Nitrous on 2.5 Drag

[Stoker boy]

Too much work to take the cover off for a photograph.

[Stoker boy]

My boat runs crisp and clean with the present fuel curve, on the rare occasions that I have changed props, it developed a bog. I’m thinking your TPS is created with vacuum I’ve been told adjusting the fuel curve with that type of system doesn’t always help a bog, I don’t know. My system uses a potentiometer so I change the curve regardless of what prop and it runs crisp again. perhaps instead of a prop change a fuel curve change might help.

As far as 120° of disparity between port and starboard, I don’t think that’s much. just barely above body temperature.

[CI STV]

[QUOTE=Stoker boy;3445695]

Just to add some clarity to the post above, I’m running my EGT probes in the two bottom holes in the exhaust divider plate, since I have the 12” F1 mid and that doesn’t use a adaptor plate, so the only place to put the probes is in those two.

I also run a 12 inch, I’m not using bolt holes 16 and 17 for EGT probe.

Where are your probes?

[CI STV]

My boat runs crisp and clean with the present fuel curve, on the rare occasions that I have changed props, it developed a bog. I’m thinking your TPS is created with vacuum I’ve been told adjusting the fuel curve with that type of system doesn’t always help a bog, I don’t know. My system uses a potentiometer so I change the curve regardless of what prop and it runs crisp again. perhaps instead of a prop change a fuel curve change might help.

As far as 120° of disparity between port and starboard, I don’t think that’s much. just barely above body temperature.

Yep, the ACU doesn’t use a TPS but uses the MAP sensor to detect throttle position and then adds fuel based on vacuum/MAP and RPM.
I have the Brucato software as well as their Steamwheel, so I can change the curve as necessary.

[CI STV]

Happy Easter everyone.
I still haven’t been able to test the boat since the last time I posted up, but since the purpose of this thread was always intended to share what I learned on this nitrous adventure, I want to make sure I didn’t miss anything.

One of the things I don’t think I shared was that prior to the last outing, I turned all of the nozzles so that the nitrous was being sprayed straight into the reed cage.

You might recall that I had raised the concern early on in the thread that if I did this, the pressure of the nitrous hitting the reeds might cause the reeds to hang open and impair performance.

I didn’t come up with that theory on my own. I read that in a nitrous forum a while back when a certain nitrous guru was telling someone to watch out fir that when injecting nitrous on a two stroke. I’m sure it’s a possible problem, but I’m glad to find it’s not that much of a problem for me. At least I don’t think it is.

I say that because I don’t know if the nitrous stream is interfering with my reeds closing, but I can tell you that it doesn’t seem to impair performance, since I was able to turn 9920rpm/92mph in 360 feet and although the EGTs are way lower than I expected (one being much lower than the other) it still hauled a$$ pretty good, even with little or no trim and running the motor low to get good prop bite.

BUT, that was with a small prop (22P) and steep gear (2:1), so it remains to be seen what it will do when I get it geared and propped properly for 800’.

BTW: there’s no doubt the the nitrous stream is hitting the reeds. FYI: I’m running 5-petal rubber “Sportjet” cages with PPE carbon reeds. Not sure what would happen with Boyeson dual stage with the thinner “primary” leaf on top.

It maybe that it makes it harder for crankcase pressure to close the reeds, ie “slowing them down” a bit, and maybe there’s a bit of HP being compromised there, but it seems the nitrous more than makes up for it.

I had been toying with the idea of changing the nozzles to the straight annular style and letting the motor pull the nitrous into the reed cages, but at this stage I think I’ll stick with the 90 degree nozzles at least until I get the other gearcase on it and make a full pass with it.

