What Actually Happens When RPMs Too Low?

[anon1]

Hello,

What, tulipped valves?? How? How low for how long?

Recently got new 4-blade prop to test due to sluggish low end performance. With original 3-blade prop engine would turn 4600rpm on the 4200-4600 recomended range.

New prop went up 1" pitch and engine lost 700 rpms, It now runs WOT @ 3900, maybe 4000. Obviously that is outside the minimun recomended. But is it that bad?

Today I did a compression test and found 124, 110, 90, 110, 110, 112, 110, 0, #3@90 went to 110 after 3 squirts of oil. #8@0 just fluttered the needle a little.

Both times, before and after the switch, the engine has always ran good, no backfires or stutters. Before would top out at 4600 @ 52mph, after 3900 @ 56mph.

Now I'm REALLY wondering if #8 @ 0psi was really the problem before the prop switch or if testing the new prop at 3900 is what caused the 0psi?

 

[Apollo75]

Re: What Actually Happens When RPMs Too Low?

Hello;

The general consensus is when replacing a 3-blade with a 4-blade to drop down 1" in pitch. Since you added 1" @ about 250-rpm per inch, that would seen right for you to lose 500+ rpm.

I would see if you can exchange it for one 1" smaller in pitch to bring your rpm up a little.

I cannot say if it would hurt anything.

Added : Sorry, missed the zero compression on #8 ---> that is probably where your extra couple hundred rpm went --- I hear big blocks only lose 100-rpm

I do not think a day with the new prop did this.

Have fun

OFM

 

[freeone]

Re: What Actually Happens When RPMs Too Low?

Now I'm REALLY wondering if #8 @ 0psi was really the problem before the prop switch or if testing the new prop at 3900 is what caused the 0psi?

prop would not cause you to loose comp. just a coincidence. now 0psi on #8 will cause sluggish low end performance. increasing the prop pitch reduces RPM levels by roughly 200 rpm's per inch of pitch. this has some good info. http://www.caravelleboats.com/propeller info.htm

 

[TilliamWe]

Re: What Actually Happens When RPMs Too Low?

You had problems before the prop switch.

The cylinder that raised when you squirted oil has a ring issue, not valves. #8 is most likely a valve issue, though. You've got more than one problem with your motor.

But to answer the broad question, tulipped valves is what happens when you "lug" an engine. And at only 3900rpm WOT, you were lugging it a lot. Did it make any noise during your testing? Noise that sounded like detonation or pinging? If it didn't, then you didn't damage the valves during testing. But if it made lots of combustion noise during testing, and you continued to run it, then you could have done it then.

Of course, confirm your tachometer is correct. Maybe you weren't really turning 4600 with the old prop, and on days when you loaded the boat up with extra gear and passengers, you were lugging the engine. If your engine can't turn its MAX WOT RPM, then you are lugging at every rpm, not just the top rpm. Make sense?

 

[Bondo]

Re: What Actually Happens When RPMs Too Low?

Both times, before and after the switch, the engine has always ran good,

Ayuh,.... But it was runnin' on 7 cylinders begining at Some point......

 

[Apollo75]

Re: What Actually Happens When RPMs Too Low?

tuliped valves -- I have never seen one --- I have seen bent, melted, valve heads that have fallen off, valves that have run very hot and got pounded into the ports by spring pressure.

I have heard of intake valves being tuliped --- well an intake valve runs cool as it gets a cool charge each cycle --- only way it could have tuliped is to run hot and the only way an intake valve runs hot is from detonation.

I say lugging could tulip a valve only if accompanied by detonation

I have just changed my prop and increased 300-rpms from 3600 to 3900. That prop has been on for years --- never hurt my engine.

More experienced opinions needed

OFM

 

[anon1]

Re: What Actually Happens When RPMs Too Low?

Apollo75: I was told that going from 3 to 4 blades would take about 150-200rpms and up 1" would take away 200rpms for a total of 350-400, not the 700 that I got.

TilliamWe: I had a shop tach on the engine when testing with the new prop. Surprisingly (based on what people say) both the tach and the speedo are very accurate. I did not have the shop tach on with the old prop but with the confirmation of accuracy I would think that it was actually turning 4600 before.

Bond-o: I'm beginning to think that that point might have been before I bought the boat. I've not had it long but it has never really performed like I thought it should.

Appolo75: Detonation from overheating just one cylinder? From an air leak or something?

Thanks.

[add] Does anyone think that this 350magnum would have run for extended amount of time with only 7 cylinders. Like if I did buy it this way, I've used it 5 or 6 times and run it for several hours at a time. Could it survive this on 7 cylinders without causing more damage and stopping the engine?

 

[TilliamWe]

Re: What Actually Happens When RPMs Too Low?

Apollo, I have seen 3 tulipped valves, all from my 1997 5.7LX EFI Gen + engine. I may still have one, and I'll try to find it. It's obvious just from looking at it, that it is deformed, but certainly was never struck by the piston. Which valves in a SBC are the bigger ones? If it's the intakes, that's the ones that were tulipped in my engine. As I pointed out, there was definately detonation going on in my engine, but to an untrained ear, you might not know what the sound is.

Anon, you would be STUNNED at how well a 350 Chevy will run on 7 cylinders. I ran my boat for over a year on 7 cylinders. (with another cylinder down about 30psi comapred to the other 6) Besides some black smoke at idle (which you didn't see if you were moving at all), it would still hit 4800rpm when the boat bottom was clean. After I repaired the heads and valves, it would hit 4950, just as it always had. It would run "forever" on 7 cylinders, actually. Mine ran for well over 100 hours that way. It did use more gas, seemingly, but other than that, not a big deal. But boy o boy, when the bottom got just a minor amount of growth, the engine lugged and detonated badly. And it burned a lot of extra gas, cause it was working so hard.

