Tell me about my trucks cam

[ian75c10]

The PO of my 75 c10 says the cam in it is a comp cam 277/282, 470 lift at .050. Its a 1967 327 bored to 331. So is this a good cam for this engine? What are the pros / cons. I have no idea how to interpret cam specs. I am trying to make my truck as much a mileage friendly daily driver as I can get it. I currently changed to a new electric fuel pump ( carter 4070 ) and added a new regulator and gauge. The gauge says 5 when the reg is set to 3.5 so I don't know which one to trust. from the gas tank its > filter > pump > filter > regulator > gauge > carb. I have notice just after doing this my gas mileage is much more horrible. Can the timing be off? If anything I thought this would improve gas mileage. Would love some input from the pros and I am loving learning. thanks

[87V20Kansan]

The first thing to know about cams is the difference in advertised duration (the amount of time a valve is open, measured in degrees), and duration at .050". Back in the day cam manufacturers used to measure wherever they wanted to in the rotation of the cam core to come up with numbers for duration. This led to "x" brand's 270 cam not being really very close to "Y" brand's cam in terms of similarity. People got confused and poor or under performing engines were often a result. As the industry evolved everyone eventually adopted the idea of measuring duration by measuring at .050 inch valve opening to moment the valve closed. Comp still for some reason likes to run "advertised" duration numbers in cam specs but provides .050 inch numbers as well. Duration numbers for .050" will always be smaller by around 20-30 degrees because we don't get to count the degrees of rotation the cam turns to get the valve open to the .050" point.

Case in point is your cam. The closest I could find on Summit was this:
http://www.summitracing.com/parts/cca-12-523-5/overview/make/chevrolet

It's advertised is 270/282 degrees with .473 lift on a 106 LSA. It's actual .050" duration is 242/250 degrees. It's an older circle track grind designed to move the power band up in the rpm range but with relatively low lift numbers so the smaller carbs in circle track class racing would be able to keep up well enough. We were limited to 500 cfm two barrels in our roundy class back in the day.

In a 331 inch small block that cam would have enough duration and overlap to produce a pretty choppy idle, and like the specs say, should come on at about 2,500 rpm and top out around 6,000 rpm provided there is enough intake manifold and cylinder head to support it. In such a small displacement it might cause poor vacuum signal and could make your power brakes act up sometimes.

That same cam in something with more displacement, say a 406 inch SBC would tame down a little. It would still pull to around 5,800 rpm but the idle would smooth out some and vacuum signal would increase.

All of that said, if you like how it sounds, how it drives, and most importantly the brakes don't lose assist and make you hit things, then it should be fun to drive. There might be a few more ponies to be had with a good set of 1.6 roller rockers to free up a little friction and get the valves to open a little further.

But you want gas mileage. That isn't a cam for mileage. Something a lot tamer like the edelbrock performer would be better from a mileage stand point. It won't develop as much peak power but the torque specs will come up a bit. Meaning you would need less throttle input to get going from a dead stop. I don't know what you are getting right now but anything approaching 14-15mpg in a 2wd in these years with a carbed small block is doing right fair in my book. Heck my lifted 3/4ton 4wd on 37's gets about 9 and I think that is fine considering what it is! My advise is to buy a beater for mileage and drive the truck when you need a truck or need some fun!

[ian75c10]

Kansan thank you so much for the input. Very good knowledge here. I checked out that link from summit and I am curious if that is close to whats in my truck. The only thing that seems different to me is the idle in my truck is very smooth. I actually like a loppy cam idle sound but that is something I do not have. I love the idea of the edelbrock cam you mentioned with a bit better milage and low end torq. I am going to look into that. If you had a link to send me for one that would be awesome! Thanks again for the help and info.

[MANCHU1-9IN]

I'm far from being an expert, but that seems like WAY too much duration for a heavy street truck with a small cubed engine. Is the low end torque not there? In other words is it a dog off the line? The lower LSA would bring the torque curve down but still seams too big. I just don't think you will be revving high enough to take advantage of that cam, it would do a lot better with 400+ cubes.

Again though, I am no expert. This is just my observation over the years, reading about different set-ups so I'm probably wrong lol.

[ian75c10]

I actually feel like it does dog a bit more than it should with this cam. Its hard to tell with the ratio of the 700r4 and the 4.10 gears. But I can tell. I had a 72 chevy shortie van with a 350 and th350 trans and that thing had tons more low end torq than this truck. I am for sure going to look into changing the cam soon. What all goes into the time to do that for a local shop? I for some reason was thinking its a fairly quick process. Of corse will have to redial carb and reset timing I am sure. I also am very tempted to put in the 3.4 rearend I have as extra. I am wondering if its easier for my mechanics to just grab the gears from it and swap them into my current housing or if it cheaper and quicker for them to swap out the whole rearend? Thanks again for the info this is great stuff.

[rich weyand]

When you change the cam you need to change the lifters.  So the carb, intake manifold, valve covers, radiator, fan, shroud, water pump, etc, etc, all come out.

As for choosing a cam, read this entire thread:
http://forum.73-87chevytrucks.com/smforum/index.php?topic=26161.0

Also read this article:
http://www.crankshaftcoalition.com/wiki/How_to_choose_a_camshaft

My own recommendation for a 350 with stock heads and normal driving is Comp Cams 12-235-2.

[87V20Kansan]

Another option is to put a looser converter in it so the engine can get up in to the power sooner. You probably don't know what is in there currently, but my guess is it's stock or really close to it. At least 2000 rpm stall should wake that thing up very well. But that would negatively impact gas mileage. So play with what you have before you throw big dollars at it. Tune on that thing for a while.

Another thing about that cam is that it might like a little more initial timing than what is currently dialed in. When you have a cam with that much duration it wants to run in the higher rpm range. In that higher rpm range it takes a little more timing advance to make them run right. Case in point, the 406 in my '78 Malibu wagon. It has about 8:1 compression and a 232/242 at .050 with .488/510 lift dual pattern cam in it. That low compression number coupled with that lumpy cam means that engine likes 22 degrees of initial advance with a total advance of 42 degrees all in at about 2800 rpm. That is a ton of initial in an SBC! Car runs great at those numbers. With a 650 carb, OD, 2.73 gear, and a tight converter it will run 19 mpg.

Check your timing with no vacuum supplied to the advance on the distributor and add a couple degrees and see if it wakes up a bit. If the timing light says 8 BTDC, then make it 10BTDC. Then settle the carb down a bit and go drive it. If it was a positive result, do it again until you get the engine to ping. When it starts to ping, back it down a degree or two and enjoy! It would also help on the gas mileage issue too as the engine will not be as lazy out of the hole, thereby using less throttle angle.

What is the rest of the engine combo? Intake? heads? Carb?