[CI STV]

Just wanted to update this thread, since I finally was able to make a a couple of ~700’ passes on it yesterday and one pass I held it down to the 800’ buoy.
I’m now running 1.78 gearcase with a 24P Yamaha.
I ran it with the Steamwheel set at +10 percent, just to be safe, and with the timing fixed at 19 degrees.
It’s launching hard, but straight as a string and it pulls like a beast through the midrange, and it hit 111.5 mph on one run, but I’m still seeing the right side EGT drop off steadily any anything above 8700, and by the end of the run it’s 60+ degrees cooler that the left side. It feels like it’s starting to lay down up top too.
I’ve cleaned injectors, swapped them from side to side and just about everything electrical has been changed, except the ACU, which I’m seriously thinking of trading for a PPE box.
But I’m starting to wonder whether, since this is obviously rpm related, I mightn’t have a weak reed on one cylinder, and that’s starting to float at 8500+rpm, and causing that bank to go rich?
Does that sound plausible?
I’m running a set of PPE carbon 5-petals on stock Sportjet rubber cages, but I have a set of billet 7-petal cages and some Boyesen reeds that I might try on the next outing.
Would love to see what this thing will do if it wasn’t going rich/lame up top.
I’ll post up some video links later, once I upload the videos on YT.

[skialot2]

I have no actual experience with this but wouldn't a floating reed make it go lean, not rich? If the reed floats you wouldn't get the crankcase pressure needed to pop the fuel mixture into the cylinder. Wouldn't that make it go lean not rich? Or am I theorizing that wrong?

[CI STV]

I have no actual experience with this but wouldn't a floating reed make it go lean, not rich? If the reed floats you wouldn't get the crankcase pressure needed to pop the fuel mixture into the cylinder. Wouldn't that make it go lean not rich? Or am I theorizing that wrong?

I honestly don’t know, but my thinking is maybe if it can’t build crankcase pressure properly at high rpm, that cylinder wouldn’t produce the same power as the others, and since less power in that cylinder will equate to less exhaust heat on that bank, that could be the culprit?

The other thing is that I have the nitrous nozzles in front of the reeds, injecting both nitrous and fuel, so if one reed is hanging open, it will no doubt blow crankcase pressure back through the reeds when the piston is on the downstroke. And I noticed that after those runs on Saturday, the inside of the cowl and the side of the block was coated with a mixture of fuel and oil (no bueno!) so I need to figure out what’s causing that and fix it.

[skialot2]

Now that sounds much more possible to me then going rich. I would set up a micro camera under the cowl to try and see where the fuel oil mix is coming from.

[CI STV]

Now that sounds much more possible to me then going rich. I would set up a micro camera under the cowl to try and see where the fuel oil mix is coming from.

That’s my next step: put a GoPro under the cowl and get some video of what’s going on.

[CI STV]

Back to this problem of EGTs dropping, in case anyone wants to offer some ideas as to what it could be?

I haven’t had a chance to do any further testing since a couple of weekends ago, but thought I would post this up in hopes someone may have any advice on what it could be.

I did try last weekend to change out the reeds/cages to the billet 7-petal cages, but that didn’t work out, since Diamond machined their manifold to only fit the 5-petal Sportjet cages. No need to discuss that on here now.

Below is a simplified version of the datalog from one of the runs I made the last time out. This is from a ~800’ run. You can see from the gradient of the green line (speed) how hard it accelerates during the first part of the run and then it tapers off at around the same time the EGT on the right bank (yellow line) starts to drop (@~8500 rpm) and then flattens out further at around 9000rpm when the left bank EGT starts to fall off precipitously.

By the end of the run, both banks have dropped by around 100 degrees from peak and you can literally feel the boat lay down a bit right before the end of the run.

Please note that this is a Diamond ported Drag motor that off has always pulled hard to 10k and I’ve turned it 10,500 before. So, I don’t think it’s port-limited for sure. The exhaust port has been raised above Drag specs, based on what I’ve measured.

Any ideas?

BTW: I’ve changed out the stator, trigger, flywheel, plugs, already.

Note: I did have the Steamwheel set on 10% on this pass and the timing is fixed at 19 degrees.

Thx.