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Discussion Starter #1
I've seen formulas to determine this, but there are too many variables to take them as gospel.

I reved the engine to about 6800 the other day in 2nd. I like to stay below 6600, but it's a lot easier to over rev the engine now that I have a T5z. It wasn't intentional to rev it to 6800, just looked down at the tach and saw it...oooops, I better shift.

I'm curious about what sort of safety margin I may have if I stay below 6600. If I reved it higher, maybe the valves would float and eat a piston before the bottom end broke...not sure.

Here's the bottom end and valvetrain specs:

Hi Nodular 331 crank with radiused mains/rod journals
Hawks H beam 289 rods with 7/16 cap screws
KB 322 hyper pistons
ARP main studs
'68 302 block
Comp 282S solid lifter cam
Pro Magnum roller rockers
Hardened moly lightweight pushrods
Comp Cams valve springs (about 125# closed and 300# open)
SS 1.94/1.60 undercut valves

Thanks
 

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I'm no mechanic, but I think a lot of that would have to do with the quality of the build. Balancing, torque specs, proper oil transfer and pre-lube, sizing of bearings, quality of the bolts, correctness of the bore, etc etc etc would all be a factor. If anything is not 100% it could come apart after a few 5K revs. Only one way to find out. http://board.moparts.org/ubbthreads/images/graemlins/devil.gif
 

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All this determined by cam spec's and heads. With that being said 6600 is a safe bet for that engine.
 

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The typical concern is valve float. At some point up the rpm band, the springs lack sufficient pressure to return the valves to a fully closed position in the time allowed by the combination of lift on the cam lobe, cam duration, and the rpm. Comp Cams tech line will generally tell you when this occurs, if you give them your springs rates and the cam grind.

The other thing to consider is that given the cam specs, at some point on the rpm range the cam simply will not make any more power. . . .e.g., "running out of cam" It makes no sense to overrev the engine beyond the point that the additional rpm makes wny more power. For that reason, a rev limiter tailored to your cam might actually be a useful thing.

The other points raised are equally valid, but refer to mechanical limitations to rpm whereas I am addressing here the engineering aspects.

enjoy the ride!
 

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Discussion Starter #5
Disregard the cam's rpm vs power capability. As I mentioned, I would like to keep rpms below 6600 (the cam is still making very usable HP there). This is more of a mechanical failure and safety margin question.

Let me ask it another way...should the bottom end of the engine handle 7000 rpm assuming everything is balanced and lubricated adequately?
 

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I agree with oldrusty, that the only way to find the rev limit of your engine is to break it. I agree with you that there are too many variables to rely on a formula for any kind of fail safe data.

The best rule would be to find out your hp peak and shift about 250 rpm higher than that. Then apply the old facetious saying, that if something breaks it needed fixing anyway. There are also some shift point calculators out there to determine your optimum shift points for the quarter mile, although since that's not my thing I can't point you to any.

Think of all the ways that an engine can fail at high rpm: rod bolt breakage, crankshaft breakage, a valve coming apart, a retainer lock failing, valve bounce, and good old fashioned bearing failure and/or oiling failure. Think of all the factors that could cause each of these failures, and you're looking at just about every component, machining operation, and assembly task there is. So it really is impossible to "calculate" or even guesstimate what redline is safe, of how long is it safe.

Regarding valve bounce, of the two kinds of loss of valve control, this is the one that kills engines. Both valve bounce and valve float happen because the spring lacks sufficient pressure to control the mass of the valve train -- control of the valve train being defined as keeping all the components (cam lobe, lifter, pushrod, rocker, and valve) in contact. But because valve float consists of the lifter launching off the top of the cam lobe, and because this happens with the piston down in the hole, unless it is very severe valve float is unlikely to cause piston-to-valve contact. In fact, Super Stock drag racers design their cams to launch the lifters off the cam lobe on purpose, as a way of increasing effective lift without exceeding the factory cam specs. And these are engines, some of them with domed pistons. If high rpm valve float were a cause of piston-to-valve contact, it would be most dangerous to a domed piston. So it's actually high rpm valve bounce that is the thing that causes piston-to-valve contact, since the piston's at the top of the stroke when the exhaust valve is seating.
 

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Not what you asked about, but definitely something to consider unless you've already got it covered.... is a scattershield. Back during my hotrodding days in the 60's, I saw a clutch that had exploded on a 55 Chev, and it was one of those impressions that last a lifetime. Looked like several sticks of dynamite had gone off, and actually tore up the dash area with shrapnel. I have no idea what RPM it was turning when it let loose but the guy was injured pretty severely.

You've probably already got the necessary protective equipment. Just thought I'd mention this since it's not something everyone thinks about, at least until they've seen the results of a clutch explosion.
 

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Yes..it will handle it. I have pushed mine up there several times but only by accident. Other things to consider at that rpm..fuel pump, water pump and any external pieces clutch, pressure plate..both my dad's sbf and mine have only had clutch failures at high rpm's.
Mine at 6300 rpm and his at around 7800. Both were at the track and we both reacted the same way....Oh s$it..LIFT!
 

