zoltan944

every other car ive had dynoed a 5-10hp max gain from the switch. not sure if the 928 is any differant? But i should also get a couple pounds out of there too

mark kibort

probably one of the easiest HP values to calculate. based on diameter and effiency of the fan blade , and a thrust value, we can get a pretty close approximation for HP required. there is NO WAY that a driven fan takes 5hp. have you ever seen a 5hp engine on an ultra light?? it will blow you over. the fan pressure and flows of a driven fan, at redline, are rev limited for one, due to the clutch, and have a moderate air flow. my driven fan dropped overheating at idle on a hot day to mid scale on the temp. the two S4 electric fans do the same electric fans also can speed up and match the incoming air flow at high speeds, for which the driven fan really cant do as well. when i start to get up in the temps, i go from one S4 fan to dual with the flip of a switch to a redundant relay system. the temp promply drops to mid scale in a race, proving its doing its job better than the driven fan.

But back to the basics. if you want to figure out the power of your fan, you can plug numbers in a simple formula. thrust= 13 (power(HP) x diameter (feet) x eff %) to the 2/3 power (^2/3)

This is something i deal with every day professionally. see what happens when you plug in 5hp (or even 10hp) to 1.5feet diameter , 50% eff, etc.

anyway, if you got 5-10hp changes from removing the fan, it wasnt due to the removing of the fan.

anyone know the speed of the fan or the ratio to the drive pulley?

Mk

mark kibort

"hand speeds"?? what kind of "considerable " gains have you seen? you really cant see the gains, as they are not there. the only real thing you remove is some parisitic losses of the belt spinning the fan and the fan itself spinning at top speed. its fairly light, less than 1lb for the blades and doesnt represent a big inertial load at all. the entire assembly of the fan was 5lbs with the bracket, bearings, bolts etc. if it was 10lbs indiamter, and direct drive, inertial forces, like on a flywheel would be less than 1hp in 4th gear, but much more in (near 20hp) but it is only 1lb. so forget about the losses there for quick reving the blades. 1lbs on a 18" diameter is not that much to "notice" let alone measure. Next, the aerodynamic forces generated by the fan at the rpms it runs at are not that great either. I dont know the top speeds, but obviously they are less than direct drive due to the clutch. it does flow some air, and probably incrementally less than the dual electric fans, but its not 5hp worth. maybe it could be in the 1hp and if you really stretch the truth, 2hp. . that means that an electric fan doing the same work would be near 4hp at 100% efficency. 4HP would be 3000watts. trust me for 3000watts, i could blow a heck of a lot harder than the belt driven fan.

its all guess work on your part here, because the physics are pretty clear and most have replace driven fans with electric with similar cooling characteristics. I for one have done this with a 10amp standard AC fan in front and a 18" spal fan in back, and totally, they were less than 20amps for both.(ie only 280watts input, and with 50% efficiency, thats only 140watts, or .18hp)
So, two fans mechanically drive .18hp of air flow rate of work (thats POINT 18 hp) If took my fan set up and did 10 x the air flow!!! that would only be 1.8hp.

if you see a difference or feel a difference, its from something else, not the driven fan removal.

yes, the aero forces go up with the square of speed, and the power required goes up with the cube of speed. (^3) so, spin a driven fan with a 1/4hp drill. bet it gets near 2000rpm to run 4000rpm, that would be 8x the power. since the power is really effectively 1/8hp output (50% efficiency of the drill) the power required to drive the fan at 4000rpm (double the speed) would be 8x (cubed) , or 1hp. this is what you are dealing with here, 1hp, not 5-10hp. Thats obsurd.


mk

zoltan944

so the weight of the mass your moving have nothing to do with anything?

mark kibort

a quick result of the formula. fan thrust (lbs) = 13( HP x diameter x eff)^2/3

If the driven fan takes 5hp at max hp that would mean it produces 30lbs of thrust!!!
if the driven fan takes 1hp at max hp that means it would produce about 10lbs of thrust. (more likely, but still high)

ever see those big fans that cool the engines at a dyno. those are 1hp fans at 2-3 feet diameter and they dont fly away, and push a HECK of a lot more air flow than our driven fans at 6000rpm engine speed (???something lower , fan speed due to the fan clutch) one obvious point, is those bearings are not rated at 30lbs axial loading by design also, the thin themo plastic blades would warp beyond belief if you spun those blades to create 30lbs of thrust. a quick test is to look at the fan statically, and then get to the outer 3rd of the blade and watch the movement by pulling on it with 1lb of force. watch how much it moves. the fans are 9blades or something, the distortion would be near 10lbs of force on those blades to make 10lbs of thrust. (80% of the force is generated at the outer 3rd of the blades) then, rev your engine to 5k, see if the blades are distorting that much. they wont be.

you can optically test the rpms of the blades too. i forgot the design speed of the fan, but its no where near as fast as engine speed.

