What is the logic behind choosing a blower drive pulley size?

**Glenn_03Cobra** · 03-03-11, 10:24 PM

On a never ending quest to mod the Cobra beyond a Cold Air Intake and Magnaflow mufflers, it looks as though fantasy is about to meet reality. Among a few mods that are planned, one engine mod that Cobra owners routinely install has me a bit puzzled.

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Perhaps some would rather choose moderate versus extreme power performance, but I wonder... since the Cobra's 4.6L motor is topped with an Eaton supercharger, Ford's SVT team built a solid bottom-end to handle 600+ RWHP. Why would one, who doesn't have a specific purpose for their blown aggressions other than the weekend twisties, upgrade to a moderately smaller than stock blower drive pulley (such as a 3.10) versus a more aggressive pulley providing another 2-4 lbs of boost.

So, I get that decreasing the pulley drive size increases the Eaton's RPM creating more heat and wear. And, I realize not everyone wants or needs maximum power from their engine, but I've met probably 100+ 03/04 Cobra owners and not one of them purchased their Terminator for a casual driving experience.

So I ask... what is the logic a GearHead uses when choosing a pulley size for a blown street powerhouse. Then I ask, on the other end of extreme, why would would one stop with the blower drive pulley? Why not upgrade the under drive pulley as well?

There are a variety of reasons that come to mind such as -

Minimizing wear and risk to component failure to the blower, motor, and driveline.
Controlling heat issues with higher RPM produced by a smaller pulley.
Or perhaps one's logic is that a moderate pulley might mean they don't have to upgrade their blower cooling system.

But... the questions still remains - What is the logic an average Cobra uses, when choosing a pulley size for a car that is not a casual ride to start with? One that never or rarely touches the track? What's in their head?

**blind** · 03-04-11, 03:51 PM

the smaller the pulley, the more likely you are to get belt slip as well, so they also require an idler pulley upgrade at the same time.

don't do it until you do the IRS Diff brace at a minimum!

**Glenn_03Cobra** · 03-04-11, 04:02 PM

Where's the fun in that LOL...
If I do any mods, I'm gonna have to tie-up Julie and stuff 'er in the closet while I order stuff

**Eric B.** · 03-07-11, 04:12 PM

Ohh, the closet. What a good idea. I need to go get something out of my trunk.

**Powershift03** · 03-07-11, 09:29 PM

"What is the logic behind choosing a blower drive pulley size? "

Basic hot rod answer, Glenn..... A SMALLER BLOWER PULLEY MAKES YOUR CAR GO FASTER

The only logic needed.

LOL

**Glenn_03Cobra** · 03-07-11, 09:43 PM

Good point LOL

I get the feeling I asked the wrong crowd to help me develop this into an article

Next topic please...

**Tetge** · 03-07-11, 11:22 PM

I wrote a long technical post that answered all your questions but, when I clicked the submit reply button it wiped it out and said I was not logged on. It took awhile to put down all the fact and figures and I guess that the board is set for a quick trigger as far as activity and logging users out goes. So, too bad.........

**Glenn_03Cobra** · 03-07-11, 11:59 PM

I originally changed the default from 5 mins to 10 'cause it bugged the crap out of me after losing a few posts myself. I just moved it out to 15 mins which messes with some other stuff too. But it's strange that happened to you as I had recently started posts, went to do other stuff then came back to finish the post about 20 mins later and never got logged-off. I'll experiment with this to see if there is another setting that causes the logout.

Tips for those that don't know:

Depending on your browser, you might be able to click the back button to get your post back.
When logging-in, be sure there is a check mark in the box that says "Remember Me" so you will be logged-in automagically if logged-out. Be cautious though... if others can access your computer, they would be able to log on as you.

**Brakelate** · 03-08-11, 10:41 AM

The big problem when increasing boost pressure is heat. Fine if you are a street driver, who can rarely go WOT, or if you are a drag racer, as in a short 11 or 12 second sprint, there is enough time in between races to allow temps to cool back down.

On a car that is taken on a track to HPDE's or driven in an environment that allows full throttle through multiple gears, or for a lengthy pull to top speed ( with the 155 limiter removed) you can easily build up enough heat to start cooking things... even that strong built bottom end.

I should know.

But, in any case, I say tie Julie up and stuff her in the closet anyway.

**Tetge** · 03-08-11, 01:59 PM

Actually, the long post that I lost said basically that the choice of pulley for a stock or nearly stock engine with a stock blower depended on intended application. For drag racing alone I think that a 2.75 or 2.76 or 2.80 works well. You have to get a good dyno tune and you should use an extra idler pulley to get better belt wrap to lessen slip. But, back in the day, I had a 3.10 pulley that was pretty sweet overall. Rick at Amazon Racing sold it to me with a mail order tune in 2002 and the tune was crap even after returning it to get it fixed. Rick told me that a 3.10 was the smallest pulley that could be used with the stock Eaton that would not rev the Eaton past its stated design limits, and I later saw charts and info that confirmed this. I finally took the car to Danny Swanson who did wizard tune on the 3.10 and it certainly ran very well. However, in the bad air at LACR, other Cobras running 2.80 pulleys were able to beat me due to their superior torque so I changed to the 2.80 and had Danny retune the car later in 2002, and it has remained that way ever since. But, I do not drive hard on the street and I only used to drag race so heat sinking and all the issues with a 2.80 versus a 3.10 are not an issue for me.

**Tetge** · 03-08-11, 02:14 PM

I went back to my dyno run files and the blue is the 3.10 tune on Swanson's dyno and the red the retune for the 2.80 pulley a few months later on the same dyno (and the numbers for the 2.80 were recently validated on RET's dyno so they, at least, are accurate). Ironically, the 3.10 pulley made better SAE numbers on the dyno for some reason, and it did run nicely. However, in the real world, the 2.80 pulley pulled harder down low and that made a difference launching a heavy car on sticky rubber down the 1320'. Still, the chart for the 3.10 does not look all that bad, and, the 3.10 does not over spin the Eaton.

And, BTW, at the drag strip I used the stock pulley; a 3.10; a 3.00; a 2.90; and my current 2.80. Yes, I have the pulley tools, obviously. And, each time I went to a smaller pulley, I went quicker. However, I was also stepping up to better tires and 4.10 gears and a proper shifter and different clutches and better technique, etc. I finally went 12.8's on OEM F1 tires at LACR with the stock pulley, so who knows how quickly I might ultimately have gone if I had stuck with the 3.10 pulley and the the impressive looking Swanson tune? I seem to recall that Carlos ended up with that steel 3.10 pulley and he alleges that he is still running it. But, seat of the pants, you certainly can feel the difference in the torque as you decrease the pulley size. I returned the car to stock a few times but could not stay that way since I almost got run down pulling into traffic because I had no grunt. And, in reality, since I shift at 6200, I am using the torque more than the ultimate high RPM HP. I finally just locked in on the 2.80 and forgot about it.

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**Brakelate** · 03-08-11, 05:18 PM

Originally Posted by Tetge

I am using the torque more than the ultimate high RPM HP.

And so I wonder why you badmouth my choice with cam timing that suits a fat torque curve, over peak HP.

**Tetge** · 03-08-11, 08:34 PM

Originally Posted by Brakelate

And so I wonder why you badmouth my choice with cam timing that suits a fat torque curve, over peak HP.

Because with a ported blower and all, you can turn more revs and make even more power up high and take advantage of the superior breathing and revving capablilities of a small displacement DOHC engine. You may as well have a pushrod 302 with big valves if you are not going to rev it up.