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Discussion Starter #9
Thanks Pete. That's all this questions was. Will the engine grenade if I over rev it to ???? rpm? Maybe I didn't make it clear in my original post but I thought I did. I don't plan on doing it intentionally, but was wondering what sort of margin am I working with. Now that I have a manual trans, maybe it's a good idea to consider a rev limiter.
 

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I am sure camachinest has more knowledge on this subject...like what happens to the rods at this rpm etc.
 

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Tracy,
I've run my 289 up to 6,900 alot(track) and it's fine. I think critical mass is somewhere just above 8,000 IIRC. I have that info somewhere packed away.
 

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Tracy,

I'd keep the RPMs under 7500 myself, but I bet it would take some 8000+ blasts a few times. Of course, a rev-limiter makes this whole issue mute :winkgrin:
 

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My Boss 302 reference book talks about the TransAm versions racing at 7,500 - 8,500 rpm redlines, while at the same time acknowledging the engines did not have stock internals. These books speak of the 302 being a high-winding engine making its torque up high, as opposed to the FE engines. Based solely on this, I suspect that running it up to 7,000 rpm on an infrequent basis is not going to gernade your engine, assuming the bottom end is balanced, is assembled with proper torque on the mains and rods, yada yada yada. YOu could add a main cap girdle to avoid main cap walk if that is a concern. the 68 block is still in the "early year" definition and is stronger than the mid-late 302 70 blocks.

good luck.
 

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You've got a good crank, even better rods, and better bolts in the bottom end. That will push you a couple of thousand rpm above stock. *However*, you've also got a longer throw on that nice crank. Pistons, bolts, and rods tend to break as the average feet per minute speed of the pistons goes above certain levels for certain types of materials, generally cast pistons shatter first, followed by hyper, followed by forged. Since your pistons are traveling farther you would have about a 350 to 450 rpm reduction as compared to a similarly built 302 which has about a 250 rpm reduction compared to a similarly built 289.

If you had forged pistons with all the parts you mentioned I would think 7,000 rpm would be fine. Since you don't I would guess a bit lower.

The only caveat here is that there is a big difference between sustained rpm and a limit you just "visit" on occasion. :) Meaning if I were you, *I* wouldn't want to take the car to the Bonneville Salt Flats and peg the engine at 7,000+ rpm for 5 miles in an attempt to set a land speed record (I'm a chicken).

All of the above is just my armchair opinion, probably not worth the electrons it took to record it. You built that engine (and a phenomenal engine it is) and probably have a better feel for it than anyone else ever could. Do whatever feels right to you. :)

John Harvey
 

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Just a quick follow-up.

This site lists some simple formulas - probably similar to what you've already seen.

Plugging your engine's specs into their formula gives a piston speed (in average feet per minute) of about 3,850 fpm at 7,000 rpm. That is in the range that is usually recommended for forged pistons - Often 3,800 to 4,000 fpm is quoted as the threshold were forged pistons are recommended.

Just a reference point.

John Harvey
 

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Regarding revving an engine you have to take into consideration your rod/stroke ratio, how well the engine is balanced and your valvetrain componants. We will assume your cam will rev to 7000+ RPM.

If you used the 5.4 rod (or longer), balanced the assembly within a gram, used 3/8 pushrods, lightweight retainers, good USA springs and valves, then 7200-7500 would be a max you could see on occasion.

If you were going roadracing, then I would put a 6600 - 6800 chip in there and stay below those ranges. One thing you have to remember is that a chip will save your engine if you ever bust a rear end gear or driveshaft. If either of those break while your foot is to the floor, your engine will see more than 9K, but the limiter will prevent that.

As far as driving on the street, I doubt you'll ever feel the power drop off, even at 7000 RPM. We would determine our gearing for oval racing by going 1000 RPM past the HP peak, before braking for the corner. You could use the same basic logic, if you have a dyno sheet, for your shift points. Your engine probably peaks between 5800 and 6200 and just add 1000 RPM and you are set........

Gary
 

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You need a rev limiter. Pertronix makes a standalone unit or you can get the new Crane XR-1 element that replaces the points. It has a rev limiter built in.
 

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re:Let me ask it another way...should the bottom end of the engine handle 7000 rpm assuming everything is balanced and lubricated adequately?
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Tracy,

Provided the valvetrain holds up and doesn't allow intimacy between valves and pistons.....your engine's RPM capacity
is contingent on Oldrustyrelics parameters....

There is one more parameter that is OFTEN overlooked.

Connecting rod bolt stretch....during an engine's assembly.

Did you just torque the bolts on? or did you torque and measure with a bolt stretch meter?

I would definltey find out where your valves start floating
and subtract 500 rpms for safety given everything else is kosher.

If you have an MSD ign I would use that rev limiter to that tolerance.
 

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You better get a rev limiter on that bad boy quick. All it might take to find out is one missed shift into second and BAM there goes all your hard work b/c the motor over revved itself.
 
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