i know this is going to sound like a dumb question, mainly because i don't know how to ask it properly but ...



what do you gain from installing lightweight rotors



i know there is a general power(acceleration) gain, but what kind and how much of a gain can be expected?



also, is there a downside to putting in lighter weight rotors?



this is what i am looking at and trying to understand

http://www.racingbeat.com/resultset....rtNumber=10006

http://www.racingbeat.com/resultset....rtNumber=10007

didn't look at your links but........





higher compression-



Same as in pistons. It's easiest to look at it as boost. A higher compression ratio packs more 02. This give the power increase.



The dissadvantages is you cannot run as high as boost levels due to detonation. You can run higher boost by cooling the air (reducing the temps lower'n detenation via IC). You can't do that inside your car. So for high boost application you want low compression and to let your turbo/ic do all the work packing the 02.



if you plan on cranking boost - low compression



if you plan normal-modest levels - high compression

I think hes talking about lighter rotors, not rotors with higher compression.

well you can't install lighter rotors with out getting higher compression (unless specifically made)



s4 would have to use a s5 which is higher compress and s5 is the lightest.......



but anyway......



lighter rotors would reduce rotational mass. This would mimic the flywheel/light weight flywheel situation.



revs/launching debate



all what you want out of your car

The lighter rotors would allow you to *potentially* build a higher revving motor. But the light rotors are only part of the formula.

i would guess 10 hp the diffrence between the s4 and s5 engines were 14hp, there have been other changes as well as compression...

thanks

I think you are on the right track to having figured it out.



The rotating mass of the engine, transmission drive shaft, rear end and wheel/brake rotor assemblies, and even tire weight, accounts for a very large part of the work that must be done by the engine when you accelerate the vehicle.



Of the above list of things that need to be spun up to accelerate, the engine is recurrent, with each gear change. So of all of the rotating items that can be lightened, engine components provide a fertile ground for improvement.



There is a less obvious benefit in the lighter rotors and that is the expanded rev range. The lighter rotors can be turned at higher revs without loosing the rotor bearings. With a good breathing rotary, the HP curve is still going to the moon when you have to call up the rev limiter and end the party.



Like walking in the fast lane of the freeway, you can do it now and then but eventually you get killed.

So you might get lucky at 9,000 plus RPM now and then, with a 13B but eventually................



So maybe with lightened rotors you could go 9,500 RPM without penalty.



HP is a statement of work. Or 33,000 pounds lifted against the force of gravity one foot in one minute, is one horse power. So simply the formula has a time component in it. The one minute. A distance component, the foot. and a force component, the weight, 33,000 pounds.



For this example suppose that the stock engine must not exceed 9,000 rpm to stay alive but you new engine is safe to 9,500 RPM.



So HP=torque X RPM /5252 So lets say that your monster port Holley fed 13B puts out 300 big ones at 9,000 RPM. But with the new light rotors you can go to 9,500 RPM. So it follows that 9,000 X 175 (foot pounds of torque) / 5252 = 300HP does it not? Well 299.8857HP, but close enough.



So then 9,500 RPM X 175/5252=316.54HP.



But what if the sponsor says go for it, and you get a set of rotors for each weekend this year. You throw them away or make lamps out of them, because they are going to be developing cracks.



Then we can turn it up to 10,000 between shifts.



So, 10,000 RPM X 175/5252=333.20 HP. And nothing changed but the weight of the rotors (and the counter weights).



There is a flaw or two in this explanation, for example the torque would decay a bit as drag will increase at the cube of velocity. But this works for now.



So you get the idea.



The two answers are that (1) the rate of acceleration will improve dramatically, and (2) the increased revs available offers improved power.



But wait, there is more. If you order right now or in the next ten minutes I will throw in this extra benefit to light rotors and extra revs.



Yes just tonight,



you cover the rpm drops between shifts much better.



Say that on the one/two shift there is an rpm drop of 1,500 RPM. In the old days before light rotors you shifted at 9,000 RPM. Or as above you leave 300 HP and drop to 7,500 RPM or (175 X 7,500/ 5252=249HP).



Now with lite rotors. The 1,500 RPM drop will be from 10,000 RPM so the result of the shift will be 8,500 RPM. So (175 X 8,500/5252= 283HP). Or 34HP more at the start of 2nd gear.



You can feel 34HP, and it feels gooood.



The picture is a Tilton three disc 5 1/2" clutch sitting on a Chevy low ground clearance flywheel. It must be modified slightly to fit a rotary. Those are Saenz tranny gears in the background. A dog ring trans that can be shifted with out the clutch.



Lynn E. Hanover

Thanks for posting that, Lynn!



As always your posts are very informative and well written.



Regards,

Frode

Lynn E. Hanover, just wondering how you got a starter motor to suit that tiny flywheel?

What trasmission were you using?

Well now, if you were running a T-10 style trans, that flywheel can be used with several of the gear drive (offset) Tilton starters along the right side of the trans.

We cannot use the outer part of the flywheel. Mazda comp makes a spider style ring gear thing that bolts to tabs left after you machine off most of the spokes on the Tilton flywheel. I will cut off all of the flywheel part. I will bore out the center hole and cut down the locating spigot on the rear counter weight. I will bolt the modified flywheel on with an old style flexplate to get a starter ring. So my rear counter weights have both Mazda and Chevy bolt patterns. I use a top mounted starter (stock Mazda) because there is a problem with suspension and header space all around.



In my case we run a either a Richmond Gear trans synchronized 5 speed, or a Saenz dog ring 5 speed.



The Richmond is a big fat thing built up on a Ford size and spacing system. It is the old Doug Nash street 5 speed. The front face has a bolt pattern for many types of bell housings. This one has a large number of choices for each gear ratio. 5th is straight through (one to one). That uses a Chevy S-10 bell housing and a Long (brand name) shifter. The trans is more like what you used to find behind a big V-8 in a TransAm car. It is actually too heavy for what we need. But they are less than $2,000 bucks new. A dog ring version is available for around $6,000, but still too heavy.



We got a deal on the Saenz, but new they are about $6,000. It uses a RX3P bell housing and a slightly shorter drive shaft. It has its own built in shifter. The Saenz is out of action right now because of a learning curve problem. The faster you shift it, the longer it lasts. One slow shift and Houston you have a problem.



Lynn E. Hanover

I have an old RX-2 top mounted starter. Is there a newer, smaller, lighter one available now? The RX-2 unit is a large weight on top of the engine.