Nitrous on 2.5 Drag

[CI STV]

Sounds like you got it covered, I’m sure you’ll work your way through it. Good luck, stay safe.

Thanks again for all your advice man. It is truly appreciated. I’m going to report back on here with the results once I get to test it again.

[CI STV]

112 octane is an advantage when you’re running high compression. What compression are you running. A stock drag motor with 26 cc heads is somewhere in the 175 to 180 cranking compression.

At that compression you’re safe to run something like 102 103 octane you can obtain that by mixing pump premium and race fuel.

Running that high of octane is really only an advantage when you’re running very high compression.
Are you running gasket heads or O-ring heads.

You keep an eye on your Pyro’s. If they get hot after the past, you can just enrich the motor injectors to cool your motor down.

Thanks. I’m running 21cc chambers right now. Haven’t checked the compression lately, but if memory serves it was around 190 psi with those and 215psi or so with the 17cc chambers I was running before I put the nitrous on it. My motor is a ported drag motor.

I have a Brucato steam wheel on the dash that I could use to dial in some extra fuel if that’s needed.

[Stoker boy]

Chuck was a good friend of mine and taught me a lot of things. He had a lot of R&D ideas and I was always testing stuff for him,. Chuck knew quite a bit about 2.5 motors. I don’t know about new modern systems, I know you can only put in so much nitrous and fuel into a combustion chamber.

There’s also a whole lot of other tricks you can do to motors.

[Stoker boy]

Thanks. I’m running 21cc chambers right now. Haven’t checked the compression lately, but if memory serves it was around 190 psi with those and 215psi or so with the 17cc chambers I was running before I put the nitrous on it. My motor is a ported drag motor.

I have a Brucato steam wheel on the dash that I could use to dial in some extra fuel if that’s needed.

I believe I’m around 215 or 220 cranking with stock Mercury 21 cc O-ring heads. the Block has been decked and I believe my squish is about 25 THO I think 35 to 40 is normal but the Pistons aren’t slapping the heads so I’m good.

I don’t run Mercury electronics.

[CI STV]

From the couple of telephone conversations I had with him back then, Chuck certainly seemed to be both knowledgeable and helpful. I remember him telling me that he wouldn’t recommend going over 100hp without using methanol with nitrous.

Earlier in this thread there was some discussion about the tuning map/graph in the Merc ECUs that tends to lean out the mixture significantly at high vacuum and between 8,000 and 8,250, which means as you come off the throttle at the end of a run and the MAP sensor sees high vacuum, the ECU will lean out the fuel mixture as your rpm drops through that range.

Every fuel graph I’ve looked at has that lean spot in it, except a couple of tunes someone sent me that were custom tunes (by James Perry, I think) which had that low fueling spot in the graph smoothed out. So I figure I was onto something that a few smarter people had already figured out.

I would bet good money that lean spot in the stock Merc tunes has fried many a motor on nitrous, and maybe a few without it too.

For ease of reference, this is what I’m talking about:

I’ve fix that problem by simply smoothing out the numbers on those cells to resemble the surrounding cells in the graph. It makes no sense to me why those are so different.

[Stoker boy]

I know some of the new systems including Mercury boxes use vacuum for throttle position.
I have an old antiquated potentiometer system for throttle position. I can adjust the fuel curve every 250 or 200. RPM. don’t recall . I have friends that use fancy exotic expensive systems and they mostly tell me my antiquate system is just not as user-friendly. If they make a prop change they really don’t have to adjust the fuel curve. If run a 22 then go to a 24 I have a bog so I have to adjust the fuel curve in the computer.

I’ve figured out the system so for me it’s easy.

[Stoker boy]

From the couple of telephone conversations I had with him back then, Chuck certainly seemed to be both knowledgeable and helpful. I remember him telling me that he wouldn’t recommend going over 100hp without using methanol with nitrous.

Earlier in this thread there was some discussion about the tuning map/graph in the Merc ECUs that tends to lean out the mixture significantly at high vacuum and between 8,000 and 8,250, which means as you come off the throttle at the end of a run and the MAP sensor sees high vacuum, the ECU will lean out the fuel mixture as your rpm drops through that range.

Every fuel graph I’ve looked at has that lean spot in it, except a couple of tunes someone sent me that were custom tunes (by James Perry, I think) which had that low fueling spot in the graph smoothed out. So I figure I was onto something that a few smarter people had already figured out.

I would bet good money that lean spot in the stock Merc tunes has fried many a motor on nitrous, and maybe a few without it too.

For ease of reference, this is what I’m talking about:

I’ve fix that problem by simply smoothing out the numbers on those cells to resemble the surrounding cells in the graph. It makes no sense to me why those are so different.

Looks like at 8000 RPM the curve is every 250 RPM. My curve looks something similar to that. Mine is every 250 RPM all the way, I can change fuel curve or TPS for Tuning.
That’s why you set your Pyro alarm, in case something goes lean on you.

[CI STV]

Looks like at 8000 RPM the curve is every 250 RPM. My curve looks something similar to that. Mine is every 250 RPM all the way, I can change fuel curve or TPS for Tuning.
That’s why you set your Pyro alarm, in case something goes lean on you.

Yep. The chart increases in 250 rpm increments from there through 10,500 rpm, whereas below 8000 rom, the cells are 500 rpm apart.

But what I was talking about is how the numbers drop significantly in the cells in those columns at the bottom of the chart, which is for lower MAP/higher vacuum.

So imagine you lift off the throttle at the end of a run and the RPMs then drop from ~10,000 down through that range and the MAP sensor is seeing high vacuum due to the closed throttle and high rpm. As it passes through that range, the fuel curve leans out and for a brief period, on every run, your motor goes a bit lean (probably a lot, given the significant drop) for as long as it takes to get through that rpm range.

