Nitrous on 2.5 Drag

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Originally Posted by Scream And Fly

I just want to say that motor looks like a scary beast! And also that this is a very interesting topic and I hope it continues to expand on this subject.

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Thanks Greg.
I started this thread on here because I much prefer this platform than Facebook, which I find quite annoying, to be honest.
So, thanks very much to you for keeping this site going.
Nitrous is a topic that is typically shrouded in mystery and misconceptions about its dangers, etc. mostly based on old experiences made from mistakes (god knows I made many) back in the day when much less was known about nitrous, and some of those ideas prevail to this day.
Yet we’ve seen over the years what nitrous could do on thee two stroke outboards in the capable hands of folks like Gordon Montague, Marty Logan, David Carver, Micheal Dixon, etc.
Hence my desire to start this thread, so that I can document what this “mythical” gas can do.
I intend to share what I learn as best I can (unless someone shares something that they don’t want me to publish) with the folks on this site, and I hope that if they see me making stupid mistakes, they’ll point it out, lol.
I know that a lot folks “in the know” are understandably unwilling to share their trade secrets, but, since we don’t really have organized racing where I live, I have the luxury of not having to worry about the competition gaining advantage from my own efforts.
And I’ve gained a lot of knowledge from some very competent people on sites like this sharing their advice with me, either publicly or privately, and so I’m happy to share what I can, and to provoke some constructive discussion.
After all, that’s the reason you created this site, isn’t it?
Thanks again for what you do.

Link works great. Very impressive!!!

I decided today that while I’m waiting on the new pump and since the wind is supposed to fall out tomorrow afternoon and then pick up again on Tuesday, I would install a Kenne Bell Boost-a-pump voltage booster on the boat.
For these who aren’t familiar, it’s a pretty simple but effective device, that works by raising the voltage going to the pump from 12V to 17.5 volts, which increases the volume of any pump.
I had one laying around in a box in my garage that I used to have on my Vette a few years back, so I wired it into the harness for the pump on the boat.
I’m not going to use the typical boost activated Hobbs switch to activate it, although I may wire the trigger into the nitrous system so the voltage only increases when I turn the nitrous arming switch on. For right now I’m just using a jumper wire to trigger it. It’s not like I’m going on a river cruise or anything, so it should be fine running on 17.5 volts for a few minutes.
I tested it this evening and with it activated, you can hear the sound of the pump increase dramatically. Hopefully that will give me the additional volume I need, which can’t be much, since the pressure only fell to just under 53psi and stayed there until I lifted (thick red line on datalog below) when I ran last Monday.
Attachment 539147
And, since it was so rich last time, I’ve also dropped down a couple of jet sizes on the fuel jets in the nitrous nozzles, to the jet spread that N.O recommends, which will reduce the fuel demand in any event.
If the weather prediction is right, I’m going to do another test hit in the pond tomorrow.
I’ll report back and hopefully I’ll have some better video to post up.

Managed to get one very short test hit this afternoon, with somewhat disappointing results.
Firstly the Boost-a-Pump bolt-on was a fail, it seems, since the fuel pressure drop was worse this time than the last.
Here’s the datalog with the fuel pressure highlighted (in red):
Attachment 539177

See how it drops almost immediately at the hit and drops down to 48 psi from 55.66 psi, then it recovers but is wavy?
You could hear and feel it surging in the boat as well. I have some videos that I’ll upload to YouTube and post a link for those later.
But man, did it leave hard out of the hole! You can see from how quickly it revved to 8100+, but once it started surging and lit up the dash, I let off immediately and put it on the trailer. I was in the throttle for maybe 100ft.
Here’s the log with the RPM highlighted (black line):
Attachment 539178
At this point, I’m not sure whether it’s the pump lacking volume or it’s the short 90-degree elbow fitting at the bottom of the fuel rail that’s restricting flow into the fuel rail, or both. But the UPS man came earlier to deliver what I need to fix those things.
I may end up having to put a stand-alone fuel system in it, but I’m trying to avoid that for now, because I’ve already added a bunch of stuff to this boat and it’s starting to look very crowded now.
To be continued…

Here’s a video a friend of mine took with his phone from the shore:
https://www.youtube.com/watch?v=zXyzdn3KS1E

Just to update this thread, I had to discontinue testing when I discovered that my transom was flexing and there were cracks that appeared in the gel coat along the bottom of the transom.

