Quick WOT RPM question...

[smiles16]

I have a 19.5' Ebbtide Catalina Bowrider with a 305 4bbl Merc and Alpha One. I have had the carb rebuilt, tuned up the engine, put fresh gas in her, and installed a brand new Aluminum Quicksilver 19 pitch prop.

I had 3 people on board (roughly 550lbs) including myself with a near full tank of gas (roughly 25- 30 gallons). I want to note my boat does not take on a drop of water and is conplerely dry and sound.

At full trim and WOT I was pushing about 4150 according to the tach.

Does that sound about right or do I need to step down in prop pitch?

 

[Scott Danforth]

What was your speed per GPS?

My searay would hit 4800 rpm at 48mph with 4 people and a 19p prop with a 5.0

 

[smiles16]

It was 50 mph per the GPS.

 

[Scott06]

I'd verify the tach before I'd change pitch. What Scott mentions is what you'd typically expect with a 19" pitch unless some one changed drive ratio

 

[smiles16]

I will try to get a hold of a shop tach for the weekend.

Would the serial on the drive be able to confirm drive ratio? Is there any other way to tell?

PO was mostly concerned with speed when he owned this boat, so ratio has been a question in the back of my mind.

 

[smiles16]

I want to add that I just used one of the online prop calculators using my current prop size (19p), 0.15 slip, 1.47 gear ratio, and 50 mph. The calculated RPM came out at ~4800. So, I do believe this is a tach issue then.

This would also possibly explain why my idle-air has been such a pain to tune since I have been tuning my carb (Qjet) at 650 RPM per my current tach.

This raises a question for me though... If my tach is 700 RPM off at WOT, how am I still reading 650 at idle? Theoretically, this means my tach should be reading -50RPM at idle 'eh?

I know there are adjustments on the tach based on the number of cylinders on the engine, but is there a way to calibrate it beyond that?

 

[Bondo]

I know there are adjustments on the tach based on the number of cylinders on the engine, but is there a way to calibrate it beyond that?

Ayuh,.... Calibrate it to the nearest dumpster, 'n slap in another one,....

If idlin' at 650 rpms,.... or even 600 rpms, so long as it ain't stallin',..... what's the problem,..??

 

[smiles16]

Well er'.... not so sure Im actually idling at 600 or 650 anymore if the tach is that far off at WOT. Been fighting a slight bog at in gear idle and chocked it up to a poor idle air setting. Now im thinking im idleing so low that the engine is just trying to stay running.

I really should just to chill out and put this thread on hold until I verify my true RPM.

I was mainly trying to figure out if the speed vs pitch was matching everyone else's numbers in case I had an engine issue.

 

[AShipShow]

Doesn't the 5.0 usually have a 1.65 drive ratio? You said above you calculated slip at 1.47 ratio...if its actually 1.65 then you definately want to check your RPMs with a shop tach cuz your way over revving if so.

 

[smiles16]

Mine is a factory 4bbl 305 (230hp), so it should have a 1.47. The 1.62 came on the 2bbl 305's (200hp).

 

[HT32BSX115]

Mine is a factory 4bbl 305 (230hp), so it should have a 1.47. The 1.62 came on the 2bbl 305's (200hp).

Unless you bought it new, you should verify your drive ratio, without an accurate ratio, RPM and speed, the prop-slip calculator is totally useless.

Pull the spark plugs, have someone turn the crankshaft by hand (in gear) and count prop blades.

1.62: 1 is 16.2 turns of the crankshaft to 10 turns of the prop OR...... 8.1 turns of the crank to 5 prop revolutions etc etc etc.....

0.1 turn is 36 degrees or a tad less than halfway between 12 o-clock and 3 O-clock.... 0.2 x 360 = 72 degrees Or just a little shy of 1/4 turn (90 degrees)

1.47 equals approx 1.5:1....... 3 engine turns = 2 prop revolutions etc.....

It's easy to verify your ratio.


Cheers,

Rick

 

[Scott Danforth]

Ratio is on the sticker on side of drive

Make sure your tach is set right. Could be set to 6 cylinder. Then at 4150 rpm, you would actually be spinning 5500 rpm.

 

[smiles16]

Im currently reading too low. Wouldn't a setting of 6 make it read higher?

 

[smiles16]

Anyway... I have purchased a shop tach and will be checking out this weekend. Will be able to update on Monday.

 

[smiles16]

Update:

Took it out last weekend after buying a Harbor Freight Automotive Multimeter that had a inductive tachometer option. Do not buy these for the tach readings, it was worthless.

So, I never got to confirm my current tach. I will be "renting" one from a local auto parts store like I should have in the first place.

To add to this post- I had 4 guys on board including myself (bachelor party) and a cooler. I was still able to reach 48.8 mph per GPS.

 

[achris]

....This raises a question for me though... If my tach is 700 RPM off at WOT, how am I still reading 650 at idle? Theoretically, this means my tach should be reading -50RPM at idle 'eh?

You are assuming the tacho is off by a constant, when in fact most inaccuracies are more arithmetic, like a percentage off. If it's off by 700 at 4800, that is reading 15% low, so at idle, 650 revs, it will show 550... Although, that said, most 'inaccurate' tachos will be accurate at one particluar speed, and read progressively higher in one direction, and lower in the other.

I know there are adjustments on the tach based on the number of cylinders on the engine, but is there a way to calibrate it beyond that?

Not without opening it up. And all the ones I have come across are plastic welded together. Even the ones I have opened up, the 'calibration' is a laser cut resistance.. Not really adjustable.

Im currently reading too low. Wouldn't a setting of 6 make it read higher?

Yes. It would increase the reading by 33-1/3%

Chris......

 

[smiles16]

Thanks Chris! I didn't think of it this way, and it makes perfect sense.

I bet the pink slip on my boat my idle issue is the engine just trying to stay alive. Which also explains why getting the idle air right has been such a bear.