Nitrous on 2.5 Drag

[CI STV]

Planning to remove and reinstall the motor today.
Going to use a 3” setback plate that I’ve made up using the original F1 motor plate and some modified HD transom brackets from T-H marine that will allow me to bolt the plate on the transom using the F1 bolt pattern.

I’ll post pictures of it up later.

So, although I’m going to obviously test it to see what works, I wanted to see whether my assumption is correct that I maybe need to utilize a lower prop shaft centerline setting with the nitrous to keep the prop from cavitating on the launch, due to the additional torque from the nitrous.

I’ve been using ~1 3/8” above the pad with the old 3” setback bracket, and that seemed to work well with the Yamaha props I’ve run.

So the question is whether anyone knows from experience whether I might need to run the motor lower with the nitrous than I did before, to get more initial bite? I’m asking because the last hit I made on it, which I aborted at like 100’, the motor screamed to ~9000 rpm in that distance. The boat launched hard enough, but it still felt like it was freewheeling a bit out if the hole. I had the motor set at ~1 3/8” on that pass, whereas the first test hot on the nitrous, I had the motor buried (~3/4-1” lower, unintentionally) so it was wallowing around badly, even though it accelerated hard.

I’m also thinking that if I’m right on this, since I have the benefit of the F1 up/down midsection, I could utilize the second timer on my trim timer to start with the engine in a lower setting to get initial bite, then delay it for a bit and then activate the up/down jack to bring it up to the optimum height at about 200’ out when the prop hooks up and the boat is packing air?

BTW: I have a bunch of 1.25” I.D. billet half-clamps of different thicknesses that I’m going to use on the up/down jack, to limit its range so it can’t exceed the max height and cause blowout.

My thinking on this is that props work just like a torque converter in a race car: the more power you have, the more initial “flash stall” (flash stall=prop cavitation) you get off the line, hence the need for a “tighter” converter with nitrous. Any of you guys who are nitrous car guys will understand what I’m saying.

Or, maybe I’m just overthinking again, which I tend to do?

[Stoker boy]

Set up a couple of buoys, have a timer set up in your boat. If you’re after ET, that’s the way to do it.

[RBT]

Set up a couple of buoys, have a timer set up in your boat. If you’re after ET, that’s the way to do it.

AIM will give you accurate times to the foot.

[Stoker boy]

AIM will give you accurate times to the foot.

I still have that old technology stuff, but it still works for me.

Either way, I found that to be the most accurate way to see what performed and what did not.

[CI STV]

Set up a couple of buoys, have a timer set up in your boat. If you’re after ET, that’s the way to do it.

That’s how I do it. I can use the Datalogger to measure the ET to any particular rpm or speed. I can also use it to evaluate differences in prop slip.
Just wanted to know if I was on the right track, so I can set the lower and upper limits of the up/down jack.

[CI STV]

Pulled the motor off the transom and reinstalled it with the new setback plate setup yesterday.
The new stern brackets are exteremely thick (5/8” on the transom and 1/2” on the sides) The new bracket is 3lbs heavier than the old one, but it will not flex, that’s for sure, lol.

Everything lined up perfectly and there’s no slop anywhere. We spent quite a bit of time measuring everything and the engine height is now back to where it was before.

[Stoker boy]

Have you considered different props? Are you running a progressive nitrous system?

[CI STV]

Have you considered different props? Are you running a progressive nitrous system?

I have a few different props, albeit they’re all Yamaha or Yamaha-style props.
I have a very simple, time-based progressive controller, which ramps in the nitrous from 40% to 100% in 1.5 seconds.

[Stoker boy]

I have a few different props, albeit they’re all Yamaha or Yamaha-style props.
I have a very simple, time-based progressive controller, which ramps in the nitrous from 40% to 100% in 1.5 seconds.

I’ve only run Yamaha drag and lightning ET props, nothing larger than 24 pitch. I’ve run 175 first stage (no progression) one second delay for 100 horse second stage, bottle pressure about 1000/1050 PSI First stage was good, but when I started running second stage, I had to run different plugs because it was not firing the charge properly.

I believe I have the same 12 inch F1 housing as you do. It has the trim in/out and vertical up down.

I would have to go remeasure, but I believe I’m running about an inch and a half above the pad.

On my hull, I believe there’s just a slight trim up and that setting works just fine.

I didn’t find any advantage on ET with regards to raising the motor height up or down. I didn’t have a cavitation problem.

