Rx8 trans in rx7 fc3s
06-17-14, 12:06 AM
#76
The gearing is great but the solid trans mount makes the car sound like ****, im getting a strange harmonic in the car at 55mph. The car is definitely faster and much more fun to drive. I'm riding on 225/45/17 and anything smaller than that might be too small. When I go turbo I will probably install the 3.9 rear. The exeddy clutch and act flywheel feel great and the 1997 civic slave works perfect.
The trans I bought feels worn compared to the one in my rx8 but that's what I get for shopping the lowest price on craigs list.
My only complaint is the noise from my trans mount. I don't feel any vibration in the shifter but the noise is greatly reduced when I push the clutch in. It's a ugly sound and my first thought was unbalanced drive shaft which I still haven't fully ruled out.
The trans I bought feels worn compared to the one in my rx8 but that's what I get for shopping the lowest price on craigs list.
My only complaint is the noise from my trans mount. I don't feel any vibration in the shifter but the noise is greatly reduced when I push the clutch in. It's a ugly sound and my first thought was unbalanced drive shaft which I still haven't fully ruled out.
06-17-14, 03:47 AM
#77
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It could be the input bearings, I've killed a few of those now, including the one in my 8.
What did you do for trans mounts? I don't think I saw a pic of them?
Don't bother with the 3.9 unless you're looking for some sort of fuel economy while cruising. I went with a 4.3 in my FC and would do it again if given the choice.
What did you do for trans mounts? I don't think I saw a pic of them?
Don't bother with the 3.9 unless you're looking for some sort of fuel economy while cruising. I went with a 4.3 in my FC and would do it again if given the choice.
06-17-14, 10:53 AM
#78
I cut the center out of the factory trans mount. Used 2" square tube to space it from the chassis. I drilled two holes in a steel plate bolted it to the car and welded it all together. I'm going to replace the 2" square tube with polyurethane block but it will take a week for the block to ship.
06-17-14, 11:02 AM
#80
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the Rx8 transmissions are pretty loud
06-18-14, 10:21 AM
#82
I cut some thick heater hose into small sheets and used it where I could on the trans mount. The vibration was greatly reduced. I think replacing the square tubing with polyurethane will make it feel perfect.
I had to change the flywheel and the drive shaft so I worry when there is a new vibration, I have to make sure I'm not damaging the motor.
The vibration doesn't change much from 3k-8k and is more noticeable in the passenger seat which makes sense to me because the engine turques that direction and the trans is bolted near the seat.
I still suspect this strange driveshaft is out of balance but the shifter doesn't vibrate at all.
Once the poly is in place I think everything will be nice and quiet like before. It's not that bad now but I don't want Any noise or vibration.
Driving this car aside form a little extra noise is 1000 times better than before. The na trans gearing is terrible and with 17" rims it was way too tall, this is much better.
I had to change the flywheel and the drive shaft so I worry when there is a new vibration, I have to make sure I'm not damaging the motor.
The vibration doesn't change much from 3k-8k and is more noticeable in the passenger seat which makes sense to me because the engine turques that direction and the trans is bolted near the seat.
I still suspect this strange driveshaft is out of balance but the shifter doesn't vibrate at all.
Once the poly is in place I think everything will be nice and quiet like before. It's not that bad now but I don't want Any noise or vibration.
Driving this car aside form a little extra noise is 1000 times better than before. The na trans gearing is terrible and with 17" rims it was way too tall, this is much better.
06-25-14, 09:59 PM
#83
Installed poly mounts, harmonic is still there but hardly noticeable. 6th gear = 60mph@3000rpm bald tires. My rx8 18" rims 65mph@3000rpm with 4.1 rear perfect 
My math says rx7 should be 64mph@3000rpm so I might install 3.9 rear.
With the 5speed na box 65mph@2500 and 77mph@3000rpm this was way too tall.
My math says rx7 should be 64mph@3000rpm so I might install 3.9 rear.
With the 5speed na box 65mph@2500 and 77mph@3000rpm this was way too tall.
06-26-14, 09:16 AM
#84
Only 65mph at 3000 rpm? I'd rather leave it stock. That's way too fast to be of any real world daily use. What will you get with that, 8 mpg? Top gear is supposed to be overdrive for cruising. I wanted to do this mod. Now I'm content with the stock box.
