sec carb is a good setup, and it will produce better fuel economy and better secondary transition than the mechanical carb, especially if you're running an automatic. If you're running a 4-spd car or if the car sees a lot of strip use and economy is not an obstacle, the mechanical carb works fine as long as you drive it and jet it in such a way that you avoid a secondary tip-in stumble.
Thread Starter
Team Owner
Joined: Jul 2001
Posts: 182,997
Likes: 88
From: Lakeland, FL
Cruise-In IV-V-VI-VII-VIII Veteran
St. Jude Donor '03, '06, '17
Re: Will my Carb work good with my new engine??? (lars)
The vac sec carb will work fine. The secondaries are not pulled open by manifold vacuum - they are pulled open by venturi vacuum. Venturi vacuum increases as airflow through the primary side of the carb increases, and is completely independent of manifold vacuum (even on a mild-cammed car, there is little or no manifold vacuum at wide open throttle, yet the vacuum secondaries open just fine, right?). If you're planning on driving the car on the street, the vac sec carb is a good setup, and it will produce better fuel economy and better secondary transition than the mechanical carb, especially if you're running an automatic. If you're running a 4-spd car or if the car sees a lot of strip use and economy is not an obstacle, the mechanical carb works fine as long as you drive it and jet it in such a way that you avoid a secondary tip-in stumble.
The car will be mostly driven on the street. However - every once in a while I want to take it to the strip... I want to get the most possible performance out of my engine when racing. I was planning to re-jet my carb too. The company that built my engine recommends a Holley 750 Double Pumper (4779) with 76 primary jets and 82 secondary jets. The carb on the mule engine was setup that way. I was thinking in changing the jets on my Holley to the same setup. I am also going to change the main body to the proform body that flows around 800 cfm... With my setup - will I be able to get close to the carb that was used on the mule engine?? If not - how many HP would I lose (approx.)??
Thanks
Olivier
