Top speed correct?

[SeaDooSam]

Hi all,
My '96 Chaparral 1830, I have a feeling the engine is not working at its full potential. It is a 4.3lx 190hp with alpha one gen + drive.
Here are the facts:
WOT RPM: with 17pitch prop 4300-4400
With dinged up 19 p prop 4200-4300
Top speed: with 17p 38 mph gps (1person)
With 19p prop 42 mph gps (1 person)

I feel that it should at least do close to 48 mph.
It just had an oil change and new carburetor.

 

[thumpar]

It calculates out right.

 

[SeaDooSam]

Thanks very much! Is there a way to calculate that? Or by calculate you just mean "sounds right"

 

[thumpar]

There are a few prop slip calculators you can use. With a 12% slip (about normal) you are dead on what it should be.

 

[stonyloam]

19 in/rev. X 4300 RPM / 12 in/ ft X 5280 ft/ mi = 1.289 mi/ min X 60 min/ hr = 77.36 mph at a 1:1 ratio. You Run 1.81 ratio outdrive. 77.36 mph/ 1.81 = 42.7 mph.
that would be no slip, 12% slip would be 37.6 mph. So your tach is probably reading a little low. Your 17 inch prop would calculate out to 38.2 mph. (No slip) So yeah you are right on!

 

[JoLin]

I Kinda get what you guys are saying, but what about the fact that the OP's engine isn't reaching full recommended rpm at WOT (4400-4800 for the 4.3)? Wouldn't that indicate that there's room for improvement? Mine top out right at 4800. Just askin...

 

[stonyloam]

I Kinda get what you guys are saying, but what about the fact that the OP's engine isn't reaching full recommended rpm at WOT (4400-4800 for the 4.3)? Wouldn't that indicate that there's room for improvement? Mine top out right at 4800. Just askin...

Well yeah, that is the whole point of doing the calculation. 4300 RPM with the 19 pitch prop comes out to 42 mph at 100% efficiency (0 slip), and you know that can't be right. At 10% (about average) slip the engine RPM would need to be 440 RPM higher than he is reading. Calculate that out at 4740 RPM, 10% slip and you get 42.4 mph, which is just about right on. My suggestion is to check the accuracy of the tachometer at WOT.

 

[SeaDooSam]

Ok thabks for all the great info. It seems to be right on the money. I wonder why many people get closer to 50 out of their 4.3l? Thanks again. Very helpful and thanks for writing out the calculation! I will try to Check the tach again.

 

[thumpar]

Ok thabks for all the great info. It seems to be right on the money. I wonder why many people get closer to 50 out of their 4.3l? Thanks again. Very helpful and thanks for writing out the calculation! I will try to Check the tach again.

Some people use the dreamometer on the dash or make it up like fisherman do on the size of a fish.

 

[SeaDooSam]

That may be true. People might count 45 as 50!

 

[JoLin]

Well yeah, that is the whole point of doing the calculation. 4300 RPM with the 19 pitch prop comes out to 42 mph at 100% efficiency (0 slip), and you know that can't be right. At 10% (about average) slip the engine RPM would need to be 440 RPM higher than he is reading. Calculate that out at 4740 RPM, 10% slip and you get 42.4 mph, which is just about right on. My suggestion is to check the accuracy of the tachometer at WOT.

Thanks. I didn't have my head in gear. The calculation is used to solve for several factors- like slip, rpm and speed. In this case, given that slip % and speed are both about where they should be, it points to a possible issue with the tach. Got it!