Author Topic: Dyno Speeds  (Read 19465 times)

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Offline isiahstites

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Dyno Speeds
« on: October 20, 2007, 12:09:00 PM »
I know the with the speed given by the dyno the bike is not moving any air and probably not the same load as you would see at the track, but I am curious how far off is the dyno speed versus what you have seen at the track?

Scott

Offline 1212FBGS

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Re: Dyno Speeds
« Reply #1 on: October 20, 2007, 12:24:59 PM »
figger 20% + or -
kr

Offline panic

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Re: Dyno Speeds
« Reply #2 on: October 20, 2007, 12:40:32 PM »
Not sure of the question. Are you asking if the air speed on the salt affects power in a way the dyno cannot simulate?
Yes.
Or are you asking if the RPM change used to record power on the dyno is accurate and linear compared to the actual conditions at high speed?
No. The typical dyno is programmed to permit engine speed to increase by either 300 or 600 RPM per second (other settings are possible), since that most closely mirrors the transients experienced in racing venues where acceleration is the key factor. Larry Widmer has frequently commented (paraphrased slightly) that peak flow CFM and peak power are not the crucial goals, and that an engine with high transient power will outperform the former in most venues.
IMO acceleration in low gear (rapid RPM change) on the salt is of minimal comparative value vs. achieving top speed, where engine speed change will be very slow and transient power less important.
Obviously, every properly geared vehicle is straining to pull high gear (viz. acceleration almost stopped just before the traps), but only the faster vehicles are really stalled in high gear because of their extremely high wind resistance
This is not the dominant factor in slower vehicles where force × mass has a high value, and is only present for a few yards in drag racing and never in sprint cars.
I don't know if the faster cars are using dyno pulls more like 100 RPM/second, but it might generate some interesting data. I would be looking at plenum volume, the trade-off of CFM for port velocity, header primary diameter, and flow-through on overlap. All these are compromises with values favoring best acceleration will frequently conflict with values favoring peak power. Sizes and shapes that move the torque peak up will have greater positive effect on top speed than the loss of acceleration, especially where traction is limited for most of the run.
The very high load and absence of residual accelerator pump discharge also means that some engines will benefit from smaller air correction (curve it rich rather than jet it rich) to reduce chamber temps, and some spark reduction for the last mile (perhaps 2-4°).

Offline isiahstites

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Re: Dyno Speeds
« Reply #3 on: October 20, 2007, 12:55:31 PM »
Just wanted to know an estimation of how far off dyno speeds were versus track speeds since you have wind resistance on the track.

Scott

Offline JackD

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Re: Dyno Speeds
« Reply #4 on: October 20, 2007, 12:59:31 PM »
"Yup , just like they said and said and said, a dyno might float yer boat, but it won't teach you how to sail." (me)
"I would rather lose going fast enough to win than win going slow enough to lose."
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aswracing

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Re: Dyno Speeds
« Reply #5 on: October 20, 2007, 01:05:49 PM »
Scott is using a Dynojet inertia dyno, which isn't programmed for any particular acceleration rate. It just has a big drum of a known mass moment of inertia riding on pillow block bearings, and a simple optical sensor that tells the electronics every time it completes a revolution. The electronics calculates the horsepower from the speed and acceleration rate of the drum, knowing it's mass moment of inertia. Since the circumference of the drum is also known, it can also calculate ground speed. If a tach pickup is used, it can also calculate torque, since it knows horsepower and engine rpm. It's kind of a weird torque number though, it's engine torque, i.e. upstream from the gearing, but including drivetrain losses.

So a first gear dyno pull will happen really quickly. A top gear dyno pull will take several seconds. Results are similar in each case, because even though the acceleration rate varies, so does the speed range. High acceleration at low drum speed (i.e. a first gear pull) indicates the same horsepower as low acceleration at high drum speed (i.e. a top gear pull).

Anyway, Scott, on my Dynojet, the speed is a little off from what I would get turning that exact same rpm in that same gear at the Salt flats, primarily due to wheelspin. Wheelspin is essentially zero on the dyno, it's never zero at the salt flats, at least not at the speeds and power levels of my bikes. Maybe for a sub 150mph bike or something you can ignore wheelspin.

The other thing about wheelspin is that it's different every time I go out there. I use a spreadsheet to calculate gearing and choose sprockets. But until I make the initial pass and get a handle on how much wheelspin I'm getting, I don't really have my arms around it.

I would advise that you gear your bike for the class record, at the power peak rpm, or maybe slightly higher, based on some wheelspin assumption. Start at maybe a 5% wheelspin assumption given the power level and speed capability of your bike. but depending on the weight of the bike, and the salt that particular day, it'll be different.

I can send you a copy of the spreadsheet if you'd like. It's in Excel.

Don't be afraid to gear it slightly short. The physics would indicate that you want to the motor right at it's power peak, but historically, my bikes have always wanted to be geared slightly deeper and taken slightly past the power peak rpm. That's where they seem to get their best speeds. YMMV.

You certainly don't want to be below your power peak rpm when the bike runs out of poop. That really makes for slow speeds, in my experience.




Offline isiahstites

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Re: Dyno Speeds
« Reply #6 on: October 20, 2007, 01:12:58 PM »
I would like the spreadsheet you can send it to stites@cox.net

Aaron, since you are talking about gearing I have a question? I am currently geared at 2.0, 24 front 48 rear and the stock gearing is 1.9 or 1.8 if I recall correctly. I thought 2.0 would be a good place to start to see if the bike can pull the gearing rather than be to optimistic and work my way down in teeth on the rear sprocket. Your thoughts??

