XR1, XR2, XR4 etc,

[Mandella69]

Can someone enlighten me on the meaning of and the difference between Mercury motors with the designation XR?. I recently picked up a used boat with a 1988 150 XR2. After searching different resource sites, I learned the motor has either a 1.87 or 2.0 gearcase, but not much else. The Mercury manual says that the rev range for the XR2 is 5500-6000 as opposed to 5000-5500 for the regular 150. Why?
Thanks

[Tom Brown]

I've never heard of an XR1 but there were definitely "special" versions of engines before the XR2. In the early 70s, there was a 'J' verson of the inline 6 that put out a tiny bit more power. The J block's older brother, the 1500xs, was rated at 155 hp, up 5 hp from the regular 1500.
The XR-4 is a 2.4 liter 150 hp engine. It was 2.4 liter when the 150 and 175 were 2.0 liters. If you look at the porting of an XR-4, it is very modest. While I've never compared it to a 200, I would guess the XR-4 to be the same or, more likely, have more mild porting than the 200.
In that sense, the XR-4 is actually a lower performance engine than the regular 150 in that it puts out less power per volume. The reason that's good is because low RPM torque is very much a factor of engine displacement and has little to do with porting.
The XR-4 hits really hard out of the hole. At least, mine does. The XR-4/150 hits tons harder than my 2.0 liter 175 on the same boat.
The XR-4 would probably be really well suited to larger, heavier boats than the regular 150. It's a lot of fun on smaller, lighter boats too, though.
I'm no expert on the XR-2 but I believe it is kind of the opposite idea of the XR-4. It probably has more aggressive porting than the regular 150 and I'm pretty sure it has larger carbs. It probably breathes a little better in other ways as well, but I'm not very familiar with the XR-2.
I assume the XR-2 has the same bottom end power as the regular 150, both being 2.0 liters of displacement. The XR-2 is said to pull pretty hard at higher RPM, though.
In that sense, the XR-2 is probably better suited to small, light performance boats. I would assume it will put out well more than 150 hp at 6500~6800 RPM.
If you get a boat with an XR-2, I would recommend you prop it to rev to the redline or better.
As far as gear cases go, the XR-4 had a special gear case. It's small; the size of the inline 6 cases. The only ratio was 1.78:1 (yes... 1.78... not 1.87). There's no need to worry about that now, though. They've all blown up years ago and have since been replaced with regular fishing engine cases.
The XR-6 is a 2.5 liter 150. I think it's the same idea as the XR-4, only taken a step further. Also, the XR-6 has the same size gear case as the XR-4 but is stronger and didn't have the same terrible track record of the XR-4 gear case.
... so I think the idea is that the XR-2/150 is a heated up 150. It's probably happier revving a little more than a stock engine. The XR-4 and XR-6 are 200 hp engines that have been dumbed down to 150 hp, plus they both have the small gear case.

[Mandella69]

I was not sure there ever was an XR1. Do you happen to know what gear case the XR2 has? One sources says the ratio is 1.87, another says it is 2.0. The motor is on a 16' Charger DL.

[Tom Brown]

My manual shows it as 2:1 but that engine is 17 years old now and could easily have had a lower unit swap or two in it's life.
You can tell by rotating the crank and counting rotations at the prop. If two crank turns equals exactly one prop rotation, it's 2:1.

[Tom Brown]

By the way Mandella, why are you concerned about the gear ratio?

[Mandella69]

Tom
I am not "concerned" per se, just curious. For a constant speed, the difference between a ratio of 1.87 and 2.0 is about 400rpm. I am told that the non XR2 150 has a ratio of 1.87. Assuming that the motor puts out the same 150HP as the non XR2, but at a higher RPM, what is the advantage? I would think that the standard motor would be able to carry a higher pitch prop than the XR2 but not rev as high. So in effect there would not be a great difference in top speed.
For instance, on my v-drive, I only would up the pitch or increase the gear ratio if the motor was upgraded to produce more HP. This assumes the original pitch and gear ratio was optimum for the motor before any changes.

[HavasuDreamin']

I assume the XR-2 has the same bottom end power as the regular 150, both being 2.0 liters of displacement. The XR-2 is said to pull pretty hard at higher RPM, though.
In that sense, the XR-2 is probably better suited to small, light performance boats. I would assume it will put out well more than 150 hp at 6500~6800 RPM.
If you get a boat with an XR-2, I would recommend you prop it to rev to the redline or better.
From personal experience, the XR-2 is a GREAT motor. I had a 1985 XR-2 on the back of my 19' Eliminator Daytona for several years. The motor ran like a bat out of hell. I spun my 22" chopper in the 6,200 - 6,500 RPM range and the motor never blinked. The powerhead responds well to mods.
The XR-2 gear is a 2:1.
Mandella........much like a v-drive, it doesn't matter where the gearcase and/or prop pitch is, as long as you prop your particular set up to hit the peak RPM band of the engine. The only time you get into a debate on this topic is when you are talking about slip factors on really tall props. 32", 34", etc. props tend to have more slip than smaller props.
Good Luck.

[stoker2001]

As far as gear cases go, the XR-4 had a special gear case. It's small; the size of the inline 6 cases. The only ratio was 1.78:1 (yes... 1.78... not 1.87). There's no need to worry about that now, though. They've all blown up years ago and have since been replaced with regular fishing engine cases.
The XR-6 is a 2.5 liter 150. I think it's the same idea as the XR-4, only taken a step further. Also, the XR-6 has the same size gear case as the XR-4 but is stronger and didn't have the same terrible track record of the XR-4 gear case.
... so I think the idea is that the XR-2/150 is a heated up 150. It's probably happier revving a little more than a stock engine. The XR-4 and XR-6 are 200 hp engines that have been dumbed down to 150 hp, plus they both have the small gear case.Tom,lots of good info in your post,but when the drag racers talk about the small diameter gearcase they refer to the Xr6 typically,even tho i think they came with both big and small cases.the small case can be real reliable with some minor mods on a 300hp+ drag with somewhat of a light boat with drag prop.i know cause i have ran one for two years on a nine second boat and it was good for two tenths off my elapsed time versus a full size case.i keep waiting for it to go KABOOM,but it keeps on ticking.you make a good point for reliability on a typical bass boat go with the big case a you will be better off