The Stovebolt Site Forums The Shop Area Engine Shop cams that can lug?

Hey guys,
I'm in the process of re-building the 235 engine for my napco suburban and am driving myself mad trying to choose a cam.
The three choices I'm looking at are:

Stock 235 cam: 168/170 int/exhaust @ .050"
My experience driving around with this was good... with my big tires, tall 3.38 rear end, and really wide gaps between ratios in the sm420 I am forced to lug the engine in a lot of situations (ie. when shifting from 2nd to 3rd especially uphill, or just going up short hills in 4th). Even down near 1000 RPM this cam has no problem with wide open throttle.

Stock 261 cam: 184/189 int/exhaust @ .050"
Tempted to buy this one, hoping to unlock more power in my ~2100rpm highway cruise mode
But will I loose torque/drivability in the 12-1500 rpm range?


Bulldog cam: 212/218 int/exhaust @ .050:
Am I just getting greedy here? Is this going to trade all my lugging ability for crank pounding up above 3000rpm?

What cams do you guys run, what type of driving, how well do they lug? In my opinion the 235 sounds like it wants to explode by the time you get to 3500rpm, what do you guys think?

The Bulldog will be too much- - - -a definite loss of torque in search of a little more high RPM horsepower. Since the 261/Corvette cam was originally intended for use in medium-duty trucks, torque is what it's all about. It will deliver better low end to midrange torque than the 235 cam while also giving you just a bit more high RPM horsepower. Getting any any more radical on duration and lift is going to start eating away at the lower end numbers.

The Elgin Industries cam is a clone of the original 261, with the only difference I can see is a slight variation on lash adjustment numbers. The one we put into John's 261 gave a noticeable improvement over the original cam, (also a 261) but that one had a lobe going flat.

A well-built 235 will run all day at 3000+ RPM, but bring a set of earplugs- - - -it's going to be noisy. I believe the factory HP was rated at around 3600. Getting much above that RPM is a fool's mission- - - -HP starts to drop off pretty radically unless you're running a full race cam and you've done a lot of porting and exhaust work. That kind of modifying really kills the low end performance- - - -wide open throttle all the time for a mediocre gain in HP. Talking about getting above 5K RPM with a stovebolt is territory where we're going to start wondering what you're smoking for even claiming that's going to happen. They're a lot happier in the 2500-2800 range and gas mileage will be better, also.
Jerry

Thanks for the input! I will have to search for Elgin Industries... the one I saw was a 'speed pro' sealed power brand.

I don't remember the Elgin part number offhand, but they list it as a "290 degree" duration. That's incorrect, as they're including the clearance ramps in the advertised duration. Actual "valve open" time is around 260 degrees or so if I'm remembering the degree job I did on John's cam correctly.
Jerry

3.38 rear end? I understand why you are having trouble with lugging under many situations which you mentioned.
Weren't the original rears 3.90?
The engineers did that for a reason. Napco trucks are heavy, I would guess that the suburbans are even heavier. In order to move that much weight out of its own way with a 235, you need 3.90 gears.
You mention "big tires". How tall? Taller than stock will add to your problems, compared to stock height.
I do not believe that any cam will make your 3.38 geared truck, with a 4 speed do what you want.
Carl

Yup, 3.9 was original. I have those on the shelf. These other gears are admittedly a bit of an experiment... I was amazed when i found the bearing kit and gears (for GM 55T axle) on justdifferentials.
Not sure about the tires, but I have 32inchers on now.
So far I've found the lugging shifts to be 'tolerable' with the stock cam, but certainly wouldn't want to sacrifice any low end torque from my cam.
The gears & tires really shine on the highway though... I can cruise 65mph with the rest of traffic on the highway & get ~15mpg. Not bad for an ol suburban

Yes, I thought the 3.38 might have been a typo. Cool find.
Tires shorter than your 32 inch ones will help reduce the lugging problem. Taller ones will only make it worse.
Selecting shorter tires will probably have more of an impact than cam selection. Combining shorter tires with a cam which gives you maximum torque in the RPM range which you are having lugging issues would be where your focus should be. With the 3.38 gears, I am guessing that the lugging starts at 2,000-2,500 range?
Like they say, "no such thing as free lunch." You can't have awesome highway speeds AND not have lugging issues.

