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Tech / General EngineIs your car making a strange sound or won't start? Thinking of adding power with a new combination? Need other technical information or engine specific advice? Don't see another board for your problem? Post it here!
I wouldn't be using XFI lobes to 7000 rpm. And then trying to use that RevKit to fix the mistake.
Rather than spending all that money for nothing, just get a proper cam that doesn't beat the crap out of the valvetrain.
For Gen 1 or 2 SBC's, I'd be talking to either Lloyd Elliot or Advanced Induction to get a custom cam. Lot less money and more performance than your current plan.
For only 110 bucks i vote yes for your motor - very cheap insurance device as it does little harm. But it does add some stress/flex to the vlv train when opening the vlv. I had thought they were much more expensive than that. I think a lot owners would use them if they knew they were that cheap - i didnt.
Sorry CardOO, your vote doesn't count. You have no experience in doing what he's trying to do. Reading magazine ad/articles doesn't count
UltRoad, I shifted my factory HR setup at 7100 rpm for six years before the bottom end of that motor gave out. Never a valvetrain issue. Stock LT1 lifters (similar to your LS7 p/n). Custom cam from AI (228/234/108, .613/.613). Manley Nextek beehive springs set up 160 lbs seat.
But, you CAN'T do that with those XFI lobes; they're nasty. Most people that have used them have found they can't control them above 6500 rpm. And several have broken springs before 20k miles.
The custom cams from AI and LE have lobe profiles that can accomplish this.
I'll take Comp's XE lobes any day over the XFI's.
For what it's worth, I have a Comp custom ground cam with XFI lobes. Lobe spec on the intake is 274 @ .050" and .574" lift. They're set up with Comps 26918 valve springs (125 on the seat @ 1.8", 367 @ 1.150") and Crower (offset trunnion) 1.6 ratio rockers. Also in the mix are Comp short travel retro-fit hydraulic roller lifters. 1st -2nd red line shift is 6500 rpm (and often inadvertently closer to 7000 rpm) and this setup has been getting beat up for close to 30 000 miles and plenty of rpm everyday.
I will admit that the valve train is noisy in that, despite the lifter pre-load being set and the valve train geometry nailed down, there's the constant tapping of the valve landing on the seat as well as the whirring of the full roller rocker.
I think the general consensus is that with a light valve train, using a modern era spring (like the Bee Hives or LS) and a light rocker arm, the problems with valve train stability aren't what they once were. Sort of renders the Rev-Kit as unnecessary.
Just my two cents based on personal experience.
Wow, skinny, I think you should change those springs. Yes, I know people that have achieved what you did, but I know more that have broken springs. An acquaintance of mine just broke a 26918 spring at 17k miles, set up nearly identical to what you did, with the XFI466 cam, 270 deg, .570" lift, and he's been shifting at 6200 rpm, probably a few inadvertent excursions above that. 6 spd car.
Sorry CardOO, your vote doesn't count. You have no experience in doing what he's trying to do. Reading magazine ad/articles doesn't count
Yes, u should be sorry. A rev kit is for high rpm and protects the engine from lifter ejections among other things like lifter bounce. And no i didnt read that in any mag article.
So have u ever used a rev kit? Com'on dont lie. Tell us u havent used one either. Then u tell us when a rev kit would be needed too. Or maybe u cant make something like that up and cant tell us anything 'bout them anyways. Put up or shut up.
No sir, I have never used a rev kit. I'm not that gullible. I've also not pulled an LT1 cam out of an F-body and replaced it with a ramjet truck cam. Do tell us all about that one please What whopping hp number did you see on the dyno. and how much slower than a stock-cammed LT1 car is yours at the strip. I already know, but please share here
This forum will eventually run you off too
That'll be checked in the next couple of weeks or so. The heads are on their way to the machine shop and the springs will go through the checker.
On a similar note, my earlier version of this engine, with Vortec heads and an XR276HR cam, (224 @.050", 530" w/ 1.6 rockers. Not nearly an aggressive a profile) had the 26918 spring go 50 000 miles before I took the heads out of service a few years back. I recently sold them (to a hot rod shop) who, prior to purchase, mag'd the heads and ran a few springs. All acceptable but I recall thinking they were a little short of the as new seat pressure. I'm waiting to see how these new springs have fared with the aggressive (although still small ) XFI lobes.