 

[Apollo75]

Re: What Actually Happens When RPMs Too Low?

Apollo75: I was told that going from 3 to 4 blades would take about 150-200rpms and up 1" would take away 200rpms for a total of 350-400, not the 700 that I got.
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.
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Appolo75: Detonation from overheating just one cylinder? From an air leak or something?

Thanks.

[add] Does anyone think that this 350magnum would have run for extended amount of time with only 7 cylinders. Like if I did buy it this way, I've used it 5 or 6 times and run it for several hours at a time. Could it survive this on 7 cylinders without causing more damage and stopping the engine?

Cylinder #8 is a weak cylinder while idling ---> if you have ever removed wires from a distributor cap to check firing then you will remember some really effect the engine and some don't. You probably never noticed the engine being rough in neutral --- may have felt something while idling in gear ??

Those are just ideas thrown out that "may" have contributed to the problems --- detonation ---> one cylinder goes then another --- engine could have a lot of hours and be worn and #8 was the first to give up the ghost.

About the exact number of rpms down to single digits --- check with hawsiii on the "prop and other topics" forum. With a cylinder out and under more load with the new prop more rpm loss will be seen than the old prop with less load.

OFM

 

[anon1]

Re: What Actually Happens When RPMs Too Low?

...
Anon, you would be STUNNED at how well a 350 Chevy will run on 7 cylinders....

And I think that might be why I didn't check it out more before buying it. I paid for two surveys on other boats that I was looking at and didn't buy. This one ran good and didn't set off any alarm bells.

 

[Apollo75]

Re: What Actually Happens When RPMs Too Low?

Yes, it could be a lean condition --- an engine under load at all times needs more fuel at all times than a car engine. Plugs should be dark tan / grey --- the side facing the intake should be fuel blasted and almost black.

Could have been a vacuum leak, bad intake gasket, carburetor, any thing that makes it lean --- and that is one of the last cylinders on the block with a long intake runner not like the middle cylinders.

These are just guesses.

I would fix it -- don't need any more damage --- I hope it is just a valve adjustment --- wouldn't that be nice

Good Luck

OFM

 

[180shabah]

Re: What Actually Happens When RPMs Too Low?

I was told that going from 3 to 4 blades would take about 150-200rpms and up 1" would take away 200rpms for a total of 350-400,

If you were trying to increase hole shot why would you want to "take away" any RPM's???

not the 700 that I got.

The estimates you were given are just that, estimates. They also assume a healthy engine.

Skip the prop question until you get your engine running properly.

 

[Apollo75]

Re: What Actually Happens When RPMs Too Low?

Apollo, I have seen 3 tulipped valves, all from my 1997 5.7LX EFI Gen + engine. I may still have one, and I'll try to find it. It's obvious just from looking at it, that it is deformed, but certainly was never struck by the piston. Which valves in a SBC are the bigger ones? If it's the intakes, that's the ones that were tulipped in my engine. As I pointed out, there was definately detonation going on in my engine, but to an untrained ear, you might not know what the sound is.

As stated, I have just never seen one --- an engine with the valves sunk down into the ports by heat from detonation may be what people call tuliped. I do not know.

Yes the intakes are the bigger valves --- I hear about this tulip problem --- It takes a lot of heat and pressure to get a valve to deform like that.


anon1's engine could have been a victim --- cannot say for sure.


Best to the both of you


OFM

 

[TilliamWe]

Re: What Actually Happens When RPMs Too Low?

--- an engine with the valves sunk down into the ports by heat from detonation may be what people call tuliped. I do not know....

Now you know, that's what a tulipped valve is.

Anon, for a while, I didn't even notice the black smoke coming from my engine. But mine was always in the water. Had I run it on the trailer, maybe I would have noticed? Finally I noticed when backing into my slip, on a wind-less day, and I smelled gasoline. Then I looked at the transom and saw a little bit of a fuel slick. At that point I said, hmmm, I've got unburned gas coming out, let's do a compression check". Voila, #2 was at 0psi, and #5 was down around 120. I did loosen the intake valve on #2 and got 60psi, that's when I was sure I had a valve issue.

 

[anon1]

Re: What Actually Happens When RPMs Too Low?

... Voila, #2 was at 0psi, and #5 was down around 120. I did loosen the intake valve on #2 and got 60psi, that's when I was sure I had a valve issue.

What engine was that on, and roughly how much $ was it to fix? Thank you very much.

 

[TilliamWe]

Re: What Actually Happens When RPMs Too Low?

1997 5.7 LX EFI Gen +

Repair done in 2005--Reconditioned head was $179 plus tax. Reconditioning the other head was $160 no tax. Mercruiser gaskets to put it all back together were $200. Labor was free, I did it myself. I suggest you do it too, cause to pay a shop $80+ an hour, is gonna cost you huge. I would think a shop would charge no less than 16 hours labor, more like 20. You do the math.

 

[45Auto]

Re: What Actually Happens When RPMs Too Low?

Appolo75: Detonation from overheating just one cylinder? From an air leak or something?

Thanks.

[add] Does anyone think that this 350magnum would have run for extended amount of time with only 7 cylinders.

As others have said, a 350 MAG running on 7 cylinders is hard to detect some times. The engine the piston below was in (starboard engine on a twin engine boat) was smooth as silk, idled at 650 RPM all day, but wouldn't turn over about 3500 RPM. Identical port engine would go over 5000 on the same prop.

All other cylinders and pistons looked brand new, looks like just the one cylinder had big-time detonation problems.