That might not be a problem for a non-nitroused motor, but on nitrous when you build serious heat in the cylinders, that would not be ideal for sure. So, using my Brucato software and tuning cable, I simply smoothed out those cells so there’s a smooth transition.

That’s all based on my understanding that the higher those numbers, the higher the fueling rate (through injector pulsing).

Another smart racer revealed on this thread that he’s added fueling in the high vacuum/high rpm cells for nitrous use to provide a similar cooling effect of a methanol “washdown” when you lift off the throttle and the vacuum goes high at higher rpm. That’s pretty ingenious, I think.

I am very grateful for all the helpful info and advice that you guys provide on these forums. It has helped tremendously.

[CI STV]

I know some of the new systems including Mercury boxes use vacuum for throttle position.
I have an old antiquated potentiometer system for throttle position. I can adjust the fuel curve every 250 or 200. RPM. don’t recall . I have friends that use fancy exotic expensive systems and they mostly tell me my antiquate system is just not as user-friendly. If they make a prop change they really don’t have to adjust the fuel curve. If run a 22 then go to a 24 I have a bog so I have to adjust the fuel curve in the computer.

I’ve figured out the system so for me it’s easy.

I hear you. Stick to what you’re used to if you’ve already figured it out. I’m that way too, but I have to admit that having the telemetry from a Datalogger has revealed lots of things I never even thought of before.

The Brucato ACU does in fact use MAP to determine throttle position. It’s a true “speed/density” system, which is the simplest form of fuel injection and it uses the same logic (vacuum) as a carburetor, actually, to give the motor the fuel that it needs. In fact, a good multi circuit carb is much more complex than this.

[CI STV]

Yep. The chart increases in 250 rpm increments from there through 10,500 rpm, whereas below 8000 rom, the cells are 500 rpm apart.

But what I was talking about is how the numbers drop significantly in the cells in those columns at the bottom of the chart, which is for lower MAP/higher vacuum.

So imagine you lift off the throttle at the end of a run and the RPMs then drop from ~10,000 down through that range and the MAP sensor is seeing high vacuum due to the closed throttle and high rpm. As it passes through that range, the fuel curve leans out and for a brief period, on every run, your motor goes a bit lean (probably a lot, given the significant drop) for as long as it takes to get through that rpm range.

That might not be a problem for a non-nitroused motor, but on nitrous when you build serious heat in the cylinders, that would not be ideal for sure. So, using my Brucato software and tuning cable, I simply smoothed out those cells so there’s a smooth transition.

That’s all based on my understanding that the higher those numbers, the higher the fueling rate (through injector pulsing).

Another smart racer revealed on this thread that he’s added fueling in the high vacuum/high rpm cells for nitrous use to provide a similar cooling effect of a methanol “washdown” when you lift off the throttle and the vacuum goes high at higher rpm. That’s pretty ingenious, I think.

I am very grateful for all the helpful info and advice that you guys provide on these forums. It has helped tremendously.

EDIT: I absolutely agree on setting the EGT alarm, which I had done from when I first putting the motor in the boat.

I have alarms set for:

< 53 psi fuel pressure;
>1175 degree EGT (on either bank);
< 10 psi water pressure above 30kph and 6000 rpm.

In the event if any of these things occurring, my dash will flash red and display which alarm was triggered.

[Stoker boy]

I’m not totally in the Stone Age, I do have computers, and a data logger on board.

Since things happen pretty fast during a pass, I go to my data logger, I can look at a particular RPM, fuel curve and pyrometers. If a particular area looks fat or lean, I can make adjustments as necessary.

[CI STV]

I’m not totally in the Stone Age, I do have computers, and a data logger on board.

Since things happen pretty fast during a pass, I go to my data logger, I can look at a particular RPM, fuel curve and pyrometers. If a particular area looks fat or lean, I can make adjustments as necessary.

LOL! No man, I didn’t think you were Fred Flintstone!�� I could tell you know what you’re doing for sure.
Data loggers work great for slow minds like mine, so I can go back and see what the hell went wrong!

[CI STV]

Just a quick update:

I tested the boat this weekend again, just a quick ~300’ blast, and it’s still dropping the fuel pressure, worse than ever, so I’m going to first reconfigure the fuel delivery to not pull fuel off the second port on the regulator and if that doesn’t work, I’m going to put a second dedicated fuel system on the boat just to supply the nitrous system, and be done with it.

The good news is that I cured the transom flex, and everything else works great and the boat still handles beautifully. I have a short on board video, which I will upload to YouTube and post a link later.

I ran it this time with my old 24P prop, rather than the 22P Tillman, and it launched hard, but seemed to cavitate more and screamed to 9,000+ rpm, but then the fuel pressure dropped and then the rpm dropped and recovered but once it started surging, I eased off.

Once I get the fuel system sorted I’m going to have to play with the setup and prop.

[Stoker boy]

I’m going to first reconfigure the fuel delivery to not pull fuel off the second port on the regulator.

I think someone here mentioned that.

[CI STV]

I think someone here mentioned that.

LOL! Yeah, some guy named Stoker something told me to do that, but I was too dumb to listen!

Actually, I did intend to do so, but I’ve been focused on putting the boat together and that turned out to involve more work than I anticipated, so when I finally got it finished with that on Sunday, I decided to throw it in the pond and make a quick hit. At least now I know it wasn’t that my old Weldon pump had crapped the bed. Might actually end up putting that back on since the new one makes a high pitched noise that sounds like a mosquito in my ear!

I’ve found all the fittings I need to re-rig the fuel supply to the solenoid without cutting any hose etc., so it should take me long to do that. I’m going to tee it right before the fuel rail. Hopefully, that will work.