Does crank pulley size matter? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

desert-rat · 03-03-2025, 02:57 PM

My crank pulley measures 7.75" diameter which drives the fan and alternator. The power steering pump is on a smaller diameter groove of the same pulley. I need to replace the pulley and I have another 7.75 " diameter but power steering pump is gonna ride on 7.75" groove. Will that be too big for the power steering pump?

**70STOVEBOLT** · 03-03-2025, 03:27 PM

It will cause the pump to spin slower, and may affect the ease of steering. What is the diameter of the smaller groove on your current pulley?

**Steeveedee** · 03-03-2025, 03:29 PM

The pump will run at a higher rpm. It doesn't blow up at 120 MPH in a car, so I'm thinking that it would work OK if slightly overdriven. What is the diameter of the pulley that drives the pump now? Maybe someone else will chime in with a good reason not to?

**70STOVEBOLT** · 03-03-2025, 04:24 PM

Quote:

Originally Posted by Steeveedee

The pump will run at a higher rpm. It doesn't blow up at 120 MPH in a car, so I'm thinking that it would work OK if slightly overdriven. What is the diameter of the pulley that drives the pump now? Maybe someone else will chime in with a good reason not to?

Not true. If you use a larger pulley, the pump will turn slower, not faster.

HO455 · 03-03-2025, 06:48 PM

Assuming an rpm of 1000 and a 6" diameter on the pump.

With an assumed diameter of the smaller driving pulley of 6.75 the current pump rpm is 1125.

With the new driving pulley diameter of 7.75 the pump rpm will be 1291.

http://www.stevegs.com/utils/beltcalc.htm

Myself · 03-03-2025, 07:06 PM

Quote:

Originally Posted by 70STOVEBOLT

Not true. If you use a larger pulley, the pump will turn slower, not faster.

Negative Ghostrider.........

Increase DRIVE pulley size and the DRIVEN side turns faster

Decrease the DRIVE size and the DRIVEN side turns slower.

The crank is the DRIVE, the accessory is the DRIVEN.

Think of it like this........let's assume (for quick math ) that the crank pulley is 3" in diameter. By the time the crank turns ONE revolution it will run through a bit over 9 inches of belt. If you had a 6" pulley, by the time it rotates ONE revolution, it will run through over 18" of belt.......MEANING with the same ONE revolution, the 6" pulley goes through TWICE as much belt.............spins the DRIVEN side TWICE as fast.

RichardJ · 03-03-2025, 07:09 PM

ho455 and stevie and "Myself" are correct. The larger the drive pulley, the faster the belt moves.

Circumference is Pi*D. 3.14 x 6" = 18.84" or how far the belt move per engine revolution.

3.14 x 7.75" = 24.335" or how far the belt moves for the larger pulley per engine revolution.

**PbFut** · 03-03-2025, 08:57 PM

A small power steering cooler might be in order depending on your use.

**70STOVEBOLT** · 03-04-2025, 05:29 PM

I stand corrected. Would I be correct if I said a larger pulley on the p/s pump would make it turn slower?

desert-rat · 03-04-2025, 05:39 PM

Quote:

Originally Posted by 70STOVEBOLT

It will cause the pump to spin slower, and may affect the ease of steering. What is the diameter of the smaller groove on your current pulley?

The smaller groove seems to be an inch smaller at 6.75"

54blackhornet · 03-04-2025, 05:52 PM

I don’t remember Sister Cabrini ever mentioning this in geometry class ?

Myself · 03-06-2025, 12:08 PM

Quote:

Originally Posted by 70STOVEBOLT

I stand corrected. Would I be correct if I said a larger pulley on the p/s pump would make it turn slower?

Yes, the PS pump is the DRIVEN side. Increasing the pulley diameter would slow it down.

Ever ridden a multi speed bicycle? Your legs are the DRIVE. When you shift into fifth gear, that's the small gear on the rear sprocket (DRIVEN). Your legs (DRIVE) slow way down but the wheel (DRIVEN) speeds way up.........5th gear!

: Shifting back to first, the largest gear, slows the wheel back down.

03-03-2025, 02:57 PM	#1
desert-rat Registered User Join Date: Jan 2013 Location: 3,018 feet above the SoCal/Mojave Desert region. Posts: 406	Does crank pulley size matter? My crank pulley measures 7.75" diameter which drives the fan and alternator. The power steering pump is on a smaller diameter groove of the same pulley. I need to replace the pulley and I have another 7.75 " diameter but power steering pump is gonna ride on 7.75" groove. Will that be too big for the power steering pump?

