Possible high altitude performance tweak

[Speedling]

Please help me understand or validate my understanding here guys. I've always read and experienced that our engines are rev limited. On the 160 hp mr1 (1,052 cc) i maxed out at 10,200 rpm at sealevel. On the 1.8L engines I think I see around 7,800 max. If this is correct than most of the bolt on performance improvements (adding spacers, making the air flow into the engine less restrictive with better or no air filters, etc.) will only help me get to the max rpm faster they won't improve my top speed.
Is this correct?

Changing the pitch of my impellers is the only way to increase top speed at max rpm. Is this correct?

Misc.: As far as I know, top speed and acceleration are inversely related when it comes to pitch changes (I.e., you can have one or the other but not both and increasing one means a corresponding decrease in the other).
Any disagreements on this?

Pretty much!
If you made 300 hp on each engine but didn't change impellers you would just get to the rpm limit quicker. Top speed would be the same.

 

[Speedling]

Oh! And conversely, if you are at a high elevation and get less air then you make a bit less power. The factory tries to give you a decent set of impellers but without making enough power you don't always get to the max rpm, which is where peak power is made on these high spinning engines. Taking some pitch out of the impellers brings the rpm back up to the max which is where the max hp is and thus you will actually get an increase in top speed!

 

[sysinu]

The factory gives you impellers (sometimes properly) pitched for sea level with a combination that at sea level can get you to the point where you're RPM limited at top speed.

The combination of reducing the restriction on air (increases power and top-end RPM) as well as changing the impellers will move that balance.

@djsquid Adam is right (as always). Spacers make a big difference and if I were to do it again, I would still change my impellers, but not from sea level to high altitude - instead just a little less pitch than they are pitched for at sea level.

 

[djsquid]

@sysinu I will be up at Flaming Gorge April 16, 17, 18. I will be able to do plenty of high altitude testing then. The spacers will be in by then, and I will probably have plenum filter of some sort in hand as well.

 

[sysinu]

@djsquid awesome! When we were up there a few weekends ago the bays were still frozen and the water was 34-36 degrees at the top. If you're going up there to go fishing, I highly suggest getting navionics and sonar maps for your phone - the contours of the gorge have a lot of really steep under water ridge/cliffs and that's where the lake trout hang out.

 

[djsquid]

@sysinu good advice. We will also be fly fishing in some of the feeder streams as it is the week of the rainbow spawn. And of course boating and seeing the beauties that the gorge has to offer.

 

[Reno Rob]

I am boating in Northern California/Nevada, Stampede reservoir (6,000 feet) and Lake Tahoe (6,225). Have a new to me 2017 Scarab 195 HO with a 250 HP supercharged Rotax 1503. Any advice on things I can do to get better speed? I understand the impeller needs to be changed, but am really not sure on pitch and that, and I also read fuel systems and ignition might need to be changed? This is in the owners manual, but that is all it says and is VERY generic.

 

[Speedling]

I am boating in Northern California/Nevada, Stampede reservoir (6,000 feet) and Lake Tahoe (6,225). Have a new to me 2017 Scarab 195 HO with a 250 HP supercharged Rotax 1503. Any advice on things I can do to get better speed? I understand the impeller needs to be changed, but am really not sure on pitch and that, and I also read fuel systems and ignition might need to be changed? This is in the owners manual, but that is all it says and is VERY generic.View attachment 132049

The impellers can be repitched by a company such as impros to help. The timing and fuel is going to be a dealer or maybe even aftermarket tuner.

 

[Speedling]

Sharp boat by the way

 

[Reno Rob]

Thanks Speedling, we love it already, just trying to get the bugs worked out on the speed/performance side.

 

[Reno Rob]

Hey all, thanks for the advice. Got a hold of Impros today and got a fix identified with an 11/16 impeller. In the Scarab manual on page 26 it says I "may require adjustments to the ignition systems, fuel systems or the impeller". But this is super vague. The guys at Impros said they thought the fact that it is fuel injected and supercharged, that the only thing needed was the impeller, since air and fuel is controlled by the computer. Any thoughts on this? Since my closest dealer is 5 hours away, hoping someone here might have some input. Really hesitant to drive all that way unnecessarily.