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Safe Rev Limit of 351 Crank and Rods?

35K views 35 replies 13 participants last post by  Jrbarnes1 
#1 ·
Given the current finishing techniques and bearing and rod bolt availabilties, what would a safe rev limit be for 351 cranks and rods, after what treatments? Would the limits vary with intended use, say forced induction versus high compression natural aspiration?
 
#27 ·
STROKEXD said:
Forced induction set-ups do not need to rev as much to produce their power.

5000 for a stocker clevo
9000+ for a mega dollar pro-stocker
6000-7000 for most well built street motors
As an note about the mega dollar Pro Stock engines, they were typically destroked to around 3.25-3.30" to fit the weight breaks based on CID that are now a part of Comp Eliminator.

It is interesting to see what that .20" does to piston speed, which is the real critical factor in determining maximum engine speed (assuming everything is built to take it) because of the material limitations are quickly exceeded by about 6000 FPM, give or take, depending on budget...a lot less for stock parts!

FPM = ( stroke * rpm ) / 6

FPM = ( 3.50 * 9000 ) / 6

FPM = 31500 / 6

FPM = 5250


FPM = ( 3.30 * 9000 ) / 6
FPM = ( 29700 ) / 6
FPM = 4950

4950 is a good point for a factory rod with all the best work done to it. I would never consider running a factory rod that hard these days when you can buy stronger, lighter rods for about what it costs to do all the work to a set of factory rods. A good set of Oliver rods at $1750 will run up to about 6000 FPM with everything else in the super-ultra lightweight category.

My 393 stroker:

FPM = ( 3.85 * 7600 ) / 6
FPM = ( 29260 ) / 6
FPM = 4877

Anytime you can run below 5000 FPM with good strong, lightweight parts, your engine will live "forever."

As a segue on the topic of calculating FPM, an instructor at an automotive school (of all things) was stupid (who said there are no stupid questions, an instructor?) enough to say:

"I do however question some of the formulas. For example I have always wondered how the piston speed formula was arrived at.

(stroke X RPM)/ 6 = fpm ???

I know its been an industry standard for eons but does that constant really make any sense or is it just me, maybe me???"

...that "industry standard" is an example of his stupidity. (Can anyone tell that I don't like this guy?)

The point is that stroke is only 1/2 of the distance that the piston travels, you see, it goes up 3.5" and then back down 3.5" on every revolution and "6" just happens to be half the number of inches in one foot!

stroke * 2 * RPM / 12 = FPM
stroke * RPM / 6 = FPM

...if you figure piston speed for a stocker with hydro lifters:

FPM = ( 3.50 * 6200 ) / 6
FPM = ( 21700 ) / 6
FPM = 3617

This is fairly safe for the material strength of factory parts, even cast pistons (though I don't recommend using them over 3500 FPM, which is exactly 6000 RPM with a 3.5" stroke Clevo)...assuming good rod bolts!

FPM @ 7000 RPM = 4083

That is a very good number for factory rods with good ARP bolts, shot peened, resized, pressed pins OK (prefer floaters) with good quality forged pistons and everything else matching. 4375FPM = 7500 RPM. This is pushing the limits of what I'd consider safe for pressed pins, though I know guys have run them even higher and longer. I just get prickly hairs when I think about it. For brief little shots using synthetics, it will probably live fine. I'd still use a 6800 RPM limiter because there isn't anywhere but the racetrack where there is a difference between 6800 and 7500 unless you happen to be motoring down the road with a 5.46:1 rear gear in first and want to see how much fuel you can burn per km. I can tell you one thing...if you do happen to pull second, it is going to really FLY.

Just so everyone's clear about my point on "unsafe" on the street at any RPM approaching 7000...with a 5.46:1 gear using a 26" tall tire at 7000 RPM it is ~159 Km/h (at 1:1 "high gear") and we all know that 5.46 isn't exactly a street gear ratio. A 3.90 would be ~224 Km/h. Multiply those numbers by the inverse of the gear ratio of the selected gear. EG: 2.46 first gear (inv=.407) .407 * 224 Km/h = 91 Km/h. Not too many places around town to run it up to 91 are there?

Anywhere on the street in any gear there is no place for 7000 RPM using any rear gear. A few young'uns would argue that you can run it up to high RPM using a mountain gear and a transmission with a really low first gear, but my reply is that (assuming not spinning the tires, which is a no-no on the street) there would be too much acceleration rate for it to be safe as other drivers wouldn't be prepared for a car accelerating at such a high rate and could change lanes, pull out from parking lots or even pedestrians step into the street because, well, you were so far away a second ago. Just not safe. Fun and exhilarating maybe...but not very safe.


:davis:
 
#28 ·
Davis thanks for the tech info, and yeah gearing is a consideration. It worked the other way for me in an old combo with 2.77 gears. I used to coast along at 70km/h with the FMX in 1st then stamp on it upto 100km/h still in 1st. Not too many cars I ran against on the street were ready for that.

