Be prepared, C-Prepared Autocross (1981 Camaro)

Discussion in 'Project Progress' started by 1981paul, Sep 11, 2008.

  1. G72Zed

    G72Zed Veteran Member

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    FWIW, this is the way I understand the whole "driver-driven" ratio thing and how I set up my ride. This was particularly focused on the "charging" system.

    I wanted complete charging at idle, no issues with low RPM cruise due to 5-6 spd OD's on the highway, but wanted to keep things from getting out of control on high RPM autocross and road course outings.

    As an example, my 1 wire Alt. needed a minimum of 2,400 shaft RPM to work and to be activated, max rpm must be limited to 18,000 rpm.

    The math of: driver / driven x rpm showed me I needed- a 5.5 driver pulley with the 2.35 Alt. pulley to get to a 2.34 o/d ratio, and that would be 1,030 RPM. At the same time, my max engine rpm's to keep the alt. shaft under 18,000 is 7,692 right at my max RPM's.

    I see many street machines and "hot rods" with pulley ratio's all over the map, Alt/water pump/power steering and a good % of them have some sort of "charging" or "cooling" issues they struggle with. When I ask what ratio's there using, things go quite.

    Worst is the one's who go and buy the pulley reduction kits/set ups because they "make more power".... I tested this on the track/dyno, most times, it's a bunch of baloney as they do not tell you what negative effects it has.
     
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  2. 1981paul

    1981paul Moderator Staff Member

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    Good thinking. My alternator pulley is 2.75". It is driven off a pulley on the water pump, which measures out to roughly 3.7". If I do that math right, the ratio currently is 1.35, so the alt is spinning at 10,090rpms at 7500 (engine rpm) - it can do up to 20,000. I think a smaller alternator pulley is in order.

    Going to a 1.75" alternator pulley gives me a ratio of 2.11 which is 15,857rpm at 7500, still well under the 20,000 limit.
     
    Last edited: Apr 13, 2021
  3. 1981paul

    1981paul Moderator Staff Member

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    Solid info, and thank you. I think you're on the right track. New alternator pulley is on the way.
     
  4. 1981paul

    1981paul Moderator Staff Member

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    I should mention that I know the speed numbers aren't quite right since mine is driven off the water pump, but I do think it will be better. Doing the math across three pulleys is a bit beyond me, at least this morning.

    EDIT - crank and water pump pulley appear to be 1:1, or close to it, so I'm probably good to go.
     
    Last edited: Apr 13, 2021
  5. C4Racer

    C4Racer Veteran Member

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    just figure out what your water pump to crank pulley ratio is. For me they are both 4" pullies, so the water pump is running engine speed. Makes the math simple.

    I ran a 1.75" alt pulley - doing the same math, and it kept it on at idle and was within the RPM limit.
    But for me it squealed like crazy when cold. So I still have that and would throw it on if I were going to do a cruise event or something, but the 2.5" pulley makes a lot less noise. Maybe a different belt would help, but that's such a small pulley and it's not a serpentine belt like the one running on the water pump in the set-up I have from KRC.
     
  6. G72Zed

    G72Zed Veteran Member

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    Looks like if you want to keep the 2.75 Alt pulley, and in order to hit 20,000 shaft rpm at 7,500 you need a 2.67 alt. ratio (20,000/7,500=2.67)

    Then you take the new alt. ratio x alt. pulley dia. to give you the required "driver" diameter. (2.67 x 2.75 = 7.3425) You need a 7.3425 driver.

    If you had a crank/water pump pulley at 1:1 with 7.3425 diameter with the current 2.75 alt pulley, your alt ratio is 2.67, so, use the required "shaft" rpm (2,400 let say) to activate the alt charging and divide by the alt. ratio (2,400 / 2.67 = 900 rpm)

    So you have full charge at 900 rpm idle, and safe at 7,500 (20,025)

    Sorry for the long math, but when setting up race engines with power steering/mech water pump & alternator with V belts that need to last the full race at constant high rpm's, not overheat, and keep charge without overtaxing any of the systems, this all needs to work together. Ya, lots of math, then the pulley choices come rolling in, but you know what is needed, lots of choices.

    I had to explain this to the March Pulley rep, he did not get it really, I had to buy 2 separate kits to get what I wanted/needed. They make nice pulley's, they do not build race engines or racecars, or at least the guy I spoke to didn't.

    Hope this helps.
     
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  7. C4Racer

    C4Racer Veteran Member

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    The problem with all that is some of these roundy round focused companies don't have all that many options on the various pulley sizes, so you have to work within that framework. Ideally a 2.5-2.75" alternator pulley is more ideal as a lower limit on sizing just from a belt engagement standpoint, so if the various ratios are available to let you use that and also drive the water pump and PS pump the right speed - that's what I would do. But to me the priority is running the water pump and PS pump at the correct speeds vs. the alternator - ideally you can make all 3 work, but with race oriented components you might not quite get there as the alternator is more of an afterthought.
     
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  8. 1981paul

    1981paul Moderator Staff Member

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    That's a good way to think about it. Right now, I'm probably not changing the crank, water pump, or power steering pulley. The car cools very well for what we do, and the power steering is fine (every once in a while you need a rev to wake things up from idle). I ordered the smaller alternator pump which should spin it faster, but not too fast. If it's an issue, I can always swap back to what I have now, because, as you said, the alt is the lower priority.
     
  9. G72Zed

    G72Zed Veteran Member

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    Some of them, yes, others, nothing is an "afterthought"

    FWIW, I worked with Jones Performance Products, top PRO level company, they have all the pulley style's and combo's you can dream of, and know how to it get all working together for your setup, even if everything does not comes from them.

    FWIW, here's a pic of my setup. Crank/water pump @ 1:1 with a 5.5" dia. pulley on both. Alternator pulley is 2.35 and power steering is 6.0. This gives a 2.34 alt. ratio and a .92 power steering ratio. All have the correct depth V groove.

    So, with a 2,400 alt. shaft rpm activation, this happens @ 1,026 engine rpm, max is 18,000, and at 7,600 chip, that's 17,784 shaft.

    Power steering setup is a Jones custom valved for the LEE 12:1 box, max shaft is 7,000 and with the 7,600 chip, that's 6,967 shaft rpm.

    Have not eaten nore tossed a belt with this setup since it's been on the road and track, and this engine see's lots of rpm all the time....

    Alt location and leading belt lengths play an important part in belt stability and gets overlooked often.

    JMO


    Iphone pic's 1054.JPG
     
  10. 1981paul

    1981paul Moderator Staff Member

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    Well, I've done what I can to get ready for the next dyno session. Smaller alternator pulley appears to be doing the trick, at least for now, and new valve springs are in. I didn't think to test a new one until it was too late, but here are how the old ones stacked up.

    [​IMG]

    You can see the whole thing, and then many reasons my back is terrible, below:

     

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