5MT vs 6MT spreadsheet

[NSFW]

Google has this spreadsheet thing I heard about a while ago, and I've been wondering about gearing, so...

 

http://spreadsheets.google.com/pub?key=pnPjNqBluy31HYtJ2uAdlRw

 

Anybody want to sanity-check the "speed @ 3k RPM" columns? I'm not sure it makes sense to have the specB at 70mph @ 3k RPM in 5th.

 

Note that the spreadsheet assumes the specB has the same overall tire size as the regular GT, which may not be the case.

[Vimy101]

^ The Spec.B definitely has larger wheel/tire circumference. @ 3000 rpm I'll be doing ~75 in the 06. Revs per mile for 215/45-18 are 811.

[f1anatic]

definitely 70 mph @ 3000k in 5th.

(2005 manual GT)

[John M]

I've attached a speedo pic from a 5spd 3000GT/Stealth twin turbo to show what we're missing out on. It's at only 2400 rpm at 70 which doesn't sound like much difference until you do the math; it'll be doing 90 at 3000 rpm which is much more useful for interstate driving. The 6spd car is even better than that by a few hundred rpm.

 

If only Subaru would buld a 6spd meant for US highways.

[edmundu]

The GT gear ratios are all wrong, including the final drive.

[Vimy101]

I've attached a speedo pic from a 5spd 3000GT/Stealth twin turbo to show what we're missing out on. It's at only 2400 rpm at 70 which doesn't sound like much difference until you do the math; it'll be doing 90 at 3000 rpm which is much more useful for interstate driving. The 6spd car is even better than that by a few hundred rpm.

 

If only Subaru would buld a 6spd meant for US highways.

 

The 3000GT is a V6. It doesn't have to turn as many rpms as a four banger. Apples and oranges comparison.

[Vimy101]

The GT gear ratios are all wrong, including the final drive.

 

No complaints from me.

 

You have to remember that the 2.5 turbo boxer has a lowered compression ratio so in order to maintain adequate power at highway speeds the rpms have to be up.

 

With the 06 OEM Spec.B 18"s (and summer gas) I have gotten 27.6 mpg over 1500 miles of mixed mainly highway (lots of secondary rtes as well) driving.

[DukeTrout]

Tested it tonight. 71 looks right for 3k in 5th for the 07 spec.B. I was still well short of 3k rpm in 6th at 80 mph.

[John M]

The 3000GT is a V6. It doesn't have to turn as many rpms as a four banger. Apples and oranges comparison.

 

It is most certainly a fair comparison. The 2.5 liter makes more than enough torque to comfortably cruise at 70 mph below 2500 rpm.

 

It's only half a liter difference in displacement; number of cylinders is irrelavant.

 

No street car needs 6 acceleration gears. We've got a broad enough torque curve to appreciate a deep overdrive in 6th. Imagine what it could do for gas mileage. It's one of the reason my 4000lb Stealth could hit 29 mpg on the highway at 80-85 mph.

[NSFW]

The GT gear ratios are all wrong, including the final drive.

 

D'oh! Thanks, I pulled them from the wrong (non-GT) column of the page I was looking at.

 

Will fix and post an updated sheet tomorrow. Maybe tonight if I have time.

[Vimy101]

It is most certainly a fair comparison. The 2.5 liter makes more than enough torque to comfortably cruise at 70 mph below 2500 rpm.

 

It's only half a liter difference in displacement; number of cylinders is irrelavant.

 

No street car needs 6 acceleration gears. We've got a broad enough torque curve to appreciate a deep overdrive in 6th. Imagine what it could do for gas mileage. It's one of the reason my 4000lb Stealth could hit 29 mpg on the highway at 80-85 mph.

 

The number of cylinders is entirely germane. There are only 4 power strokes per revolution vs 6 for a 6 cylinder therefore the 4 banger has to turn 50% more rpm to achieve the same power output (all things being equal). The 2.5l turbo boxer also has a lower compression ratio in order to cope with the FI.

 

I agree that having a taller final ratio on the 5MT for cruising would be useful although being able to punch it at 3000 rpm (75 mph in the 06 Spec.) and getting good power without having to downshift is very, very convenient. But then you get into extra load from aerodynamic drag that increases approximately with the square of your speed which of course requires increasing engine power correspondingly.

 

And who can for get the old adage, "There is no replacement for displacement." A 3.0l is 20% larger than a 2.5l.

 

I believe SoA was after modern 3.5l 6 cylinder na performance from a 2.5l 4 banger. IMHO, they were very successful. However, we also have AWD and its inherrent mechanical drag. Add 10% to your present mpg to ballpark comparable fuel economy to a comparable FWD vehicle which has a very short drive train and less weight to boot.

 

Supposed to snow here very soon.

 

GO BEARS

[Prod]

My N/A 2001 Outback (4EAT) would do 80 mph @ 3000 rpm. My 5EAT LGT will do the same. My LGT makes 85 hp more than the Outback (stock for stock). I think our gears could be taller without a problem.

[Vimy101]

^ I think so too but then it wouldn't be an ass kicking MT LGT. I can't even imagine owning a 5EAT. If LGTs only came in EAT AWD, I'd be driving a G35x right now.

[PhilT]

The number of cylinders is entirely germane. There are only 4 power strokes per revolution vs 6 for a 6 cylinder therefore the 4 banger has to turn 50% more rpm to achieve the same power output (all things being equal). The 2.5l turbo boxer also has a lower compression ratio in order to cope with the FI.

