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Old 09-25-2015, 01:11 PM   #1
willis68
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Default another sohc to dohc


i have a oil burning 95 sl1, that i am considering to do a sohc to a dohc swap. i have heard all of the basics but was wondering if the little things will need to be swapped. my car is the sohc with a manual, abs and cruise. i know the trans, clutch and flywheel will work. what about a/c, p.s., exhaust down pipe, air box, motor mounts, misc stuff like coolant hoses, and the electronics. with the electronics i know the ecm is going to have to be changed. the harness i can do with. my biggest concern is if i found a obd2 car and if the serial data circuits between modules are different. like the cluster and abs module. i know there is alot of gm cars that didnt have the abs and cluster on the class 2 data bus back with the early obd 2 years. i plan on getting the parts at a you pull it yard. basicly i will be starting out with a long block with the manifolds and a ecm.

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Old 09-25-2015, 01:52 PM   #2
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need dohc trans to match motor or you will not harness the fun of the extra powered. Geared very differently. Look up SOHC vs DOHC parts on rockauto.com to check for part no overlap. Never actually looked at an SOHC upper torque mount but I suspect the mounting points and mounts are the same.

You make your life potentially very difficult trying to swap an obd II based system into the car unless you plan to pull and take the entire main harness as that's the only way it is gonna have a chance of working.

Then there is the question of dash compatibility---at some point they went digital so if you don't swap the dash, then no go.

95 is an oddball s car year. Replacing w a 95 DOHC motorfull harness + PCM w reflash is the easiest path.

No one is going to stop you from making it more difficult on yourself......

Plus unless you want to replace the downpipe + cat with the obdII version with 2 O2 bungs, I'd cancel out the cross obd swap
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Old 09-26-2015, 12:28 AM   #3
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so the trans in it will bolt up but the gear ratio is different. if i went with a odb1 dohc setup is the data transfered to the abs airbag and cluster the same
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Old 09-27-2015, 04:40 AM   #4
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I believe it is, but I rather someone who knows for sure chime in.
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Old 09-27-2015, 07:58 AM   #5
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why are there two threads on this?

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Old 09-28-2015, 01:15 PM   #6
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i messed up and noticed the s series forums after the fact
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Old 09-28-2015, 04:04 PM   #7
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Quote:
Originally Posted by willis68 View Post
so the trans in it will bolt up but the gear ratio is different. if i went with a odb1 dohc setup is the data transfered to the abs airbag and cluster the same
You do NOT want an SOHC tranny on your DOHC; it nullifies 1/2 the point in doing the swap. The goal is to transfer the power of the engine to the wheels at the appropriate rpm, through the tranny and torque converter.

Putting an MP2 (SOHC trans) on a DOHC instead of an MP3 (DOHC trans) is likely going to give you a mess w r t shift points and power.

It's EXACTLY why there IS an MP3. Because the MP2 performs like crap when paired with the DOHC. They designed the MP3 to, when paired w a DOHC, perform well in all around conditions. If the MP2 worked well with a DOHC, the MP3 wouldn't exist. If your concern is cost of replacement for the tranny, rest assured you will be horrendously disappointed with the performance if you leave an MP2 in there w the DOHC.

Analogy: 10 speed bicycle

If 1st gear sprocket isn't big enough (diameter), you have no chance to power your way up that super steep hill, no matter HOW POWERFUL your legs might be. You can't overcome physics.

If 1st gear sprocket was bigger, you'd be able to pedal easier compared to the first scenario IF your legs were strong enough to propel you and the bike up the hill.

If 1st gear sprocket is absurdly large (relative to the 1st 2 scenarios), even with strong legs, you'd be moving up the hill but your rpm would be crazy high and you likely wouldn't be getting very far very fast.

No difference with a car. Engine = legs, gear sprockets = tranny sprockets, rod = road.

The goal is the same: optimal power transfer using the gear ratios that match the engine output the best

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Kinda confused on the airbag/ABS data on an OBD I setup question. You ARE on an OBD I setup. Did you mean OBD II?

As for dashes, 95 was the last year of OBD I. However, I've seen many posts lamenting that for example, the 93 and 94 dashes in an SW1 don't interchange. Why? Reconfig of circuits, pins swapped around......they may look identical from the front but don't function correctly/totally). Some of the connectors may be different. Just find a 95 SC2/SL2 and prob SW2 dash and avoid all that.
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Old 09-28-2015, 08:48 PM   #8
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It'll work just fine.

You'll need new intake/exhaust manifolds (and downpipe) and matching power steering/ AC brackets.. everything else will work.. hoses, mounts, etc.
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Old 09-29-2015, 12:11 AM   #9
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i am really not looking for performance. if i was, i would remove the hot air intake off my car. a little bit more power would be nice. but i would like to keep the mpg up. i know there is different ratios for different engines and tire sizes. i am happy with the ratios that my car has. the only thing that i question is the vss pulses per mile. a lot of the 90s gm cars was 5000 per mile. the vss is an anloge input to the ecm. i think i might need to hook up my scope on the circuits to the cluster and compare to the vss to see if the frequency is the same hertz. i am thinking of staying with the obd 1 electronics but upgrading wouldnt be too hard if i had the modules and connectors and at least i would have gm's service information to use too.
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Old 09-29-2015, 06:44 AM   #10
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Kinda wish you had brought up the "I'm not looking for performance" part alot earlier -- I wouldn't have spent all that time and effort trying to convince you to use an MP3.

I'm in no way being sarcastic, but if you're not looking for performance, why not just rebuild the 95 SL1 SOHC motor? Do you just want the HP kickup from the DOHC for increased pep?
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