Inliners International Forums General Discussion Engines max substained RPN

Rate Member posted 09-28-2007 04:37 AM
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Somewhere I read that the max substained RPM for long stroke engines was 85% of rated horsepower RPM. The 218 cu flathead Plymouth engine falls into the long stroke engine catagory. The horse power is rated at 3600 RPM which limits the top driving RPM to 3060 RPM. Does anyone know if the 85% rule is a true statement for long stroke engines and where does it come from? Somewhere someone has a manual for engine design which probably has this information and I would like to know more about the subject as I have been cruising mine up to 3400 RPM and just replaced all of the piston pin bushings and all of the bearings. The bearings all showed signs of destress and two cylinders had the top ring broken. It was suggested that I had run the engine at too high of an RPM range for long periods of time and maybe to much advance. The top half of all four main bearings showed more wear than the bottom halfs did, what gives here? The engine had less,than 8600 miles from total rebuild, but I did a 4600 mile trip in August which probably did the most damage as we cruised between 65 and 75 MPH for hours on end (3200 to 3400 RPM).

Dear Jim;

I've never heard of that one. Hard to say where it came from etc.

Sounds like low oil pressure, or to thin an oil could have been used. The distance (4,600) is not long or the speed (3,300) to high. I doubt that your trip/operation was the cause.

How many times was the oil changed during the 'break in' period?? What about after/during the trip?? Were you using multi-grade oil??

Did the 'rebuilder' make the suggestion?

Check with Mike G. He may have some info on this subject.

Good luck.

If Mike G. doesn't have the answer, I'd bet Tom Langdon at Stovebolt Engineering does.

I'd think that new piston pin bushing and rod bearings would lower the end compression height of the piston at the the end of the stroke, but... Did the ring land get removed? I'm assuming new piston rings or,,,

Larry

I built the engine myself, this is the first engine failure that I have ever had. The oil pressure always ran at 40 to 50 at 60 MPH. 20 at idle at 500 RPM. Have always run 40W to 50W oil, Delo 400 for first 1200 miles changed to Mobil 1 but changed back when it engine would not quit using oil. The 4600 mile trip used three gals of Delo 400. The pistons were not hurt just two broken top rings. The new rod bearings plastigaged out to .0015 on all six rods. Just got the engine running again and retorqued the head.

J;

You may have cracked the rings in instalation, but that's really rare. This accounts for the oil consumption.

Why were you using such heavy oil, during break in??

Two thing break top rings. Insufficient end gap (.004" per inch of piston diameter required) or detonation. caused by too much timing or not good enough fuel. Top oil ring has nothing to do with using oil. It relates to compression pressure.
Nothing wrong with 40 wt oil. It is extra protection.
Lots of chrysler engines were used for wooden launches and are infact still running some without even an minor overhaul in 40 years. They are twin carb engines for the most part. They run usually at idle or wide open most of thier life. One hing that really is differnt is tey have huge oil sumps compard to a car engine and they run cooler of course with a constant supply of coolant. DId youb re te engine or just re ring it. D. Which rings did you use , wnat brand?
That is what I would wnat to know.Then we could go from there if you wish.
Re stroke length. Engine design principles generally be live that piston speed is the limiting factor. Under 3500 is good under 3000 is safe. That is not RPM it is feet per minute.
Take the rpm x the stroke in inches X 2 /12 = feet per minute.. I am sleepy so i will recheck this inthe morning to make sure i didnt error but as long as you are under this there is no reason to expct engine failure even at sustained running. given sufficient oil volume (oil being the bearings coolant ) and sufficcient cooling by the rad and water passages.
D
D

I am running 10 degrees advance at idle and I am deaf to the point that I have not been able to hear a car ping except when the 70's cars would run on. I got some low octane gas,low 80's in Calif. About the engine,I had the head milled .090 and had to releive over the valves for .060 clearence and the cylinder bore is .040" over stock. I am runnung dual exhaust and intake with Carter YH carbs. and a mild cam. Car converted to 12 volt.

D;

There WAS a "ring problem" as two broke and there was NO mention of any leakage.

New/rebuilt engines usually run SAE 20 oil during 'break in' and then heaver, depending on the type of operation planned etc. Using SAE 40 or 50 during that period is NOT 'normal' either.

With 40+ pressure all the bearings should have run just fine. Why there was a failure there is a mystery, I guess.

As far as the pin bushings go; this may be due to the higher compression and advanced initial timing, (higher combustion temp) hard to say. They may have been to tight to start-off & the heavy oil just didn't get in there enough.

Those MOPAR flatheads ran forever (almost) like you say, till they were replaced in the 60s.

Sometimes "stuff happens".......

Good luck to all.

