fastlane40

Need some help here.I currently have a 502, 8.75:1 compression with a Weiand 177 blower running 5 psi.I now have a Dyers 871 i plan to fit.If i drive the 871 at 5 psi will there be a problem with detonation due to the increased volume of air?What horsepower gain should i see.I have 550hp now.I also have a superchiller to go with the 871.

mcollinstn

a 177 stuffing 5 psi into a 500 inch motor is adding more heat than the 8-71 will add. And the 8-71 at 5 psi will be adding more heat than the 8-71 with chiller will add (assuming all pressure measurements are taken from the intake manifold..).

In each case above, the measured VOLUME of air passing into the motor will be the same. But the hotter charge will have less molecules, or molecular volume. That's where the volume thing comes into play.

If you are currently running an actual 5 psi of boost, then add the chiller and 8-71 and re pulley for 5 psi in the intake while underway (so the chiller will be doing its part). Be sure to re jet for your application.

Your hp gain will be in the neighborhood of 20 to 30 hp, with much lessened risk of detonation (you were concerned that you would INCREASE your risk, but the opposite will occur). Your existing setup may have been running less than optimal ignition timing. You should be able to take a look at optimizing your timing with the 8-71 and chiller.

fastlane40

Am i right in assuming that the differences in volume between the 2 blowers can be compared to a garden hose at 5 psi and a fire hose at 5 psi.But is the limiting factor the volume of the engine.Whereas the 2 hoses have no restriction,forgetting the heating of the intake charge is 5 psi the same with a small or big blower?

boatn70

i really dont know much in this area fastlane, but id be thinking along your line too, or, why would a bigger motor require a larger blower??? not just for the air temp difference i would assume the volume to fill the cylinder also.
im here to learn with you though, hope you get some good replies here, im listening!!

BBQ70

What Mcollinstn said is correct, at equal boost the 8-71 will have a cooler more dense air charge that will make more HP. What you have to remember is that the 177 blower has to spin over twice as fast to move the same amount air that the 8-71 will move. By spinning the smaller blower that fast you do generate more heat. The best way to explain heat and air density in a simple form is to take a box that is 1 cubic feet of volume. If you put a pressure gauge in the box which reads 0 psi and the box is sealed so no air can leak out, you know you have 1 cubic foot of air in the box. If you then heat the box and the air inside, the pressure will increase but you are not adding any more "volume". This is sort of what is happening with the smaller blower. So 5psi of boost at a cooler charge is actually more air than 5psi of hot air. If boost stays the same you will be less prone to detonation with a cooler air charge even though there is "more volume" of air.

Griff

Also, realize that the 871 will take more hp to spin than the 177. You could easily bump the boost up a pound or two and still be safe.

Plain & simple, hotter air takes up more room than colder air. The actual volume of fuel/air with be larger at the same boost, but it will also take more hp to spin the 871.

Turbojack

That is if you are spinning it at the same speed. If it is running at a slower speed the HP cost may be equal or less to the smaller one spinning faster.

fastlane40

So i could see little gain in HP and speed after adding the extra weight and power required to drive it.Basically unless you want more than 5 psi you are wasting your time with a large blower.The obvious thing to do is lower the compression add a few more psi and then the 871 is the go.By the way if any of you guys are interested in a new 671 and up Dyers blower,check out Ebay user ID pop88.Well priced and a great bloke.

mcollinstn

A blower consists of two rotors, two gears, 6 ball bearings, 5 seals, and a case. Some street blowers also have teflon tip seals.

A larger blower has more inertia than a smaller blower.

But at a steady state speed, you have a given frictional loss that is based on the sizes of the bearings, seal area, and gears, along with the viscosity of the lube you are running. A 8-71 will have a higher frictional loss at a given rpm than a 177. At HALF the rpm, the 8-71 will be pumping more true volume than the 177, though, so you must compare a 177 spinning twice as fast as the 8-71 to compare frictional drive losses. The winner of this comparison will be very close to a toss up. The main reason will be the larger gears on the 8-71 and the 90 weight lube in it. So - we call this one a TIE.

Second, pumping losses. This part is simple. It takes effort to pump air. For an air pump to move a certain volume of air at a certain pressure, the work performed results in a combination of two things: air pumped AND heat. The unit that pumps the same amount of air with the least amount of heat is requiring less energy to do the work. Winner: 8-71.

So from the standpoint of which blower requires more HP to deliver the same endresult temperature-compensated intake charge: the 8-71 is the winner.

Only if you want to compute inertial lag times in a freely loaded vroom vroom contest to see which motor can hit 6000 rpm the quickest, will the 8-71 come out the loser. It is the clear winner in all other comparisons.

Now, if you already HAVE a 177, then at what point do you start determining if it is WORTH the effort and money to throw away the 177 and go with the 8-71. That's a different story altogether. Now you're talking dollars and cents and it is a choice that only you can make./

But the conclusion that an 8-71 is only better with big boost is incorrect. The 8-71 is better on that motor under the conditions described.