Mk

mark kibort

the mass of the fan blades are 1lbs. the inertia of the fan blades is very small. think of the effect of an 18" diameter disc added to your clutch assembly. first of all, the HP effects will be rate of speed change dependant, so more effects if you rev the engine in neutral, and less effects in each gear. most dyno runs are done in 3-4th gear, so the rate of change of the rpms is something like 4-8 seconds for 2000rpm to 6000rpm change. this is SLOW, and the effects of even 10lbs on a flywheel diameter is insignificant. But, we are talking about the mass of the plastic fan blades which weigh less than 1lb and diameter of the center clutch area is only a few engines, and thats where most of the weight is. anyway, we are talking some very very low values in HP to accelerate this kind of mass to any driven speeds.

the main factor here is the thrust (mass flow) of the fan. its in the 10lbs range at the biggest stretch of your imagination, and thats going to be less than 1hp. In otherwords, insignificant. However, i pulled mine too, not for the 1hp, because of its 7-15lb weight getting rid of the brackets and smog pump as well as the fan.

does that answer your question regarding the "weight of the mass you are moving"? the biggest factor is the HP requried to generate the air flow at the high rpms. the fan is clutch limited and the fan itself is not designed to move massive amounts of mass flow. if it was, the fan blades themselves would be made more rigid, and they are not. they are pretty flimsy.

Mk

Imo000

ok.. so in theory, removing the fan, belt and the air pump will have no significant gain?

mark kibort

so the answer of the main question could be found with the following formula.
Velocity = 550 x eff x power/ thrust (assuming 50% eff, and 1hp power)

so if we use my 10lbs of thrust, with 1hp, we get about 35mph or near 50f/second. at 18" diameter, thats 254 sq" or 1.75 sq' and at 50 feet/second velocity thats near 90cf/s. or 5404 cfm.

so, when you guys run two 2500cfm fans back to back, you are getting close to the performance of the driven fan, assuming its 10lbs of thrust and about 1hp of mechanical energy required to drive it.
Its probably less, but for our discussions, this is pretty accurate.

MK

James-man

I agree that, cumulatively, you should have something in terms of measurable gains. Fan alone, doesn't sound like that much, but the package deal should add up to at least a little something.

I think that either some over-isolation of variables has occurred or gains have erroneously been attributed to only one of several pieces.

mark kibort

exactly. nothing you could write home about. we could tune the dyno with the things we DO know about the fan, belt, and air pump. have you ever spun the air pump? its got a very low flow rate and friction load. the belts and bearings have some drag, with effect on hp going up with rpm proportionally. measure the torque of the fan and pump together, and the twisting force that you could generate with your hand stays relatively constant and HP required will go up with speed proportionally. whats the hp of a few oz/in x 6000rpm?? negligible.

the main reason to pull all this stuff is for weight if you are talking race car.

you cant measure the gains that would be made by the removal of all this stuff, due to the fact that the forces generated by the driven fan are so small, even at high engine rpms. inertia of the pump, the fan blades and the fan pulley are so small that they are insignicant as well. simple physics here, aside from the forces on the fan blade, for which ive shown the slightly more complicated equations to figure out ball park forces and power required. net net of it all is in the 1-2hp range at best.

Here is a reality check for you. ever cut your lawn with a power lawn mower? i used to have a 1.5hp briggs and stratten engine.. imagine attaching it to the fan blades of a 928. (and the smog pump via a belt) it would produce so much more air flow and thrust than our system, it wouldnt even be comparible!

have you seen those motors driving the fans shown on this thread? they are only 15amps . the motors are little 200watt motors .13hp net . they move a ton of air. Imagine what 1hp net would do!!!! certainly not 5 or even 10hp!!

MK

mark kibort

Actually, the fan is the biggest factor. the smog pump and pulley friction is fairly fixed and minimal. again, picture a lawnmower attached to the fan set up driving the fan, smog pump via the belt. it would blow more air than you could imagine!

mk

69gaugeman

Big Dave,

I want to do this to mine as well. Maybe we could have a fan replacement party at my place.I have a hoist to make things easier.

Imo000

Well, maybe the total gain that the aftermarket claims is, overall area gained under the HP curve. Perhaps possible thats where the 5-10hp gain is calculated from?

On mine the bearings were shot and that was the main reason for the switch. 6 months later I build a supercharger system so at that point it would have been necesarry to switch for an electric fan anyways. The airpump had to be removed to make room for the supercharger head unit.

With the original fan, the engine had a " large displacement" sound. If was the fan making all the moise. After the switch I can hear the individual injectrors slightly ticking away. With the electric fan its much easier to listen to the engine tp make sure everything sounds right.

James-man

OK, don't know about the rest of you folks, but I am convinced that you should have some other driving factor to do this swap. You should have some reason other than potential HP gains. It is too much work and too much money to get a 1 or maybe 2-sh HP gain.

You may have saved me several hours of work that may have been otherwise been someday wasted.

Imo000

Here's a driving factor....... the electric fan is great at the drag strip. All I have to do is jump the thermo switch after each run to quicly cool down the engine!

I strongly suggest that if your original fan is in good working order, keep it. If the clutch needs filling of the bearings are worn, perhaps a switch to an electric fan would be a good idea.