09-06-2011, 08:53 PM #1
- Join Date
- May 2010
Props and Nozzles... the relationship between efficiency, RPMs, and speeds
My FZR is running almost 80 every hit (79.8 all day) in moderate heat (90+), with 1/2 to 3/4 tank of fuel. The rpms are right around 8360-8380. With a 85mm nozzle it will turn near 8600.
My question is this: is there a theory to finding the sweet spot for this combo (or any other)? Its well stated that 8400-8500 is the target range for an R3 setup with stock valvetrain. Is it better (more efficient) to open up the nozzle and leave pitch in the prop, or use the tighter nozzle and take pitch out of the prop?
How did 8500 become the number of choice? You can run your car in first gear but it does not necessarily go faster b/c the rpms are up.............
I know in the end its testing, testing, and more testing, but I am curious if someone has some in depth knowledge of the "how and why" these statements are true, and how to seek more MPH.
I want to see 82ish. Several say I should be there with R3/C3, but why am I not? I know there is some left, but how much?
09-06-2011, 09:09 PM #2
09-06-2011, 10:03 PM #3
Phillip, you are on the right track brother! That is a very good point you hit on and I wil give you a little insight on my experience with building and racing these things for a living a few years back. There are always two ways to skin a cat and whether you choose to go with a smaller nozzle and smaller pitch or vise versa is totally a personal preference. I always went with the larger nozzle and taller pitch as it gave me better PTO and since the taller pitch would give me more distance per revolution, the top speed was always respectable. There are people who try the large pitch and smaller nozzle but alot of factors come into play and as long as the pump design can process the water properly, it will work but that is mostly trial an error as you don't truly know how much your particular pump and setup will handle until you hit the limit whether it be maximum recorded speed or the pump oversuffs.
Every engine and hull is different no matter how much you try to make them identical, there will always be subtle differences and just because this guy runs this at that speed doesn't mean yours will have the same results. Ideally you would want to dyno the engine and find out what max hp and torque is and where it lies in the powerband. That will give you a better idea of where your target rpm should be. I used to hook up the tach I was using in the boat to the engine on the dyno and compare the numbers on the dyno to what the tach read so I knew exactly where peak hp and peak torque were. The peak hp and peak torque were usually within a few hundred rpm of each most times and that is where you would see your differences in not only other members crafts but in your own craft.
If you elect to run smaller pitch, you will depend more on peak hp for sustained rpm and speed and if you run a larger pitch, you will depend on torque to pull it. That is why you see one guy running a 13/21 at 8550 and going 80mph and another guy with a similar craft and setup with a 13/24 at 8300 also going 80mph but both crafts will have a different feel.
Regardless of how close you try to build an engine to be identical with the next, there will always be differences between the two. For example we built 2 identical 1200cc triple super stock engines and both made 280hp with similar torque numbers and rpm bands but both engines responded to different setups. We built the first one and it ran like a gang buster and then dynoed the second with the exact same setup and it was down 30hp but in the end, it came around but required stinger restriction where as the other didn't, it also responded to reed spacers where as the other didn't respond to them along with a few other things ie: compression ratio, expansion chamber volume, header length etc.
So in closing, there are going to be differences between engines themselves stock for stock and I am willing to bet there will be differences in engine responses from the aftermarket equipment they are paired with and without dynoing and knowing exactly what the engine is doing and what is working and what isn't, you never truly know and you just have to keep throwing ideas at it and test them until you achieve the results you desire and not all results will be the same boat to boat.
09-06-2011, 10:17 PM #4
On my experience I will always get better holeshot with a larger nozzle while getting better top speed a smaller one keeping in mind we are only dealing with only a few millimeters apart, Julio...
09-06-2011, 11:01 PM #5
To make it as simple as possible: more pitch/larger nozzle = better acceleration...less pitch/smaller nozzle = better top speed. There are lots of variables that will give slightly different results: prop design, pump size & design, cone size & hp of the motor; for the most part the above will stay the true though.
The physics of it are not hard to understand. Forcing the same amount of water through a smaller hole will increase its velocity & decrease thrust & visa versa. Think of it like putting your finger over the end of a garden hose...the more you restrict the flow, the faster the water sprays. More thrust(large nozzle) = better acceleration & more velocity(small nozzle) = better top speed.
The mistake many people make is they don't do proper tests. Lots of people will put on a bigger nozzle, gain rpm(& speed) & assume that it's a better top speed setup. The ONLY way to accurately find out what nozzle size yields the best top speed is to repitch the impeller to get the rpm's as close as possible with each different nozzle. The biggest challenge with dialing in rpm is changing weather, especially since these boosted motors can gain/lose 300+ rpm with a 20* change in air temp. Not everyone wants to invest in multiple props for different weather, which is the optimal thing to do. Having different nozzles is cheaper & a more convenient swap. What I tell most people is to use a larger nozzle in summer & pitch their prop to that, & have a smaller nozzle on the shelf for when cooler weather shows up.
For most of our(rec rider) purposes, the top speed vs acceleration trade-off is the only thing we think about. For someone who's racing, keeping the pump loaded & hooked up is a major goal...they don't really give a shit about top speed. Adding pressure behind the prop with larger cones & more veins in the stator will help keep the pump loaded, but also negatively affects acceleration.
As for your question why most people target 84-8500, it's because it gives you a safe margin for higher revs during unhooks. If you're propped to turn 86-8700 with a stock valvetrain, you're kinda on the ragged edge of what the motor can handle when the revs spike during unhooks.
09-07-2011, 06:48 PM #6
- Join Date
- May 2010
09-08-2011, 07:36 PM #7
84-8500 rpms' is the sweet spot because supposedly the oem cams make peak power around that rpm range, if you notice most of the SC wheels claim their boost ratings around this rpm range, and also when the reflashes came out i believe they specified to target this rpm range.
as far as prop and nozzles goes, theirs alot to it but to keep it simple, different props have different prop designs and very important root angles, bigger root angles like bigger nozzles, and props with smaller root angle like smaller nozzles, if you put a too big of a nozzle on a prop with a small root angle the prop cannot pressurize the nozzle and you loose performance everywhere. thats why the SD guys always had best performance with the smaller nozzles cause their props have small root angles compared to the props we run on the SC yamahas. also increasing the diameter of the props acts as a much bigger root angle thats why the guys running the 160mm props have great results with the bigger exit nozzles as big as 86-87 mm.
for your setup using a 155mm skat swirl i recommend you stick to the stock nozzle, leave the leading edge at 13 and pitch the trailing edge of the prop for around 8500 rpm's
hope that help.
09-08-2011, 09:19 PM #8
With aftermarket SC wheels I've always had better top speeds testing small nozzles...just have to be careful with plate angle because it's more likely to overstuff. This might not hold true if using a stock SC wheel because the boost pressure after 8K rpm flattens out big time & it probably won't like the extra load.
Personally, I'll never leave a small nozzle on my ski because I think it accelerates like dogshit, but talk to Hitman & he'll tell you what kind of speeds JD1 is getting out of low boost skis with small nozzles.
09-09-2011, 09:02 AM #9
Sticky. Definitaly one of the better threads that all people who mod their ski should read.
09-09-2011, 10:29 AM #10
Please remember, when switching to smaller nozzles, take it slow and only decrease one size at a time. I dont want anyone making the same mistake that I did.-------Stay safe.
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