350 with 305 Heads...Cam selection

[cburke]

Hello everybody, i am putting together a motor for a project truck(1988 chevy lwb 5 speed) with some SBC parts i have laying around. I have a stock bottom end 2 bolt main block with stock dished pistons, and a set of 601 heads(305 heads) that i have done some minor porting to, stock valve size. I am guessing the motor will be 10-1 compression maybe a tad more. I am undecided on which cam choice to go with and i need some help choosing one. I currently have this camshaft but i am worried it might be a tad too big(engine pro mc1993):

Duration @ .050": Intake: 230 Exhaust: 230
Advertised Duration: Intake: 287 Exhaust: 287
Valve Lift: Intake: .480 Exhaust: .480
Lobe Separation: Intake: 106 Exhaust: 110
Power Range: 2000-5000
Idle: Rough
Lifter Part #: 817

I have also been looking at the old crane 274H06 Grind (summit 1785) as an alternative,

Basic Operating RPM Range: 1,800-5,200

Intake Duration at 050 inch Lift: 218

Exhaust Duration at 050 inch Lift: 218

Duration at 050 inch Lift: 218 int./218 exh.

Advertised Intake Duration: 274

Advertised Exhaust Duration: 274

Advertised Duration: 274 int./274 exh.

Intake Valve Lift with Factory Rocker Arm Ratio: 0.450 in.

Exhaust Valve Lift with Factory Rocker Arm Ratio: 0.450 in.

Valve Lift with Factory Rocker Arm Ratio: 0.450 int./0.450 exh.

Lobe Separation (degrees): 106

Which cam would you choose? With the 305 heads i figured the truck will have tons of low end torque and the bigger cam will help it breathe up top. Or will i not notice a difference between the two? Thanks for the help

[Captainfab]

If you are keeping the TBI you will not be able to use either cam. You will need a EFI compatible cam.

[Marshy]

In general, I think those cams are too large for those hesds. Those heads stock do not flow really well. Who ported the heads and did you have them flow benched? I have a roller cam in my engine with 206/212 duration @0.050" lift and it has a strong mid range. Larger duration and you are trading low end for top end. If I were you, I would be looking for something in the upper 190 for in take duration and low 200 for exhaust duration. That will keep your dynamic compression high. If you are worried about too much compression you could get it ground with a low LSA to bleed off some of that dynamic compression and give you a more lumpy idle. I also wouldn't bother with too much lift, .450 is probably good. I was in your similar position a short while ago and decided to ditch the 305 heads and go with some Vortec heads. Even sold them I'm the parking lot of the junk yard for more than the core while I was going to pull the Vortec heads.

[cburke]

Thanks for the responses. The truck will not be staying efi, i will be converting it to carb/HEI distributor for simplicity.

[Tom]

230@.050 is much too big. The 305 heads will indeed give you the low end torque but no cam is going to make this thing breath good on the top end. The ports and valves are just too small to work with this big of an engine. They don't even work great on 305's. What gas ya running? 10:1 compression, iron heads with bad combustion chambers, and small cams all equal detonation.

[DMT]

Those heads would work just fine with the 230/230 cam. The extra duration would help bleed off some of the higher compression ratio you will have with the smaller chambers.

"The 601 305 heads respond very well to porting";

http://www.crankshaftcoalition.com/w...evy_305_engine

The above is a good resource for the different 305 heads.

For anyone thinking that cam is "to big" should know that the super stock guys that are lift limited use huge duration cams(much much larger than 230/230) to help a factory un-ported head fill the cylinders to launch one of those cars. Also @.480 lift that is still below the max lift of where those heads will flow. So saying the cam is "to big" is a misconception.

Better gearing will help out of the hole, mid range with those heads and that cam is where that combo will shine, should rev to and pull up to 5,800 RPM.

[Tom]

Originally Posted by DMT

For anyone thinking that cam is "to big" should know that the super stock guys that are lift limited use huge duration cams(much much larger than 230/230) to help a factory un-ported head fill the cylinders to launch one of those cars. Also @.480 lift that is still below the max lift of where those heads will flow. So saying the cam is "to big" is a misconception.

A bigger misconception is thinking that what works for class specific racers will work well on a daily driven street vehicle.

[Marshy]

I'm curious how he's getting 10:1... Seems a little high to me without knowing the specifics.

[DMT]

Originally Posted by Marshy

I'm curious how he's getting 10:1... Seems a little high to me without knowing the specifics.

The 305 heads have smaller chambers than 350 heads. for example your 350 Vortec heads have 64cc chambers while those 305 chambers are around 58cc making it easier for them to make more compression.

Using the 230/230 cam will help keep the engine out of detonation as well because it will have more overlap to bleed off some of the compression to allow the use of pump gas.

The super stock example was to show that more duration @ .050" helps flow more air even in an un-ported factory head. Besides when did a .480"/.480" lift, 230*/230* @ .050" duration become a "big cam" for 350 cid? The higher side of medium sized would be a bit more accurate I would say, but is WAY better matched with 10:1 compression with iron heads than a smaller one no matter what iron heads he runs, if he wants to run pump gas. A smaller cam(depending how small) could bring the dynamic compression to high for pump gas causing the need to run octane booster or mixing in race fuel with premium to stave of detonation.

At around 238* duration @ .050" with 10:1 compression is when a cam starts getting big with 350 cid requiring more gear to keep it from being a dog from a dead start.

Would running the 230/230 cam with the iron 305 heads be the optimum combo, I would say no. But running any cam smaller then that 230/230 is asking for detonation running pump gas.

Using the cam with the 218/218 @ /050" with 10:1 compression would need to have the timing below optimum to keep it out of detonation bringing the combo down on power, or run more octane.

If the OP is set on using the 305 heads then the cam choice is dictated by static/dynamic compression ratio and the type of fuel he wants to use, not so much the flow characteristics of the head getting bolted on in this situation. If he wants to run pump gas, the dynamic compression needs to be around 8.5:1 or lower with premium for the 8.5:1 to around 8:1, below that down to 7.5:1 a 50/50 mix of midgrade and premium, below that straight midgrade down to 7:1, below that regular 87.(IIRC, going by my CRS memory so I could be off slightly but should still be pretty close, for a more accurate recommendation Google is your friend)