Can someone please explain to me what would happen (theoretically) if, say, you had a cam with quite a bit of lift and descent duration, but didnt have the heads to back it up? My thought would be that the engine would 'choke'. If the cam is trying to pull in lots of air/fuel but the heads cant keep up, would the engine not run like crap?
I mean, of course its not gonna run rightously, thats why everyone gets aftermarket/ported heads right? What I'm trying to ask is, will it HINDER performance rather than increase it? Like a decrease in performance over a smaller cam that could potentially 'keep up' with, or were more of a match for the heads?

I've just been kind of wondering about this.

The reason I ask is, I installed my passed-on 305 cam, which was hefty for the 305 even, on to my 350 TBI Caprice (193 swirl heads). I plan on replacing these heads sometime when I get money, but I'd like to get the car running as well as I can for now, with what I have. Seems like the car is 'gasping' for air when I get on it. Dont bog or anything, I have everything on the carb set about right I believe (cc QJet, B hangar, CK rods). Plenty of potential air flow through the cleaner/air box. Exhaust is all 3", no cat, with headers. The car will rev fine from a stop to floored, it just wont pull real hard. Throttle response is good. It's only when you floor it, such as when your cruising on the freeway and floor it. I can floor it going 65, it'll downkick instantly and rev to 4K no problem. Timing is good (~10*-11*). Car will spin the tires straight off the line descently. Like I said, its just from higher up RPMS that it wont pull.

Sorry for the long post, just trying to get some details in there so someone can help me out. Can anyone explain to me whether my idea is right or wrong here? And if not correct, why?

Thanks
-Ben

 

Anyone?

 

do you know the specs on the cam ?

 

sig

 

That cam should work fine in your setup...- the heads are poor performers.....you may also need to upgrade your ingnition system... I think its the heads that are holding you back... Just save some $$$ and dont expect the impossible...invest in a good pair of performance heads....thats where its at.

 

I'm being very insistent on getting this running the complete best I can with what I have. So far I've gone through and replaced everything in the ignition (ignition coil, ignition module, wires, plugs - and gapped at .045", Pick-up coil, cap, rotor button, as well as check all wiring). Everything in the ignition is up to par. Timing like I said is at 10-11 degrees. I've set my carb idle mixture screws at 3 or 4 turns out, although I can't get a steady duty cycle reading on my multimeter of the idle air bleed screw. I've tried many combos, and it still jumps all around the place. I may post a new thread on that as I dont want to go into it. But that should only affect idle and off slightly off idle performance, correct? O2 sensor is new as of this winter and I can get varying readings between 0 and 1 volt in closed loop. I have the rich stop screw all the way up, lean stop screw all the way down (temporarily of course) untill I get the IAS set correctly. I dont think the MCS will wear THAT fast. So anyways, everything is up to par as far as I'm concerned.

Perhaps I was expecting too much, straight 'out of the box' ? I dont know.

I have a set of 081's (58cc) ported/polished that I'm thinking of having 1.94/1.6 valves installed, I think it would be a pretty drastic upgrade over the 193's but no one could give me a real direct answer on that in my thread covering those head differences. I guess I'll just have to see...

 

Does the ecm still have anything to do with your timing curve?
I have heard that if you have your base timing to high the motor will start to ping and then the ecm will pull out craploads of timing, turing it into a dog.

 

I still have the ccQJet and distributor. I had the timing just high enough where it would audibly predetonate, but I pulled it back just enough as to where it wouldnt. But I've heard that the knock sensor can pick up inaudible detonation before it gets audible, perhaps it did some. I'll back the timing off a little and reset the ECM and see if that helps.

Thanks.

 

The heads are garbage. 305=boat anchor

 

I dont have a 305, its a 350. Are you talking about the 193's being garbage? You may be talking about the 081 (305) heads?? These (I hear) are good heads for porting, even for a 350, with larger valves. I dont have money for an aftermarket set right now, so I want to use the best of what I have. If you're talking about the 305 cam that was passed on, it wasnt the original, its the cam in my sig.

-Ben

 

I would go with the 081's..they can be worked to perform well on a 350....with that much lift on that cam..I would look into better springs and screw-in studs..while you are at it...that may be some of the problem you are having now....thats really pushing things on stock heads...
A lot of people use 305 heads on 350's..good way to bump up the compression cheap...

 

The issue with matching a cam to heads, besides whether the hardware on the heads will fit with however much lift the cam produces, has to do with the RPM range they each optimize the motor for.

If you have swirlie "peanut port" heads, they simply won't flow enough air to support any significant power above 5000 RPMs or so. If you put too big of a cam in the motor, it will try to raise the engine's optimum operating range, at the expense of lower-RPM torque. So what you'll end up with is a motor that has no bottom end because of the cam, and no top end because of the heads. In other words, a slug.

Match the head flow to the engine's requirements at the RPMs you intend to operate it at; and pick a cam that goes along with that same flow-based RPM. It's real easy to put too much cam in a motor, since they all cost about the same for a given type (roller, flat-tappet, street, steel billet, etc.).

"There's no such thing as too much cam, there's only too little motor".

 

305 heads are among the very best GM heads to port and polish. We have dozens of people on this forum who've done it and will testify to their great flow characteristics.

I'm one of them.

 

Thanks guys. RB, that clears a lot up and I understand the whole idea a little better.

Riley, I already had the studs pinned for the larger lift (it supposedly works quite well and is a lot easier on my wallet). I intend to get the right springs to go along with the bigger lift. If I do it I'm gonna have them right. I know its pushing it right now with the lift on the stock 193's but comp claims this cam is compatable with all stock 305 and 350 heads, or at least for my 87 305 roller block, and newer. I checked for spring binding, and there was none, but its close...possibly a little too close. Oh well, these heads are gone in a little while. It's a shame I had already had the 081's done up with a nice valve job, if I'm gonna go with larger valves.

Oh yeah, I think this this cam was for off idle to 5000 or so rpms, so I thought it may work well with what I had.

Two questions.
1) can anyone tell me if the heads would need machining for larger springs for the lift, or can there be springs bought that are the same width/hight that will accept a larger lift, assuming it would have the correct seat pressure and rate.
2) supposing these 081's had a good bowl blend and mild porting done with 1.94/1.6 valves, correct springs, cam I have now, stock aluminum dual plane intake (which I think is a descent intake), 4bbl, headers, would it be wishful thinking to be expecting anything near 280-290hp at the flywheel with this combo? Thats kind of what I was expecting.

Thanks again for all the good responses.
-Ben

 

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