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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?
 
#3 ·
My NA 10.5:1 comp 351 running 351 crank and rods has seen 8500rpm and it's done plenty of work since.

Although I'd personally say 7000-7500rpm. A cleveland has quite large connecting rod bolt etc. So it is quite strong. How the engine is preped also makes a big difference. The main difference being longevity of the engines life.

I agree with StrokeXD with the force induction comment. Forced induction motors don't need to rev.

Brenden
 
#36 ·
My NA 10.5:1 comp 351 running 351 crank and rods has seen 8500rpm and it's done plenty of work since.

Although I'd personally say 7000-7500rpm. A cleveland has quite large connecting rod bolt etc. So it is quite strong. How the engine is preped also makes a big difference. The main difference being longevity of the engines life.

I agree with StrokeXD with the force induction comment. Forced induction motors don't need to rev.

Brenden
I agree my 351C q code reved 8500 many times. I'm not saying it did any good to rev it that high at all just that it held together. Max rpm in a practical usable and useful sense was around 6000 rpm. Development of hp at low rpm and keeping it there through max rpm is what the Cleveland was designed for. Cam life is low. Three valve springs created wear. These engines loved to run and breath.
 
#5 ·
TICK4D said:
7000-7500 on standard parts is rubbish.
STROKEXD is right on the money.

Not only the crank and rods, but anything above 5500-6000 and your valve train won't last.

Also brenx your 8500 rpm must have been an over rev. 8500 pahh
Sorry oh mighty god. Who mentioned valve train?? You should re-read the post as poster also asked about what prepared parts would do. He only asked about crank and rods. Prep them and the rpm is able to be done. I did ;)

It was on a dyno at the time. It did three 8500rpm runs.

Theres plenty of 7500->8500rpm clevo's around running nothing fancy rod wise. It's all in the balancing and prep.

[edit] Yes, I am surprised it didn't go bang. I am also very happy that I now know I have a strong bottomend that is capable of rev's.

Brenden
 
#6 ·
Clint : Shot peen the rods/run ARP rod bolts etc/run light weight quality pistons/balance and clean up the rotating assembly and your well on your way.

My bottomend un-assembled was $2872 three years ago.

Brenden
 
#7 ·
Clint said:
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?
Factory 351C rods with ARP (or similar) rod bolts after magnufluxing, shot-peening and resizing, will live well to about 7500 RPM depending on total power output and the weight of the pistons and pins used with them (lighter is generally better).

Factory 351C crank underground 10/10 and re-nitrided will live well to about 7500-8000 depending on total power output and the total bob weight hung on it. I wouldn't recommend going over about 6800-7000 RPM using an externally balanced stocker/worked crank/rods setup, but I know guys who regularly do...and a few that shift at 7800 with them. Also, total power output should be about 450 HP or less with a stock/worked bottom end, but as everyone knows, Pro Stockers ran destroked (offset underground on the rod journals) factory cast iron Cleveland cranks upwards of 9000 RPM back in the day.

You can easily run a stock crank/rods setup to 7000 RPM safely with everything else matching, especially valvetrain like a nice solid lifter cam and double coil springs matched to the cam. However, at those RPM, you need to start looking at other places where the added weight will be "bothersome" like the intake valves (on 4V heads) and the pistons/pins, even pushrods will need to be chrome-moly to shave the weight while adding strength.

A good set of lightweight forged pistons with coated skirts in a flat top design with a set of reworked factory rods with ARP bolts and tapered wrist pins, a factory crank turned 10/10 under and (straighted as needed!) re-heat treated with a cam like this:

Isky Cam

...with a FunnelWeb 4V, modestly ported 4V CC heads with screw-in studs and guideplates and an 850 DP will make 470 HP at 6500 RPM and 410 TQ at 5500. It will spin well to 7000-7500 but power will begin dropping off very quickly after 7000-7300 RPM. It will run fine externally balanced with a SFI dampener and flexplate/flywheel. Use oil restrictors to keep the oil in the mains and run a crank scraper to keep oil down. If I was planning to build this engine for the street, I would probably keep the MSD set at 6800 as a maximum RPM for it. I would probably buy a set of chrome-moly I-beam rods for it with full floating wrist pins for a regular "track" car that was also driven on the street.

