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#41
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The weight of the associated parts above the lobes are a MAJOR player here. When we actually start testing some of these parts we start finding problems associated with them that will not show up in dyno simulations, cam doctor reports, etc.
Comp's XE lobes are horrible when it comes to seat timing events in terms of problems with high RPM's and the typical weight of the parts used in a Pontiac engine build. I've watched them go "dead" on the dyno WAY before they should have when the RPM of the engine exceeded the ability of the springs to effectively control the valve action. This even happens with the "hefty" spring pressures they often recommend for those cams. I'm not the only one who has troubles in this area. Back when the XE lobes first came out Jim Weise, owner of the Buick board and very accomplished engine builder tried them. He replaced a T/A cam he'd been using a with a Comp XE cam. Not only did power very abruptly END on the dyno just shy of 5000rpm's he was nicely rewarded with a couple of broken rocker arms. He replaced the XE cam with a T/A cam and the engine made power right past 5000rpm's w/o issues and no more rocker arm breakage. He also noted that the T/A cam made quite a bit more power as it didn't experience RPM related issues. I watched a dyno session with a Pontiac 455 engine using a Comp XE284 cam and ported 7K3 heads. They had nothing but troubles with from the very first pull. The engine quit pulling at 4800rpm's like you put a two-step on it. Wasn't making chit for power anyhow, and they worked and worked to get past 400hp/500ft lbs. Initial pulls were barely 400hp and just shy of 500ft lbs torque. I was waiting in line for the dyno so got to watch it all play out. I'd add here that at the time the cam was unknown as they were just dynoing the engine for a customer who wasn't present. After waiting for several hours while they worked frantically to get some power out of it they finally ended up with a whopping 530hp and just a tad over 500 ft lbs. HORRIBLE numbers for the combo as it had good flowing heads, single plane intake and 850 CFM carburetor......IMHO. For OP here I'd stay as far away from the XE274 cam as I could. The HR cam is a nice upgrade anyhow. That particular cam doesn't have a lot of lift and doesn't need a butt-ton of spring pressure either. Butler added it to the list of "custom ground" cams along with the next step up 236/242 @ .050" cam. Below is a dyno chart from one of my customers who used the larger cam in a 455 build with dished pistons and #16 heads. It delivered pretty decent power for the combination of parts and no RPM associated issues on the dyno pulls either.......
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If you can read this, thank a teacher. If you can read this in English, thank a Veteran! https://cliffshighperformance.com/ 73 Ventura, SOLD 455, 3740lbs, 11.30's at 120mph, 1977 Pontiac Q-jet, HO intake, HEI, 10" converter, 3.42 gears, DOT's, 7.20's at 96mph and still WAY under the roll bar rule. Best ET to date 7.18 at 97MPH (1/8th mile), |
#42
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The sharp drop off some people report with the XE hyd flat tappet cams could be directly related to either a geometry issue or the need for more rate in the valve spring.
All too often the Comp Tech people, and depending on which work cubicle the call gets routed to, just regurgitate their standard recommendation for a valve spring and it is taken as gospel. Often too low ! And then you hear the crap.... BUT we shouldn't HAVE TO run big spring pressure in our cams ! It would be of interest to know how many un told thousands of people out there in the up to date modern hot rodding world that are using a bit more spring pressure to control things without any issues at all with longevity. I have spoken with two other engine builders featured in the back of Jim Hand's book about this VERY subject, along with numerous well known engine builders outside our community about it. None have any qualms about it. And let's make sure it's known that the XE cam bashing is involving a hydraulic flat tappet cam and not a hyd roller cam. Jim Butler uses XE hyd roller lobes as does Dave Bisschop. .
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'70 TA / 505 cid / same engine but revised ( previous best 10.63 at 127.05 ) Old information here: http://www.hotrod.com/articles/0712p...tiac-trans-am/ Sponsor of the world's fastest Pontiac powered Ford Fairmont (engine) 5.14 at 140 mph (1/8 mile) , true 10.5 tire, stock type suspension https://www.youtube.com/watch?v=qDoJnIP3HgE Last edited by Steve C.; 05-16-2022 at 12:33 PM. |
#43
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Quote:
Stan
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Stan Weiss/World Wide Enterprises Offering Performance Software Since 1987 http://www.magneticlynx.com/carfor/carfor.htm David Vizard & Stan Weiss' IOP / Flow / Induction Optimization - Cam Selection Software http://www.magneticlynx.com/DV Download FREE 14 Trial IOP / Flow Software http://www.magneticlynx.com/DV/Flow_..._Day_Trial.php Pontiac Pump Gas List http://www.magneticlynx.com/carfor/pont_gas.htm Using PMD Block and Heads List http://www.magneticlynx.com/carfor/pont_pmd.htm |
#44
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It's not my dyno sheet, just sent here by a customer. Power numbers are good for the combination of parts. Where the rubber meets the road that's all that really matters and I've mentioned at least once that it is my belief the dyno operator didn't run enough fuel pressure and sucked the carb bowl a little low causing a slight "dip" in the graph. Of course they don't call you the day of the testing, so I have no doubt the dyno operator like most others don't like Q-jets, understand them, and still believe old/outdated information on them like not liking high fuel pressure, among other things........
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If you can read this, thank a teacher. If you can read this in English, thank a Veteran! https://cliffshighperformance.com/ 73 Ventura, SOLD 455, 3740lbs, 11.30's at 120mph, 1977 Pontiac Q-jet, HO intake, HEI, 10" converter, 3.42 gears, DOT's, 7.20's at 96mph and still WAY under the roll bar rule. Best ET to date 7.18 at 97MPH (1/8th mile), |
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