 

 

You might want to look up the difference between 2 stroke and 4 stroke engines, and then do the maths again

 

 

Otherwsie I agree with your 4 vs 6 statement.

[Vimy101]

^ Ya, I thought about that after I wrote it but the ratio remains the same so what the hell.

[SLegacy99]

Im at about 3100 in 5th gear doin 70 mph. Guess the gearing hasnt changed much.

[John M]

Do you people honestly think that if our cars were geared to run 2500 rpm at 70, they wouldn't be able to hold speed on the highway? That's ridiculous.

 

The only aspect with bearing on how the car would drive is the engine's power curve. If it makes enough torque at that rpm (it does) and power at that rpm (it does), the car will drive normally. The NA 4 cylinders don't make this low end power and thus need the higher rpms.

 

Top gear, whether it's 5th or 6th, is not for acceleration. If you're cruising and need to speed up more than a few mph, downshift. Going WOT in the top gear (unless already redlining the previous gear) puts more stress on the bearings than anything else you can do. You may be able to pass people in 5th now but it's not good for the engine.

 

That's why it's not important that the Stealth happens to make 315 torque at 2500 rpm; you're never using that much to maintain speed. Instead of going WOT to make max power you're going go downshift anyway.

 

I guess I could dig up a stock dyno to prove my point further. The LGT makes more than enough power to cruise at 2500 rpm. Regardless of the gas mileage savings, I'd like it just for the decreased engine noise.

[NSFW]

The number of cylinders is entirely germane. There are only 4 power strokes per revolution vs 6 for a 6 cylinder therefore the 4 banger has to turn 50% more rpm to achieve the same power output (all things being equal).

 

So, all other things being equal, a 2.5L 6cyl would make 50% more power than a 2.5L 4cyl? Even though both burn the same amount of fuel per revolution? (Remember - same displacement because we're keeping all other things equal.)

 

This makes no sense to me.

[LittleBlueGT]

So, all other things being equal, a 2.5L 6cyl would make 50% more power than a 2.5L 4cyl? Even though both burn the same amount of fuel per revolution? (Remember - same displacement because we're keeping all other things equal.)

 

This makes no sense to me.

 

I'm with you buddy, cause you are right.

 

6cyl 4cyl, 2 cyl, it doesn't matter at all, except the amount of power it makes it that rpm.

 

Screw loose?

[NSFW]

http://spreadsheets.google.com/pub?key=pnPjNqBluy31HYtJ2uAdlRw

 

Updated with GT gear ratios.

 

John, if you can install the 3.272 differentials from the GT AT, you'll get 90mph in 5th gear at 3k, so 75mph would be a little under 2500rpm. You'll lose some acceleration in 1st gear, but you could gain about 20% in fuel economy.

[Vimy101]

I'm with you buddy, cause you are right.

 

6cyl 4cyl, 2 cyl, it doesn't matter at all, except the amount of power it makes it that rpm.

 

Screw loose?

 

The discussion had to do with RPM. A 4 banger has to spin more to accomplish the same work because it has fewer cylinders. A 3.0l six in the original complaint has more displacement than a 2.5l boxer. To get the 4 banger to match a 6, FI is used to cram in more fuel but the 4 cylinders still have to cycle faster. A very simple concept really.

 

There are reasons why additional cylinders are mechanically advantageous when increasing displacement.

[Vimy101]

So, all other things being equal, a 2.5L 6cyl would make 50% more power than a 2.5L 4cyl? Even though both burn the same amount of fuel per revolution? (Remember - same displacement because we're keeping all other things equal.)

 

This makes no sense to me.

 

By equal, I meant equivalent cylinder size which should have been obvious to you by the 50% ratio in the post.

[LittleBlueGT]

The discussion had to do with RPM. A 4 banger has to spin more to accomplish the same work because it has fewer cylinders. A 3.0l six in the original complaint has more displacement than a 2.5l boxer. To get the 4 banger to match a 6, FI is used to cram in more fuel but the 4 cylinders still have to cycle faster. A very simple concept really.

 

There are reasons why additional cylinders are mechanically advantageous when increasing displacement.

 

Discplacement is the key not # of cylinders.

 

 

I have actually heard it said that more cylinders with same discplacement actually makes slightly less power, all things being equal.

[PhilT]

Discplacement is the key not # of cylinders.

 

 

I have actually heard it said that more cylinders with same discplacement actually makes slightly less power, all things being equal.

 

It's not as simple as that, otherwise everyone would be making 6 litre single cylinder engines.

 

The greater volume the cylinder, means you have to get a larger volume of air/ fuel mixtures into it in a short period of time. This greatly affects acceleration, even though you may have a large amount of torque at low revs, it doesn't translate into power as you can't get the rpm to increase fast enough.

[LittleBlueGT]

It's not as simple as that, otherwise everyone would be making 6 litre single cylinder engines.

 

The greater volume the cylinder, means you have to get a larger volume of air/ fuel mixtures into it in a short period of time. This greatly affects acceleration, even though you may have a large amount of torque at low revs, it doesn't translate into power as you can't get the rpm to increase fast enough.

 

I know it is not as simple as that, but the poster above seems to think that our car need to spin fast at highway speeds because it is a 4-banger.

 

It doesn't.

 

My car could pull a 40 foot truck at 2800 rpms. It would do just fine at 2200 rpms as well, though it wouldn't mush you in the back of the seat like it does now when you ease in to the throttle.