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Old 02-13-2005, 03:40 AM   #1
Dubhlachan
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Default Manual Trans SOHC -vs- DOHC

From what I understand the SOHC S-series cars have a different gear ratio. Would it be possible or advantageous to use a SOHC trans with a DOHC motor? Curious.
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Old 02-13-2005, 04:36 AM   #2
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Default Re: Manual Trans SOHC -vs- DOHC

I've driven both and didn't notice a difference at all. Not sure there is one, seems to be some extra bucks forked out by Saturn to do nothing.

The main reason behind the SOHC is fuel economy; changing gear ratios would also have helped so why do both?
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Old 02-13-2005, 05:00 AM   #3
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Default Re: Manual Trans SOHC -vs- DOHC

SOHC have taller gears. If you have SOHC tranny in a DOHC, you'll probably have slower acceleration and slightly better gasmilage.
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Old 02-13-2005, 05:43 AM   #4
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Default Re: Manual Trans SOHC -vs- DOHC

Yup. If youre looking for performance and good gas mileage, get 1st-4th gear from the mp3 (dohc) and 5th gear from teh mp2. That way youre when youre cruising at 70 youre not at 3k freakin rpms.
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Old 02-13-2005, 06:23 AM   #5
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Default Re: Manual Trans SOHC -vs- DOHC

My first car is a SC1, I always loved how I didn't have to shift quite as much and how it kept RPMs low, but acceleration isn't so hot. If I had a long spool turbo maybe it'd be good.

That said, I love the spacing in my SC2's and just wish 5th was nice and tall so I had low RPMs on the freeway, so I'm with bert...

My Miata's is way too close, but not quite close enough to do every other gear, and 80mph = 4k rpm, rather annoying....
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Old 02-13-2005, 07:00 AM   #6
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Default Re: Manual Trans SOHC -vs- DOHC

I've also wished for a taller 5th gear. Ever been shifting at 3k or wherever you habitually shift at... and try to shift into a non-existant 6th gear when you hit 3k rpm in 5th? I wonder if anyone's ever changed thier gear ratios.
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Old 02-13-2005, 05:03 PM   #7
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Default Re: Manual Trans SOHC -vs- DOHC

Not that Im aware of. It just makes more sense to get a 5th gear offa mp2. Less time and money.
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Old 02-13-2005, 09:22 PM   #8
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Default Re: Manual Trans SOHC -vs- DOHC

Quote:
Originally Posted by Dubhlachan
I wonder if anyone's ever changed thier gear ratios.
i think one guy changed fifth, and if i ever get my engine/tranny rebuild done it is on my list.
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Old 02-14-2005, 12:59 AM   #9
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Default Re: Manual Trans SOHC -vs- DOHC

I don't think anyone mentioned it, but the reasoning for someone to use the DOHC engine with a SOHC trans (mp2) is for turbo applications because of the longer gearing it is ideal for the turbo spooling...vice versa with a supercharger...mp3 is better because it doesn't spool...if you aren't doing forced induction, it's better to stick with your mp3.
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Old 02-14-2005, 01:11 PM   #10
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Default Re: Manual Trans SOHC -vs- DOHC

First and Second are the same in both, so you're not losing anything there. Its third and up that are taller. I've taken a liking to it (I have an MP2 in my SC2), as the car likes being dropped down into third on the highway much more than it did with the MP3. Not to mention my gas mileage has improved by about 5 miles/gallon.

If you are going to switch out 5th though, make sure they're compatable. the shafts were changed at some point, as I was trying to make a hybrid (MP2.5?) and it totally didn't work. The shafts wouldn't go in straight together, and the 5th I had from the MP2 wouldn't fit on the MP3 shaft. I think the MP2 was from a 95, and I know the MP3 was a 97.

Also, if you manage to just switch out 5th, you lose cruise control, I'm guessing because the computer isn't seeing what it wants to in terms of RPM:MPH, due to the different gearing.
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Old 02-14-2005, 05:13 PM   #11
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Default Re: Manual Trans SOHC -vs- DOHC

Quote:
Originally Posted by Pimped92SL1
I don't think anyone mentioned it, but the reasoning for someone to use the DOHC engine with a SOHC trans (mp2) is for turbo applications because of the longer gearing it is ideal for the turbo spooling...vice versa with a supercharger...mp3 is better because it doesn't spool...if you aren't doing forced induction, it's better to stick with your mp3.
Not true.

You want longer gearing with a turbo to put the boost you make down for a LONGER period of time, hence why using an MP2 is ideal.

And an mp2 with a supercharger would be retarded, as you have to get into high rpms as fast as possible to make boost, FAST. It would take forever to make good boost with a supercharged MP2 DOHC>
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Old 02-14-2005, 05:16 PM   #12
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Default Re: Manual Trans SOHC -vs- DOHC

i said mp3 for s/c...ok longer period of boost being put down with the mp2, so i screwed that part up...

but i said mp2 for turbo and mp3 for s/c...no reason to have a mp2 in a stock dohc unless you want better fuel economy
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Old 02-14-2005, 05:18 PM   #13
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Default Re: Manual Trans SOHC -vs- DOHC

MP3 with a MP2 5th gear.

Mmmmmmm, Im working on it right now, hehe.
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Old 02-14-2005, 05:21 PM   #14
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Default Re: Manual Trans SOHC -vs- DOHC

Do the MP2 and MP3 trannies have the same Final Drive?
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Old 02-14-2005, 05:23 PM   #15
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Default Re: Manual Trans SOHC -vs- DOHC

Yes they do.
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Old 02-14-2005, 09:51 PM   #16
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Default Re: Manual Trans SOHC -vs- DOHC

i dont know why everyone talks about the 3k+ rpm at highway speeds.

i got great mileage averaging 75mph which is about 3400 rpm in 5th on a 5 hour drive one way to visit my gf this past weekend.
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Old 02-14-2005, 09:57 PM   #17
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Default Re: Manual Trans SOHC -vs- DOHC

Quote:
Originally Posted by Qlara
Do the MP2 and MP3 trannies have the same Final Drive?

nope. that's why you can't change them...

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Old 02-14-2005, 10:13 PM   #18
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Default Re: Manual Trans SOHC -vs- DOHC

Quote:
Originally Posted by TheSaturnKid
Yes they do.
http://www.differentracing.com/tech_...gearratio.html
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Old 02-15-2005, 12:02 AM   #19
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Default Re: Manual Trans SOHC -vs- DOHC

I just checked out your link Saturn Kid and there doesn't seem to be a great deal of difference between the two 5th gear ratios. 0.643 -vs- 0.730. Still a 5mpg increase on the highway is attractive. I typically drive 70+ highway and I do Long roadtrips sometimes. The kind where you have to change your oil in the middle of the trip. Something to think about I guess when down the road I eventually have to do my clutch.

It might be easier/better to just swap the trannys out instead of tearing into mine because I don't have the knowhow/experience/tools to tear into a transmission.
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Old 02-15-2005, 12:18 AM   #20
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Default Re: Manual Trans SOHC -vs- DOHC

it doesn't seem like a huge difference looking at the ratios, but i think it was something like 500 RPM at 70 MPH. It might be counteracted in the MP3 though, as you spend more time in 4th at lower speeds. The MP3 could easily be in 5th at 32 or 33 MPH, and it pretty much chokes the engine at that speed with the MP2. But since 4th is also taller, I think its not as bad in the MP2, fuel mileage wise.
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