TruBlu yeah am definately looking forward to having a chat with Sandro. The new red box is MSD's adjustable timing controller. It's working great - wind up the advance for around town to get over the big cam issues down low, and wind it back when I'm going full tilt. Can put any fuel in it on the fly also if I get stuck away from home and the drum of BP racing.

xbgs351 said:
Not likely. A 108 or 106 LCA would be needed for a motor of that capacity.
What cam program are you using. Does it take into account inlet and exhaust tuning?
Dyno2000 and that LCA got calculated after punching in individual inlet and exhaust events to give the overall best HP & Torque curves. Agree that 115LCA sounds odd-ball, but that's what it shows as the best cam. And that's why I was wondering if anyones actually running a cam like that.
 
#29 ·
STROKEXD said:
Davis thanks for the tech info, and yeah gearing is a consideration. It worked the other way for me in an old combo with 2.77 gears. I used to coast along at 70km/h with the FMX in 1st then stamp on it upto 100km/h still in 1st. Not too many cars I ran against on the street were ready for that.
Yeah? You really ought to try it the other way sometime! There is nothing like winding up a (US) BOSS 302 to 8000 RPM in first gear with a 4.88 or fatter gear along side of someone...when you hit 2nd, the other guy disappears! With a high gear like a <3.0 you're moving away, but without the level of umph that a big o' nasty stump puller gear will. A 6.14 with a 30" tall tire at 9000 with a roller cam and a Yate's headed R-block 302 on injected alcohol will scream up through the gears like a cat with its tail afire. Now that is where a close ratio 6 speed is really handy! Banging at 9 and only dropping to 7. The guy (previously) next to you won't know what hit him but after about a half second, he won't be able to see your rear bumper on the horizon! Just remember, light car + high revving engine + stout gear = ultra quick.

:davis:
 
#30 ·
STROKEXD said:
PS Whilst on the cam profile topic I recently experimented with specific valve event timing on the same software until I got the meatiest torque and HP curve (not just peak HP); based on a 378 stroker with 4V heads, single plane inlet and 11:1 compression.

Interesting results: @ 50" lift 240 deg inlet, 251 deg exhaust, .610 inlet and .650 exhaust 115 deg lobe centreline.

Anyone out there running a cam like that?
Ran the specs through Engine Analyzer Version 3. With 116 LCA cf 108 lca it lost 15 ftlb of torque, and made the same horsepower, but only after revving it out another 600 rpm. ie the cam sucks.

It is also a lot of lift.
 
#31 ·
STROKEXD said:
With this dyno software I've been playing with it is sure interesting seeing the HP curves and where it falls off by trying different cam profiles. Sure takes a monstor cam to justify extreme revs.

Brendan your cam peaks at exactly 6500 by the way and declines steeply after that.

I've loaded up all the biggest cams the manufacturers offer and none I've found peak any later than 7500-8000 revs. So one wonders what those pro-stockers use...
Stroke ,what program you running?I have the Performance Trends version and have found that it is probably a little generous in its predicted values.Have found a discrepancy in the cylinder head file data pertaining to 2v head port volumes.From memory i think they list port volume at over 200cc which is way off the mark. Anyone else experienced in this field?
 
#32 ·
russxr67 said:
Stroke ,what program you running?I have the Performance Trends version and have found that it is probably a little generous in its predicted values.Have found a discrepancy in the cylinder head file data pertaining to 2v head port volumes.From memory i think they list port volume at over 200cc which is way off the mark. Anyone else experienced in this field?

The heads in the program are way off. They are closer to 170 cc.
 
#33 ·
Desktop Dyno2000 is the program.

Stroke, here's a recent post of a dude in the US running a 408 cleveland and big cam to suit - little bit bigger than the specs you were looking for. Pulled a chassis twisting 10.8@122 with a 1.48 60ft time. He says '79 Fairmont - guessing XC?? Mines a Mar 79 XD.

258/268 @ .050" .630/.658 lift............hmmmm, good daily driver??

http://www.network54.com/Forum/message?forumid=119419&messageid=1083288863

Cheers
 
#35 ·
TruBlu351 said:
Desktop Dyno2000 is the program.

Stroke, here's a recent post of a dude in the US running a 408 cleveland and big cam to suit - little bit bigger than the specs you were looking for. Pulled a chassis twisting 10.8@122 with a 1.48 60ft time. He says '79 Fairmont - guessing XC?? Mines a Mar 79 XD.

258/268 @ .050" .630/.658 lift............hmmmm, good daily driver??

http://www.network54.com/Forum/message?forumid=119419&messageid=1083288863

Cheers
The US fairmont of that Vintage are what the fox mustangs were based on. Much lighter than an Australian fairmont.

If you run a wide LCA you lose the ability to tune the inlet and exhaust system. You miss out on forcing the air/fuel in on the inlet side and scavenging on the exhaust side.
 
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