Scott

aswracing

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Re: Dyno Speeds
« Reply #7 on: October 20, 2007, 01:19:33 PM »
"Yup , just like they said and said and said, a dyno might float yer boat, but it won't teach you how to sail." (me)

I'll tell you what, I've found the dyno to be an enormously valuable tool for getting prepared. Not only does it help you get the tune spot-on, and work out things like exhaust configuration, intake tract length, and so on, but it also has a way of shaking out bugs. I'd much rather have something go wrong on the dyno, at the shop, than at the salt flats. I can't tell you how many times the dyno has shown me something that would've been a problem at the flats.

No, it doesn't get you 100% of the way there, but 90% easily.

One of the big things it doesn't do for you well is show you fuel delivery issues. A carb bowl may run empty when you lean on it for miles, but you never see that on a 5 second dyno pull. I fought that at Finals. Finally figured it out just as the wind came up.

I've found that my bikes want little or no difference in jetting between the dyno and the flats. 'Course, I live at 5400'. Seems like every time I try to fatten it a tad to go down to the flats, I end up putting it back.

The dyno helps me get the timing spot-on, too. I establish where the power falls off on each side (adv & ret), and center it right between them.

I see so many guys attempt to tune at the event. That's just really hard to do, because to get feedback on a change, you've got to wait in line. I find it's much better to have your shit together just as well as possible when you roll it out of the trailer, and then go into a fine tuning mode as quickly a possible.

aswracing

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Re: Dyno Speeds
« Reply #8 on: October 20, 2007, 01:27:18 PM »
Scott, I can't answer that question without a whole bunch of other information. Then I plug all that into the spreadsheet and let it tell me what sprockets to use.

Engine rpm is critical. I've got to know the engine rpm of the power peak, or better yet, a range of rpm over which it makes essentially the same power.

Then primary ratio. Yours is probably 1.6, with a 35 on the crank and a 56 on the clutch basket.

I'll assume you want to run in 5th, which is 1:1. Your gearbox has the fewest losses in 5th.

Then front and rear sprockets.

Then rear wheel circumference. I like to measure this by putting the bike on a swingarm stand and taping a piece of wire to the tread, dead center. Wrap the wire around the tire and cut it off at the exact circumference. Now peel it off and measure it.

Finally, wheelspin assumption.

From all that, you can see the resultant speed, or range of speeds over the rpm range. The spreadsheet lets you put in different sprocket combinations and see resultant speeds.

The spreadsheet will also calculate horsepower required for a given speed. This calculation is based on the cubed relationship between hp required and speed increase. So you have to put in a reference point, i.e. my bike can go 160mph on 130hp. It calculates hp required for other speeds from that reference point.

Let me throw it together with numbers from your bike and send it to you.

Can you tell me the power peak rpm range, and also the rear tire circumference?

Offline isiahstites

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Re: Dyno Speeds
« Reply #9 on: October 20, 2007, 01:35:44 PM »
Let me throw it together with numbers from your bike and send it to you.

Can you tell me the power peak rpm range, and also the rear tire circumference?

Aaron,

            If by power peak rpm range you are speaking of what rpm the motor made its highest hp numbers it was at 7300 +/- and the rear tire circumference should be very similar to Susan's bike as it is the same rim as hers and I have on the stock z rated tires which are 24.5" tall.

Thanks,
Scott

Offline revolutionary

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Re: Dyno Speeds
« Reply #10 on: October 25, 2007, 09:50:28 PM »
We'll know this weekend how our dyno speed compares with track performance.  We know we made 622hp/559tq on an engine dyno in the same configuration and ran 179 at Maxton last week.  We'll be running Sat morning on a Mustang chassis dyno to see how close the numbers are.
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Offline interested bystander

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Re: Dyno Speeds
« Reply #11 on: October 25, 2007, 09:57:09 PM »
Holey toledo, just wandered on 2 this post and realized you are talking about MOTORCYCLE horsepower!

Mercy sakes alive!

Wouldn't send my kid out on that rocket!
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Offline bvillercr

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Re: Dyno Speeds
« Reply #12 on: October 25, 2007, 11:31:33 PM »
Holey toledo, just wandered on 2 this post and realized you are talking about MOTORCYCLE horsepower!

Mercy sakes alive!

Wouldn't send my kid out on that rocket!

If he was talking about a motorcycle that had 622 hp it better go faster than 179.
« Last Edit: October 25, 2007, 11:33:53 PM by bvillercr »

Offline Harold Bettes

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Re: Dyno Speeds
« Reply #13 on: October 29, 2007, 11:50:30 PM »
Howdy All,  :-D

Since you mentioned 179mph and the initial thread was about dyno speeds. :?

I have gone 179mph on a bike and although it was relatively easy. :| It scared the stuffing out of me and I don't spook too easily. :-P

Of course, this was only on the chassis dyno and I am well convinced that one of those streaks would skin me up in a heartbeat as my skill level would be overcome before the thing got out of second gear! :roll:

You guys on the crotch rockets continue to amaze me and my hat is off to you all.

Regards to All,
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As iron sharpens iron, one man sharpens another.

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Re: Dyno Speeds
« Reply #14 on: October 30, 2007, 03:12:43 AM »
I would like the spreadsheet you can send it to stites@cox.net

Aaron, since you are talking about gearing I have a question? I am currently geared at 2.0, 24 front 48 rear and the stock gearing is 1.9 or 1.8 if I recall correctly. I thought 2.0 would be a good place to start to see if the bike can pull the gearing rather than be to optimistic and work my way down in teeth on the rear sprocket. Your thoughts??

Scott

 ME TOO PLEASE :wink:! kazracing@cox.net