Calculating supercharger belt size after pulley change.

[jimxms]

Today I decided to change my supercharger pulley as I had a new one sitting on the shelf for a couple of years. The only issue is that I'm unsure what belt size i should be shopping for.

My old belt was 1115mm diameter on a 3.5" pulley, and my new pulley is 2.8".

My man math is telling me that as the new pulley has a 0.7"/18mm smaller diameter, i should half this number (as the belt only contacts one side of the pulley) and subtract that 9mm from the diameter of the belt, meaning I'm looking for a belt of around 1106mm? Does that sound right?

Friends keep telling me to use a piece of string for measurement, but the current belt would pass this test. However it doesn't come close to holding enough tension not to slip like crazy. I need to go smaller, but I don't know by how much - and choices of 8PK belts this short are limited in the UK meaning I need to make a fairly accurate guesstimate for a USA purchase.

Thoughts?

[Saleen836]

Silly question but...
You have had the car mapped/have a new tune for the smaller pulley,yes?

[jimxms]

Saleen836 said:

Silly question but...
You have had the car mapped/have a new tune for the smaller pulley,yes?

I will have the laptop on my lap as i drive down a *ahem* private road to tune it

[The Rotrex Kid]

I had the same Issues when I changed pulleys. The simple and best answer is to get a few belts and try them. You will find what works with the changes and keeps the best tension required.

Not much help with your maths but real world answer.

[tendown]

What diameter is the other (unchanged) pulley?

Is there a tensioner or is it a simple belt between two pulleys?

Edited by tendown on Tuesday 11th August 22:14

[jimxms]

tendown said:

What diameter is the other (unchanged) pulley?

Crank pulley? I dont know in all honesty. Is that needed for calculation? I assumed not as there's too many other things at play here to work out the belt based on the size of the pulleys and the distance between them. There's an idler that provides extra belt wrap, and a tensioner in there too that both alter then length.

EDIT: just seen your edit. So yeah, not a simple(ish) calculation

[talksthetorque]

With the same pulley centres you would need to deduct half the circumference difference wouldn't you?
Have you tried online belt calcuators like this one?
https://www.bbman.com/belt-length-calculator/

[stevieturbo]

jimxms said:

Today I decided to change my supercharger pulley as I had a new one sitting on the shelf for a couple of years. The only issue is that I'm unsure what belt size i should be shopping for.

My old belt was 1115mm diameter on a 3.5" pulley, and my new pulley is 2.8".

My man math is telling me that as the new pulley has a 0.7"/18mm smaller diameter, i should half this number (as the belt only contacts one side of the pulley) and subtract that 9mm from the diameter of the belt, meaning I'm looking for a belt of around 1106mm? Does that sound right?

Friends keep telling me to use a piece of string for measurement, but the current belt would pass this test. However it doesn't come close to holding enough tension not to slip like crazy. I need to go smaller, but I don't know by how much - and choices of 8PK belts this short are limited in the UK meaning I need to make a fairly accurate guesstimate for a USA purchase.

Thoughts?

plenty of belts available.

And if you dont like string, cut a belt and measure it.

[jimxms]

stevieturbo said:

plenty of belts available.

And if you dont like string, cut a belt and measure it.

If I cut the current belt I'm using, it would measure up fine. But unlike measuring a v-belt for accessories, the supercharger i'm using needs the belt SUPER tight - to the point of snapping.

Easy answer is I need to go smaller. But if you look around for 8pk belts that are in the range of 1100-1110 there arent many this side of the pond.

[Dave.]

What car?

Shirley you're not the first ever person in the world to do this, so what are other people doing.

Assuming there's a tensioner, you'll be ok with a range of belts, it's not millimetre precision.

Also, having the belt "to the point of snapping" won't do the'charger much good, and will possibly rob you of power (I think, happy to be corrected).

[slybunda]

at the coventry motofest i saw a drag racer car that had a corvette v8 7.1 ltr engine i think and the charger belt was toothed like a timing belt. are you able to do such a conversion to a toothed belt?

