Under Drive Pulley's Performance
Posted: November 12th, 2006, 2:23 pm
Looked around a little but wanted to know if the UDP for a 2.5 Litre will fit on a factory 1.8 Litre? Asking because until I do my swap, I want it for my K8.
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I went out in the stock room and found a block of aluminum the right size and turned it on a lathe in the old prototype shop in the R&D dept. So as far as I know that are not commercially available for the MX-3 although I am starteing to see them for other new cars.Bling Bling wrote:Where might I find an O/D pulley for the alternator? Or is it a HUGE deal?
had mine on my car without a OD pulley for over a year, never had a start issueBling Bling wrote:Where might I find an O/D pulley for the alternator? Or is it a HUGE deal?
The OD alternator pulley with the U/D under drive pulley basically cancels each other out so you have a the original drive ratio or maybe just slightly underdriven. Underdriving the alternator does not free up as much power as the P/S or water pumps as people think. The alternator only creates a lot of mechanical loses when it is charging with the UDP the alternator is making less than 12V at idle so you are running off the battery. Also the stock alt. pulley ratios are set up to spin the alt to be most efficent at 3000-3500rpm where the engine will spend most of it time. EFI engine need a lot of juice to run at peak power even more for things like the lights and ICE. If the alternator is not charging 100% the regulator will ask the alternator to try to supply more current increasing the mechanic power load on the engine. Sometimes this load is often greater that the savings of spinning it slower with a UDP and the additional load remains even after the alternator is driven at it optimum speed until the batterey is fully charged. If the battery is old and has lost a cell or developing internal resistance this will also place a load on the alternator so U/D it here is a handcap as well. To a point the faster it spins the less work it will do. Where as the water pump is also optimized for the same 3000-4000rpm to achieve the best cooling but once you pass this speed it is fight back pressure in the cooling system which only increasing the faster the engine revs. The P/S pump is slightly different it has a pressure relief valve that allow it to work over a larger rpm range but is still over pressures at + 6000rpm. Unless it is a pure race car you are better of not U/D the alternator at all. On my SOHC for example I removed the water pump and thermostat completely for a gain of between 10-13hp over the UDP on the water pump itself. There was little difference between on the UDP on the alternater and the stock pulley once the engine was over 4500rpm but there was a big difference on the bottom end where the the stock pulley had a slight advanages the longer the engine sat at idle.Nd4SpdSe wrote:wound't getting OD pullies to compensate void the benefits of the UD pully to begin with?
You wouldn't have a starting issue the car will start and run as low as 10V The question is would there be any performance benefit and how much with a UDP and I was just saying that underdiving the alternator can change the gains depending of the charging state of the batterey.papa roached wrote:had mine on my car without a OD pulley for over a year, never had a start issueBling Bling wrote:Where might I find an O/D pulley for the alternator? Or is it a HUGE deal?