06-26-14, 11:00 AM
#85
Anything under 2800rpm at 65mph and you're probably overloading the engine and wasting fuel.
This more about improving 1-4th but fuel economy is still a concern.
I was at 2500rpm@65mph its the equivalent of constantly driving up hill which also wastes fuel.
2800@65 =3500rpm@81 in my car less than that and ur wasting fuel and power.
3000rpm@65mph feels great and my rx8 gets 21mpgs average tracked over 3 months with cell app so econo drivin I probably get 25. It's much better on fuel with the 4.1 rear.
My goal is 65mph@3000rpm that is where it will preform best and the gearing will work best for acceleration and mpg. Best of both worlds.
This more about improving 1-4th but fuel economy is still a concern.
I was at 2500rpm@65mph its the equivalent of constantly driving up hill which also wastes fuel.
2800@65 =3500rpm@81 in my car less than that and ur wasting fuel and power.
3000rpm@65mph feels great and my rx8 gets 21mpgs average tracked over 3 months with cell app so econo drivin I probably get 25. It's much better on fuel with the 4.1 rear.
My goal is 65mph@3000rpm that is where it will preform best and the gearing will work best for acceleration and mpg. Best of both worlds.
06-26-14, 12:04 PM
#87
My S5 na which runs horribly pig rich stock gets 23 mpg on the freeway now. My old S4 which was tuned averaged 23 in my normal city driving. On the freeway it could get around 27 and once on a long flat drive got in the 28's. I'd do better to just lean out the S5. You aren't overloading anything at that speed and rpm. If you were, you couldn't still accelerate in 5th gear cruising and we all know that is easily possible. The RX-8 can't hold a candle to an FC in the fuel economy game due to it's horrible rear end gearing. Of course it's extra weight isn't helping either. I understand your logic that the engine is (potentially) in a higher efficiency range but there's no direct proof of that. Unless of course you can hit 30 mpg. Then I'll believe it.
I do agree that you should be able to increase acceleration in the non overdriven gears but 6th gear should correspond to 5th in the RX-7 tranny or nearly so. The only point of high gear is cruising. If your sole focus is purely acceleration then I see the desire to use this.
I do agree that you should be able to increase acceleration in the non overdriven gears but 6th gear should correspond to 5th in the RX-7 tranny or nearly so. The only point of high gear is cruising. If your sole focus is purely acceleration then I see the desire to use this.
06-26-14, 12:20 PM
#88
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I do agree that you should be able to increase acceleration in the non overdriven gears but 6th gear should correspond to 5th in the RX-7 tranny or nearly so. The only point of high gear is cruising. If your sole focus is purely acceleration then I see the desire to use this.
the Rx7 is geared for economy.
06-26-14, 04:03 PM
#89
gearing is a catch 22, engines burn less fuel at lower rpm but to run lower gearing you need more power which means you need more fuel. I've been playing with gearing and my motors like 2800 to 3100@ 65mph anything lower than that and you stressing the motor wasting fuel and power.
06-26-14, 05:28 PM
#91
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For what it's worth, that's just like my Honda Prelude in fifth gear. 3k at 60 mph, 4k at 80mph, and so on. Average about 27 mpg. My '06 RSX-S in sixth is down about 200 RPM at 70 (~3300). Average MPG is 31.
I bet the RX7 is more fun than both the Hondas.
I bet the RX7 is more fun than both the Hondas.
07-08-14, 11:27 AM
#93
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Anything under 2800rpm at 65mph and you're probably overloading the engine and wasting fuel.
This more about improving 1-4th but fuel economy is still a concern.
I was at 2500rpm@65mph its the equivalent of constantly driving up hill which also wastes fuel.
2800@65 =3500rpm@81 in my car less than that and ur wasting fuel and power.
3000rpm@65mph feels great and my rx8 gets 21mpgs average tracked over 3 months with cell app so econo drivin I probably get 25. It's much better on fuel with the 4.1 rear.
My goal is 65mph@3000rpm that is where it will preform best and the gearing will work best for acceleration and mpg. Best of both worlds.
This more about improving 1-4th but fuel economy is still a concern.
I was at 2500rpm@65mph its the equivalent of constantly driving up hill which also wastes fuel.