03-03-2025, 03:27 PM	#2
70STOVEBOLT Senior Member Join Date: Jun 2005 Location: Winona Lake, IN Posts: 7,066	Re: Does crank pulley size matter? It will cause the pump to spin slower, and may affect the ease of steering. What is the diameter of the smaller groove on your current pulley? __________________ 70 C/10 SWB 402/TH400/3.73 "The Needy Beast" 200,000 Mile Club Disc Brake Club Owner installed options: Front Sway Bar Power Steering Power Brakes Cigar Lighter Courtesy Lights Deluxe Side Markers Wiper Delay Sliding Rear Window Power Windows Power Locks Sniper EFI 2015 Silverado 1500 LS 4.3/6L80/3.23 lowered 2" front & rear

03-03-2025, 03:29 PM	#3
Steeveedee Ram-A-Lam-A-Ding-Dong Join Date: Jan 2013 Location: Simi Valley, CA Posts: 11,902	Re: Does crank pulley size matter? The pump will run at a higher rpm. It doesn't blow up at 120 MPH in a car, so I'm thinking that it would work OK if slightly overdriven. What is the diameter of the pulley that drives the pump now? Maybe someone else will chime in with a good reason not to? __________________ ~Steven '70 Chevy 3/4T Longhorn CST 402/400/3.56 Custom Camper Simi Valley, CA

03-03-2025, 06:48 PM	#5
HO455 Registered User Join Date: Feb 2016 Location: Portland Oregon Posts: 12,411	Re: Does crank pulley size matter? Assuming an rpm of 1000 and a 6" diameter on the pump. With an assumed diameter of the smaller driving pulley of 6.75 the current pump rpm is 1125. With the new driving pulley diameter of 7.75 the pump rpm will be 1291. http://www.stevegs.com/utils/beltcalc.htm __________________ Thanks to Bob and Jeanie and everyone else at Superior Performance for all their great help. RIP Bob Parks. 1967 Burban (the WMB),1988 S10 Blazer (the Stink10 II),1969 GTO (the Goat), 1970 Javelin, 1952 F2 Ford OHC six 4X4, 29 Model A, 72 Firebird (the DBP Bird). 85 Alfa Romeo If it breaks I didn't want it in the first place The WMB repair thread http://67-72chevytrucks.com/vboard/s...d.php?t=698377 Last edited by HO455; 03-03-2025 at 06:52 PM. Reason: Forgot link.

03-03-2025, 07:09 PM	#7
RichardJ Registered User Join Date: Apr 2010 Location: So Cal Posts: 1,557	Re: Does crank pulley size matter? ho455 and stevie and "Myself" are correct. The larger the drive pulley, the faster the belt moves. Circumference is Pi*D. 3.14 x 6" = 18.84" or how far the belt move per engine revolution. 3.14 x 7.75" = 24.335" or how far the belt moves for the larger pulley per engine revolution. __________________ '67 GMC 2500, 292, 4spd, AC

03-03-2025, 08:57 PM	#8
PbFut Senior Member Join Date: Feb 2022 Location: Ca Posts: 1,026	Re: Does crank pulley size matter? A small power steering cooler might be in order depending on your use.

03-04-2025, 05:29 PM	#9
70STOVEBOLT Senior Member Join Date: Jun 2005 Location: Winona Lake, IN Posts: 7,066	Re: Does crank pulley size matter? I stand corrected. Would I be correct if I said a larger pulley on the p/s pump would make it turn slower? __________________ 70 C/10 SWB 402/TH400/3.73 "The Needy Beast" 200,000 Mile Club Disc Brake Club Owner installed options: Front Sway Bar Power Steering Power Brakes Cigar Lighter Courtesy Lights Deluxe Side Markers Wiper Delay Sliding Rear Window Power Windows Power Locks Sniper EFI 2015 Silverado 1500 LS 4.3/6L80/3.23 lowered 2" front & rear

03-04-2025, 05:52 PM	#11
54blackhornet Registered User Join Date: Jul 2008 Location: Eastern Oregon Posts: 2,481	Re: Does crank pulley size matter? I don’t remember Sister Cabrini ever mentioning this in geometry class ? __________________ 71 C-20 07 Magnum RT AWD



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