WOT speed on a '83 20' SF w/200

[VirginIslander]

Hey dljohn,
Looks like we're neighbors. I'm in Newberry and work in Gainesville.

Your numbers are kinda what I was expecting. The performance I'm experiencing is more what I would expect from a 150. My expectations are based on my 10+ year old memory of a friend's Potter 20 MA with a 150 on it. I'm still trying to figure out to what extent it's because of differences in the hull, or because I'm running this Frankenmerc on it.

I took it to P&J Marine and they stuck it on their Dyno. The torque value they gave me was 95% of the minimum for a 200, but I don't know what rpm they measured it at, so I can't calculate horsepower. If it's a linear conversion, and merc's minimum torque spec for a 200 is at 200hp, then mine is developing 190 hp.

Anyway, thanks for the input. Every extra data point helps. I'm gonna keep tweaking. BTW, how high is your motor mounted? Where is the cavitattion plate in relation to the bottom of the hull at level trim? Mine is about an inch above.
Thanks,
Dave

Max torque will be at a lower RPM than max HP.
Max torque is the RPM where the motor has the best volumetric efficiency.

Torque and HP are mathmatically related: HP equalls (torque (in #/ft) times rpm) divided by 5600.

If you look at the torque - HP plot from a dyno, you'll see they always cross at 5600 rpm. There's more ft/lbs of torque than HP up to that point, and then there's more HP than torque above that.

If the motor doesn't turn that high, then there'll always be more torque than HP.

If you're getting 95% of the torque spec for a 200, you're probably getting close enough to 95% of the minimum HP spec.

They aren't linearly related, but they're close enough for what you're figuring.

Even if you had the rpm where they measured the peak torque, it would only give you the HP at that rpm, not the max HP.

Darlene

[Blue_Heron]

Max torque will be at a lower RPM than max HP.
Max torque is the RPM where the motor has the best volumetric efficiency.

Torque and HP are mathmatically related: HP equalls (torque (in #/ft) times rpm) divided by 5600.

If you look at the torque - HP plot from a dyno, you'll see they always cross at 5600 rpm. There's more ft/lbs of torque than HP up to that point, and then there's more HP than torque above that.

If the motor doesn't turn that high, then there'll always be more torque than HP.

If you're getting 95% of the torque spec for a 200, you're probably getting close enough to 95% of the minimum HP spec.

They aren't linearly related, but they're close enough for what you're figuring.

Even if you had the rpm where they measured the peak torque, it would only give you the HP at that rpm, not the max HP.

Darlene

Thanks, Darlene,
As I understand it, the dyno they used is programmed and all they do is enter the model number for the engine they're testing, run it up to max recommended WOT rpm (5500)and see if the torque curve matches up. I don't think they calculate the max HP or do any further analysis on the numbers, but I'm just guessing. I don't know if the torque value they gave me was max torque, or torque at 5500 rpm.

I got most of what I know about the relationship between torque and HP here:

http://www.revsearch.com/dynamometer...orsepower.html

[VirginIslander]

That's a good page on HP/torque. It also made me realize I was thinking of something else when I posted 5600, it is 5252 rpm. (and it's too late to edit my original post to correct that)

Torque is usually given as the peak torque number, just as HP is given as the peak HP. Both are usually referred to at specific RPMs in knowledgable circles, but often just the HP is used in general reference.

For a "collection of black parts", you seem to have at least a reasonable facsimile of a 200 merc.


Darlene