My first DYNO session!

[N20LT4Bird]

I was slightly dissapointed with my numbers, as I had calculated them using weight and trap speed at about 525 HP to the wheels. The guy that ran the dyno said the the "mustang" dyno gives VERY conservative numbers and if I ran it on another dyno, it would be about 18% higher. So, my numbers arent too bad then.

First N/A run: 343 HP @ 5495 RPM and 357 lb/ft @ 3400 RPM

Second N2O run: 458 HP @5300 RPM and 524 lb/ft @ 3818 RPM

Third N2O run: 461 HP at @ 5300 RPM and 561 lb/ft @3400 RPM

Take a look at the complete runs, I have them on file. What do you guys think of my air/fuel ratios? Ian @ madz28 said he would dial the tune in once I had it dyno'ed. Is there room for improvement in either my N/A or N2O runs? Look at how the motor leans out right as I hit the nitrous and then goes real rich. Is this normal?

[94 formula]

Look at your torque curve though that is whats impressive. Both NA and on the bottle just as your car starts to loose torque your HP picks right up. Its alot better than getting 20 more horses but not having anything till after 5000rpms. I've always been a fan of keeping the powerband broad and not just uptop.

[Injuneer]

Can't seem to open the attachments. Nonetheless, those are good numbers.

I have a calculator that looks an awful lot like the ones that people "claim" are for "rear wheel" HP. I have simply tweeked some of the constants so that the formula gives results matched to actual ENGINE (flywheel HP) numbers that I have for several 4th Gens. My calculator indicates your 122MPH is produced by a 527HP engine. Depends obviously on the race weight I use - how does 3,600# sound. Also indicates that you could be running in the low 11's with that kind of HP/MPH.

If you assume you are at 461 SAE corrected rwHP, and that you have a 12.5% drivetrain loss (I also have those numbers, measured directly from engine and chassis dynos for my car with the T56, Street Twin, 12-bolt, etc.), you are at 461/0.875 = 527 flywheel HP.... all checks out.

Hot Rod magazine did a 5 dyno comparo, and found that the Mustang dyno produced results that were about 3.5% lower than a Dynojet. That was corrected to "standard" conditions, which is slightly different than "SAE". Unforunately, they used a HP-challenged Mustang GT, so it was only making 220-230rwHP, so not sure how a higher HP car would compare on the Mustang dyno. The good news is.... the Mustang dynoo appears to report about 11.2% low on the rear wheel torque, so that thing is laydown huge torque numbers.

I know everyone feels that the online calculators give you "rear wheel HP" and some of them actually say that. But is I put 127.7MPH in, at a 3,765# race weight (both actual numbers, with the car weighed on an NHRA verifed scale following the run), it says I am making 612HP. There is NO way my car makes 612rwHP on a 125-shot. I have the engine dyno, and it makes 633 FLYWHEEL HP on a 125-shot, and was making 505 REAR WHEEL, for a 20.2% drivetrain loss on the TH400. Just the ambient weather conditions account for the difference between the 612 and 633HP.

I know there's one of the regulars here who challenges my "model", and my claim that it shows flywheel HP, not rwHP.... but you can't argue ith successful results. The models were developed by Chrysler drag racers, 35 years ago, when the concept of "rear wheel HP" was not very popular. The reason is can produce good results, for a low-loss manual tranny, or a high-loss automatic, can be accounted for by the fact that the automatic can dramatically increase "area under the curve" with a high stall, high slip, high torque multiplication setup. And its "area under the curve" that produces the ET and MPH, not the peak HP.

[N20LT4Bird]

Can't seem to open the attachments. Nonetheless, those are good numbers.

I have a calculator that looks an awful lot like the ones that people "claim" are for "rear wheel" HP. I have simply tweeked some of the constants so that the formula gives results matched to actual ENGINE (flywheel HP) numbers that I have for several 4th Gens. My calculator indicates your 122MPH is produced by a 527HP engine. Depends obviously on the race weight I use - how does 3,600# sound. Also indicates that you could be running in the low 11's with that kind of HP/MPH.

If you assume you are at 461 SAE corrected rwHP, and that you have a 12.5% drivetrain loss (I also have those numbers, measured directly from engine and chassis dynos for my car with the T56, Street Twin, 12-bolt, etc.), you are at 461/0.875 = 527 flywheel HP.... all checks out.

Hot Rod magazine did a 5 dyno comparo, and found that the Mustang dyno produced results that were about 3.5% lower than a Dynojet. That was corrected to "standard" conditions, which is slightly different than "SAE". Unforunately, they used a HP-challenged Mustang GT, so it was only making 220-230rwHP, so not sure how a higher HP car would compare on the Mustang dyno. The good news is.... the Mustang dynoo appears to report about 11.2% low on the rear wheel torque, so that thing is laydown huge torque numbers.

