i have aholley 670 in my 69 ss 350 . i was looking to upgrade the springs but dont know which one ot go with. what springs come in these from holley? and which springs are the best for all around streetabilty and performance? do changing the springs really make much of a difference or am i wasting my time tryin to change them
The springs most certainly make a difference... Just changing springs helped find the last 12hp on the dyno for the 383 in my wife's convertable. I used the lightest spring Holley makes to get the secondaries open sooner. It all depends on how you want the car to react. Lighter springs will open the secondaries sooner and you will see and feel the performance, however it comes at the cost of economy. Believe it or not, some guy's will setup the secondaries to come in late to save on their fuel bill at the cost of performance. It all depends on what you are looking for?
1967 Prostreet Camaro BBC TH 400 w/4,500 stall 4:56 gears,narrowed and tubbed...
The opening rate of a vacuum secondary system is controlled by the diaphragm spring located in the vacuum secondary diaphragm housing. A “lighter” spring will allow the secondary throttle plates to open more quickly. A spring assortment kit, Holley P/N 20-13, is available to help you “tailor” the secondary opening rate to your application.
From my Holley carb book by Urich & Fisher
Using a 350 CID Engine
Spring------start to open------ full open
Black---------2720-------not fully open at max air flow
These rpm numbers go down as cubic inches go up and an air cleaner causes these numbers to go down as well... There was no air cleaner used when they tested this.
To find the right spring, start with the plain one and test drive. If you feel the sec's kick in when you go wot they are opening too soon. If you don't feel a kick in then try the next lower spring. If you feel the kick in go back up one. You get the idea...
Not feeling the sec's kick in may seem wrong but on-track testing has shown if you feel the sec's open you are running slower with a vac sec carb...
I got same 670 on my 406 68 SS the secondaries do come in late with stock springs but I don't think I will change it as it runs great and fuel mileage (all though not checked yet) doesn't seem bad and spark plugs are burning just right color and the performance is not bad feels like it could be a mid 13 second car with some tuning. I have only got 300 miles on it and will see if the carb needs tuning after engine is broke in.
Edd Gordon 68 Camaro SS350 now SS 406 96 SLP SS Camaro