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.

 

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.

 

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.

 

Money spent $$$$
$100 for Tll drive shaft
$20 for trans mount
$20 slave cyl
$400 trans & starter
$260 act flywheel
$150 exedy clutch
$120 for shifter parts

$1070 total.

 

the Rx8 transmissions are pretty loud

 

John I think part of the issue is your solid mount. I put poly mounts on my car and now I get a vibration around the same RPM. I need to put the rubber mounts back in and see if the vibes go away or not.

 

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.

 

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.

 

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.

 

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.

 

With this gearing top of third will be 100mph perfect or the 1/4 mile and with turbo 4th will be 130@8500

 

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.

 

the Rx8 is geared for acceleration. the Rx8 gears are basically the same thing as the competition transmission from the early 80's, except the Rx8 has a granny gear first.

the Rx7 is geared for economy.

 

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. Its just too much load on the motor to be efficient and that fact is probably hidden from you turbo guys because you won't feel it as much in the peddle but look at your vacuum when in 5th.

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.

 

the rx8 is 3k at 60, 4k at 80 5k at 100, and so on. while i would love 3k at 80mph, it would be about at good as you can get and only new 2-3mpg over 3k at 65.

i go no speedo, but i usually stay under 4k rpm.

 

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.

 

The usdm cars had the better 5th gear, jdm cars had a real short 5th gear sadly. My vert with a 4.1 would do 64mph at 3k(gps backed up speedo reading).

 

Than you really have no idea how physics works.

Best would be a totally different 6th gear.

Apples and oranges when comparing fuel economy from the 7 to the 8.... here's something to ponder.
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

Thankfully someone understands this.

There lies the key right there.....

I'm pulling about 40kpa cruising around..... @80mph and 4k on the highway.... and getting about 19-20mpg.... with lots of abuse.

Now if I were to get a different 6th gear and drop those "cruising" rpm down I could probably pick up a few mpg....

It's that physics thing I mentioned before.... that whole concept that can't be avoided in this world.

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.

 

you live up to your name very well sir.

 

That was me being calm

 

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.

 

Wrong - VE - learn about it

Means a higher MAP value which will require more fuel per RPM to maintain a proper AFR.

Higher engine speeds with throttle restriction means lower MAP values which means less fuel per RPM to maintain a proper AFR.... add in the VE variable and you see where I'm going with that....

See above...

Because of VE yes, but not because of the reasons you're eluding to.

There's no free lunch..... higher revs, less MAP vs lower revs and higher MAP, pumping losses, frictional losses etc etc etc.... you're still dealing with constants. Constant VE because the same engine, constant drag, constant mass, constant car... it's going to take a certain amount of energy - fuel - to move that constant the same distance.

 

the biggest difference that everyone is missing the fc has small intake ports/runners which have higher velocity at low speed, the rx8 has much larger ports and runners and lugs as low as 2200 rpm. something i've never felt in the fc.

 

I have a ported 13B-RE.... only bigger ports would be a bridge or a P-Port yet I still pull 15" @ idle and over that @ cruising speeds which are 4k @ 80mph and I still get close to 20mpg.....

but all those valves don't do anything at 2k with an engine that revs AND holds torque to nearly 9k

 

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.