What Alternator?

My first GT40 build. Got the engine (302) in and now working out how and what alternator to use. Space is tight so have ordered a single groove March crank pulley to keep things back from the bulkhead. I am looking at getting the alternator low on the right hand side on a purpose made bracket. A Wosp 300 looks as if it will do the job. I am thinking 95amp but there is a 175amp equivalent. Even with electric power steering pump, electric vacuum pump and all the usual fans etc I can't really see the need for 175amps. What do others use?
 
If the car has EFI, get the higher output alternator. If not, I'd still use the higher output alternator. I messed around with lower output alternators in my Cobra and in the end had to put in a high output alternator. My Cobra has EFI. I ended up using a late model GM alt which when the case was reclocked, bolted right in to the standard 302 alternator location and brackets. You might need a different location for your GT40 but my experience is that having a bit too much alternator capability is a good thing.
 
Too much alternator will only rob horsepower.
What you need can even a 60amp alternator handle with ease so a 95amp will be more then enough.

175amp, well if you have dvd sytems in the backseats and kids & their playstations on it.. big high end sound system.. otherwise don't bother.

EPS requires amps on heavy steering only but your battery will be standing by if in need.
EFI doesn't need heavy amps also.

Optima redtop 50amps battery.
 
If your car will be street driven, there's another factor beyond the max output amps in selecting an alternator. It's the rpm range where that amperage is generated. Early alternators had to be spun up to higher rpms to reach max output. Modern alternators ramp up the amperage at much lower rpms. The driving scenario that forced the alternator change in my Cobra was stop and go traffic on a hot day after dark. Cooling fan and lights draw the power but engine is either at idle or if moving below 2000 rpms. So even though I had an alternator that tested as having 100 amp output, it wasn't keeping up with the demand because it was producing about 50 amps or so in this situation.

There's been about three generations of alternator technology since first introduced. For a street driven car, you want something that's at least 2nd generation, probably better if 3rd. In other words, an alternator that produces majority of amperage by about 2000 rpms or so. You can determine the max output needed by adding up the draw from major consuming parts (e.g. cooling fan, electric fuel pump, lights, etc.) and adding on a 20% safety margin, but use a modern alternator so it will actually produce those amps during real world driving conditions.
 
If your car will be street driven, there's another factor beyond the max output amps in selecting an alternator. It's the rpm range where that amperage is generated. Early alternators had to be spun up to higher rpms to reach max output.
You'r talking dynamo with seperate rectifiers, from the 70's on they are all 3 fase rectifyers wich will work fine on street aplications. 60amp iss suffiecent enough. All my daily's ( I have 12) are 70's cars with alternators ranging from 45amp to 60amp. My F250 is converted to 70amp cause of it bumper winch.
EFI, twin electric fans, electric powersteering, electric brake pump, all working fine with a 60amp.
 

Keith

Lifetime Supporter
The real “determining factor” for alternator selection is the wire gauge size between the alternator and the battery at the alternator output. If it is a 10 gauge wire a 60 to 70 amp alternator is all you should run, a 100 amp could ”cook” a 10 gauge wire if the battery is low and the alternator is putting out 100 amps to recharge the battery and keep up with the other electrical demands (fuel pump, ignition,lights,etc.) If you want to run a 100 amp make sure you have a 8 gauge circuit back to the battery. For a 175 amp alternator even an 8 gauge wire is on the marginal side and you may want to go larger.
The alternator only puts out the power required to power the systems and charge the battery so a 60 amp alternator and a 100 amp alternator will use the same hp to output 20 amps if that is your running power consumption. The only time the 100 amp alternator will use more hp is if the running power consumption goes over 60 amps.
Keith
 
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