Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: philward on February 16, 2022, 02:37:02 PM
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I'm currently converting back to electric staeter on the CR750 Replica.
For authenticity (as near as you can expect for a road going replica) I used a race alternator that was narrower than standard in conjunction with a narrower cover. This allowed me to use a nearer to original fairing without cutting a hole in the fairing for the alternator. Down side was that the kit junked the starter ring/clutch (hence no starter motor)
the Electrex World race alternator kit included a voltage regulator/rectifier that I have mounted under the seat with the battery and all other electrics (the inclusion of the replica central alloy oil tank creates space issues). I want to retain this reg/rec (if possible) as I've squeezed it into position and don't want to re-position/make new brackets.
BEAR WITH ME, I'M GETTING THERE!
The Electrex race alternator I'm told (by Andy at 'GoodBits' who sold me the system), that the alternator only kicks in to charge at 3000 rpm. Not an issue while racing.
Question for the electricery experts, is it the reg/rec that controls when the alternator starts charging or the race alternator.
I need to know this so that when I put the original alternator back on, the system charges as per the original Honda spec - as I can't/don't want to always ride the bike constantly above 3000 revs. It hasn't mattered to date as the battery doesn't get taxed as much without the electric starter motor
Pending the answer, I will be able to retain the rec/reg that on the bike or change it to one thats made for standard 750.
Here's the Electrex race alternator spec
https://www.electrexworld.co.uk/acatalog/RPK-1750-Honda-CB750-SOHC-Alternator-System.html
Thanks in advance
PS, I have found a high cranking amp battery that will fit in the current custom battery box under the seat
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The alternator controls when it starts charging Phil, the reg/rec does the converting the AC to DC and the switching on/off of the power coming from the alternator when the voltage gets too high, does this by switching some to an earth IIRC.
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That was my understanding Ken, so is the current regulator switching the charge to the battery starting at 3000 rpm or is the race alternator not producing any charge until 3000 rpm?
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The answer is both, the generator needs to produce over battery voltage to charge and this depends on speed, number of windings and input to field coil
Reg controls input to field coil, best to ask supplier if it will work on a standard system
A standard system with lights on doesnt charge till over 2500rpm!
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What about without lights on Bryan? Never really got involved with electrics but I worked on quite a few problems back then so maybe I've just forgotten as I got older.
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Memory has gone Ken but there is a charging test page in the manual with loads and revs etc
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Phil, dunno if this helps you… but it helped me!
Simon
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…but wait… there’s more!
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Thanks Simon
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The honda manual output chart shows 12.4 volts at 2000rpm (just about reaching parity with a reasonable battery) which could be interpreted as "switch on point" as the rpm is producing output that can charge a low battery.
Specifies 14.5 volts @ 4000rpm to reach full output.
Also 13.2 volts @ 3000rpm, not a huge amount over a decent battery.
You can see the gradient from that, and why low rpm accompanying high draw would leave the battery depleted.
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I've seen that graduation from the manual and it seems that if the race alternator and its rec/reg isn't that much difference. So, keeping the rec/reg from that system when I convert back to standard alternator should be ok.
Andy at 'Good Bits' has developed a race alternator cover that accomadates the stator in the cover (as opposed to bolting on to were the starter gear fits on the Electrex system) but due to the cost including the Electrex system is over £1000 he's hesitating to produce me a cover (about £320 to me as I have the Electrex system already). This would allow the starter to be installed and retain the narrow profile of the race alternator.
Based on the race alternator appearing to have similar characteristics to the standard system, (Electrex spec sheet says output is 25 amps - IS THAT SUFFICIENT?). I might push Andy to make me a cover as a one off.
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25A = circa 300W. Should be enough I would have thought?
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I think it was Andy I was chatting to at the last DS open day. He had his circuit race 750 there and trouble to get it firing so we could hear it, got it in the end though and sounded good.
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K2 - K6 , here's mine shortly after initial start up - has baffles fitted and before Smiths sorted calibration of rev counter. Made few mods since.
https://www.youtube.com/watch?v=YmL8YYWeUMo
PS, if you look at the digital speedo, you can see the charging voltage (bear in mind that the rev counter is reading 1000 rpm too much at the time)
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IIRC Phil the standard system only pushes out 200w. So if the Electrex system pushes out 300w that's a big improvement.
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Should be ok then Ken - Just spoke to Andy at Good Bits and he's had 5 modified covers (of his smaller race alternator covers) made now that relocates stator into the cover allowing the stater gear to be re-inserted. So ordered one and saves adding width using the standard alternator casing and cutting fairing to accomadate
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Seems like a good way to go as that race type alternator cover is a big marker in replica terms, and not having to cut the fairing.
The manual data for original states 12volt at 15amp rectifier output, 180 watts baseline as I understand it.
The bike looks fabulous by the way Phil.
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I thought the later 750s got 200w alternators.
I read the other day the 500K0 was originally supplied with a 200w alt but they stopped doing that about the same time as they changed the crankcase and went with 150w afterwards, not a great decision.
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I think it's how they write it as the above quote is from early 750 manual in which the quote "at 12volt battery voltage" so looks like a baseline/minimum of 180 as most system are going to run above that notional 12volt.
So pure technical supply is able to be supported at 12volt 15amp but reality is most likely higher in normal use for "marketing purposes"
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Realized my post wasn't clear as the manual I've got doesn't state the total wattage, just the data above and noted it's conditional on the battery supply voltage as to the total watts. Effectively it's variable within a range rather than single figure across all operational conditions.
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Phil said his alternator was a 25A output unit…..300W?
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That's the new compact one, the other/original from manual data is to compare the two.
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The 750 auto had a higher output alternator
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So if Phil fitted an Electrex alternator I wonder if they ever made one for the 500/550.