Pulled the motor off and found out the plywood coring in the transom was completely rotted, apparently for many years, which is no doubt why I’ve always seen the trim change on the datalog on every run I’ve made.

You can literally see the motor push in and spring back out when I let off the throttle on the video I posted of my motor when I tested the nitrous the first time.

It was rotted so bad I could rip most of the plywood coring out with my bare hands, and as far as I could see, the only reason the transom didn’t come apart completely was because the boat was well built and the hull was laid up using some sort of Kevlar weave. It was built to handle a 600hp nitroused V8, so I figure that helped.

I cut it all out from the inside and removed the rotted plywood and I’ve now rebuilt it using Coosa coring, biaxial and West Systems epoxy. It’s pretty stout now. I don’t think I’ll have a flexing issue again.

I’m also now thinking I may get rid of the 3” setback bracket I had on it and mount the F1 clamp bracket on the transom, since it was carrying the bow well even with the motor pushed in such that it was running probably as much as 2 degrees of negative trim. The extra power from the nitrous will tend to pick the nose up quicker now anyhow.

I’m hoping to have it back in the water in a couple of weeks, and will recommence testing and update the thread again then.

Just as a side note, to illustrate what I’m talking about with regards to the datalog showing the change in trim angle from transom flex, here’s a snapshot of a datalog I made back on a test pass in Sept of last year, before I had nitrous on the boat.

Attachment 539692

The heavy pink line shows the trim position. You can see (the red circle) the trim start to reduce from when I nail it and from there it slopes down and stays down until I lift, then it springs back up immediately, so much so that it exceeds what the set trim position was. At first, I thought the trim cylinder had air in it, and I bled that several times, but it still did same thing, as it’s always done. Now I know what was causing that: transom flex.

Note: On this pass, I set the trim at 36mm, which equates to about 1 degree of positive trim, and I did not use the trim timer or touch the trim buttons during this pass. I did hit the trim button after I made the pass, which is why you see the small triangle in the pink line on the right hand side of the circle. During the run, it pushed the trim indicator cable in to the point where it was at 34mm, which is around 1-1.5 degrees of negative trim. Then it sprung out when I lifted so violently that it exceeded the set trim position. Some of that is probably due to the slop in the cable though, tbh.

BTW: in case anyone is wondering, I’m measuring trim with an AiM linear gauge, mounted on the PPE trim limiter switch. Very cool setup that Brendan came up with. The trim is set to max out at 36mm (+1 degree) on the trim timer, but I can manually set trim (and engine height) in any position with the bottoms on the wheel.

Attachment 539693


Funnily enough, the boat has always run fine with that trim. From my experience, STVs don’t need the kind of trim that, say, a Mirage does, to run fast.

To be honest, I suspected for a while that this might be the problem, but was kinda in denial about it for a while, until the nitrous made it very clear what was going on.:(

Race boat life.

Glad you caught it in time. Would be a shame to sink that much work, or worse yet get hurt.

Finished the glass work on the transom yesterday.
Just need to roll on some gel coat on the exterior of the transom to cover the holes we had to patch in the outer skin, since this STV has had several engines on it over the years, including Monty’s big looper V8, an F1 V8 and more recently my Merc on a standard bolt pattern.
Here’s a few pics to show how bad it was and what we did to replace the transom and reinforce it at the same time.
This is what we found when we cut out in the inner fiberglass:

Attachment 539842

We replaced the plywood core with two layers of 3/4” Coosa, starting with a layer of Biaxial on the outer shell, then a layer of mat, then the first Coosa panel, then another layer of biaxial and mat. We used West Systems epoxy for everything. Clamped that all together and let it cure for 24 hours.