Nothing but "mostly stock" parts are needed with attention paid using a good set of Manley (or similar) one-piece taper-neck stainless steel valves, 7* single groove keepers and I'd probably spend a few extra bucks on titanium retainers, since they don't cost that much more. I'd definitely use 3/8" guideplates and pushrods with the recommended valve springs matched to this cam. I'd run a set of Crane "Gold" extruded aluminum roller rocker arms and a Jomar stud girdle. I would use ARP studs and main cap supports in the bottom end and ARP head studs on the top end.

I would probably recommend using a set of the AFD 4V aluminum heads (due out soon) if the wallet was open wide enough for them. Matched up, you'd have an easy 500 HP on a very budget minded Cleveland that would sing smooth like the wind and howl past the competition like a banshee.

This is the perfect combination for a lightweight, early Mustang with a 9" rear end and a Mustang II front end conversion kit (to give plenty of room for the extractors!). I'd use a 5-speed Richmond transmission with a 3.50 rear gear and a 10.5W slick or DOT slick like the ET Street. An 8-point chrome-moly cage and you'd have a slick little car that would run very low 10s--all engine that you could drive to and from the track.

I'd keep a very close eye on my clearances and every dimension during assembly and try to start out with a 4-bolt main block if I planned on using the AFD heads as it would be very near 550 HP by the time everything was done. I would also use a Mellings "regular volume" oil pump and a Milodon chrome-moly oil pump drive shaft with a Milodon street/strip pan like the 30927 part.



I would also weld up a pan-protector cross-member as added insurance in keeping the deep-sump pan safe on the street.

A set of Hooker 6211-1HKR extractors might fit, but I'd probably build my own from a Stahl kit.


....sorry...what was the question again! :wnc:

:davis:
 
#8 ·
brenx said:
You should re-read the post as poster also asked about what prepared parts would do. He only asked about crank and rods. Prep them and the rpm is able to be done. I did ;)

It was on a dyno at the time. It did three 8500rpm runs.

Theres plenty of 7500->8500rpm clevo's around running nothing fancy rod wise. It's all in the balancing and prep.

[edit] Yes, I am surprised it didn't go bang. I am also very happy that I now know I have a strong bottomend that is capable of rev's.

Brenden
Maybe you should re-read the thread.... Safe Rev limit.
Ask anyone who builds Ford engines regulary Mick Webb, Ford muscle Parts, Shanes race engines, Herrods.....anyone.
7000-7500 is not safe.....
Why go to 8500 rpm on the dyno, surely you are not trying to say yours makes power all the way to 8500 rpm.
The load on a dyno is hardly anything like the load when driving on the road.

Why don't you or who ever built your magic engine to start building Pro-Stock engines, you can teach these boys a thing or two by the sounds of it....LMAO
 
#9 ·
TICK4D said:
7000-7500 is not safe.....
I think that it is safe to say that running any engine that makes as much power as a well designed/built Cleveland is unsafe on the street at anything approaching 7000 RPM in any gear! However, the engine won't mind as long as you build it properly EVEN with stock (reworked) rods and crank. Of course, everything else has to match.

As I said, I'd probably run a limiter at 6800 RPM for it with stock rods. I would probably do a set of chrome-moly I-beams with full floaters on anything where I was really planning on spending a lot of time "ripping" it at those RPM.


:davis:
 
#11 ·
TICK4D said:
Maybe you should re-read the thread.... Safe Rev limit.
Ask anyone who builds Ford engines regulary Mick Webb, Ford muscle Parts, Shanes race engines, Herrods.....anyone.
7000-7500 is not safe.....
Each builder has their own opinion of safe operating rpm. They are entittle to that. In many forms and applications clevelands have gone well beyond 7500rpm. Intended "application" is the key here. Pavtek does an engine of that nature.

Why go to 8500 rpm on the dyno, surely you are not trying to say yours makes power all the way to 8500 rpm.
The load on a dyno is hardly anything like the load when driving on the road.
No-one including me said nothing about the power. Not even suggesting it. Why? Nothing wrong with a little test here and there. Just wouldn't do it on a regular basis. Beauty of my bottomend is I am aware of it's capabilities, as it's been there.

I'm sure you've even pushed an engine a tad harder than your supposed to in your time. Stock engine or not.