2800@65 =3500rpm@81 in my car less than that and ur wasting fuel and power.
3000rpm@65mph feels great and my rx8 gets 21mpgs average tracked over 3 months with cell app so econo drivin I probably get 25. It's much better on fuel with the 4.1 rear.
My goal is 65mph@3000rpm that is where it will preform best and the gearing will work best for acceleration and mpg. Best of both worlds.
My S5 na which runs horribly pig rich stock gets 23 mpg on the freeway now. My old S4 which was tuned averaged 23 in my normal city driving. On the freeway it could get around 27 and once on a long flat drive got in the 28's. I'd do better to just lean out the S5. You aren't overloading anything at that speed and rpm. If you were, you couldn't still accelerate in 5th gear cruising and we all know that is easily possible. The RX-8 can't hold a candle to an FC in the fuel economy game due to it's horrible rear end gearing. Of course it's extra weight isn't helping either. I understand your logic that the engine is (potentially) in a higher efficiency range but there's no direct proof of that. Unless of course you can hit 30 mpg. Then I'll believe it.
I do agree that you should be able to increase acceleration in the non overdriven gears but 6th gear should correspond to 5th in the RX-7 tranny or nearly so. The only point of high gear is cruising. If your sole focus is purely acceleration then I see the desire to use this.
I do agree that you should be able to increase acceleration in the non overdriven gears but 6th gear should correspond to 5th in the RX-7 tranny or nearly so. The only point of high gear is cruising. If your sole focus is purely acceleration then I see the desire to use this.
Loaded FC with driver and full tank of fuel = 3,000lbs.
410rwhp @ 14psi
RX8 trans
4.3 rear end
255/40/17 rear tires
20mpg on the freeway while CONSTANTLY beating the **** out of the car.
Oh.... I cruise my car at a horribly pig rich .88L
That is true when looking at a factory rx8 with factory rear gearing but with taller rear gears you'll find the tranny has a tall first gear and close range 2nd thru 6th. My rx8 with 4.1 rear feels amazing while my rx7 is now geared exactly like a factory rx8. I'm going to replace the tires and then maybe install my old 3.9 rear. Gearing is a preference like driving style but you guys running 2500 rpm@65mph I'm sure you are wasting fuel and performance.
Now if I were to get a different 6th gear and drop those "cruising" rpm down I could probably pick up a few mpg....
gearing is a catch 22, engines burn less fuel at lower rpm but to run lower gearing you need more power which means you need more fuel. I've been playing with gearing and my motors like 2800 to 3100@ 65mph anything lower than that and you stressing the motor wasting fuel and power.
I know you're not comparing the fuel economy of a hondah 4-cylinder to a rotary, right? I guess I could put in my experience of last week driving a 2002 S2000 2500 miles cross country and cruising @ 4400rpm and 80mph and AVERAGING 87mph across texas and getting 24mpg.... because it's totally relevant.
07-08-14, 02:59 PM
#96
An engine does not become more inefficient at lower engine speeds for a steady cruise. It may be unacceptably unresponsive, but not more inefficient.
Part of understanding physics is to understand what an internal combustion engine is at its most basic - which is an air pump. One must also understand that the power required to maintain a given speed in a given car is constant - for most cars to maintain highway speeds requires only 10-20hp. The throttle plate (or variable valve lift in some newer piston engines) is used to control power output to maintain a given speed in a given gear, by adding a restriction that makes the engine pump air less efficiently.
Lower engine speeds but less throttle restriction make the same power using less fuel than higher engine speeds with higher throttle restriction to manage the power output, because especially when the power demands are low, the proportion of the engine’s output that goes to overcoming pumping losses becomes higher. To maintain a given speed, an engine will be more efficient with larger throttle opening, but lower engine speed used to generate the required amount of power. The limit of this is the point at which the engine, even at WOT, can't make enough power even at that engine speed to overcome the car's drag and mass.