I know everyone feels that the online calculators give you "rear wheel HP" and some of them actually say that. But is I put 127.7MPH in, at a 3,765# race weight (both actual numbers, with the car weighed on an NHRA verifed scale following the run), it says I am making 612HP. There is NO way my car makes 612rwHP on a 125-shot. I have the engine dyno, and it makes 633 FLYWHEEL HP on a 125-shot, and was making 505 REAR WHEEL, for a 20.2% drivetrain loss on the TH400. Just the ambient weather conditions account for the difference between the 612 and 633HP.

I know there's one of the regulars here who challenges my "model", and my claim that it shows flywheel HP, not rwHP.... but you can't argue ith successful results. The models were developed by Chrysler drag racers, 35 years ago, when the concept of "rear wheel HP" was not very popular. The reason is can produce good results, for a low-loss manual tranny, or a high-loss automatic, can be accounted for by the fact that the automatic can dramatically increase "area under the curve" with a high stall, high slip, high torque multiplication setup. And its "area under the curve" that produces the ET and MPH, not the peak HP.

Interesting info. I have always read that the horsepower calculators calculate rear-wheel horsepower. I dont know how they can give you engine horsepower unless you tweak some of the constants as you say. The formula that I have seen is: hp = weight * (speed / 234)3. This gives me 510HP using 3600#. (With me in the car, its really more like 3800#). You are right in that these numbers do sound high for rear wheel power. What is the formula your using?

The 12% loss on torque on the mustang dyno is interesting. That means over 600 lb/ft of torque at the wheels That would explain my traction problems

Now, I would like to get my PCM dialed in exact and maybe jet up to a 125 shot

[Injuneer]

I'm using the same format you are, but have adjusted it to:

HP = WEIGHT X (MPH/222.79)^3.18816

[azkkn]

your air fuel seems to be right on the money on the two N20 runs, any thing richer would be an issue, and its nice and safe being at 11.5 ish so no worrier there. all in all i would say your current tune is pretty good

[PUREPWR]

I'm using the same format you are, but have adjusted it to:

HP = WEIGHT X (MPH/222.79)^3.18816

Does this MPH mean the trap speed in the quarter mile?

[N20LT4Bird]

Does this MPH mean the trap speed in the quarter mile?

yes it does.

[Craig 94 TA GT]

I created a 3D model of Fred's formula, and the curve looks very good. I plotted the Z axis for horsepower, X for weight, Y for trap speed. If anyone is interested in viewing the actual graph, I can post the program (need OpenGL libraries)...with any X,Y,Z limits. You can rotate the graph to any position, but it's a bit hard to see the actual results of each plot. However, it's really good for viewing the general curve of the model. The program is still on BETA version though, so it has a few bugs.

BTW Sean nice numbers

[Joe 1320]

you want the a/f mixture to be a hair on the rich side. That is what is protecting the motor. If a little too lean, instead of burning air and fuel, piston material starts next. You could have it dyno tuned for a few extra ponies, but I don't think I would do it. Too many bad things happen with that much oxygen when things go lean.

[Injuneer]

With only a 100-shot, he's probably OK with the 11.5-11.7:1.... but as the hit goes up, I would recommend running it richer than that. It will not hurt you to get closer 11.0:1. You reach a critical point when N2O power equals 50% of NA power (e.g. 400NA + 200N2O) because the oxygen content of the mixture is at a point where its getting really hard to control the combustion speed.... the higher the O2 content, the faster and hotter it burns. You need to account for the significant difference in stoichiometry between air:fuel and air:N2O.

In general to get a starting point, figure your NA HP at 12.8:1 and your N2O HP at 7:1. Do a weighted average, and you will find that you should be seeing 10.9:1 at a 2:1 NA:N2O mix (400 + 200). At a 4:1 NA:N2O you would be looking for 11.6:1. (400 + 100, as he tested it). You may see a wisp of black smoke on the track at those levels, but as Joe points out, its protecting your engine.

[N20LT4Bird]

Good info guys. Thanks. Im still trying to sort out all of what Fred was saying, but I will get there. Interesting. Bottom line, as my nitrous power reaches it's theoretical limit of 50% N2O power, I need to richen it more and more. I dont think im gonna go with too much more nitrous until I get my bottom end beefed up a bit more. Now that I have a baby on the way, that will be in about 18 years

My biggest concern is the fact that it is leaning out (14.5:1) as soon as I hit the button, then its richening back up quickly. Is this because the N2O is at a higher pressure than the fuel, and its reaching the cylinders faster?

Also, my air/fuel ratio flutters while on the nitrous, especially in my last run, richening up on occasion. I assume that this is the nitrous bottle not being full, and its cutting in and out.......

[Injuneer]

The delayed hit on the fuel may be due to the relative length of the lines, from the solenoids to the nozzles. While pressure obviously plays a part, you can't really change the pressure a whole lot. You could try moving the fuel solenoid(s) closer to the nozzle.

