I'm looking for advise from anyone familiar with running a Comp Cams Magnum 286H in a small block.
I'm currently running their XE268H, based on the raves I'd heard about this cam, but have since heard that it's not recommended for higher compression ratios. I'm running about 10.3 to 10.4:1 with iron heads (Vortecs), tight quench, and all the sharp edges broken to reduce detonation, and, though I've not heard any detonation, or seen any indication on the plugs that it's a problem, my engine seems to nose over above 5500 rpm or so, and 1) I'd like it not too!, and 2) I'm concerned that I may be having some detonation that I'm not otherwise seeing that's contributing to the performance loss.
I'd like to have it run stronger up to, or even beyond 6000 rpm, and if I lose a little bottom end, that's not such a huge problem. The car has a vacuum cannister, so that shouldn't be a huge issue either. But, even though I want to maximize performance, it's still primarily a street car, and I don't want something that's absolutely MISERABLE around town. Comp's recommendation was the 286H (got this same advice from a couple different customer service techs), but I'd like some input from people that might be running it on the street in their own cars. I'd probably also run the Pro Magnum lifters with it.
P.S. I've run some Dragstrip Plus comparisons, and they look favorable (higher peak hp, and a broader powerband, too), but I don't trust their rather limited head selection options. I don't know if I've got them modeled accurately or not, since it doesn't allow me to enter flow numbers. They're a lot better than '70's "smog heads", ported or not, and they flow, unported, as well as most aftermarket heads, 'til you start getting up into the higher volume runner heads (AFR 195's, etc.) I really distrust this aspect of the program, which they state themselves as being the "most critical" modeling criteria.
So I don't know if the 286 looks good 'cause I've overstated the capabilities of the heads, or if it would really work that well. I'm getting somewhere in the neighbor hood of 450 hp.
Nov 5th, 01, 10:30 AM
thats a tough call, you may want to drop it down to the 280h magnum. it has a little less duration, so it should be better on mid range torque/hp.
Nov 6th, 01, 08:18 AM
That's kind of what I'm going back and forth between - the recommended 286, or the safer and more streetable 280. And wondering if it's really the cam that has it feeling like it's nosing over above 5500, or something else.
Nov 6th, 01, 11:59 AM
I'm running the xe274 in a 355 and the chassis dyno shows peak hp just below 5500. Mine also stops pulling between 5500-6000. I shift at 5700 and that seems to work really well. I've got 76cc iron heads with stock type pistons (flat top with valve reliefs), my combo would probably run slightly better with the xe268. The xe274 has a pretty rought idle, but makes a ton of torque. If it's a steet/strip car I'd go with the xe274 based upon the rest of your engine combo.
Nov 7th, 01, 10:34 AM
Are you sure you do not have an ignition or fuel delivery problem? My engine is much milder than yours and it still revs pretty happily to about 6200. Although with the compression you are running you could do very well with a slightly larger cam. Personally I would go with a dual pattern cam since the intake/exhaust relationship of the Vortec heads is not the best. Just my .02 cents.
<BLOCKQUOTE>quote:</font><HR>Originally posted by Huck:
That's kind of what I'm going back and forth between - the recommended 286, or the safer and more streetable 280. And wondering if it's really the cam that has it feeling like it's nosing over above 5500, or something else.<HR></BLOCKQUOTE>
Nov 7th, 01, 10:42 AM
Could you be running out of air? How big is your carb?
1970 350 four bolt, .060, Speed pro hyper pistons, stock crank and rods, '77 vette GM manifold, edelbrock 600cfm, comp magnum 280H cam, '993' 75cc heads, comp springs, manley valves 1.96"-- In 1972 Nova, aka Camaro in grocery getter's clothing.
Nov 7th, 01, 11:24 AM
Jimmybyrd - don't think I'm running out of air - I'm running a 750cfm Performer and a Performer RPM manifold.
Rubeng442 - not as certain about the fuel supply. I'm running the stock 5/16 fuel lines and high pressure in-tank pump, regulated to 6-7 psi for the carb. It's not that it won't rev there, but it does seem to be losing steam. Is there any good way to check this?
Of course I am running a solid flat tappet cam, but my little 282s pulls very hard up to 6300 RPM. 236* duration on a solid cam is pretty small by most standards BTW. Lots of vacuum at idle, 14 - 15" in park 11 - 12" in drive.
My point - your XE268 should pull hard in a 350 to about 5500 RPM's as you state. I'd also look at a little bigger cam, but that 286H might be a little on the big side IMO, maybe an XE274 or a 280H?
68 Camaro, 383 small block with TH350 trans. 11.98's at 111mph and never trailered.
Nov 9th, 01, 05:04 PM
personally i'm going with the 280H, it seems to be just right on the duration for a built up 355.
Nov 9th, 01, 05:09 PM
5500 seems to be about the power peak with this combo (mine is almost identical to yours...just a bit less compression) but mine will go to 6000 easily. If you do decide to go with more cam, just remember to watch the retainer to valve guide clearance with those vortecs...unless they have been machined down some you are right at the edge. Erson offers a number of cams with .472 lift with durations up to 240@.050...might be worth checking out.
375hp 78 Chevy truck
77 Chevy Nova
95 Chevy Lumina 3.4L
and building a 78 Nova
Nov 12th, 01, 06:42 AM
Thanks for the input, guys. I also got Boodlepoof to run some desktop dyno numbers for me. The general trend suggests that the improved high rpm torque doesn't make up for the loss in the bottom end. The area under the curve, comparatively, suggests the 268 is pretty much optimum, as long as I'm not planning on running up in the 6500-7000 rpm range. So I guess I'll hold off, for now, keep an eye out for signs of detonation, and try to optimize what I've got before I start trading away that stump-pulling bottom end. Maybe Rubeng442 was onto something with the fuel supply questions.
Anyway...thanks for all the input. I'm sure I'll be done modifying this car by the time I quit driving it! http://www.camaros.net/forum/biggrin.gif
Huck, put a solid cam that is comparable to the XE268H hydraulic cam AND I guarantee that you will be able to wind her to the moon w/ the right springs!!!
I used to wind my little homemade 301 w/ the -097 little Duntov (that came out of a junk engine, lifters and all) to 7000+ rpm all the time.
Oh, and Eric68 said it best. IMHO. pdq67
Nov 21st, 01, 10:24 PM
I think the problem may be with the heads, they have small intake runners(165cc?) or around there and arent really designed for high rpm.
Nov 22nd, 01, 03:44 AM
Well, I have old double hump heads with the small runners but also have a solid lifter cam and it will wind to the moon as pdq says. I suspect it is more the lifter design. I've been following these threads with quite some interest as my next cam, emotionally, would be a solid roller. Decisions, decisions.
69 RS Conv,355,M20,4.10's and I don't worry about stone chips.
Nov 22nd, 01, 04:50 AM
Something else to consider besides head intake runner size when picking your cam. If you go with a cam larger than the 280H or XE 274 you might also want to consider a single plane intake to match the cam's higher RPM band IMO.