Right on.
Here's my rpm vs speed vs gear spreadsheet in case you're interested:
rpm vs speed vs gear
Download button should show up in the upper right corner.

Lugging is perceived below 1500rpm when stomping on it. Usually when accelerating and shifting from 2nd to 3rd... I don't actually have a tach, so this is all estimated from the spreadsheet, but I'll accelerate just over 20mph (3000rpm) in 2nd, and then hold the clutch in for 2 or 3 seconds while that 30lb flywheel is slows down enough to shift without grinding. By the time I'm in 3rd i've slowed slightly & the speed works out to 12-1300rpm. This is where I lug my way up to ~25mph (~1500rpm) before it sounds happy again. Not an issue on flat stuff, but annoying going up hills. Hence my interest in cams that increase low end power below 1500rpm, if I had different final gears (or tiny lowrider tires) maybe the bulldog would be more feasible.

In 4th gear I can go slow as 35mph ~1300rpm as long as I don't mash the gas

Thanks for the replies!
Logan

I believe the camshaft Jerry is talking about is "Elgin Camshaft E-293-S for Chevy Straight 6 235 1942-63 Mechanical".
Todd

I'm assuming you're running a SM-420 4 speed, right? If the gears are grinding on an upshift, you need a new set of synchronizers. There's absolutely no reason to delay that long between gears other than a downshift to the non-synchronized 1st. gear which must be done from a full stop. I've never heard of taking several seconds to upshift a properly-working SM 420. Fix the transmission and you'll have a lot more luck avoiding lugging down even with that ridiculously high-geared rear end. Did you also change the gears in the front drive axle?
Jerry

Hotrod Lincoln,
That's a good point... another good reason to stop procrastinating the transmission work (or to get better at double clutching). Yup, it's the 4 speed with synchros except on 1st.
Surprisingly the the same gears worked for the front an the back axle... at first I was worried about backwards driving the front ring/pinion, but that's how the original set was too! The 3rd members are in fact interchangeable.

Todd,
Thanks for the part #, I'll check it out.

I have run the 261 cam in both the 235 and 261 engines, both instances I was happy with the idle quality which was set at 475rpm. The cam has good low end grunt and mid range torque too, a happy balance if you ask me for a truck. After putting a couple hundred miles on the 261, I'm pretty much in love with everything about it, best inline I have owned.
Steve

Sometimes there's a simple fix for gear grinding with a SM-420. Some folks think "newer is better" when it comes to lubricant and they use super-slippery synthetic gear lube like 85W-140 instead of the specified 90 weight mineral oil. That prevents the synchronizers from working properly to put the brakes on those heavy gears during shifting. If you're using synthetic lube, drain it, flush the trans by idling for a for a few minutes with a couple of quarts of Diesel fuel in it, then drain and refill it with 90 weight. You might be surprised at how much better it shifts.
Jerry

Jerry is right, last year I was experimenting with gear oils in the sm420s and found the same thing, the 90wt mineral oil seems to work the best if you are having issues with either the 3rd or clutch gear synchonizers

I'm running redline MT-90 which is supposedly good for sychros... but perhaps I'll try out mineral oil.
The other synchros except for 3rd seem to work great, I have a feeling that 3rd was the most useful gear throughout it's life.

That MT-90 should work as well as mineral oil- - - -the difference in mineral oil and "multi-purpose" gear lube is the extreme pressure additives that are needed for hypoid rear end gears that's in the multi purpose lube. "MT" probably stands for "Manual Transmission" which is just a fancy way of suggesting it's straight mineral oil. Sometimes just letting the clutch pedal out momentarily while in neutral on an upshift will take some of the strain off a weak synchronizer until you get a chance to dig into the box.
Jerry