Yes, not surprising to see the springs do well with the XE lobes of that XR cam. They never cause problems as long as you set them up right.
Even if the springs off your current XFI setup check ok on pressure (I'm sure they'll show a little bit of loss), I'd throw them out anyway. The damage they incur doesn't show up in spring rate. It's fatigue damage, no way of seeing it until actual failure.
Thanks guys for your suggestions. I've a couple threads where it's been suggested to get a custom cam grind. Also I've seen in a few videos where guys were like "what's that tapping noise". Starting to realize that sound is probably the roller lifter "falling off a cliff" and nailing the top of the valve, and that was at idle! This is a situation where a rev kit might actually make worse since there's additional spring pressure on top of the lifter. A custom cam is sounding more and more like the smartest way to go...
Maybe I'll see if I can return the XFI292 or sell it. Anyone interested? I paid $297.
..may hit 7000rpm sometimes... using a XFI292 cam ....
My question to the OP is why? I understand how it happens. I touch off of 7000 rpm now and again but I'm not making any power up there (and I only get there when the limiter is off and I'm being careless).
And byComp Cams reckoning....
..neither will you.
Showing an upper range limit of 6200 rpm and allowing a few hundred rpm for engine overspeed puts you at 6500. I would think that following the spring recommendations (ie. small mass spring assembly like the 26918 specification) you would be OK. Certainly in the valve train stability category. As for a daily existence, like a race only engine that sees nothing but high RPM all the time, it's prudent to be regularly checking all these components anyway.
Edit: Or is this a case of , as LG4Bird suggests, just not a good idea?
Comp's "RPM range" in this case is just plain ridiculous, or it's an error. Even with a 2-plane manifold, come on: look at those duration numbers! They claim their "RPM range" is based on a 350 CID....lol! Even a 383 will want to peak near 6500 with it.
OR.......maybe it's their roundabout way of saying it's safe (valvetrain stability-wise) with their recommended springs up to only 6200 rpm. Whether they intend that as their hidden message or not, that's about the truth of it.
People have run it successfully to 6500 rpm, but not with those 918 springs. I'd want 150 lb on the seat with a beehive and at least 160 with a dual spring.
Ult, what type of intake manifold are you going with?
Yeah, that thing is really gonna want to rev. It'll pull well from 6200 to ~7000. Get in touch with Lloyd Elliott www.elliottsportworks.com and get a proper cam for it.
If I were stuck choosing from among Comp's cams, I'd rather go with the XR294.
Comp's "RPM range" in this case is just plain ridiculous, or it's an error. Even with a 2-plane manifold, come on: look at those duration numbers! They claim their "RPM range" is based on a 350 CID....lol! Even a 383 will want to peak near 6500 with it.
OR.......maybe it's their roundabout way of saying it's safe (valvetrain stability-wise)
You know I thought the same thing and have before looking at some of Comps dyno graphs. Interesting thing about that 292XFI though is that it actually has less overlap than the XR288HR. Looking at simulations of the two of them with the same compression and cylinder heads, they both peak around 6200. Of course different heads and intake will move those peak numbers around but the wide LSA of that 292 takes a lot of the bite out of it.
My 383 build uses 6in forged H-rods for durability...
I've read that overlap is good for relieving cylinder pressure on high compression engines, i.e. 11:1 or less on 93 octane fuel injected engines.
You know I thought the same thing and have before looking at some of Comps dyno graphs. Interesting thing about that 292XFI though is that it actually has less overlap than the XR288HR. Looking at simulations of the two of them with the same compression and cylinder heads, they both peak around 6200. Of course different heads and intake will move those peak numbers around but the wide LSA of that 292 takes a lot of the bite out of it.
Yeah, I'll give you that; the XFI cams' "wrong for SBC" 113 LSA does result in a bit less overlap than more properly spec'd cams would give.
Nevertheless...peaking at 6200 rpm would mean shifting about 6700 rpm with most trans ratios if you're interested in optimizing your use of the powerband.
Ult, overlap doesn't relieve cylinder pressure. Later IVC event does that.
02-03-2016, 06:22 PM
