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lightweight engine parts effect performance

Old 04-03-14, 11:07 PM
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lightweight engine parts effect performance

I'm gathering quite a few engine parts throughout the years and I'm thinking about building another engine.

I was wondering about how much the weight of the internals effect the engine. I have a s4 t2 reciprocating assembly right now, but I really dream about shaving that weight in the rotors from an s5 and an rx8 eccentric shaft.

I know it would be hard to compare the weights because compression ratios change also, but I'm still sure there are a few of you guys out there that have dealt with a few engine builds where you could tell the diference.

I have spare s4 t2 rotors and counterweights, so I'm trying to think if it would be worth buying s5 rotors and counterweights for the weight savings. rx8 eccentric shaft is a good price, but I have to search again for what else I would need in order to run that in my engine

I know the s4 na 9.4:1 rotors are not high in demand, but I was also thinking about a 9.4:1 turbo setup. I really like that compression ratio number for the 91 octane fuel in CA, but knowing that the s5 parts weigh a lot less is killing me.

BTW I saw on Mazdatrix website that the s5 parts are 1/2lb lighter, and this other website I just saw said the s5 parts are 1lb lighter!!!!! Which is correct?
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Old 04-03-14, 11:37 PM
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i can check tomorrow but i would be surprised if they are even .5lb in difference.
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Old 04-07-14, 10:36 PM
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Originally Posted by RotaryEvolution View Post
i can check tomorrow but i would be surprised if they are even .5lb in difference.
Updates?
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Old 04-08-14, 01:06 PM
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Rotor weight makes no effective difference in power. The major difference is high RPM stability: lighter parts can rev higher with less stress on the e-shaft.

The Rx8 e-shaft is a good deal since it is not only lighter but also stiffer, and also much cheaper than a rx7 one. The problem with this is that you will have to have the rotating assembly balanced.

Unless you are planning to rev over 8k regularly, just stick with the s4 parts and put your money into something else.
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Old 04-08-14, 07:29 PM
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both stripped E weight rotors (heavy for comparison sake, A-C are lighter)

s5 t2= 4330g (9lb 8.8oz)
s4 n/a= 4580g (10lb 1.6oz)

so i guess the both are correct, the disparity is 8.8oz or just over .5 lbs each rotor or 1lb for both.


an equally stripped renesis 10:1 rotor is 4210g (9lb 4.4oz)


eshafts:

1986-2002 6790g (14lb 15.6oz)
2003+ renesis 6580g (14lb 8.2oz)

so theoretically you could shave off over 1.5lbs from an S4 to an S5 assembly with a MSP renesis eccentric shaft, and relatively cheaply. the question is, does your goals support the compression ratio?

these 3 parts make up 90% of the rotating mass inside the engine, upwards of 35lbs for the rotors and shaft not including weights, oil pump, gear , etc. question is: how much does 1.5lbs make a difference to 40lbs when spinning? the clutch flywheel is actually a much simpler place to shave off some serious weight comparatively!

in the end my proposal is to worry less about the rotors, pick your compression ratio and use your flywheel to determine your inertia coefficient for the engine itself. rotor weight is a factor when you've exhausted all other areas to lower that mass, a light flywheel can remove over 10lbs from your rotating mass alone and it's cheaper than anything else. keep in mind the results will also be more dramatic since the inertia is 1 to 1(relative to crankshaft speed).

Last edited by RotaryEvolution; 04-08-14 at 07:53 PM.
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Old 04-08-14, 10:32 PM
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Thanks for the info! Your right. Right now I have s4 t2 parts, and I bought some used s4 na rotors and I'm keeping my fingers crossed that they are good candidates for a rebuild if I need one in the future.
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