Scion tC Forums banner

1 - 18 of 18 Posts

·
Registered
Joined
·
372 Posts
Discussion Starter #1
about 10 days ago, I was making a high speed run from picking my daughter up from college. I was averaging about 80. Something did not feel/sound right. After several minutes, I realized that it was the engine. At 80 mph, the engine was turning over at @4200 rpm.

I let the clutch out and eased it back in. Still 4200 rpm (or thereabouts). I did not think of dropping it to 4th, but I sure as heck knew it was not in 3rd. The sucker would be screaming! Besides, 5th is very noticeable due to its placement.

This may have happened one more time since then.

Yesterday and today, I was making the run back and forth to the airport. So, I ran it at 80 mph with and without the AC on. Both times, the engine was turning over at 3600 to 3700 rpm. Sounded right as well.

Given that it is a 5-spd, is there software (PDM, ECU or whatever you call it) that would determine rpm on a gear? Was the 5th gear not properly engaged? I would put it down as being touched in the head, but my ears said "high" and my eyes did see the gauge at 4200 rpm.

FWIW, the gauge cluster sucks in terms of visibility for me, but I am quite sure I was not pegging 90!!


Any thoughts?

Dan
 

·
Registered
Joined
·
936 Posts
Defintley a "had to be there" situation. If you put the effort in and find the final-drive ratios of the manual gearbox, ill do the math on RPM/per gear. Sounds like you might have been going 90 and not realized it!!!!!!!!

On Edit: I was playing around with some numbers for fun.... the way I figure it is that based on calculations if your car was in 5th gear which is a 1:1 ration and the transmission ratios on the manual tC are 2.7:1 (which they arent) then your RPM would be 4802.02xxxx

Once you find the real transmission ratios ill do the final calculations and post them up! - t
 

·
Registered
Joined
·
1,425 Posts
At around 85, or 4k rmps, the engine make more noise and vibration, this i believe is b/c of VVt-I. There is nothing wrong with the transmission or the engine, that is all normal. It does hit 4200 when going 90 in order to give us more power in 5th at high speeds. IDK if VVt-I gives us more power or better fuel enconomey, hopefully both...
 

·
Registered
Joined
·
372 Posts
Discussion Starter #4
Originally posted by TCrazy@Sep 1 2004, 06:40 PM
<<snip>>Sounds like you might have been going 90 and not realized it!!!!!!!! - t <<snip>>
You may be right. Like I said, I have a hard time seeing the dash (will definitely go aftermarket when one comes out), but 90 is pretty far over compared to 80. Ugh, I tend to drive based on the speed of the traffic and the position of the speedometer needle since I can't read it that well. May have to do 90 one of these soon to see where it pegs.....

Thanks!

Dan
 

·
Registered
Joined
·
4,694 Posts
with our tire size we have 845 revs per mile, so at 80mph the tires spin at 1125rpm (80mph / 60minutes * 845revs per mile).

So if your tach was reading 4200, then the final drive ratio would have been 3.73...

Our differential gear ratio is 4.235, 5th gear is 0.775, so real final drive ratio in 5th is 3.28. At 4200rpm it is 1280 wheel revs per minute, which is 90.8mph.

So buddy, you were going 90mph. Mathematically proven
You should know that nobody goes 80 on I-285, or God forbid I-75/I-85 - you'd get ran over...

For 80mph you should be in 3700rpm range.

For the reference:
Differential gear ratio: 4.235
Gear ratios
1st / 3.538
2nd / 2.045
3rd / 1.333
4th / 0.972
5th / 0.775
Reverse / 3.583
 

·
Registered
Joined
·
936 Posts
nice Zolitz.......I was searching around for those ratio's to complete my equation (see above post) but i couldnt! Props to you, Im impressed
. - t
 

·
Registered
Joined
·
4,694 Posts
After I looked at these numbers I want to get 215/50/17 rubber even more. It's going to make 2mph difference on the speedo (which will put it right at the correct reading), but wheels will look better and the revs will be almost 200rpm lower for 80mph. I would lose some acceleration, though, but nothing to be too scared of.
 