I hope my point on A/F ratio isn't confusing. All I am saying is that you need to consider the fact that nitrous, because it is richer in O2 percent, requires a lot more fuel.

Air:Gasoline stoichiometry (ideal combustion) is 14.65:1 air:fuel. For every 1# of fuel you burn, the engine needs 14.65# of air. To make max HP and torque, most engine like to see a rich mixture. 12.8 - 13.2:1 is traditionally quoted as the best for peak HP and torque. Each engine is different, but you can see that to make max power, you need to run maybe 15% excess fuel. That's a 12.8:1 A:F ratio.

Nitrous:Gasoline stoichiometry is 9.6:1. You need 1# of fuel for every 9.6# of nitrous. Also, there is less N2 that simply passes through the combustion process unaffected. And, experience has shown that you may want to run nitrous at a rate of 40-50% excess fuel. That means you should be adding the nitrous and the fuel in the ratio of about 6.0:1 to 7.0:1.

Since you are making HP using both the air/gasoline and nitrous gasoline, the total fuel requirement is a function of both the target A:F ratio for air:gaso and nitrous:gaso. The larger the portion of the HP that is produced by the nitrous, the richer the "blended" A:F ratio needs to be.

When I first had my engine built, the shop told me that they generally wouldn't spray a nitrous shot more than about 50% of the NA HP. I had heard that "rule of thumb", but couldn't understand the limiting factor. After poking around a bit, I came to the conclusion that its related to the burn speed. As O2 content, as a percentage of the total content of the combustion chamber goes up, the burn speed of the mixture and the heat released go up exponentially. Apparently, at the O2 concentration represented by a 2xNA:1xN2O power production, you are getting close to the point where you simply can't control the combustion... go a little lean for an instant, and you're going to detonate.

The "50%" thing is a rule of thumb. I've tried to push the limits, spraying 300HP of juice on a motor that barely makes 500HP NA. But, to do this, at least with the blessing of the shop that tunes the engine, we need to run VP C16 highly leaded racing fuel, doped with a smidgen of VP Air Race. As much as it would appear to be possible to just keep spraying more and more nitrous on an engine, the riskier it gets as the ratio of NA:N2O goes above 50%.

If you think about it, a stock engine shouldn't see more than about 150HP of nitrous. There are people who have sprayed 200HP on a stock engine, and somehow they got away with it. But if you have an engine that's making 500HP, you can spray 250HP with less risk than putting 200HP on a stock HP level engine. My thinking is that I need to go to a more radical cam (and probably a little more head flow) and push the NA HP levels closer to 600HP - then I can look at spraying 350HP of nitrous.

Also keep this in mind. The calibration of the typical wide-band O2 sensor appears to be based on air:gasoline stoichiometry. The sensor develops a voltage, based on the partial pressure of O2 in the exhaust, and that voltage is "calibrated" to an A:F ratio. I have asked the question of whether the fact that nitrous, because of the different O2:N2 ratio, isn't also affecting the calibration of the wide-band sensor. So far, no one has been able to answer that question for me. I know when you burn propane, methanol, etc., you have to change the meter calibration to reflect the stoichiomtry. That leads me to question a little the precision of the wide-band when you start seeing 10.0:1 ratios running nitrous.

I'm also a little fascinated by the current developments in nitrous : propane applications.

[N20LT4Bird]

The delayed hit on the fuel may be due to the relative length of the lines, from the solenoids to the nozzles. While pressure obviously plays a part, you can't really change the pressure a whole lot. You could try moving the fuel solenoid(s) closer to the nozzle..

I have thought about this, but wonder if its really worth it, it's definately leaning out for a split second, but the question is - is it really that detrimental to the motor?

I hope my point on A/F ratio isn't confusing. All I am saying is that you need to consider the fact that nitrous, because it is richer in O2 percent, requires a lot more fuel..

It took me a couple reads to get it, but I grasp the concept. Makes perfect sense.

Also keep this in mind. The calibration of the typical wide-band O2 sensor appears to be based on air:gasoline stoichiometry. The sensor develops a voltage, based on the partial pressure of O2 in the exhaust, and that voltage is "calibrated" to an A:F ratio. I have asked the question of whether the fact that nitrous, because of the different O2:N2 ratio, isn't also affecting the calibration of the wide-band sensor. So far, no one has been able to answer that question for me. I know when you burn propane, methanol, etc., you have to change the meter calibration to reflect the stoichiomtry. That leads me to question a little the precision of the wide-band when you start seeing 10.0:1 ratios running nitrous.

This is something I have not thought about before......you would think that it would HAVE to be off, because the normal O2 sensor is calibrated on a ratio of 14.7:1. The A/F ratio you get from the wide band would be correct based upon 14.7:1 stoichiometry, but as you have already pointed out, this cannot be applied to a nitrous mixture. So, a normal wide band reading probably couldnt be trusted with larger nitrous shots.....or at least it would have to be interpreted. You could probably come up with a formula based upon the percentage nitrous use.