hi i have some 1967 98cc bbc heads their instaled and i dont know the intake runner size. i did some looking around but no luck. also ive got a dual plain intake if i put an open carb spacer on it  will it perform better or worse. thanks jeff

Need some more information here, casting number of the cylinder heads will help but even volumes of the castings varied a lot as do obviously the volumes of the good and the bad ports.

Spacers are tuning aids and nothing more. A 1 inch open spacer on your dual plane might help pick up the top end slightly.

thanks for the info. the casting#s on the heads are 3904390 they were on a 396 when i got them they have had some porting done to. how much i dont know. thanks jeff  

no one here without knowing everything about your motor and transmission can tell you if it will perform better or worse.  go to the 1/4 race 2 or 3 rounds, throw on a spacer and run another 3 rounds.  average the times before and after.

you are not running a head that can reproduced by anyone because it has been ported.  All a spacer does is increase the intake runner size which leans out the car tuning.  leaner (to an extent) makes more power.  generally carbs run rich through the entire RPM range so when you add a spacer it leans it out through the entire range, getting it closer to 14.7:1 "perfect" ratio.

I added a spacer on my 305 (carb, intake, headers, gears, shift kit, ignition) and it cut off an average 0.15 seconds off my 1/4.

just go try it, they are cheap and all you would be wasting it a couple of gaskets.