Hello all, from time to time I have someone ask me how to figure a blower drive ratio or how to choose pulleys. I just wrote a long explanation to a gentleman in an e-mail, and I thought perhaps I'd post it here as well in case it helps anyone here. If the subject has been beaten to death, please ignore. If the tech archive can use this info, feel free to move this there. here goes: This discussion is a bit long, but will be worth it because you will know what you are doing by the end of it, in terms of either determining what drive ratio a particular pair of pulleys will create, OR what pulley tooth counts you will need to obtain in order to create a specific drive ratio. Note: the original question had to do with how to get 10% underdriven. But the following explanation is thorough enough to allow you to figure virtually anything out regarding tooth counts and drive ratios. Now, you can utilize lots of different tooth counts to get approximately-- say, 10% under, or whatever ratio you are shooting for. 10% underdriven is a ratio, so I'll give you a quick lesson so you can figure out your setup for yourself. Actually, the lesson itself should be quick, but I am going to be very thorough in explaining it all because I don't know your level of understanding any of this, or your math skills, and I want you to know how to do this quickly. It really helps when having to figure this stuff out over and over again! If you get lost, print it out and look at it all again. Before we begin the lesson on ratios, the 1st thing to be aware of when setting up a custom drive setup... you'll have to be prepared to be flexible. That is, just because you bought 2 pulleys doesn't mean that you'll find the right size belt to fit it all. But, you'll have a baseline that you can use to get further toward the end goal. It's good to get a baseline going, whether or not you wind up having to select different size pulleys later. Don't fret that too much for now. It's almost normal to have to switch pulleys or belts or whatever at some point. So let's talk about your top and bottom drive pulleys, which you probably should have before you bother looking for a belt... 1st thing here is that your tooth counts will be drastically different depending on what tooth style you use (such as the old vintage 1/2" pitch, or the 8mm, or the 13.9 or 14.0 mm). For instance, an 8mm pulley of the same exact diameter as a 14mm pulley will have a much higher tooth count than that 14mm pulley will. This is because the teeth are smaller, and so there will be many more of them on a pulley of the same diameter. Don't get too hung up on a 35-tooth pulley vs. a 69-tooth pulley. What REALLY matters is the ratio. After that, what matters would be little things such as whether or not your bottom pulley clears your timing pointer, or if your belt fits, etc. etc.). 2nd thing is that you should make sure that your two drive pulleys and your belt all have the same tooth style... this should be obvious, but I'll say it anyway. That means, the two drive pulleys and the belt must all be either 1/2" pitch, OR 8mm, OR 13.9mm, OR 14.0MM. And if the setup is one of the metrics, then you also need to watch out for whether or not you are dealing with GT-style or not. And of course, all must be of compatible widths as well as lined up properly together. (starting to see why there are so many different drive parts out there, in different sizes and styles, plus offsets and spacers?) One other thing I should mention is that some guys say that you can mix and match 13.9mm and 14.0mm, and others say "don't". I'm going to say "don't", as there is no room for tossing a belt with everything perfectly compatible-- whereas if you mix and match, you MIGHT. As for GT style or not, I do not know if those can be mixed and matched. But again I am going to say "don't". Now on to the ratio lesson. If the top and bottom pulleys have identical tooth pitch and count, then your ratio is exactly 1 to 1. If you divide 1 into 1, you get 1... or 100%. That means 0% underdriven, and 0% overdriven. How to figure out your drive ratio even before you get your pulleys, should you be trying for something other than 1 to 1? Simple math. There is a little formula for it. The actual formula dictates that you must remember whether or not you should be dividing the top into the bottom, or the bottom into the top. Then you have to subtract a value of 1. And then you have to be able to interpret a negative number vs. a positive number. The person that doesn't understand what they are trying to arrive at will quickly get confused trying to use the exact formula. I am going to simplify the concept in order to make it much quicker and easier for you if I can.... so here we go: T = top pulley tooth count B =bottom pulley tooth count Concept #1: we already mentioned it above, and that is that if T and B are the same, then you are at 1 to 1 and are neither underdriven nor overdriven. At 1 to 1, you will be dividing 1 by 1 or 1 into 1, and the result will show as 1... which is 100%. But what if you are wanting to be underdriven or overdriven? Well, anything over or under 100% will be either underdriven or overdriven. BUT remember that I said that the actual formula requires you to remember whether you should be dividing T/B or B/T? That makes all the difference in the world. I know I can never remember which to do... and yet I know how to very quickly determine a drive ratio on sight. And this is what I'm going to show you. Concept #2: If your top pulley is smaller than your bottom pulley, then the top pulley must spin around more to keep pace with the bottom pulley. This is overdriven. Highly overdriven supercharged engines have a much smaller top pulley (fewer teeth) than the bottom pulley. Conversely, underdriven supercharged setups will use a top pulley that is larger in diameter (higher tooth count) than the bottom pulley. Just remember this concept and you are well on your way to quickly being able to figure out what your drive ratio is, WITHOUT having to remember the actual formula. Now, a quick