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What operating rpm can a 300ci. hold safely?

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Wesman07
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What operating rpm can a 300ci. hold safely?

Post #1 by Wesman07 » Thu Apr 19, 2018 11:51 pm

I know it’s safe to hold the engine speed in the peak torque range. Can it hold close to peak horsepower ( 3,000 - 3,400rpm)?
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Re: What operating rpm can a 300ci. hold safely?

Post #2 by jason832 » Fri Apr 20, 2018 7:07 am

I spin mine many times a day to 5k, for about a year now. 4barrel and intake, cam, and aftermarket pistons. Rod main and head bolts are stock.

I think ford has the redline at 5k or 5500rpm. 3400 is fine. I think peak torque on a stock 300 will be at less rpm though.

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Re: What operating rpm can a 300ci. hold safely?

Post #3 by WorldChampGramp » Fri Apr 20, 2018 7:25 am

Wesman07 wrote:I know it’s safe to hold the engine speed in the peak torque range. Can it hold close to peak horsepower ( 3,000 - 3,400rpm)?



General answer is YES. I assume you are probably talking about operating the 300 engine in a vehicle in an over the road situation such as pulling a trailer and would like to limit your RPM's to the stated 3000-3400 range. As long as the engine is NOT in detonation and or NOT overheated these rugged 6-cylinder workhorses will run for 1,000's of miles and cross or great country coast to coast with ease. At least that’s what my Ford experience has been… way back when… which applies to now, despite what some of our Forum members may opine. Gramps

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Re: What operating rpm can a 300ci. hold safely?

Post #4 by Wesman07 » Fri Apr 20, 2018 9:11 am

Thanks for the reply guys.

Gramps, you are correct with the application. I run 89 octane, which I believe is enough. The engine temps remain surprisingly LOW. In fact it actually runs colder when being run WOT at 65mph with a load, then without a load and at half throttle.

I had initially thought my thermostat failed open, but it sits right where it should if I let it idle.
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Re: What operating rpm can a 300ci. hold safely?

Post #5 by THE FRENCHTOWN FLYER » Fri Apr 20, 2018 9:31 am

Interesting.
Maybe its because at WOT you are into fuel enrichment strategy, i.e., around 12.5:1 whereas at cruse mode you are closer to 14.7:1.
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Re: What operating rpm can a 300ci. hold safely?

Post #6 by Wesman07 » Fri Apr 20, 2018 9:37 am

FTF, That was the only possible explanation I could think of. The 5mpg drop would confirm that more fuel is being used.
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Re: What operating rpm can a 300ci. hold safely?

Post #7 by THE FRENCHTOWN FLYER » Fri Apr 20, 2018 9:43 am

Wesman07 wrote:I know it’s safe to hold the engine speed in the peak torque range. Can it hold close to peak horsepower ( 3,000 - 3,400rpm)?

We used to run what is called an F.I.E. dynamometer durability test. It consisted of 300! hours of nearly all WOT operation, cycling between max torque peak and max HP peak, with occasional excursions of no-load operation at 12% over the HP peak speed (free rev).
Brutal.
Those are the kinds of tests you may see where the exhaust manifolds are glowing red hot, oil temps are at max, and stuff likes to break.

My roadster pickup truck drag vehicle has 1268 runs on it - using a stock short block - and the only work we've ever had to do on the bottom end was to repair a leaky oil pump pickup tube. That's 1268 WOT quarter mile passes.
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Re: What operating rpm can a 300ci. hold safely?

Post #8 by Wesman07 » Fri Apr 20, 2018 11:04 am

THE FRENCHTOWN FLYER wrote:
Wesman07 wrote:I know it’s safe to hold the engine speed in the peak torque range. Can it hold close to peak horsepower ( 3,000 - 3,400rpm)?

We used to run what is called an F.I.E. dynamometer durability test. It consisted of 300! hours of nearly all WOT operation, cycling between max torque peak and max HP peak, with occasional excursions of no-load operation at 12% over the HP peak speed (free rev).
Brutal.
Those are the kinds of tests you may see where the exhaust manifolds are glowing red hot, oil temps are at max, and stuff likes to break.