Why don't you or who ever built your magic engine to start building Pro-Stock engines, you can teach these boys a thing or two by the sounds of it....LMAO
Why? Because he did a shit job of the topend and it took forever to get the engine back. Nothing worse that having to do what I did. That is fix and pay for someones screw ups yourself.

Brenden
 
#14 ·
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...
 
#15 ·
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?
 
#16 ·
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?
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?
 
#17 ·
When i get my AFD headed clevo going in the next month or so, i will have a 6700rpm pill in my MSD. I have stock standard crank and rods, that are not shot peened, but the only mod is good bolts and balanced. These crank and rods have been through 2 builds with me, and in a stock street car before that. I have spun mine to 7000 rpm many hundreds of times, and over revved several times to 7500, and it still fine. My new safe rpm for the future is 6700 rpm, because it probably wont make more after that anyway.
 
#19 ·
what safe rpm will my combo handle>
351 crank 10/10
302 rods, arp bolts, standard- no shot peening etc
full engine balance.
2v heads, 4v stainless valves, machined collets, moly retainers, screw in studs, rr's.
10.7:1

cam in it is close to a crane 238, so not a real rev cam.

would 7000 be safe? ask me anything ive missed
 
#21 ·
STROKEXD, I think that Sandro's red 10sec XD is running a big cam similar to those specs. He'll give you the exact specs if you see him at the Motorplex.

(hey - whats that red box on your passenger shock tower?)

Don't think anyone mentioned good APR/Miloden bolts on the mains.
 
#24 ·
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...
The cam hallam performance are installing for me is 272/280 duration they say its good for 78000 rpm, hopefully i will find out soon. :fraz:
 
#26 ·
TruBlu351 said:
Didn't get mine done, but I hear a "nitrided" crank really hardens the bearing surfaces - increasing strength and reducing wear from hi revs.
Nitriding is essential for a factory crank to live a long and productive life in a maximum performance application. Any forged "racing" crank you buy new will be nitrided. It dramatically hardens the bearing surfaces.

Undergrinding the mains/rods removes the nitrided hardening. Typical crank preparation should include: Clean, magnuflux, straighten, chamfer oil holes, undergrind, (touch up chamfering as needed being very careful not to nick the bearing surfaces), polish, heat treat.

Exacting dimensions are required in order to properly maintain bearing clearances with common bearing sizes AND so that proper oil control is obtainable.

Alternatives for street performance applications exist and Tufftriding is generally the process used. It is much less expensive and takes less time to produce a suitably hard finish necessary for lower-RPM/power applications. (The 85-percentile group of street/some strip performance engines)

A crankshaft without any heat treating may live forever in a low RPM, relatively low HP application. I would recommend anyone involved in this forum (high performance oriented) to consider heat treating their crankshaft to be a minimum requirement. Some lower cost forged steel cranks are Tufftrided rather than Nitrided as a cost savings approach. This is fine for many thousands of typical street/strip users, though nitriding is essential for the maximum effort (or most paranoid!) gang.

Of course, there is always a story to tell about heat treating cranks...some guy I talked to about his crank once decided to take my advice and get his crank heat treated. As soon as he got it back, he immediately whipped out his emory cloth and worked viciously at polishing his main and rod jounals. The next day, after he'd rang me up to excitedly tell me that he got his crank back, I drove over to have a look. Imagine the difficulty one finds in telling someone that they'd just rubbed off all of the material thickness of the heat treating to make it look smart. Heat treated cranks look like someone dipped them in dull, brackish salt water (nitrided) or jet glossy black (Tufftriding). Don't touch the bearing surfaces or try to "clean" it off. I can assure you that (if the heat treating place is reputable) it was very thoroughly cleaned prior to the process! It did have a mirror-like finish on the bearing surfaces after he'd finished with it and it did look sharp...but it was a waste of good hard work and money.

Moral of the story: Any time something appears "different" than what you expect, especially after returning from a "special process" that you're not very familiar with, use your head and call someone you can trust to give you good advice/support BEFORE touching it.

I made him send it back in for re-heat treating, to which they said that they probably get one or two a week back for the same reason! Of course they don't put a little tag around the end of the crank saying STAY OFF THE HEAT TREATING...bad for business you reckon? Makes you wonder how many guys did the same thing and didn't catch it?



...don't you just want to run over to the crank and start cleaning it up!?


:davis:
 
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