Cadillac Owners Forum banner

1 - 20 of 22 Posts

·
Registered
1992 Cadillac DeVille 4.9L
Joined
·
203 Posts
Discussion Starter #1
Has anyone looked into underdrive pulleys for the 4.9?

They get large gains for a small amount of money... are there any made for the 4.9? or would they have to be custom made?

Just a thought..
 

·
Registered
Unicycle
Joined
·
11,009 Posts
No underdrive pullies for the 4.9.

Although with the recent increase in after market for the 4.9 (still nearly nothing at all though) I woudln't be surprised to see them soon....

although with some issues I have been haivng with the alternator output at idle (dips low at times... nothing wrong with charging system or battery) I would be hesistant to have the alternator spinning even slower....
 

·
Registered
'98 Buick Regal LS - '91 Caddy Seville - '87 Caddy Seville!
Joined
·
4,425 Posts
No, no underdrive pulleys available yet, if at all...

However, you can always go to a junkyard and get some assorted GM pullys from a few cars, then find a Cadillac there and play a game of mix 'n' match with them. If something fits, then go for it and see how it works out. Then the next thing to do is find a smaller drivebelt to make it really work out. I wouldn't expect a large gain because the Cadillac still has several items on the drivebelt, but it might work out a little.

As for the alternator spinning more slowly with smaller pullys, I am confused, because I thought that much like a supercharger with a smaller pully, it spins faster, thus making more power...:hmm:
 

·
Registered
1992 Town Car Cartier & 2014 Accord LX MTX
Joined
·
34,087 Posts
not to sound like a dumbass, but what exactly does an underdrive pulley do?
Isn't that what they put on the S/C 3800's to give them more power?
 

·
Registered
1992 Cadillac DeVille 4.9L
Joined
·
203 Posts
Discussion Starter #5
They put smaller pulleys on S/C'd cars to increase boost because the smaller pulley spins just as fast, and is shorter, thus making the belt move faster.

Underdrive pulleys on NA cars go to the accessories. They are lighter, as well as larger in diameter, I believe, in order to increase mechanical advantage, this drops a load of the engine. Camaros and stangs get something like 12-15 horsepower from underdrive pulleys.
 

·
Registered
1992 Town Car Cartier & 2014 Accord LX MTX
Joined
·
34,087 Posts
ah neat good to know :thumbsup:
 

·
Registered
Joined
·
192 Posts
I use to run a double underdrive kit on my Fiero 2.8 that was done to the tits, would spin 7000rpms. http://www.aspracing.com I spun the alternator at 50% of normal speed and the water pump at 75% of normal speed. It was great for a motor that at 60mph was turning 3000 rpms. I would drive between 2500 and 4500rpms just cruising around town. ASP has a great supply of underdrive, overdrive you name it. My pulleys were made from billet aluminum to reduce weight as well.

See where this is going. Highs revs are great for under drives, however the 4.x series is not exactly happy spending its life always over 3 grand and I would pass out if I ever saw a semi stock 4.9 hit 7000 rpms before explosion.

I love the 4.9 in my Fiero now, and it spends its life in 5th gear cruising around town at 1000-1500 rpms. You just can't get away with an underdrive on a low rpm motor like the 4.9.

Just out of curiosity, what is the best route for changing the OEM alternator. I have the 140amp CS version in there now. But I think it might be failing. It is only 13 or so with a mild load at idle. IE Heater on high and headlights on. If the rad fan kicks on it drops down for a moment to 11 and then shoots back up to 13v and sits there.
 

·
Registered
Unicycle
Joined
·
11,009 Posts
From what I know, underdrive pullies make the items spin slower (hence the name)

In my case, with the "problem" I am having with the electrical system (yet EVERYTHING is in perfect spec... and rebuilt) I woudln't want to turn the alternator any slower then it is.

Aren't underdrive pullies larger diamter pullies that go on the accessories? If they were smaller it would cause them to turn faster, but require more work...

I dunno, in a sports car they are good, but for my use, I really don't want the alternator, power steering, compressor and water pump turning any slower then they are... the 4.9 isn't that choked by the accessories.

smaller, low-torque, high revving 4bangers DO benifit alot from underdrive pullies though, atleast from what I have seen.
 

·
Registered
Joined
·
192 Posts
Re: Under drive pulleys?

Just to add in.

