Roots Blower Sizing
10-23-2014, 10:43 PM
#34
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Location: Northern Indiana
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Nice informative thread Joe. I'd like to throw a little input in. Some guys including me don't have an open check book and have to shop for deals. If your stuck with a smaller blower that's not really the proper size for the application there is a way to make it work. Instead of running very low compression and over spinning the blower which will heat the charge and kill power. Raise the initial compression of the motor slow down the blower and keep the charge cool. In essence the outcome is the same for the most part. What is really being accomplished with the blower is adding cylinder pressure which makes more power. With higher compression in the motor you don't need as much added air from the blower as you would with lower initial compression. Hope this makes sense. Will a high compression low boost make the same power as low compression high boost? Not on pump gas. But you are better off putting more compression in the motor if you have a small blower and not spinning the heck out of it... Hope that may help some that have an empty wallet like me.
10-24-2014, 08:32 AM
#37
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More oxygen molecules means more than intake charge compression / cylinder pressure.
Obviously, there is a certain semi-fine line to that.
However, cylinder pressure does mean alot.
If your engine can not be made to take in more oxygen molecules, the next step is to raise physical compression.
====================
Edit in: raising compression by means of physcal parts / dimension changes does not change how much air is comming into the motor.
Adding more air into the same combustion space adds cylinder pressure.
Same air, but reducing combustion chamber size (larger dome pistons, smaller cyl head chamber, etc,etc) also adds cylinder pressure.
2 Different things.
Last edited by SB; 10-24-2014 at 08:37 AM.
10-24-2014, 08:44 AM
#38
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Ya know....I want to, but.
I am not sure how much longer I will keep the boat. Right now it is very attractive to dealers and some buyers because the mods are internal, so it still appears to be stock. Stock improved I guess
But there is no hiding an 871 blower. In it's current form Peir 57 would buy it outright at a good price if I wanted to get out and move to the mountains and live off the land hahah.
So...I go back and fourth. Bigger blowers, surface drives and paint. Or trade in for #6 fountain or OL. I just know once I get too deep I will prob own it for life.
I am not sure how much longer I will keep the boat. Right now it is very attractive to dealers and some buyers because the mods are internal, so it still appears to be stock. Stock improved I guess
But there is no hiding an 871 blower. In it's current form Peir 57 would buy it outright at a good price if I wanted to get out and move to the mountains and live off the land hahah. So...I go back and fourth. Bigger blowers, surface drives and paint. Or trade in for #6 fountain or OL. I just know once I get too deep I will prob own it for life.
10-24-2014, 09:44 AM
#40
I think what Black Baja was trying to say, was that when using too small of a blower for the application, the best bet is to raise the static/dynamic compression a bit. Since the small blower is only capable of so much, getting the compression up a tad, helps the engine make more power on "motor" so to speak.
Lets say you have a stock 525sc. What you have there is a 454, that made a whopping 1.15HP per ci. Thats hardly impressive, as today we can make that number easily without a blower and be marine reliable. Turning the boost up, to make more power on a stock 525sc, was of little gain. Becuase the blower spinning so fast simply superheated the air. You could never add 150+HP to a 525sc with pulley swaps and a chiller, without seriously redesigning the entire engine package, if you wanted to keep the 177.
Now, lets say you take a 525sc engine, but want to make more power out of it, but keep the same small blower. If you were to raise the static compression from 7:6:1, to around 8.5:1, camshaft upgrade, maybe a cylinder head upgrade, now you can pick up 50,75,100, or more hp, but at the same, or even less boost than stock. Simply because its making more power on "Motor".
Now lets talk about a 600sc merc. That had a 420 B&M blower, 7% underdriven. It also was low 7.6:1 or whatever compression. I know of guys who simply took that stock merc engine, added a superchiller, and cranked the boost up to 9-10psi, and picked up tremendous power without touching the engine internals. Simply because the larger blower was capable of making 9-10psi without extreme overdriving and took advantage of that low static compression.
Lets say you have a stock 525sc. What you have there is a 454, that made a whopping 1.15HP per ci. Thats hardly impressive, as today we can make that number easily without a blower and be marine reliable. Turning the boost up, to make more power on a stock 525sc, was of little gain. Becuase the blower spinning so fast simply superheated the air. You could never add 150+HP to a 525sc with pulley swaps and a chiller, without seriously redesigning the entire engine package, if you wanted to keep the 177.
Now, lets say you take a 525sc engine, but want to make more power out of it, but keep the same small blower. If you were to raise the static compression from 7:6:1, to around 8.5:1, camshaft upgrade, maybe a cylinder head upgrade, now you can pick up 50,75,100, or more hp, but at the same, or even less boost than stock. Simply because its making more power on "Motor".
Now lets talk about a 600sc merc. That had a 420 B&M blower, 7% underdriven. It also was low 7.6:1 or whatever compression. I know of guys who simply took that stock merc engine, added a superchiller, and cranked the boost up to 9-10psi, and picked up tremendous power without touching the engine internals. Simply because the larger blower was capable of making 9-10psi without extreme overdriving and took advantage of that low static compression.