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Dont think so Ken, i have seen one somewhere but it was nearly a grand
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Just found one on the Electrex website Bryan, RPK1550, fits 400/500/550. 25amp output, cost around £550. Improved HP and quicker throttle response as it replaces the flywheel as well, I'd imagine less torque as a result?
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Less inertia surely. Same torque.
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Yes, same torque. Might improve engine response and net horsepower if the savings in gross crankshaft mass (inertia) exceed the increased alternator flux coupling forces.
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Electrex website says 3 - 5 HP increase for 750 (I have 836 standard compression piston kit as well which Hondaman says is about 5 - 8 hp increase), so useful overall gain without high tuning/revs.
The good thing about the 500 (from my limited once only rebuild knowledge of a 500) is theres no starter gear issues in fitting the Electrex alternator that I am aware of hence none of the issues I'm having. Its worth checking how it affects the fitting of the cover though as I had to source the narrower cover seperately. (Electrex don't supply with the kit). The standard Electrex alternator on the 750 uses a machined alloy plate thats about 15mm thick that fits between the narrower alternator cover and the crankcase. If the 500 version uses the same setup and you use the standard alternator cover, it would widen the engine width. I'm only assuming this is the case based on the 750 setup as havn't looked at 500 version. Worth the pre sale research considering the cost.
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Bhp is only a calculation of torque at X rpm, which means that if the torque remains the same at the same rpm (it's just a function of combustion effectiveness) then the bhp can't alter.
Rotating mass affects the time to accelerate the crankshaft, making it accumulate revs faster (when not moving vehicle mass) but not change the total.
Capacity change increases torque as the combustion total energy is larger for the same rpm, minus the increaes in heat and friction between the two in comparison.
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I seem to recall reading that Harleys have greater torque due to the inertia of their flywheel being some big and heavy, which would make sense as it would take more load to slow down such a mass of moving metal, would also explain why they rev so slowly.
So I'd have imagined the same would have happened by removing the mass, it's quicker to speed up but just as quick to slow down. So if the engine is slowing down on a hill faster for example and your needing to change down to keep up speed then that's the same as having less torque?
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I wonder how they justify a 3-5 Hp increase when the alternator output increase is only say 100w. The hp increase could only be realised, generally, by fewer gross losses or increased rpm?
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IIRC a 500/550 alt only produces 150w so the Electrex alt doubles the output. I'm wondering if the 12a battery can cope with that or whether fitting a higher ampage per hour battery of the same dimensions would be better.
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IIRC a 500/550 alt only produces 150w so the Electrex alt doubles the output. I'm wondering if the 12a battery can cope with that or whether fitting a higher ampage per hour battery of the same dimensions would be better.
One is a store (battery) one is a production line (alternator) that obviously do different things. If you can run all demand out of a bigger store, within the capacity of that store, then you'll not run out of power. Likely that's never going to be the case though. Racers may do this though and tun without power generation.
You could almost get away no battery-ish though, as with enough output from alternator will run everything continuously. Just couldn't start with electric though.
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I wonder how they justify a 3-5 Hp increase when the alternator output increase is only say 100w. The hp increase could only be realised, generally, by fewer gross losses or increased rpm?
I don't see that either, at full output it should be consuming more torque to give that output.
The torque peak is essentially controlled by camshaft timing, ie optimised flow dynamics to fill the chamber and make the most productive burn to push the piston down.
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Not done huge miles and not revved to redline yet as still in run in period but throttle response compared to my 750k2 (that is physically standard but also has the same big bore kit fitted) is 'quicker'. If you look at that video I posted, you can see it to some extent when revving in neutral. Can't judge increased power in real terms as not rode a standard 750. Both mine seem reasonably torqey at lower revs which may be the big bore kit.
This may be due to the motor not having to drag the weight of the original rotor?
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Came across phils post which prompted me to try and give an understanding of the generator I sell.
I use the winding and moffset reg/rectifier that is also sold by electrex in their race alternator kit. The advantage is that the rotor is very small, which gives very low crankshaft inertia, no flywheel effect. ideal for us racers.
there is always a compromise though, the magnets in the rotor rotate past the stationary winding, to generate electricity, basically the faster the magnets pass the winding the more electricity is created, but because the rotor/magnets are on a very small dia, it means they pass the windings at a much slower speed for a given rpm. compared to a normal motorcycle generator alternator. (ie inside of a wheel rotates slower than the outside)
this is ok on my race bikes as I am not sitting about much and the bike is sat at high rpm most of the time,
and it works really well as the bike has no starter either, so does not drain the battery starting.
My big mistake was re-designing the special cover to allow use of starter motor, I have found that people think they ride their cafe racers on the road and think they will not be using lights and because they ride fast they will be revving high a lot of the time.
But I think the reality is that when you use the starter to fire up the bike you tend to use a lot of the battery capacity, and so putting back the charge used will need a good high rpm run without interuption to replenish charge.
for example, I had a customer bike fitted with my latest kit, with starter etc and my low profile cover and generator, I have been setting suspension and been bedding brakes in and doing runs between villages before handing over build, and I have found that general pottering about with squirts up dual cariageways etc stopping to fill with fuel etc just was'nt enough. the bike cruised along lovely, but generally speaking probably half the time it was running below the 3500 rpm minimum needed to put some charge back etc.
the bike would always start on kickstart, but the electric start would begin to struggle.
Its not the reg/rectifier that causes this, and the race reg/rectifier will not be suitable for a std honda system either.
hope this helps, the covers are expensive to make and coupled with the rest of the kit its a big spend, but saves that hole in the side of fairing, and saves you clouting the ground with the stock engine cover.