My roadster pickup truck drag vehicle has 1268 runs on it - using a stock short block - and the only work we've ever had to do on the bottom end was to repair a leaky oil pump pickup tube. That's 1268 WOT quarter mile passes.


Wow! Brutal is right!

What would commonly break or wear out under those testing conditions?
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Re: What operating rpm can a 300ci. hold safely?

Post #9 by arse_sidewards » Fri Apr 20, 2018 11:53 am

THE FRENCHTOWN FLYER wrote:We used to run what is called an F.I.E. dynamometer durability test. It consisted of 300! hours of nearly all WOT operation, cycling between max torque peak and max HP peak, with occasional excursions of no-load operation at 12% over the HP peak speed (free rev).



That's like one trailer park eviction per business day for a little more than a year or evicting just one person and leaving his double wide on the opposite coast. :rolflmao:
Last edited by arse_sidewards on Fri Apr 20, 2018 11:55 am, edited 1 time in total.
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Re: What operating rpm can a 300ci. hold safely?

Post #10 by WorldChampGramp » Fri Apr 20, 2018 12:21 pm

My experience was limited to some Dyno testing at the E & F (Engine and Foundry) Dyno wing and my real world in the Field "see if you can break it" U-Haul fleet sales engineering experiences. For years U-Haul trucks used the 300 engine as did most of the UPS fleet. Head gasket failures were prevalent and occasionally one or two cast piston skirts would wind up in the pan but the engine still ran, failed skirt and all.
The availability of quality Fel-Pro gasket technology especially the Fel-Pro 1024 gasket solve this problem on damn near any application assuming clamping loads, block deck & cylinder head thicknesses + flatness guidelines are adhered to. Anybody what to guess how much clamping load a 4-bolt combustion chamber needs for very routine High Performance usage? Gramps

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Re: What operating rpm can a 300ci. hold safely?

Post #11 by Wesman07 » Fri Apr 20, 2018 12:49 pm

I’ll take a stab at this.

I was once told the combustion chambers see roughly a ten times the number you get from a compression check ( this cold be totally bogus) (could have been a hundred times?)


15,000 psi?
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Re: What operating rpm can a 300ci. hold safely?

Post #12 by Stonebreaker » Fri Apr 20, 2018 3:29 pm

Wesman07 wrote:I’ll take a stab at this.

I was once told the combustion chambers see roughly a ten times the number you get from a compression check ( this cold be totally bogus) (could have been a hundred times?)


15,000 psi?

maybe you meant 1,500 psi? I usually see 120 to 160 psi when doing a compression check, depending on the static and dynamic compression ratios of the particular engine I'm checking.

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Re: What operating rpm can a 300ci. hold safely?

Post #13 by THE FRENCHTOWN FLYER » Fri Apr 20, 2018 10:32 pm

1000 psi is not uncommon for a stocker.
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Re: What operating rpm can a 300ci. hold safely?

Post #14 by WorldChampGramp » Sat Apr 21, 2018 7:38 am

THE FRENCHTOWN FLYER wrote:1000 psi is not uncommon for a stocker.


As you would expect FTF is right there with a stock 1000 psi combustion pressure number. Here are some extra facts to hopefully influence your decision making for a successful on or off road Ford Big six build.

Consider this: In an engine with four inch cylinder bores and peak combustion pressures of around 1,100 pounds per square inch, each cylinder exerts about 13,827 lbs. of pressure against the cylinder head at full throttle. In fact, head bolts may have to handle loads of more than five tons or 10,000 pounds per bolt at wide-open throttle!
This clamping load that is typically about three times the peak pressure exerted against the head [called the “lift-off” force]. As a result, the bolts around the combustion chamber have to exert a combined force of about 41,500 lbs. to hold the head in place. An engine such as the 6 cylinder has four head bolts around it, each bolt has to exert a clamping load of 10,375 lbs. If there are five head bolts per cylinder, the load needs to be 8,300 lbs. per bolt. If there are six bolts per hole, then the load required drops to 6,916 lbs. per bolt.
In a performance engine or diesel, the loads are even higher. With peak combustion pressures of up to 1,400 psi or higher in a performance engine, or 2,400 psi in a diesel, the load on the head bolts is far greater. Consequently, the clamping force on the head bolts is even more critical than those in a stock engine.
Head bolts are truly one of the most critical fasteners in any engine. Think of these important facts next time you build your ultimate answer to the Ford inline doubters. MAKE sure all your efforts are not wasted with a blown head gasket or worse. Research and choose quality controlled studs or bolts. It’s extremely important that all the head bolts be in perfect condition, and are torqued in the proper sequence. Gramps