·
Registered
Joined
·
372 Posts
Discussion Starter #9
Originally posted by zoltiz@Sep 2 2004, 05:47 AM
<<snip>>So buddy, you were going 90mph. Mathematically proven
You should know that nobody goes 80 on I-285, or God forbid I-75/I-85 - you'd get ran over...<<snip>>
D'oh!

I guess I have been outed. Note to self, check out speedo real close next time!

And yes, you are correct about the speed limits on I-285 and 75/85. 400 can be even worse. The 100 Days of Heat seems to have had little effect IMO.

Thank you for taking the time to do the calculations. That is some good info!

Cheers!

Dan
 

·
Registered
Joined
·
4,694 Posts
GA_tc - join us for our litte Scion meets. Since 95% of people are from Lawrenceville we usually meet up out there, but last time had a (second) tC guy from Marietta. I was the first tC and I'm from Conyers - 35+ mile drive for me
 

·
Registered
Joined
·
793 Posts
Originally posted by TCrazy@Sep 2 2004, 11:10 AM
nice Zolitz.......I was searching around for those ratio's to complete my equation (see above post) but i couldnt! Props to you, Im impressed
. - t
ROFL...you're impressed and I"m like...

HUH?

Hehe.

 

·
Registered
Joined
·
936 Posts
Originally posted by LoopyLady_1+Sep 3 2004, 03:11 PM-->QUOTE (LoopyLady_1 @ Sep 3 2004, 03:11 PM)
<!--QuoteBegin-TCrazy
@Sep 2 2004, 11:10 AM
nice Zolitz.......I was searching around for those ratio's to complete my equation (see above post) but i couldnt! Props to you, Im impressed
. - t
ROFL...you're impressed and I"m like...

HUH?

Hehe.

[/b]
Hey loopy:) good to see you around! I was just impressed with how fast he got those done, and that he knew how to do it. As more of our members start showing their brains off, i get more and more excited to read these posts! - t
 

·
Registered
Joined
·
4,694 Posts
Well, 3rd grade math came in handy (all you need in this excercise is division and multiplication)
 

·
Registered
Joined
·
4,694 Posts
Another car geek post from me:

Code:
Rolla   tC 5M   tC 4A
======  =====   =====
12.477  14.983  10.804
 7.503   8.661   6.020
 5.163   5.645   3.872
 3.488   4.116   2.795
 2.857   3.282   -
These are final drive ratios for 5-speed corolla/matrix (and maybe Celica GT), tC 5-speed and tC Auto. Basically if you divide your engine RPM by this number you will get revolutions per minut of your tires, which in turn can be transferred to your speed.

To say that tC 5M is aggressively geared will be an understatement...

With this kind of gearing Mazda3s will not be a problem at all, even knowing that Mazda3 is lighter.

If it was up to me I would opt for a differential with lower gear ratio, though. I'm not much into racing and going at 4000rpm on the higway does not seem reasonable to me...
 

·
Registered
Joined
·
11 Posts
Originally posted by zoltiz@Sep 7 2004, 07:47 AM
Another car geek post from me:

Code:
Rolla   tC 5M   tC 4A
======  =====   =====
12.477  14.983  10.804
 7.503   8.661   6.020
 5.163   5.645   3.872
 3.488   4.116   2.795
 2.857   3.282   -
These are final drive ratios for 5-speed corolla/matrix (and maybe Celica GT), tC 5-speed and tC Auto. Basically if you divide your engine RPM by this number you will get revolutions per minut of your tires, which in turn can be transferred to your speed.

To say that tC 5M is aggressively geared will be an understatement...

With this kind of gearing Mazda3s will not be a problem at all, even knowing that Mazda3 is lighter.

