What cam?

[Danny Pease]

What cam works best on a low compression motor (383, 8.7:1, stock heads, mild cam, single plane intake, 1 3/4 in headers). Hughes claims you cannot get good performance out of a high duration cam without higher cylinder pressure. I don't swallow that because the stock class engines are limited to factory lift specs and I a sure they run high duration and lots of overlap. What cams make the power for stock class engines?

Danny

[Christopher]

The stock class engines make up the difference with compression.If you were to take the average 383 4bbl engine,and blueprint it,the compression usually gets into the 11:1 range.

[Danny Pease]

Christopher, I looked up the NHRA blueprint specs for both of the engines I have, I can't get any more than 9.2:1 compression on either a 69 383 or a 72 400 (both 2bbl). I am sure the smog motors like Mike Cotten's 73 360 aren't any better. I agree the HP motors from the late 60's would have decent compression are blueprinting, but I don't see the newer engines making the horses with compression. I realize that most of the stockers run a lot of rpm (stall at 5000 and up) to make the engine build cylinder pressure with their stock legal cams, but what are the typical cam specs for the low compression stock motor?

Danny

[Christopher]

If you use the specs on the 69 383,you are right it's advertised at 9.2:1,but if you put the heads at 79.5cc's and deck the block to spec,you will get 11.1 compression ratio.The Lunati and Comp cams are the ones most of the stockers are using.I used a old old Cam Dynamics stocker cam in a 400 big block we used as a test mule engine a few years back and that thing made a bunch of power for a hydraulic cam.

[Danny Pease]

I must have my math screwed up then. I come up with a 1 cylinder displacement of 47.8786 cubice inches (4.25 bore, 3.375 stroke) that equals 784.59 cc. Combustion chamber is 79.5 cc and deck height of .075 plus gasket of .021 makes 101.817 cc. That makes a total of 886.407 cc. 886.407 minus 784.59 should equal 101.817. 886.407 divided by 101.817 equals 8.706:1 compression ratio. Where did I screw up my math?

Danny

[23T-Wedge]

Danny,

Your math is right, I think maybe you and Christopher are plugging in different numbers. For instance,the 68 RR 383 had +.020 deck height and advertised 10-1 CR. If your deck height is -.075 your CR would have to fall in the 8 1/2 range. Since you mentioned the NHRA specs, is this a class motor?? Being a 2 bbl. engine the compression height on your pistons (assuming stock cast flat-tops) is going to be less than an HP 383 piston. You would have to deck the block .095 just to get up to the 335 horse compression range. Anyway, all this talk about compression has kind of sidetracked us from your original post about cams which I probably am not the best one to talk to about, since I tend to be a little conservative on cam choices. Without knowing what the usage is I really don't know what to suggest. Looking back at your first post, keep in mind that the rpm levels on an average v-8 class car probably don't fall below 5000 on a run so you could conceivably have a long duration cammed, lower compression engine that still made good horsepower although the power and torque band would be fairly narrow. Like Hughes said, it's hard to get good performance without cylinder pressure. I think what they are describing would be more like a good bracket or pro-street type cam that had a broad torque band that was real drivable. This is a good complex question, with a lot of possible answers. Wish I could come up with a better one.

[Danny Pease]

Thanks for the reply, 23T. I am not going to run a class motor but am trying to make a stock short block a little stronger with a cam change. I am looking for something that will be compatible with my old 383 2bbl short block. I also tend to be a little conservative on camshaft selection since I don't like turning any more rpm than really needed on any engine, must less a stock short block. I guess what I am asking is how much duration is too much? I can change converter but would rather not mess with any short block changes on this old motor (would rather spend engine money on new motor I am planning) but I don't mind spending money on drivetrain that I will still be able to use with the new motor. My plan has been to build the chassis and driveline up to what I want and once it is sorted out, put a better engine in. I also want to be able to bracket race it several times a season, while I am doing this. The car is a 72 E-body that weighs over 3700 lbs with driver, 4.30 gear, and an 11 in converter that stalls about 2500. Currently I use BF Goodrich drag radials that measure 25.5 in high but I have a pair of old 10 by 29.5 slicks that are too much for the current combo. I am shifting at 5600 and runs a little over 6000 at the end of the 1/4. I don't really want to turn the engine much tighter but am looking for a little more midrange. The engine is stock 2 bbl. short block (around 8.7:1 compression), stock 346 heads, Torquer intake, Carter 750 and 1 3/4 in headers through 3 in turbo mufflers. The current camshaft is of unknown origin with .445 measured lift at the valve (no idea of duration). I am sure it is not the original camshaft but previous owner does not remember the specs (he thinks it is a Crane Energizer). The car currently runs around 14.00 at 98 mph. I am fishing ideas about the best cam for this low budget setup.
Thanks for all the input.

Danny

[23T-Wedge]

Danny,
You might look at something like the Lunati Bracketmaster, the rpm range they show is in line with what you what to run. 292 duration is probably a little much for your compression ratio but you can cover a lot of sins running 4.30 gears and a 2500 convertor so you could probably live with it. The .480 lift is real compatible with the stock non adjustables and pushrods and stock springs should be adequate as long as they're in good shape. With that much duration you could advance the cam 2-4 degrees to regain some cylinder pressure, probably leaning toward 4 degrees. If you were running closer to .000 deck height you would need to pay closer attention to valve to piston clearance the more duration and/or cam advance you ran but since you're .075 down you shouldn't have a problem. I'm a little hesitant to recommend something I haven't used although I used a Crane hydraulic with almost identical specs (292 duration-.485" lift)in my 68 RR years ago and was really pleased with it. I had pretty much the same pieces you describe for your engine except for the difference in compression. I'm sure there are a lot of other cams that would work as well or better, I just try to look at the whole combination. Good combinations are a whole lot more fun to play with than bad combinations!!!
...Good luck

[PRO]

Danny if your present cam pulls to 6 grand then I dont think you really want more cam based on your current compression,here more probably wont be better but you can do this take a cranking compression test and if its around 105-110 psi that is about as low as you can go if you get under 100psi then your too low for any kind of performance motor if it is considerably higher then pull the cam and ID it so you can compare duration #s,a good rule of thumb is every added degree of duration will drop your cranking comp 1.25 degrees.....PRO..

[Danny Pease]

Thanks, guys. The cranking cylinder pressure is 115 to 120 psi. I have another question. I already have a set of 1.6 ratio roller rockers for my planned motor. Will they help enough to justify using them on the current motor? I will have to buy pushrods but that is all I need to install them. Current valve lift is .445, will go to .475.

[23T-Wedge]

Danny,

The 1.6 rocker idea has some possibilities, you gain a little lift and pick up a little valve acceleration which should improve your mid-range which is what your were trying to get, plus you won't lose any cylinder pressure like you would with a longer duration cam. 1.6's don't change duration like I've read in some posts on here, but they do open the valve faster. Adjustable pushrods are cheaper than a cam and lifter kit and you have to buy them eventually anyway if you upgrade to a serious hydraulic or solid. I doubt that you would see a major improvement but I don't think it would hurt you either. Since you've already got the 1.6's the idea deserves some thought.....