Under drive pulleys are usually smaller crank pulleys, and or larger accessories pulleys.

In the 60 degree V6 motors. They commonly use a 25% crank pulley, and an additional 25% alternator pulley.

This slows everything down by 25% and slows the alternator even more so it drops to 50% of normal speed.

At a 1000rpm idle it would think it was only at 500rpms and at 6000rpms it would charge like I was at 3000rpms.

This was good in a way as it saved my alternator and water pump bearings from serious abuse.
 

·
Registered
Unicycle
Joined
·
11,009 Posts
ok, so I was right then... I wasn't sure.

As I mentioned... in the Caddy, I have no desire to do such a thing. I am not going to kill any of the accessories with the massive 4,900RPM shift point of the 4.9 :)
 

·
Registered
'98 Buick Regal LS - '91 Caddy Seville - '87 Caddy Seville!
Joined
·
4,425 Posts
Now I'm confused for sure! Instead of 'underdrive' pulleys, let's say 'smaller' pulleys make things go/spin faster, do they not?
 

·
Registered
1989 Sedan DeVille is now just a fond memory ....
Joined
·
10,429 Posts
illumina said:
Now I'm confused for sure! Instead of 'underdrive' pulleys, let's say 'smaller' pulleys make things go/spin faster, do they not?
Yes, the smaller pulleys increase the rpms. For an "underdrive" pulley system you either use a smaller crank shaft pulley or a set of larger accessory pulleys.

That's what I put on my 5.0 HO Mustang, a smaller crank pulley.
 

·
Registered
'98 Buick Regal LS - '91 Caddy Seville - '87 Caddy Seville!
Joined
·
4,425 Posts
Kev said:
Yes, the smaller pulleys increase the rpms. For an "underdrive" pulley system you either use a smaller crank shaft pulley or a set of larger accessory pulleys.

That's what I put on my 5.0 HO Mustang, a smaller crank pulley.
Ahh...Okay Kev, Thanks for clearing that up for me. I guess I learn something new everyday, huh? And I thought I knew everything...:hide:

Now with that out of the way, I wonder if I can get away with that on my 4.9 liter? I guess I should do what I suggested earlier: go to the junkyard and play the mix 'n' match game :devil:
 

·
Registered
Joined
·
192 Posts
Re: Under drive pulleys?

Think back to a your old 10 speed bike.

Smaller pulley on crank
Bigger pulley on rear sprocket/ (accessories)

One rotation of the crank with a big pulley means the belt moves more. But if you put a small pulley on the crank one rotation moves less of the belt.

Back to bike. The bigger the sprocket on the front/crank the faster your rear wheel / accessories go.

But if you make the rear wheel / accessories smaller they turn faster.

Does this make any sense to you. I think I have just blown a fuse in my poor old head.
 

·
Registered
1989 Sedan DeVille is now just a fond memory ....
Joined
·
10,429 Posts
Re: Under drive pulleys?

Capt Fiero said:
Think back to a your old 10 speed bike.

Smaller pulley on crank
Bigger pulley on rear sprocket/ (accessories)

One rotation of the crank with a big pulley means the belt moves more. But if you put a small pulley on the crank one rotation moves less of the belt.

Back to bike. The bigger the sprocket on the front/crank the faster your rear wheel / accessories go.

But if you make the rear wheel / accessories smaller they turn faster.

Does this make any sense to you. I think I have just blown a fuse in my poor old head.
You have the right idea,

10 speed bike;
Larger front sprocket/smaller rear sprocket = peddle slow, bike goes fast

Smaller front sprocket/larger rear sprocket = peddle fast, bike goes slow

Auto engine;
Smaller crank pulley/larger accessory pulleys = accessories turn slower, use less torque/HP

Larger crank pulley/smaller accessory pulleys = accessories turn faster, use more torque/HP.
 

·
Registered
'98 Buick Regal LS - '91 Caddy Seville - '87 Caddy Seville!
Joined
·
4,425 Posts
So then when someone places a larger pully on a supercharger, then they're underdriving it, thus making more power?
 

·
Registered
Unicycle
Joined
·
11,009 Posts
yeah, example is the s/c 3800

the popular pully mod is a smaller diameter, it makes the supercharger spin faster, thus creating more boost. Yes, it is putting more of a load on the engine, but the extra power benefit is so much worth it.
 
1 - 20 of 22 Posts
Top