hey guys im pretty new to the gasoline motorsi have always had a diesel so if you hear me say something that deosnt sound right check me on it im eagar to learn as much as i can.......I resently bought a 1985 z28 the car is lots of fun i love learning on it but i need some help....the car has a 305 with 350 vortec heads a summit racing manifold and a 450 input 450 out put cam and a holly 600 carb....i was wanting to build the car up a little more without hurting in it has the origainal 5 speed tranny and the same rearend that it was born with.. Right now im thinking about taking the 600 off and putting a holly 750 dubble pumper on it ......... would that help me out much? Would the 350 vortec heads flow enough air? im not rich or anything but this car is my new hobbie and i have a good job and not much to spend money on so im really excited to get going on this im just not sure what to do any ideals or tips would be greatly appericiated

 

I hope you shaved the 350 heads down, that is a big compression drop on a 305. Do you have a vacuum gauge? That is the quickest and easiest way to tell if you need a bigger carb. Hook it up, go do a WOT pass and if you pull vacuum when you hit redline or your shift then you know your carb is too small. I'm willing to bet no in this case. I could see a 750 on it being soggy.

 

ruben, welcome aboard thirdgen.org.

Think of an engine as an air pump. How much it can pump is dependent upon displacement and RPMs. The basic formula for carb size is CFM = CID X RPM/3456, where CID = cubic inch displacement and RPM is the max the engine will be turning (determined by the cam, mostly). The also assumes 100% volumetric efficiency, which is pretty uncommon for a street engine. For a 305, and I'll assume 5500 RPMs max (we would need more details on your cam such as advertised duration and lobe separation angle) that formula comes out to 485 CFM - you already have plenty of carb. All a 750 would do is make throttle response sluggish, and probably cause bog going to wide open throttle. The Vortec heads would flow plenty of air, but that isn't what is limiting the capacity of your 305 - it's the fact that it's a 305 that is limiting it.

By the way, I'd want to make sure those really are "Vortec" heads. On the Summit intake manifold, are there two bolts goings straight up & down on each corner of the manifold, or are they at an angle with bolts toward the center as well? It's pretty common for people to say heads with center bolt valve covers are "Vortec" (which Vortec are), but as often as not they are cheap, poor-flowing "swirl-port" heads.

 

drac
i do not have a vacum gauge when they put this motor in the took every thing out its just a motor under the hood the is hardly any thing elcetric no hearter ac any of that suff im thinking its just what the motor needs to run......where could i hook a gauge up at? i do not know if they shaved the heads for i bought the car just like it is all i know is what the fella i bought it from told me.... i think that he just fliped the car for he didnt reall give me much info just what i told yall would the 750 be too much due to is just dumping a lot of fuel and the 305 not being able to burn it properly because lack of air fire and chabmer space? would boring it out be the anser to this? i may be way off like i said im learning the basics...... a freinds opinion was to put the bigger carb and go in with eddlebroc heads... but im not so sure if he knows a little if any more than i do.... i could have a good set up for what i got im just not sure...
five seven kid


the bolts on the manifold are angled
here are my cam specs off of the paper ill give then to you im not sure what most of it means but i would like your opinon....

comp cams
Engine : cs 268h-10
part# 12-210-2
Grind number: cs 268h-10
intake exhaust
Valve dajustment hyd hyd
gross valve lift .454 .450
duration @
.006 tappet lift 268 268
valve timing open close
@ .006 int:28 btdc 60 abdc
exh:68 bbdc 20 atdc
These specs are for cam installed
@106.0 intake center line
intake exhaust
duration@ .050 218 218
lobe lift .3020 .3020
lobe seperation 110.0


981-16 Springs required
Valve spring specs furnished
with springs



This is what it reads what do yall think?