Bigger Cam/Springs?

[JGibson]

My son wants to replace the stock cam in our truck with a bigger one to get a little better sound out of it. Question is, what size should we go to and do we Have to replace the valve springs if we go bigger? I'm fairly new to building engines other than going back stock...I don't want to go to big, just enough to get the "lopey" sound.

1985 305

[rich weyand]

One thing you can do is download the Comp Cams dyno simulator here.  You can input your engine info and other details of your setup, and try different cams.
http://www.compcams.com/downloads/register.asp

One thing to watch: a "lope" at idle means the engine is tuned for horsepower at the high end.  On a heavy vehicle like our trucks, that kind of tune can give you really anemic torque numbers down low, which means your "hole shot" will suffer.  So you have to be careful, and look at the torque curve.  A big giveaway is if the cam spec recommends a high RPM stall converter.  That's to get the engine into it's torque curve before the tranny gets power.  You would have to replace the stock torque converter as well, then, and it's harder on the transmission.

Given what you want, the Comp software flags the 12-256-4 as your best fit in the "Excellent In Daily-Driver, Slight-Mild To Choppy Idle" category.  Looks like 212 hp @4500 and 285 ftlbs @3500.  1200 to 5200 rpm range and 1400 to 2400 rpm torque converter, so you could keep the stock torque converter.

Note that I have never used that cam or had a 305, I'm just trying to help out getting you started, so I would suggest you google that cam number and "305" to see what other people say about it.  If they don't like it, they will probably also recommend something else.  Best advice will always be from someone with the same goals who likes his cam, whatever it is.

Whether you should change the springs or not depends on the mileage you have on that engine.  Any cam you choose is probably going to have higher lift than stock, which puts more demand on the valve springs.  If you have more than 5,000-10,000 miles on that engine, I would change out the springs as well.  And A-L-W-A-Y-S replace lifters when replacing a camshaft.  The cam and lifters bed into each other in the very first hours of operation.  If you don't replace the lifters, even ones with low mileage on them, they'll eat your new cam.

[JGibson]

Cool thank you very much. I'll give the program a go and see what I come up with.

[nlauffer]

Another thing to remember is that when people and companies talk about a cam, it is very rarely in conjunction with the 305. A SBC cam is most of the time referenced to a 350. The same cam will act like a much bigger cam when used in a 305 compared to 350.   Like the cam I am using, comp 12-238-2, is a good bottom end cam, but would act like a little bigger cam in the 305.   I didn't look at the specs of that other cam, but you should compare the two on the simulator and you'll be able to see which direction you want to head. What I mean is the "correct" cam for a 350 might be a 268 but on a 305 might be a 256 and on a 400 it might be a 280. The is all just comparative and not actual suggestions.
Also, a 305 engine with stock heads really isn't a power producer but does well in the low end torque. So don't go for a cam in the higher RPM ranges cause you'll lose the low end where 305s do there best. Just my opinion.


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[nlauffer]

I just looked that cam up. If you want a good cam. Use the one Rich recommended. If you or your son just want the sound, go with the Thumpr. But with the Thumpr beware. I have heard of poor performance and the need for a converter.


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[frotosride]

Not to undermine Rich but Even if the engine has had a fresh build on it you will want to go ahead and replace the valve springs for the ones the cam manufacture recommends for the cam you choose. This is simply due to the seat and lift pressures of individual springs and the higher lift combined with the speed of the lobes ramp style. If the current springs do not meet the designed pressures for the cam select you can easily run into valve float, compression and combustion pressure loss due to valve train instability. All of these things can occur at different rpm's and are not limited to high rpm engines as most would like to think. Just a little food for thought. plan on changing the springs.

[frotosride]

I just looked that cam up. If you want a good cam. Use the one Rich recommended. If you or your son just want the sound, go with the Thumpr. But with the Thumpr beware. I have heard of poor performance and the need for a converter.


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Keep in mind also with the thumper cams that the Lobe Separation Angle (LSA) is what gives these cams their names. They typically have a smaller LSA. Why this is important is a lower LSA means more overlap on intake/exhaust valve timing events leaving them bothe open at the same time. This will bleed off cylinder pressures at lower rpms as well as affect vacuum which can compromise the brake boosters ability to do it's job.

[nlauffer]

The Thumpr cam in my opinion was made for all the people who wanted "the sound" out of a cam swap. I was one of those people until I got researching and talking with people on this sight. Sound and performance don't go hand in hand unless you have a manual transmission, light car, and/or are racing. It's pretty crazy once you get looking into all the extra work and parts you need to get the sound. But it can be had if you want it.

I'm still trying to figure out how to get this swapped into mine.


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[rich weyand]

Not to undermine Rich but Even if the engine has had a fresh build on it you will want to go ahead and replace the valve springs for the ones the cam manufacture recommends for the cam you choose. This is simply due to the seat and lift pressures of individual springs and the higher lift combined with the speed of the lobes ramp style. If the current springs do not meet the designed pressures for the cam select you can easily run into valve float, compression and combustion pressure loss due to valve train instability. All of these things can occur at different rpm's and are not limited to high rpm engines as most would like to think. Just a little food for thought. plan on changing the springs.

Yeah, I agree.  I replaced the cam with less than 5000 miles on a new engine, and the lift was the same, so I didn't bother.  But going from a stock lift to a higher lift, you'd be better off changing them while you're in there.