Attachment 539844

Then I made some templates and cut out two “wedges” of Coosa to act as gussets to reinforce the transom at the sides:

Attachment 539845

I then prepped those and the area where they would go to make sure they sat flat against the knees and the transom and then mixed up some thickened epoxy and glued those on, clamped them and then I put about 18 screws in each one to hold them into place. That transom isn’t going to push inwards at the corners for sure.

Then we glassed over the whole thing with two more layers of biaxial wrapped over the top of the transom. Rolled all the excess epoxy out of it and let that dry.
Attachment 539846

Finished sanding and prepping the whole interior area in the rear of the hull and rolled in some black tinted epoxy and then splashed/sprinkled some white tinted epoxy over that, to try to replicate the original finish, although should’ve practiced my technique on a piece of cardboard first, and I got a little carried away with the brush, lol.

Attachment 539849


Just need to do the gel coat on the exterior now and then it will be good to go.

Then I can slap it all back together and resume testing.

So, for reasons which I’ve adverted to in any earlier post on this thread, I’ve decided to mount the motor straight to the transom without any setback, and last night, after a lot of measuring and adjusting, I drilled the holes for the F1 clamp in the transom, using an old clamp as a template/drill fixture.

I’m hoping that when I get the motor back on the transom, I will be able to get it low enough to set the gearcase centerline where I need it with 3” less setback.

Last night I came across this screenshot of a copy of post of David Carver’s description of his setup on the Skywalker, and I’m wondering if when he said he set the bullet @ .325” above the pad, he in fact meant 1.325”?

Because 0.325” sounds low, no? My boat always handled badly if the motor was set anywhere near thst low, although it did have 3” of setback before.
Hopefully someone can chime in and let me know.

BTW: As far as I can see, my hull bottom is identical to the Skywalker hull, which if it was 625lbs bare hull weight, was heavier than my hull, by a bit.

I obviously will test it myself once I get it back together, but would appreciate if anyone could weigh in with their experience.

Here’s the screen shot, which is one of the coolest posts I’ve seen on these forums:

Attachment 539995

There's that Earl Moorman name again. :cool:

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Originally Posted by BUZZIN' DOZEN

There's that Earl Moorman name again. :cool:

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Yeah, I noticed! Don’t know the man, but he clearly knew what he was doing with nitrous on a two-stroke.
That Skywalker boat was one bad MFer! If I can just get half the performance of that machine out of my boat I’d be happy!

Started putting everything back together yesterday. Hoping to bolt the motor back on the transom this afternoon.

Will post pictures tonight of my progress.


For now, here’s the Racetronix 525LPH Walbro “Hellcat” pump, mounted on the fuel tank rack:

Attachment 540027

Although the added volume of this pump should cure my fueling issues, if it doesn’t work, I’m thinking of teeing off the main fuel line before it gets to the rail and using that to supply the fuel system. I have all the fittings to do that, if necessary.
Or, I may just use a secondary dedicated pump for the nitrous and connect that to the other -10 bung on the tank that I currently have capped.

Encountered a setback today. After installing the motor on the transom this afternoon, we took some x-dimension measurements and discovered that the motor is mounted too high (2.25” above the pad), or rather, the transom is too high. Yeah, I know, my stupid mistake in ASSuming that the transom was cut down low enough that with the hydraulic up/down F1 mid I could put the motor height where it needed to be.

Well, it turns out it’s higher above the pad now in its lowest setting than it was before with it all the way up on the jack.

So, it’s back to square one on that. I was even contemplating pulling the motor off and cutting the transom down some more, but decided there’s no way in hell I want to generate any more fiberglass dust or do any more fiberglass work.

So, I’m going to pull the motor off the transom and reuse my old 3’ setback bracket. Hoping to get that all of that sorted next weekend so I can get back to testing sometime soon.