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Re: What operating rpm can a 300ci. hold safely?

Post #15 by THE FRENCHTOWN FLYER » Sat Apr 21, 2018 10:49 am

I went thru "a few" gasket iterations before I found a cure for my persistent head gasket failures. Still using 7/16 fasteners - have not upgraded to 1/2 yet. This with 14.3:CR, 112 RON.
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Re: What operating rpm can a 300ci. hold safely?

Post #16 by Lunatic Fringe » Sat Apr 21, 2018 8:46 pm

THE FRENCHTOWN FLYER wrote:I went thru "a few" gasket iterations before I found a cure for my persistent head gasket failures. Still using 7/16 fasteners - have not upgraded to 1/2 yet. This with 14.3:CR, 112 RON.


Were you losing the gasket between chambers or to the water jacket? Most pullers don't run water, if it's to water we'd only drop 1 cylinder- not 2.

We have a 4x8 sheet of .040 copper that will be pressed into service if necessary. Did you "O" ring the block and head?

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Re: What operating rpm can a 300ci. hold safely?

Post #17 by THE FRENCHTOWN FLYER » Sun Apr 22, 2018 1:45 pm

Mostly between cylinders. Sometimes to water.
Use O-rings if you are using annealed copper head gasket.
If using FelPro 1024 DO NOT O-ring as the gasket has an internal O-ring and the two will not be compatible.
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Re: What operating rpm can a 300ci. hold safely?

Post #18 by arse_sidewards » Mon Apr 23, 2018 11:11 am

It seems like after you get the head to breath you run into problems keeping it on the block

Are there any practical sealing options that don't involve a gasket?

Plenty of hydraulic applications seal faces with nothing more than an O-ring and high quality flatness/surface finish. Gaskets have gone extinct in a lot of applications. It's not the kind of thing that requires much trial and error in this day and age. You have an O-ring of known mechanical properties, shape and cross sectional area being crushed into a groove of known shape and cross sectional area by a known amount of force.

We should be able to spec out a copper O-ring and groove combo that results in the head and block crushing the O-ring to the point where they bottom out on each other at some specific torque value. From there you spec out rubber O-rings for the oil and water and size the receiving grooves to crush the rubber O-rings enough to keep the water out of the oil.

I'm not a professional mechanical engineer but based on the subject matter knowledge I've acquired over the years I don't see why that's not possible, sure machine work is expensive but you could probably put all the machined feature in the head and just surface grind the block to ensure it meets them near perfectly. The hard part would be ensuring that all the machined features are in the same plane and identical on each chamber.



THE FRENCHTOWN FLYER wrote:I went thru "a few" gasket iterations before I found a cure for my persistent head gasket failures. Still using 7/16 fasteners - have not upgraded to 1/2 yet. This with 14.3:CR, 112 RON.
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Is there enough room in the block to support M16 threads?

APR's marketing literature 210kpsi yield strength for their chromo product line. Grade 12.9 is 170kpsi which is little under 81% of that. ARP's website doesn't list anything larger than 1/2-20. McMaster-Carr sells M16x1.5 socket head cap screws in grade 12.9.

1/2-20 has a 11.1423mm minor diameter (0.4387") and 31.04mm^2 area which comes out to a 5587.2lb yield at

Whereas M16x1.5 has a 14.16mm minor diameter and 50.126 mm^2 area (61% more area than 1/2-20).

1.61 * 0.80 = 1.28, so the M16 bolt has a little over 25% more yield strength at a comparable price.

