13bpower

Rx7store.net is selling the Unorthodox Main Underdrive pulley for $189. It claims to add 12-16 hp. I'm guessing that would be at the flywheel on a turbo II. Dose anyone have any advice on this product. Anyone have it on a N/A? Let me know if this product is worth while! I'm guessing it would be a 5hp gain to the wheels on an N/A. If so just one more thing to make my vert a little quicker.

bcty

race track yes!!
street no!!

thats the final say on underdrive pullys

FC3AZ

Why did you come to that verdict. I used to have an underdrive pulley on my Focus I had, and it did fine for over 25,000 miles. It was a nice 7fwhp gain too.

13bpower

Isn't power power? If it helps on the track why not the street? Do they help only on high end or only low end? Anyone else have an opinion on them?

FC3AZ

They help everywhere. The dyno graph for my Focus showed a 7hp and 9lb-ft of tq improvement over the whole rev range.

6port

convensional wisdom says that with the under drive pulleys installed the components that they drive will spin slower. If you are not racing at high revs on a track then your alternator or water pump(whichever the pulley is on) may not spin fast enough. This can cause a lot of problems. If its the water pump pulley you may over heat. If its the alternator pulley you may not charge the battery enough. I have not actually used the under drive pulleys myself and cannot say for sure if they will work for street use, but many on this forum and on other sites advise against it.

FC3AZ

My friend in Texas, his name on here is rx7_rage, has had underdrive pulleys for a looonnng time on his TII now. He has had only 1 problem. He did have an alternator go out, but that might have just been it giving out, we dont know. He did notice a gain with them, I believe. He might chime in here later tonight.

13bpower

It's kinda wierd, I've seen the Main underdrive pully on rx7store (underdrives everything?). Also rx7.com has a completely different setup of pulleys that underdrive the alternator and sumthin else (i can't remember right now). That would seem like a more reasonable conversion for street use because it wouldn't be underdriving vital engine conponents. Also less gain? Anyone have the Main underdrive on a street car?

scathcart

the water pump and alternator are on the same belt, connected to the crank shaft. You can just underdrive the alternator with a alt pulley, just underdrive the water pump with a water pump pulley, or underdrive them both with a crank pulley.

The benefits of underdriving include more horspeower by less accessory parasitic loss, and less cavitation of the water pump at high rpms.

The negative side is you're decreasing the amount of charge coming out of the alternator, placing strain on an already tired and over-stressed system. On a race car, this is not a problem, since there is less electircal load, and the rpms are kept high. But on a street car, where headlights, signals, and high-wattage stereo are the norm, as well as a lot of low-rpm cruising, you can count on replacing a lot of alternators.

I doubt you'd see 5 rwhp from pullies. Maybe one or two, not enough to notice, anyway. I didn't notice a thing when I took the belt off my water pump (only crank-driven accessory I have is the alternator).

IMHO, not a worthwhile mod unless its a racing motor, or you have a system designed to be underdriven.

scathcart

Nope. Power is not power. Think about periphereal port engines: nothing on the bottom, great power on the top. Makes no sense on the street.

Think about electric water pumps. Makes more power, but not sensible for the street.

Think about carbon apex seals, 30+ psi of boost, and full tube chassis cars. Makes power, but not sensible for the street.

Think underdrive pullies. Make power for high-rpm racing, but not sensible for the street.

deadRX7Conv

You need a reasonable % change in power to feel a difference. If this is one of multiple mods, it will help. If you do this as a stand alone, you might not notice a difference. On some cars a pulley change is noticeable because the weight and ratio of the pulley is changed. I remember one nissan that I swapped to a UR pulley----There was a ~4lb difference in pulley weight. Just like knocking 4lbs off of a flywheel. It was noticeable and is felt throughout the rpm band. It also underdrives the accessories which add to accessory life.
If you buy the UR or Rx7.com pulley, just remember to bump your idle by ~100 rpm or so. The ~10% increase in idle will make up for the reduction in accessory speed at idle and eliminate so called charging problems.
I've never seen a underdriven pulley cause charging or cooling issues. But, if you are experiencing problems now with either, you will compound the issue.
If your alternator is old and has lots of miles, it will need to be replaced soon anyway. Sames goes for the battery, radiator, waterpump, thermostat..........
The pulley will exaggerate problems that already exist.