After installing a 1/2" spacer on my 302 w/600 Edelbrock carburetor, is it a rule of thumb to re-set, re-adjust the air/fuel mixture ? Thanks
Cant hurt though as you are changing rate of atomization even though it is so slight with just 1/2" and what is it going to hurt
Exactly right Mike. The biggest benefit to a true separated plenum dual plane manifold design is its ability to essentially cut the primary side of the carb in half . This produces greater venturi action/twice the "pull" or volume demand in each primary bore since each piston only see's one barrel instead of both primary's. Now I will say this first. There are some engine builders and racers out there who will say that ANY amount of spacer added to a dual plane design ruins the torque building characteristics and defeats the entire purpose of running the design in the first place. They will try to sell you on running a single plane. While there is some level of truth to that, it comes in various degrees of overall effect depending on the camshaft and overall engine design. IOW's, if you have a stock cam with slightly too big a carb?.. DO NOT BREAK THAT PLENUM DIVIDER with an open spacer design. If you're looking to pump up the plenum volume for bigger CID or RPM?.. would be best to use a spacer that has the same exact divided plenum design, Edelbrock makes one for their manifolds. OTOH, if you have a bigger cam, only moderately sized carb, with shorter rear gear?.. you can easily trade smaller amounts of throttle response and lower rpm torque production for a fatter torque swell in the mid-range. You'll never turn the dual plane into a single plane from a plenum volume/rpm capability standpoint no matter how big a spacer you bolt on top but you can "tune" the power curve to some degree with different spacer designs and heights. So, I would say yes.. the engine should be re-tuned for best performance increases possible. Carb's are demand based flow devices and you will essentially change their metering characteristics via manifold design(dual/single). Generally speaking, when you kill the divided style plenum and share the airflow wealth amongst all barrels.. you'll need to advance the timing slightly and sometimes jet up 1 number or at least adjust the air bleed sizing to compensate. Ideally you would leave the mains alone and adjust the power valve restriction to supplement the additional fuel where needed under heavier throttle positions but most don't want to work that hard at carb tuning. BTW, the main reason that you'll see notched dividers in "rpm influenced" manifold designs is the fact that air/fuel distribution issues arise when pushing this style of manifold into the upper rpm ranges. The notch helps smooth out the cylinder to cylinder fueling variations. Using additional spacer height to compliment that same inherent limitation beyond a certain point will stifle the original dual plane design/performance intentions but will usually pay off in the uppermost rev ranges. Just keep it simple and use spacers to shape the power for your intended rpm range. There will be a slight trade off in every direction you go, so it's all about balancing out the pro's and con's to what works best for your combo of parts.
I put a one inch spacer on mine I have a holley 600 and came with 65 jets and I had to put 75 jets but I have 302 roller cam from 1989 mustang it's running great ..
