I Suck at Math

[Rick Iverson]

Gents;

It is my intention to mount a Kubota dynamo atop a Pinto ICP valve cover, and driven by a pulley mounted to the face of the cam pulley. I spoke with tech support at PowerMasters Racing Alternators, only to realize there are several variables in determining the two correct pulley sizes. Dave Weitzenhof addressed the dynamo speed needs to be at or less than 4,000 RPM. Questions:

• What is the ratio between crankshaft RPM and cam RPM (read: cam RPM at 7,000 engine RPM)?
• To keep the dynamo at or less than 4,000 RPM, it needs to be under-driven at 57%, right?

So I need help with determining either the two pulley ODs (Poly-V) or teeth count by anyone who is not math challenged, please.

[Purple Frog]

The cam turns at half the speed of the crank

[Daryl DeArman]

Your math is good. I can't ignore "attention" vs. "intention" though.

[Rick Iverson]

I suck at spelling and grammar as well.

Your math is good. I can't ignore "attention" vs. "intention" though.

[Rick Iverson]

What I surmise then, at 7,000 RPM, the cam is turning at 3500 RPM. I could basically do a 1:1 ratio and be well within the safe zone. I need to re-read Dave's thread on this, to see if 4,000 is a safety deal, or optimum performance output of the dynamo.

The cam turns at half the speed of the crank

[Gary_T]

Unless I'm getting completely the wrong end of the stick here - you can drive the dynamo at 1:1 from the cam.

Engine RPM = 7000
Cam RPM = 3500
dynamo driven by cam at 1:1 = 3500

[Daryl DeArman]

I suck at spelling and grammar as well.

Me too.

[TimH]

I also.

[BeerBudgetRacing]

If I recall the documentation on the dynamo, there was a minimum speed as well.

Probably not an issue but IIR it won't generate power at idle.

[DaveW]

The dynamo highest RPM should be 4200. That's ~6/10 of 7000 engine RPM, or 1.2 times 3500 cam RPM. That value is to not overheat and destroy the regulator which regulates by converting excess dynamo output to heat.

The dynamo starts putting out over 12V at ~1800 dynamo RPM or 3000 engine RPM if set up as I said above.

Link: http://www.apexspeed.com/forums/show...7&postcount=29

[DaveW]

Unless I'm getting completely the wrong end of the stick here - you can drive the dynamo at 1:1 from the cam.

Engine RPM = 7000
Cam RPM = 3500
dynamo driven by cam at 1:1 = 3500

1.2 x cam RPM to start charging at as low an engine RPM as possible (3000) w/o screwing up the regulator.

[DaveW]

What I surmise then, at 7,000 RPM, the cam is turning at 3500 RPM. I could basically do a 1:1 ratio and be well within the safe zone. I need to re-read Dave's thread on this, to see if 4,000 is a safety deal, or optimum performance output of the dynamo.

See my previous 2 posts.

[Rick Iverson]

Now that’s what I’m talking about. Weitzenhof, YOU are the man.

The craziness of all this is, while preparing to take the MCAT, I completed two (2) semesters of Organic Chemistry. I was generally referred to as an idiot sauvant by faculty and fellow students alike. Have a severe math anxiety (and spelling, grammar......).

The dynamo highest RPM should be 4200. That's ~6/10 of 7000 engine RPM, or 1.2 times 3500 cam RPM. That value is to not overheat and destroy the regulator which regulates by converting excess dynamo output to heat.

The dynamo starts putting out over 12V at ~1800 dynamo RPM or 3000 engine RPM if set up as I said above.

Link: http://www.apexspeed.com/forums/show...7&postcount=29

[Rick Iverson]

Actually, my mind was moving so fast, that my fingers couldn’t keep up.

Your math is good. I can't ignore "attention" vs. "intention" though.

[Purple Frog]

Here's a thought.
When you are actually on the track you should be keeping revs between 5,200 and 6,900 for the whole session. So the cam is turning 2,600 to 3,450. If you turn Kabota at 2,000 you'd be fine.

I would drive the device slower than the cam:
1. less wear and tear
2. you'll get enough charging
3. less drag on the engine.

YMMV

[Rick Iverson]

Frog, good stuff Pally.

Here's a thought.
When you are actually on the track you should be keeping revs between 5,200 and 6,900 for the whole session. So the cam is turning 2,600 to 3,450. If you turn Kabota at 2,000 you'd be fine.

I would drive the device slower than the cam:
1. less wear and tear
2. you'll get enough charging
3. less drag on the engine.

YMMV

[DaveW]

Here's a thought.
When you are actually on the track you should be keeping revs between 5,200 and 6,900 for the whole session. So the cam is turning 2,600 to 3,450. If you turn Kabota at 2,000 you'd be fine.

I would drive the device slower than the cam:
1. less wear and tear
2. you'll get enough charging
3. less drag on the engine.

YMMV

You also have to account for pace, double yellow, and cool-down laps, where you may not be turning over 4K RPM, plus restarts after black-flag-alls, pit visits, and spins while the engine is hot and harder to start. I don't use a jump battery. So I go for maximum safe dynamo output to keep the battery as well charged as possible. My Kubota dynamo/regulator setup (6/10 engine RPM) has worked perfectly w/o issues since 2009, and my battery usually comes off the track at ~90% charge level. I drive it with a 3L size belt off the Zetec water pump. Dynamo durability is not an issue.

You can make your own decision on a lower drive ratio if you want to save a minute amount of HP.

[jrh3]

Gents;

It is my intention to mount a Kubota dynamo atop a Pinto ICP valve cover, and driven by a pulley mounted to the face of the cam pulley. I spoke with tech support at PowerMasters Racing Alternators, only to realize there are several variables in determining the two correct pulley sizes. Dave Weitzenhof addressed the dynamo speed needs to be at or less than 4,000 RPM. Questions:

• What is the ratio between crankshaft RPM and cam RPM (read: cam RPM at 7,000 engine RPM)?
• To keep the dynamo at or less than 4,000 RPM, it needs to be under-driven at 57%, right?

So I need help with determining either the two pulley ODs (Poly-V) or teeth count by anyone who is not math challenged, please.

Ever consider running it directly off the back of the cam?
B&C has been doing it for many years on aerobatic engines.

[Jnovak]

What is the purpose of this exercise?

[Rick Iverson]

How do you mean, Jay?

What is the purpose of this exercise?