What pitch prop?

[mikjas2]

I have a 1985 70hp evinrude on a 1994 mirrocraft 16' aggressor, prop is a 17 pitch now and seems to get up fairly quick but only has a 35mph top speed.The 40 I just took off would do 25mph so just thought it would be alittle quicker, am I wrong is that about normal? would going to a 19 pitch be too much for it? Looking for ideas?

 

[jimmbo]

Re: What pitch prop?

10mph for 30hp sounds pretty good to me. What year what the 40hp? What RPMs are you getting out of the 70? range is 4500 - 5500

 

[metriccrescentwrench]

Re: What pitch prop?

Actually you're doing quite well at 35 mph for that power increase. Doing the math, in theory you should get roughly 33 mph when going from 40 to 70 hp. You have to remember that drag increases roughly in proportion to the square of speed, so it takes a tremendous power increase to go faster and faster.

 

[1kruzer1]

Re: What pitch prop?

Your concern should not be absolute top speed, but rather getting the motor propped to run at the high end of its RPM range at WOT. These are great engines but need to rev up to survive long term on today's fuels. You need to borrow or somehow obtain a tach to use for this testing, and your goal is to prop for 5500 RPM minimum. 5500 - 5800 RPM is ideal. You can do a search in this forum on this topic and you'll see lots of posts regarding the fact that these 3 cyl. motors perform best and last longest when running at the highest end of their RPM range at WOT. I suspect on your boat that the 17 inch pitch prop is pretty close, but check with a tach before blindly changing props. Good luck.

 

[metriccrescentwrench]

Re: What pitch prop?

You need to borrow or somehow obtain a tach to use for this testing, and your goal is to prop for 5500 RPM minimum. 5500 - 5800 RPM is ideal. You can do a search in this forum on this topic and you'll see lots of posts regarding the fact that these 3 cyl. motors perform best and last longest when running at the highest end of their RPM range at WOT.

Good point. I go along with going for the top of the rpm range. That'll also give you the best hole shot and it's easier on the engine because it'll have lower peak combustion pressures, which means less chance of detonation and less chance of sticking the rings with carbon.

ALSO, as to the idea of gaining speed by blindly going up in pitch - that usually doesn't work out. There's a somewhat broad rpm range where your engine is putting out roughly maximum horsepower at WOT and you're going to go just about the same speed whether you're propped to turn 5400 at WOT or 5900 at WOT. Given that, you're better off going with the prop that gets you to the top of your max rpm range.

 

[444]

Re: What pitch prop?

Good point. I go along with going for the top of the rpm range. That'll also give you the best hole shot and it's easier on the engine because it'll have lower peak combustion pressures, which means less chance of detonation and less chance of sticking the rings with carbon.

How do you figure lower combustion pressures when you're running an engine at its peak rpm where it's making max hp?

 

[jimmbo]

Re: What pitch prop?

It is at max torque where the peak combustion pressures occur. Hp is torque x rpm. Max hp occurs at a higher rpm even though torque is dropping. Over revving too far past rated rpm has torque drop so much hp drops too.

 

[mikjas2]

Re: What pitch prop?

Ordered a tach today so I will know soon what RPM its running at
Looking through some stuff on the internet says it should be around 5000 at WOT or is the 5500-5800 more accurate?
Thanks for all the replys

 

[jestor68]

Re: What pitch prop?

The 70 'Rud is rated at 70 hp at 5000 rpm. The full throttle operating range is 4500-5500.

 

[1kruzer1]

Re: What pitch prop?

The 70 'Rud is rated at 70 hp at 5000 rpm. The full throttle operating range is 4500-5500.

True, but a quick search on this topic will return several opinions from experienced pros on the subject. The common point is that these 3 cylinder loopers give best performance AND longevity by propping them to the high end at wide open throttle. Indeed, in subsequent years OMC updated the recommended operating range to 5000-5800 RPM with no redesign of the engine. The general concensus is to target for 5500 RPM at a minimum, with 5800 RPM being ideal. And, yes, you do have to watch that you don't over do it. Opinions very on the max RPM, but it seems most agree as long as you don't exceed 6000 RPM the motor will be fine.

 

[jimmbo]

Re: What pitch prop?

Indeed, in subsequent years OMC updated the recommended operating range to 5000-5800 RPM with no redesign of the engine.

Actually the later engines that were rated at higher revs were different. 56.1 inch blocks vs. 49.7 inch blocks. Totally different engine

 

[1kruzer1]

Re: What pitch prop?

I'll have to crack out the old books to be sure, but I'm "pretty sure" the rpm range went up a couple model years before they increased displacement. Also, I am very certain the 49.7 75 HP model was always rated to 5800 RPM, and internally the 70 and 75 are identical.

 

[jimmbo]

Re: What pitch prop?

No they are not identical, different heads and the block is ported differently. The 75 was rated at 5500 with a 5200-5800 range. In later year once the bigger block was out the 49.7 block was 4500-5500 except perhaps for some racing versions( can't forget about them )

 

[1kruzer1]

Re: What pitch prop?

Same pistons, same connecting rods, same crankshaft, same bearings, same bore, same stroke, same piston velocity and kinetic forces and any given RPM. My point is the 2 engines will have about the same tolerance to RPM, and as you stated, the 75 was rated to go to 5800.

 

[1kruzer1]

Re: What pitch prop?

Same piston velocity and kinetic forces AT any given RPM. (Sorry, typo.)

 

[1kruzer1]

Re: What pitch prop?

OK, opened the books to find Jimmbo is correct that the 49.7 70 RPM range went to a high of 5500 through the '85 model year, and the 49.7 75 went to a high of 5800 through that same year. In '86 the 70 came out as a redesigned 56.1 displacement with a max RPM of 6000 as Jimmbo referred to above. In that same year the 49.7 was available as a 60 horse with the same rpm rating is the previous 70, and as the Sprint which was a short shaft version designed for a specific competition class, and carried the same rpm rating as the previous 75. I stand corrected before Jimmbo, and damn, I hate being wrong!

 

[jimmbo]

Re: What pitch prop?

Don't sweat it. There are so many variations over the years. You think OMC has variety, Mercury, with all the changes to the inline 6s alone requires huge data base.

 

[mikjas2]

Re: What pitch prop?

Ok so that was quite the debate!
I gather from all that I should be looking to get to 5500 rpm

 

[jimmbo]

Re: What pitch prop?

Yes, for best overall performance, and engine longevity, 5500 would be the goal.

 

[1kruzer1]

Re: What pitch prop?

Our friendly debate was more about what OMC rated for what RPM and when. Everyone has an opinion, and here's another from a similar earlier thread:

Gonna touch on a sore spot here that keeps showing it's ugly face and many of us that have experimented, some for many, many years have found that the "RECOMMENDED RPM RANGE FOR A 1976 60, 65 AND 70HP TRIPPLE IS 4500-5500" was just fine....in 1976 for a few years. Since our fuel is 1/3 the quality of the 1976 fuel, several manufacturers and several of us tearing powerheads down through the years have PROVEN that with current day fuels, this engine SHOULD NOT be set-up for anything lower than 5500-"this figure" is what we now recommend as a minimum rpm. This engine will last much longer if you target about 5700, even 5800.