Pulley size for high altitude

Need help!!! What would be the right size pulley for high altitude(5000-6500 ft) compared to a 3.75 pulley at sea level. We have 91 here in Denver. I'm being told anywhere from 3.125 -3.625. FYI stage 2.gen3

67cobra67 said:

Need help!!! What would be the right size pulley for high altitude(5000-6500 ft) compared to a 3.75 pulley at sea level. We have 91 here in Denver. I'm being told anywhere from 3.125 -3.625. FYI stage 2.gen3

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Im running 3.5 and told its the max for 91 octane, run some racefuel or e85 and go 3.25 or 3”. Hard to tell what exactly it is compared at altitude, Im getting 10psi with the 3.5” gen 2 , longtubes o/r

I sent an email to Whipple and got back the suggestion of going up one size to make up for the elevation and the loss. Im 6000 at my house, but its a mile in Abq. The only issue with that is if I drive the car at sea level or near it itll probably be a problem. I will drive my car to Austin a few times a year for the races so Ill most likely error on the size of reliability and just live with not the max hp available for my elevation.

Thanks for your help. Ill be running the car only locally. I guess my question is, how much boost can I run on 91. Then I can adjust the pulley size to match that. I spoke to a local tuner and he said they run , I think the 3.125 or 3.375 pulley to get 10-11 pounds.

67cobra67 said:

Thanks for your help. Ill be running the car only locally. I guess my question is, how much boost can I run on 91. Then I can adjust the pulley size to match that. I spoke to a local tuner and he said they run , I think the 3.125 or 3.375 pulley to get 10-11 pounds.

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Reach out to whipple. Be real carefull with tuners at our elevation. Make sure they have a very good track record. The 20% loss in air makes any hint of a lean condition cause serious damage really quick. Its kind of funny, because so few have much experience driving cars at our elevations and what it feels like(or the lack there of, lol).

If your quick at high altitude your doing something right. Not knocking guys at sea level because there some bad asses but if you can do it up here , you can do it down there. What do you guys think about running 10-11 lbs with 91 or whats max on 91.

I don't have a boost gauge, but I live in Colorado and I use the Whipple tune. FWIW I get positive knock values with Shell 91 octane with both the 3.625 pulley and the 3.75. Highest I've seen so far is +4, but it's typically in the +1 to +2 range. I did a 50/50 mix of 100 octane with 91 and the knock was negative everywhere. I think the 91 around here just sucks.

67cobra67 said:

If your quick at high altitude your doing something right. Not knocking guys at sea level because there some bad asses but if you can do it up here , you can do it down there. What do you guys think about running 10-11 lbs with 91 or whats max on 91.

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It depends more on fuel quality on your area, If I see knock I add octane booster and I wouldn’t run more then 10-11 at that altitude. Im at 6700’, da sometimes goes up to 10k so seeing 10psi on a 3.5

I guess Ill Run torco or octainium. 100 octane fuel just isn't cost effective. Ill see how it runs now but well go to e85 when the funds allow it. Can I see the knock value on the whipple tune or do I need to get a scanner or tunner. Again thanks for the help.Ruiner46 what part of Colorado do you live?

I ran 3.5 and the Od crank pulley for over a year until i sold the car. New owner still going a year later. I lived in Denver then. PBD tune and E85.