rv cam questions

[81sask]

So looking pull my 76 engine and am wanting to put an rv cam into it. This will be my first engine pull and first time peeking am engine up. I'm completely new to buying a cam, and not sure what I am looking for. Talking to people you hear about rv cams all the time bit looking at my summit book there's obviously no"rv" can options. What exactly is an rv cam and what should I be looking at getting?

 

[rich weyand]

RV cam, sometimes called a marine cam. Lots of torque in the bottom of the range, may not wind as tight as a normal cam.

Install the Comp Cams 12-300-4. Put in new springs; the stock springs will work if they are new. Put in new lifters; you MUST put new lifters in with a new cam. They bed in within the first ten miles or so. If you run a new cam on old lifters, you will ruin the cam.

This is the cam I have in my truck. With a dual-plane manifold and headers, you are looking at 275 hp and 425 ftlb of torque. That's a lot of torque, and it comes on early. The horsepower is limited mostly by the fact that it runs out of steam about 4600 rpm.

But torque is what you drive, horsepower is what you race. For the street, torque all the way.

The 12-300-4 is called a marine cam, a.k.a. an RV cam. I simulated cams on the stock low-compression engine until I found the highest torque curve at the lowest RPM, and that's it. It's a stone gas to drive.

I've written a lot about my cam and engine experiences in various forums. This will get you started: https://www.google.com/#q=weyand+12-300-4

 

[rich weyand]

BTW, if you get that cam, and you have 3.73:1 gears, you will need at least a 31x10.5 tire in the back to hook up. With 29" diameter tires, you are going to need something like a 275 or 285 tire, like a 285/60R15, unless you like lighting them up on dry pavement.

 

[81sask]

Right now there are 3.08 gears in er and 33s for tires

 

[rich weyand]

Ouch. That's a lot of tire diameter for 3.08s.

(29/33) * 3.08 = 2.70 effective gear ration. That is, it would be like factory 2.7 gears on factory 29s.

 

[MikeB]

Yeah, definitely use a small cam with that axle ratio and tire height. Something around 194/204 duration @ .050" tappet lift would be about the max. Here's an inexpensive cam & lifter set that would work: https://www.summitracing.com/parts/sum-k1101

Stock springs are OK with this cam, but if your engine has high mileage, I'd spring for new ones, as Rich suggested.

And it's very important to lube the cam lobes and lifter faces with moly lube, and then use a good break-in oil like Driven BR30, or at least a break-in additive like CompCams P/N 159.

 

[rich weyand]

The Summit SUM-1100 is torquier than the SUM-1101.
https://www.summitracing.com/parts/sum-1100

Both of those Summit cams have very slow ramps. The difference between full duration and .050 duration is 76 degrees. What you get with the Comp 12-300-4 is faster ramps, of 48 degrees. Faster ramps means more time either open or shut, and less time in transition. That means more compression/power-stroke time, and more breathing time.

Yes, the Summit cams are cheaper, but you only get what you pay for, and sometimes less.

 

[MikeB]

Rich, we know Comp specs their advertised duration at .006" tappet rise, but who knows what Summit uses? Crane uses .004"; GM some crazy low lift like .001". That's why you might see a fairly mild GM cam with 288/278 duration, or even higher.

I think the Summit 194/204 cam is made by one company and sold under many brands -- and many specs! I want to say it's Sealed Power that shows it as 246/256 advertised.

However, I agree that it's an older design, and Comp's new stuff does have more aggressive lobes. I've always thought their 246PE would work well in a truck with a 3.08 axle. But probably not with 33" tires!
http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=69&sb=2