Perhaps I got lucky when I was setting everything up and the boat is balanced real well.

[CI STV]

I’ve only run Yamaha drag and lightning ET props, nothing larger than 24 pitch. I’ve run 175 first stage (no progression) one second delay for 100 horse second stage, bottle pressure about 1000/1050 PSI First stage was good, but when I started running second stage, I had to run different plugs because it was not firing the charge properly.

I believe I have the same 12 inch F1 housing as you do. It has the trim in/out and vertical up down.

I would have to go remeasure, but I believe I’m running about an inch and a half above the pad.

On my hull, I believe there’s just a slight trim up and that setting works just fine.

I didn’t find any advantage on ET with regards to raising the motor height up or down. I didn’t have a cavitation problem.

Perhaps I got lucky when I was setting everything up and the boat is balanced real well.

Thanks for sharing all of that, man. What type of hull is it? Is it an STV? If so, are you running any setback?

I’m nowhere near where you’re at in terms of power, since all I’m running is a single stage with around 100hp jetting. I’m hoping the fuel pressure problem goes away now, so I can jet it up a bit. I don’t intend to go any higher than 150hp jetting and I’m not going to use a second stage.

I’ve only ever used around 1 degree of positive trim, or even slightly less, and the boat ran fine within that. I’ve seen as little as 6% - 8% prop slip with a couple of different Yamaha props, and the boat handles beautifully, unless I have the motor set too low, so I’m pretty sure it will run fine once I get the bugs out.

I have a newly rebuilt XR with all new internals, 1.78 gearing and a Geiger billet carrier that I intend to run, along with a brand new Billy Eaves 24 P Yamaha that was worked for an STV (thanks, Alex!) that I haven’t tested yet.

Once I’ve solved this fuel pressure drop issue, I’m going to throw those on and see what it can do in 800’.

I hear you on the plugs. Once combustion pressures reach a certain height, you either need more spark or a tighter gap for the plugs to fire. Or a better plug. I’m running Denso plugs, since their iridium fine wire design can fire in just about any kind of combustion pressure. I typically ran IW24s, but I’m using IW27s now with the nitrous.

We got most of the stuff re-installed and reconnected yesterday, so I should be able to test it again soon.

Thanks again. Greatly appreciated.

[Stoker boy]

From what I can see, you’re running secondary fuel for the nitrous. Methanol or gas?
I may have missed it, but is your fuel issue with the nitrous secondary fuel or the injector fuel for the operation of the engine?

I’ll tell you a little more about my set up when I get a little more time.

[CI STV]

From what I can see, you’re running secondary fuel for the nitrous. Methanol or gas?
I may have missed it, but is your fuel issue with the nitrous secondary fuel or the injector fuel for the operation of the engine?

I’ll tell you a little more about my set up when I get a little more time.

Thanks.

I’m not running a secondary fuel system as yet. I’m pulling the fuel for the nitrous off the second port on the regulator, downstream from the fuel rail, which is why I’m surprised that the fuel pressure was dropping. I’m hoping the new pump will solve that. I also had a restrictive 90 degree elbow on the bottom of the rail, which I’ve not changed to a speedflow -8AN fitting, to try to increase the flow through the rail. I know this isn’t ideal, but I can’t see why it shouldn’t be enough for a 100hp shot of nitrous.

I’m not running, nor do I intend to run, methanol as the augmentation fuel for the nitrous. .

I’m hoping to avoid running a second fuel pump, but if I have to, I can easily do that by connecting it to the additional -10AN bung on my tank that I currently have capped.

[Stoker boy]

What I’ve done successfully is use the motor fuel injectors hit a button and pick the amount of richness I want. No, second pump no methanol no extra lines nothing, just nitrous.

I run a very small battery. I tried running a big weldon pump, which is more than enough for anything, but it drained the battery. I just tried the weldon pump for fun.

I’ve never tried this, but if you’re running a 56 pound regulator, you might be able to put a restrictive orifice in the return line. Perhaps that would give you the pressure you need. You might be able to use a solenoid in conjunction with nitrous activation to reduce the fuel through the return line momentarily.

Make sure you have your Pyro’s and fuel pressure gauges visible on the dash.

[108]

Methanol is the way to go. Just sayin'.

[Stoker boy]

Methanol is the way to go. Just sayin'.

It does work well, mixed with a little benol.