example of how I do it, and you're on your way to quickly doing the same thing. I don't bother trying to remember if it's T into B or B into T. I simply take the smaller tooth count and divide it into the larger tooth count. This is the easiest way to get a number that is more than 1, allowing me to skip a potential subtraction process. For instance, if I'm using a 40 tooth pulley and a 45 tooth pulley, 40 divided INTO 45 (that's 45 divided BY 40) gives me 1.125. That is a ratio! This means the smaller pulley must go around 1.125 times as much as the larger pulley needs will. How to interpret this number quickly is next. Well, your crank (bottom pulley) is going to go around one revolution and the supercharger (top pulley) will go around a certain number of times in relation to that. The blower will either spin faster or slower than the crank (or exactly the same if at 1 to 1 ratio). So... if we divided 40 into 45, and we get 1.125, then it's the .125 that we are looking for. The smaller pulley will go around .125 times as fast as the larger pulley, or 12-1/2%. So is the engine overdriven or is it underdriven? Remember concept #2. If your supercharger pulley is smaller than the crank pulley, then you will be at 12-1/2" OVERdriven, but if your supercharger pulley is larger than your crank pulley, then you would be at 12-1/2% UNDERdriven. That's pretty much it for quickly figuring out a drive ratio from 2 existing pulleys... But now how do you choose potential pulley counts (pulleys that you don't yet know the numbers of) when you have an exact drive ratio that you're targeting? Especially if you desire to make precise tuning changes to the engine? Then do what I did-- make an X-Y style table. That's columns and rows. From left to right across the top in a row, write your tooth counts in ascending order (low to high). This will be your top pulley (label that side T). Next, from top to bottom along the left side in a column, write your tooth counts in ascending order (low to high). This will be your bottom pulley (label that side B). Everywhere the same number meets the same number, you will be at 1, or 100%. You can just put "0" in these boxes, as that will mean zero under and zero over. Everything else must be divided. This is where I deviate from the above discussion. I don't ever seem to remember whether or not I should be dividing the top into the bottom or vise versa, but it does matter in this chart. So, test it out. Try dividing a small top pulley into a large bottom pulley and see what you get. It should be a number more than 1 (for instance, if 1 = 100%, then 1.1545 = 15.45% overdriven). The same tooth counts elsewhere on the chart, but with the values switched around for T and B will also receive that same number, but you'll need to somehow label it as underdriven). Honestly, a colored highlighter or colored pencil will make quick work of this part... those 1-1 boxes (which have zero's) will be one color, while everything above that diagonal line will be a second color, and everything below it will be a third color. Mark your 2nd and 3rd colors off to the side as either over or under, and you'll be on your way to making a nice quick-reading chart that you might even like to laminate in plastic. For what it's worth, the actual formula now that I am thinking of it, is B / T - 1. That is, bottom divided by top and then subtract 1 from the result. And to interpret that, all positive numbers mean overdriven while all negative numbers mean underdriven.... Also for what it's worth, because 8mm has much smaller teeth, there are many more opportunities for extremely small drive ratio changes. Probably why a lot of serious racers went to 8mm. But, not everyone cares to do this, and some may even have a specific reason why they might want something other than 8mm (1/2" pitch is vintage; 13.9/14.0 teeth might match the fins on the valve covers, etc.). First decide what style teeth you want, and then make a chart up for that one style. No sense making charts for something you'll never use. Hope this helps you guys that haven't gone through all this stuff before.
That is a great example of the type of home-made chart I was trying to describe. But a couple caveats on that particular chart there in that picture: 1) It only gives whole percentages, and doesn't give percentages to to one or two decimal places (which would possibly be of interest to serious engine tuners/racers) 2) It doesn't give any info for pulleys smaller than 32 teeth or larger than 43 teeth. For anyone who wants to use 8mm pulleys, chances are you'll have tooth counts greater than 43. And for racers, chances are you'll want a top pulley smaller than 32. But that chart is exactly what I was trying to describe, thanks for posting it.
I'm not in the shop right now, but I'm pretty sure BDS has the charts for several different belt sizes and matching types of pulleys.
Yes, they do. here they are: http://www.blowerdriveservice.com/pitchpulley.pdf (for 1/2" pitch) http://www.blowerdriveservice.com/8mm.pdf http://www.blowerdriveservice.com/14mm.pdf Their charts go to the tenth, which is enough for most engine tuners. The OCD-influenced racing engine tuner might still want to go to the hundreth, who knows. Their charts also have most pulley tooth counts on them, but I've still seen pulleys both larger and smaller than what's on their charts. It's still good to know how to quickly do this stuff in the field. Divide small tooth count into big tooth count. 1 to 1 is 1. Anything over 1 is either an overdrive or underdrive ratio. Refer to whether your top pulley is either smaller or larger than your bottom pulley. Bam, you're done. And all you need is a pocket calculator to read to the hundreth or more. But a chart quickly helps you find potential pulley combo's that give you a few decimals on the ratio one way or the other, at a glance. If you want to read to the hundreth, you'll probably have to make your own chart. Maybe nobody bothers to go to the hundreth, who knows. But there's the info.
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