I haven't run the numbers on thread engagement surface area but if there's enough meat in the block (striping block threads with the more coarse 1.5 pitch might be the limiting factor ) but if there's room M16 could be an option for people who want to go overkill on clamping.
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Re: What operating rpm can a 300ci. hold safely?

Post #19 by CNC-Dude » Mon Apr 23, 2018 11:53 am

ARP has head bolts and studs for the 429/460 engines which are 9/16. The deck should be able handle this upgrade. The 1/2" bolt conversion for the Ford 6 is an easy conversion and can handle anything you could ever build forced induction or N/A. So a 9/16 upgrade would last forever.
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Re: What operating rpm can a 300ci. hold safely?

Post #20 by arse_sidewards » Mon Apr 23, 2018 1:13 pm

CNC-Dude wrote:ARP has head bolts and studs for the 429/460 engines which are 9/16. The deck should be able handle this upgrade. The 1/2" bolt conversion for the Ford 6 is an easy conversion and can handle anything you could ever build forced induction or N/A. So a 9/16 upgrade would last forever.


That's good to know. I didn't see 9/16 on their site but I didn't look all that hard. I just browsed fasteners. I didn't bother searching through kits.
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Re: What operating rpm can a 300ci. hold safely?

Post #21 by Lunatic Fringe » Mon Apr 23, 2018 10:51 pm

CNC-Dude wrote:ARP has head bolts and studs for the 429/460 engines which are 9/16. The deck should be able handle this upgrade. The 1/2" bolt conversion for the Ford 6 is an easy conversion and can handle anything you could ever build forced induction or N/A. So a 9/16 upgrade would last forever.


Here's some 9/16 studs for our next puller. Bottom end is 11/16 to go into an aluminum block. About 9" long, goes down to the bottom of the cylinders.
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Re: What operating rpm can a 300ci. hold safely?

Post #22 by Wesman07 » Tue Apr 24, 2018 5:17 am

When you increase the fastener size, are you increasing the torque spec too?
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Re: What operating rpm can a 300ci. hold safely?

Post #23 by Stonebreaker » Tue Apr 24, 2018 7:55 am

Wesman07 wrote:When you increase the fastener size, are you increasing the torque spec too?

Yes. Normal torque specs are usually from SAE tables such as this one: http://www.engineershandbook.com/Tables/torque.htm. It's only when somebody has unusual threads that they have a custom torque spec, like the modern torque to yield head bolts GM uses on its LS and LT series engines.

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Re: What operating rpm can a 300ci. hold safely?

Post #24 by Wesman07 » Sun May 13, 2018 9:38 am

I successfully made the I-90 continental divid pass in Montana last night. I held 3400 rpm in 2nd gear with a 12,500lbs rig. Took about fifteen minutes to make it to the top. Temp did not rise.

This motor is so close to retirement and I never asked so much from it.
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Re: What operating rpm can a 300ci. hold safely?

Post #25 by BigBlue94 » Mon May 14, 2018 11:13 am

I run mine at 3000 to 3300 up the highway for 30 minutes at a time. Drove it 2 hours at 3000 when i brought her home. All while turning 37" tires that weigh about 100 pounds each mounted and balanced.

E5TE block, .040 over, stock cam, 4bbl manifold and 390cfm carb, and headers. No fan shroud, just the crappy top guard on the stock thin radiator. Never a problem.
1985 Bronco. 309ci I6, NP435, 4.56 gears, Detroit locker and tru-trac, 4" lift, and 37" swamper tires. The 309 is 9.75:1 CR with a Schneider 140H cam, 4bbl, roller rockers, larger valves, and headers.

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Re: What operating rpm can a 300ci. hold safely?

Post #26 by Lazy JW » Mon May 14, 2018 12:05 pm

Wesman07 wrote:I successfully made the I-90 continental divid pass in Montana last night. I held 3400 rpm in 2nd gear with a 12,500lbs rig. Took about fifteen minutes to make it to the top. Temp did not rise.

This motor is so close to retirement and I never asked so much from it.


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