MAy have been tght to begin with is oftea roblem in rebuilds. An engine use in severe service which a hi perf engine is should have nothing snug . I make sure my pins will fall through the pistons and the bushings if full floated on thier own when turned sideways. if they dont I make them so they willl but you probably already did that.
I do not suscribe to the ligtt pil for breakin club. I use in my engines and have for years 40 weight and if it is in the hogh 90F degrees when we are racing 50 weight. The most wear occurs when frst started so that is really when you need the most protection. I prelue the engine before starting anf amake sure it fires immediately by double checking timing and pefilling the carbs with a syringe down the vent tubes so it will fire immediately.
I haveri un accross an idea that os going around that sugggests one roll the engine over on the stater till oil pressure comes then start. This is the worst oine could do and has runed many a motor. (and camshaft as well) I guess all you can doi know is clan it up and sart of=ver with fresh rings and bearings. If it was a frsh rebore Moly ring are best. hastings is my preference. Nobody makes a better oil ring/ Chrome rings just as a note are notoriuos for burning oil. Another problem can be the gentle breakin idea. I tell my cstomers once I have fired it broken the cam in a 2000 plus for 20 minutes, then changed oil and filter while it was hot r to remove Assembly greases and any dust and such that got in during the assembly period to then drive it like they stole it or as the Ring manufacturer says drive as it would be drivin i its intended use. Rings need a bit of abuse to seat in . Babying a new mtor for a 1000 miles or so is a sure fire way to have an oil burner.
Yeah you are right too occasionally but not very often crap seems to just happen.
Anyway I wish you the very est . You have a very interesting project there andI see it is no trailer queen but is driven as it should be.
Best wishes
D

I sent Tom Langdon an e-mail asking him about the max substained RPM and this is his reply.

"The 85% is a rough guideline but that is all. Bearing material and oil
temperature also can have a dramatic influence on the "Safe" speed."
Tom

So I still do not have any hard and fast rule for a guide line on max substained RPM for long distance driving. Probably the 10% advance at idle caused the problems.

The Mopar engine has a few oiling issues. When the main bearings are installed into the block attention has to be paid to the alignment of the hole in the block and the bearing. They do not align perfectly. Either the block or the bearing has to be modified to get full flow . The bearing has to be modified also. The groove in the main is too shallow and too narrow. This also needs to be addressed. Another issue is the rod oiling system. The #2 main supplies oil to #2 and #3 rod. The #3 main supplies oil to the #4 and #5 rod. There is only one hole in the crank to supply oil to the two rods. One rod will get shorted of oil at some time. The crank needs to be crossdrilled where the rods will get oil all the time from the main. These mods will make the life of the engine a lot longer.
I have a 230 in a pulling tractor. It runs at 5500+ rpm. It had been doing it for some years now and have had no problem out of it. After the first year of running, I dropped the pan and checked the bearings. There was no wear on any of the bearings. Haven't been in it sense. It runs today as good as it did the first time I fired it. It has been running 7 years now.
Hope this helps.

A lot of these old inlines where used in big trucks equipped with a governor to limit rpm.Generally,that engine speed is a good indication that the engine speed the manufacturer felt the engine would have a "reasonable" life span and reliability at that rpm.

Dear Jim;

I just remembered those governors & Tony's correct. They were on (commercial vehicles) and trucks that went "cross-country".

Perhaps an OD transmission is in order.

Happy trails.

Looking through old factory repair manuals I see for instance the 216-235 splash oil Chevy's,the maximum recommened sustained rpm is around 3000 rpm,the later 235-261's and GMC's,3200 rpm.The other period inlines are most likely similar.And this isn't just crusing down the road,the throttle is for the most part wide open pulling heavy loads.

T;

The early Chevy's had 4:11 rear ends & 3,000 was 60 MPH and they ran for years too.

The roads then weren't like today, so ???

Happy trails.

Jim, you mentioned that you were running 10 degrees initial advance; at 3200 - 3400 rpm the centrifugal advance will be all in AND at sustained speeds the vacuum advance would also be in....if you total all of those up I'll bet you are too far advanced at cruising speeds causing detonation.....that could be the cause of the ring damage....normally the TOP half of the rod bearing will show more wear when subjected to detonation and I have seen it severe enough to blacken the rod behind the top half bearing...I'm a bit unclear as to why the top half of the mains showed more wear....perhaps the rotation of the crank forced more oil to the lower half...if thats the case then the volume(not necessarily pressure) was insufficient ...a buck and this information might get us a cup of coffee but it might help diagnose your problem....good luck

3000 for a late 235 is about right for max sustained rpm.

I got a 3.55 rear and take a 250 mile round trip on the freeways, every other month.
I found at 2500-2800rpm the engine runs near thermostat temp consistently.
If you go 3000rpm or higher or pull up mountains, engine temp creeps up a bit, especially during summer in the desert.

One of the last super hot days in September, I brought my new rebuilt 261 home on a 3500ft mountain climb of 100ft/mile.
- Never seen my old 235 that hot around 1500-2000rpm.

Max sustained rpm definetly depends on the weight and terrain.

I know my new 261 with the same driveline, but 1200 lbs. lighter weight will stay cool and allow higher sustained rpm.

I have an overdrive in my 55 Chevorlet Pick Up with 3.9:1 rear and 235 75 15 tires on 8" rim with Fenton headers, small body HEI And a Clifford manifold with a 390 CFM Holley and I have trouble holding 70 mph in overdrive. Can run 60/65 and hold more than 10 inches of vacuum but at 70 5 to 0 inches of vacuum. I have an dual 1 barrel manifold that I an thinking of installing instead of the Clifford 4 barrel manifold. Will be using Carter replacement carbs for the Rochesters which are now on my 48 Plymouth.

One thing to remember;when guys drove those trucks flat out they were a lot newer.Now ,60 years later,with millions of cycles on the crank and rods,who know if you'll run over the crankshaft if and when some sort of metal fatigue kicks in.