[tendown]

I was incorrectly thinking there might not be tensioners/idlers. You're right the diameter of the other pulley is irrelevant

If you can accurately say how much of the pulley the belt is wrapped around (you say 50%) then I reckon it should be (approximately):

If it is 50% then the reduction in length required will be
(3.5" - 2.8") *pi * 50%. = 27.93mm
Then take this off the belt if it is quoted as a diameter then
((1115 * pi) - length ) / pi =1106mm

So yes I reckon you're right, but do check the 50% thing, is the belt parallel as it comes off?

Is the belt really quoted as a diameter rather than a length, seems odd to me.

[shtu]

I reckon you might find a belt if you go via bearing suppliers, eg, https://simplybearings.co.uk/shop/Belts-Multi-Ribb...

As for length, isn't it the reduction in circumference you need, rather than diameter?

I'd have a stab at it being "(the difference in circumference between each)/2"

diameters
2.8" = 71.72mm
3.5" = 88.90mm

radii
35.86
44.45

circumference
225.32
279.29

difference
53.97

divide by 2
26.985 mm less length.


Trouble is, there's also a difference brought about by the slightly different belt path, so likely a bit tricky to be sure 100% from that.

[jimxms]

tendown said:

I was incorrectly thinking there might not be tensioners/idlers. You're right the diameter of the other pulley is irrelevant

If you can accurately say how much of the pulley the belt is wrapped around (you say 50%) then I reckon it should be (approximately):

If it is 50% then the reduction in length required will be
(3.5" - 2.8") *pi * 50%. = 27.93mm
Then take this off the belt if it is quoted as a diameter then
((1115 * pi) - length ) / pi =1106mm

So yes I reckon you're right, but do check the 50% thing, is the belt parallel as it comes off?

Is the belt really quoted as a diameter rather than a length, seems odd to me.

I will have to check that 50% in the morning, I was just really going from memory of the route it takes.

You're right about the belt specs tho - the 1115mm is length not diameter! Does this mean that I would only need to take away half of the difference? So it would be closer to 1111?

Edited by jimxms on Tuesday 11th August 22:49

[jimxms]

slybunda said:

at the coventry motofest i saw a drag racer car that had a corvette v8 7.1 ltr engine i think and the charger belt was toothed like a timing belt. are you able to do such a conversion to a toothed belt?

I think I may have to go this route soon as v-belts slip too much. I'm running specially treated pulleys (they are sorta gritty), but they wear through belts fast!

[GreenV8S]

jimxms said:

I think I may have to go this route soon as v-belts slip too much.

Have you tried belt dressing spray? Can't be right that it's needing so much tension. Can you confirm that the tensioner is on the unloaded side of the belt run?

[tendown]

jimxms said:

tendown said:

I was incorrectly thinking there might not be tensioners/idlers. You're right the diameter of the other pulley is irrelevant

If you can accurately say how much of the pulley the belt is wrapped around (you say 50%) then I reckon it should be (approximately):

If it is 50% then the reduction in length required will be
(3.5" - 2.8") *pi * 50%. = 27.93mm
Then take this off the belt if it is quoted as a diameter then
((1115 * pi) - length ) / pi =1106mm

So yes I reckon you're right, but do check the 50% thing, is the belt parallel as it comes off?

Is the belt really quoted as a diameter rather than a length, seems odd to me.

I will have to check that 50% in the morning, I was just really going from memory of the route it takes.

You're right about the belt specs tho - the 1115mm is length not diameter! Does this mean that I would only need to take away half of the difference? So it would be closer to 1111?

Edited by jimxms on Tuesday 11th August 22:49

That would be
1115- 27.93=1087mm
Pi's not needed as it's circumference already not diameter

[jimxms]

GreenV8S said:

Have you tried belt dressing spray? Can't be right that it's needing so much tension. Can you confirm that the tensioner is on the unloaded side of the belt run?

I used to use that on a previous car, but found that it was just messy and didnt last more than a few hours. Good for a single run down the drag strip, but not on a daily driver.

[rustednut]

Probably a silly question.

Are you sure using a smaller pulley is ok for the system ?

Regarding belt sizing, it will be cheaper and easier to find a 3/4/5/6 groove belt in the sizes easier. Use that to find the optimum size belt and then order the wider 1.

[jimxms]

I’ve loosened off the tensioner completely and now I can get a better feel for how much belt I need to loose.



The belt on the previous pulley barely went on. I I need to loose almost all of that slack