10% under drive or a 2.75”

[Terrencemitch88]

Hey guys,

looking for around 18 or 19 psi for my 2011 gt500. Have a 2.9 whipple gen3 with a 3” with a stock under drive making right now around 16 pounds.

what would be a better benefit going with a 2.75” or 10%over drive.

 

[Catmonkey]

I don't know about better, but it would be a whole lot cheaper and a lot less work to get a smaller upper pulley

 

[SCGallo2]

...what would be a better benefit going with a 2.75” or 10%over drive.

I would install an ATI 10% over drive balancer and not go smaller than a 3.00" upper in an effort to prevent belt slip.

 

[sleek98]

Both would work. I would rather throw on the upper and leave the stock lower on. I dont think there would be an issue with belt slip on a 2.75. I never had a problem when I ran a 2.6.

Edit: make sure you are running the right belt size if dropping down to a 2.75. I think most guys with whipples run a belt to fit a 3.25" which would be way to big to get a 2.75 to not slip.

 

[Terrencemitch88]

I don't know about better, but it would be a whole lot cheaper and a lot less work to get a smaller upper pulley

I heard that the 10% under drive pulley would make torque come on sooner please correct me if I’m wrong tho!!!

 

[Terrencemitch88]

Both would work. I would rather throw on the upper and leave the stock lower on. I dont think there would be an issue with belt slip on a 2.75. I never had a problem when I ran a 2.6.

Edit: make sure you are running the right belt size if dropping down to a 2.75. I think most guys with whipples run a belt to fit a 3.25" which would be way to big to get a 2.75 to not slip.

I have a metco Auxiliary Idler setting in my garage Would that work if I went with the 2.75 upper and keep the same belt?

 

[sleek98]

I have a metco Auxiliary Idler setting in my garage Would that work if I went with the 2.75 upper and keep the same belt?

If your running a stock sized belt yes. You shouldnt have issues, I have gone down to a 2.4 with a metco aux with no slip.

 

[Catmonkey]

I heard that the 10% under drive pulley would make torque come on sooner please correct me if I’m wrong tho!!!

I've heard that too, but it's a mechanical ratio, so I can't see there's any real truth or benefit to it. There might be an inkling of belt slip on the hit that might account for that contention. Now the crank to blower ratio will be different. An ATI 10% will give you another 0.8% more rpm on the blower over the 2.75" pulley, and the IW will you 2.6% more rpm on blower speed over the 2.75" pulley. The IW 10% is more like 12% overdrive.

 

[scotsam]

The a.t.i. stock size balancer turns out to be about 6% overdrive. And yes there was a noticeable difference down low.

 

[Radron2626]

I am running 10% lower and 2.8 upper on my 2.9 Whipple. I would recommend the IW 10% lower first.

 

[Terrencemitch88]

I am running 10% lower and 2.8 upper on my 2.9 Whipple. I would recommend the IW 10% lower first.

How much boost are you running?

 

[Radron2626]

I have the car tuned for at 18 psi with this set up. Plenty of power for the streets.

 

[69b302]

I have the car tuned for at 18 psi with this set up. Plenty of power for the streets.

What is your elevation? Your boost numbers are way low for that combo.

 

[Radron2626]

I agree, I am in Denver, 5280 elev. The car was originally doing 23psi, with this combo but it was leaning out with the Lund tune. So we tuned it at 18psi with the same combo, and it put down 751whp/746tq sae smoothing 5, on the Dynojet. With the Lund Tune at 23psi sae smoothing 5, was same number roughly. I do have a full fuel system, pumps, rails, injectors.

 

[1Kona_Venom]

I've heard that too, but it's a mechanical ratio, so I can't see there's any real truth or benefit to it. There might be an inkling of belt slip on the hit that might account for that contention. Now the crank to blower ratio will be different. An ATI 10% will give you another 0.8% more rpm on the blower over the 2.75" pulley, and the IW will you 2.6% more rpm on blower speed over the 2.75" pulley. The IW 10% is more like 12% overdrive.

you were one of those smart kids in elementary school weren't you? lol
but, very good break down though!

 

[Catmonkey]

you were one of those smart kids in elementary school weren't you? lol

Nah, I've just been around the block a few more times than most of you here.