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

Blue_Heron · #1 11-01-2006, 07:59 AM

Thanks, everyone, for the feedback.
Dave

Bigshrimpin · #2 11-01-2006, 09:52 PM

Sounds like a good plan.

When you get to the point where you're adjusting the timing.
I'm pretty sure the 2.5L Spark Advance acts like a rev limiter and automatically retards the timing once you hit 5800rpm (2.0L/2.4L mercs the spark advace module will retard the timing @ 5600rpm) People will often remove those units and manually set the timing up to 24/25 degrees.

Do a search on screamandfly.com . . . there's lots of good tips for getting the most out of your Merc.

This is a good site too.
http://www.biggerhammer.net/mercury/

lost2a6 · #3 11-02-2006, 01:31 AM

Just thought about something. Make sure that the engine has good compression and if it is carbureted that the carbs are clean. If it's fuel injected make sure that the injectors are clean. Basically make sure that the engine is in perfect running condition.

Blue_Heron · #4 11-02-2006, 09:14 AM

Quote:

Make sure that the engine has good compression and if it is carbureted that the carbs are clean. Basically make sure that the engine is in perfect running condition.

Ah yes, the engine. That's really what my question about top speed is all about. Compression is good, carbs are clean, plenty of spark, all that's good. But the engine itself is what my mechanic calls a Frankenstein motor.

It was basically thrown together using a bunch of black parts. Not sure who built it, boat was sold to me by a small dealer who had it on consignment. The powerhead is a factory reman '03 200 EFI with carbs and reed block off a 150XR6. I don't know about the mid and lower. I've been researching the differences between the 150XR6 and 200, and it looks to me like the differences are mainly in the carburetion and exhaust tuner.

The XR6 had either large bore carbs like the 175s and 200s, or small bore carbs like the standard 150s, depending on year of manufacture. Mine are the large bore WMV-3s. I hypothesized that if I rejetted the carbs with the same mains and backdraft vent jets as a 200, I would boost horsepower. Nope. No difference. I don't really want to pull the powerhead to check the exhaust tuner. Maybe I'll look at it from the other end when I pull the lower to change the impeller.

The bottom line is that if I'm already getting the performance expected of a 200 on this hull, I don't want to waste any more time trying to squeeze another 4 or 5 mph out of it. If not, I'll keep tinkering. Just don't want to spend a couple bills on 200 HP carbs and not get anything out of it.

RUSTYNTABATHA · #5 11-02-2006, 11:54 PM

The xri usualy had the smaller prop shaft and differant gear sets, just a thought. also when you get way out there and need a realy meaty wheel i have a 4 blade 27 pitch ballistic for a 200 merc from my basscat i cant use. long story on why its on the shelf but it is new in the box.

as for the motor choice look on almost every race boat and youll find theres nothing faster than mercury

Blue_Heron · #6 11-02-2006, 08:11 AM

Quote:

I'm pretty sure the 2.5L Spark Advance acts like a rev limiter and automatically retards the timing once you hit 5800rpm (2.0L/2.4L mercs the spark advace module will retard the timing @ 5600rpm) People will often remove those units and manually set the timing up to 24/25 degrees.

I already disconnected the timing module, now I'm experimenting with max advance. I Already did this on the 150XR4 on my flats skiff and picked up 5 mph top end and had to go up an inch in pitch on the prop to get the revs back below 6000.

I'm still not sure what is a safe max advance for a motor turning under 6K. The guys advancing 25 deg. are running well over that. One mechanic I talked to said 21, another said 23. We'll see.

dljohn69 · #7 11-04-2006, 11:34 PM

Hey Blue Heron I think once you finish tweeking that 200 you should easily get to 50 mgh. I have a '76 20'MA I just repowered with an Evinrude E-Tec 175 and at 5350 rpm I am at 51-52 mgh according to Garmin with a 21 pitch prop. Good Luck

Blue_Heron · #8 11-05-2006, 08:41 AM

Quote:

Hey Blue Heron I think once you finish tweeking that 200 you should easily get to 50 mgh. I have a '76 20'MA I just repowered with an Evinrude E-Tec 175 and at 5350 rpm I am at 51-52 mgh according to Garmin with a 21 pitch prop. Good Luck

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

VirginIslander · #9 11-05-2006, 06:39 PM

Quote:

Hey Blue Heron I think once you finish tweeking that 200 you should easily get to 50 mgh. I have a '76 20'MA I just repowered with an Evinrude E-Tec 175 and at 5350 rpm I am at 51-52 mgh according to Garmin with a 21 pitch prop. Good Luck

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 · #10 11-05-2006, 11:05 PM

Quote:

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