In more practical terms, a trade-off gets made between having more reserve power available easily (higher revs) vs. increased pumping efficiency, and therefore fuel economy (lower revs/larger throttle opening). Higher revs with more throttle restriction mean the engine is already being held back – and removing the restriction of the throttle plate means it can quickly make more power – say to pass or to maintain speed up a steep grade without a downshift. It also means secondary losses to friction consume a larger portion of the engine’s output. Running the engine at larger throttle but lower engine speed means the engine is closer to its theoretical maximum for that engine speed, and so can only increase power through a proportionately smaller removal of throttle restriction to maximize the power output for that engine speed, or a downshift to increase engine speed to a range where it can make more power. But regardless, to the degree slower throttle response can be tolerated, the lower the engine speed, with correspondingly larger throttle opening, the more efficiently an internal combustion engine works to maintain a given speed.
Part of understanding physics is to understand what an internal combustion engine is at its most basic - which is an air pump. One must also understand that the power required to maintain a given speed in a given car is constant - for most cars to maintain highway speeds requires only 10-20hp. The throttle plate (or variable valve lift in some newer piston engines) is used to control power output to maintain a given speed in a given gear, by adding a restriction that makes the engine pump air less efficiently.
Lower engine speeds but less throttle restriction make the same power using less fuel than higher engine speeds with higher throttle restriction to manage the power output, because especially when the power demands are low, the proportion of the engine’s output that goes to overcoming pumping losses becomes higher. To maintain a given speed, an engine will be more efficient with larger throttle opening, but lower engine speed used to generate the required amount of power. The limit of this is the point at which the engine, even at WOT, can't make enough power even at that engine speed to overcome the car's drag and mass.
In more practical terms, a trade-off gets made between having more reserve power available easily (higher revs) vs. increased pumping efficiency, and therefore fuel economy (lower revs/larger throttle opening). Higher revs with more throttle restriction mean the engine is already being held back – and removing the restriction of the throttle plate means it can quickly make more power – say to pass or to maintain speed up a steep grade without a downshift. It also means secondary losses to friction consume a larger portion of the engine’s output. Running the engine at larger throttle but lower engine speed means the engine is closer to its theoretical maximum for that engine speed, and so can only increase power through a proportionately smaller removal of throttle restriction to maximize the power output for that engine speed, or a downshift to increase engine speed to a range where it can make more power. But regardless, to the degree slower throttle response can be tolerated, the lower the engine speed, with correspondingly larger throttle opening, the more efficiently an internal combustion engine works to maintain a given speed.
07-08-14, 03:42 PM
#97
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Means a higher MAP value which will require more fuel per RPM to maintain a proper AFR.
In more practical terms, a trade-off gets made between having more reserve power available easily (higher revs) vs. increased pumping efficiency, and therefore fuel economy (lower revs/larger throttle opening). Higher revs with more throttle restriction mean the engine is already being held back – and removing the restriction of the throttle plate means it can quickly make more power – say to pass or to maintain speed up a steep grade without a downshift. It also means secondary losses to friction consume a larger portion of the engine’s output. Running the engine at larger throttle but lower engine speed means the engine is closer to its theoretical maximum for that engine speed, and so can only increase power through a proportionately smaller removal of throttle restriction to maximize the power output for that engine speed, or a downshift to increase engine speed to a range where it can make more power. But regardless, to the degree slower throttle response can be tolerated, the lower the engine speed, with correspondingly larger throttle opening, the more efficiently an internal combustion engine works to maintain a given speed.
07-08-14, 04:00 PM
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but all those valves don't do anything at 2k with an engine that revs AND holds torque to nearly 9k
07-09-14, 11:22 AM
#100
With a small timing adjustment I've recovered the one mpg I've lost while doing this swap. I believe once I get the rear gearing right I will pick up at least 2mpg while still being considerably faster than before. This was the case with my rx8 and the inspiration for me to do this swap.
For now I'm broke do to my surgery and won't be doing anything with the 7 for a few weeks. I'm saving up for some coil overs a good diff for the 3.9 gearing and a wastegate and intercooler so I can install my turbo. I hope to have a nice Tig welder by February and next year I'm shopping for fuel injection and so far I like the MS3PRO. Then off to my next build.
For now I'm broke do to my surgery and won't be doing anything with the 7 for a few weeks. I'm saving up for some coil overs a good diff for the 3.9 gearing and a wastegate and intercooler so I can install my turbo. I hope to have a nice Tig welder by February and next year I'm shopping for fuel injection and so far I like the MS3PRO. Then off to my next build.