If it was up to me I would opt for a differential with lower gear ratio, though. I'm not much into racing and going at 4000rpm on the higway does not seem reasonable to me...
hey zoltiz,

I was jus checking your numbers, and I got a little different calculation. You said that with our wheel size, we have 845 revs per mile. But I got 820 or so, the 3rd grade calculations below


Tire size is 215/45/17: 45% of 215 mm, which is 96.75 mm, or 3.81 inches. Diameter of the tire 17 in. plus (3.81*2) 7.62 in. = 24.62 inches. Then the circumference is 77.31 inches. ( pi * d). 77.31 inches is .0012201 of a mile. So in one mile, there are 819.6 or 820 revs per mile. 25 less extra revs per mile, means at 75 mph, instead of 3500 or so rpms, it's 3400. Not that big of a difference, 100 or so rpms.

Aaaannnnnywaaays, just wanted to run my numbers by you, maybe you know why there is slight difference, maybe i did something wrong. But I agree, high reving on a freeway does suck, but 4000 rpm....that's some fast highway driving (90 mph by my calculations) Just wondering, at about 75 or so in 5th, is the rpms at 3400? And if you do change tires to 215/50/17, or bigger, does that mean that gas mileage would improve due to lower rpms, but at the cost of acceleration?

Just curious about all this because I'm a prospective tc / mazda 3/ rsx buyer and I got to due my research.


tc @ 75 mph = 3400
m3 @ 75mph = 3200
rsx @75 mph = 3300

p.s also any update on the moonroof pop / hatch rattles? any tsb out yet?
 

·
Registered
Joined
·
4,694 Posts
Originally posted by edng16@Sep 7 2004, 11:28 AM
Tire size is 215/45/17: 45% of 215 mm, which is 96.75 mm, or 3.81 inches. Diameter of the tire 17 in. plus (3.81*2) 7.62 in. = 24.62 inches. Then the circumference is 77.31 inches. ( pi * d). 77.31 inches is .0012201 of a mile. So in one mile, there are 819.6 or 820 revs per mile. 25 less extra revs per mile, means at 75 mph, instead of 3500 or so rpms, it's 3400. Not that big of a difference, 100 or so rpms.

Aaaannnnnywaaays, just wanted to run my numbers by you, maybe you know why there is slight difference, maybe i did something wrong. But I agree, high reving on a freeway does suck, but 4000 rpm....that's some fast highway driving (90 mph by my calculations) Just wondering, at about 75 or so in 5th, is the rpms at 3400? And if you do change tires to 215/50/17, or bigger, does that mean that gas mileage would improve due to lower rpms, but at the cost of acceleration?

Just curious about all this because I'm a prospective tc / mazda 3/ rsx buyer and I got to due my research.


tc @ 75 mph = 3400
m3 @ 75mph = 3200
rsx @75 mph = 3300

p.s also any update on the moonroof pop / hatch rattles? any tsb out yet?
I did not personally do tire math - got my info from http://www.1010tires.com/TireSizeCalculator.asp

This site - http://www.powerdog.com/tiresize.cgi shows 819 rev per mile. So I guess 1010tires (being a canadian website) displays canadian revolutions, which are a little different than american revolutions.

Anyways. I do cruise at 90 mph a lot of times - that's the unofficial speed limit in Atlanta area - ask anybody from GA on this board


As for rpm at 75mph - pretty high. I think it's around 3500 - If you want to know exactly, I'll go on the freeway on the way to work tomorrow just for that


Thanks for looking up the tire info. Any chance of measuring the actual wheel? (circumference or diameter) so that we have our own solid point of calculation?
 

·
Registered
Joined
·
116 Posts
knowing that the tC engine is from the Camry, I wonder if the tranny is too? So, the cheapest way to take "off the shelf parts" and increase performance would be to use a radical differential.

Just sold my '67 Camaro with a 4.88 rear end. Talk about high rpms...
 
1 - 18 of 18 Posts
Top