Swapping DOHC for 3X00

[jmgtp]

Yikes.. I've been so devoted to the DOHC all these years and here I am looking to ditch it. Don't get me wrong, it runs great and has given me no serious mechanical problems.

As you may have read in a few of my other posts my herd has gotten bigger and my stable smaller. The GP is the oldest and sadly least reliable car. The Vette is the toy now, the Mazda6 the daily driver and the GP is stuck somewhere inbetween.

I just don't want to worry about a 20 year old motor breaking down on me in the dead of winter. Plus this motor is modded a whole lot - its still streetable but it has a loping idle and touchy throttle and just a lot of attributes that I don't want to deal with in the winter - it'd make snow driving harder. I'd rather have a smooth idle and lots of low end torque.

It would break my heart to part with the GP so I am putting some serious thought into a 3x00 swap since that is a newer, less miles, simpler and yes more reliable power plant. I'm really just concerned about it breaking in the winter more than anything else.

My main concern about a 3x00 swap is will it bolt in as far as motor mounts go? Does it need to be from a W car? My Grand Prix is a 91 if that matters. I'd like to go 3400 or larger but if the I can find a good price 3100 I'd consider it. Where and for how much can I expect to find a low mile 3x00. Obviously the newer/lower miles the better. Preferably 50k miles or less. What kind of fuel mileage could I expect out of a stock 3x00 / getrag 284 combo?

What can I expect to spend on this kind of swap? Keep in mind I would do it all myself so this is a parts only cost. I can make my exhaust work, I have the welding/fab skills to do it. I have a spare good condition 284 that is going in it along with a spare clutch.

Need to know things like:
-wiring issues - I'm keeping this OBD I 7730
-alternator wiring issues?
-motor mounts
-assuming my 284 flywheel works no prob
-power steering lines?
-mods to the 3x00 to make it 1991 W body compliant?
-reasonable motor cost
-hidden costs for things I'm not thinking of?
-what about my a/c compressor? will the dohc compressor work on a 3x00 - I'd like to keep that system since the r12 is still ice cold
-plumbing for fuel lines, water lines, radiator, etc?

The rest of the car is coming up on 20 years old soon but it is in great shape. I've maintained it well and there is literally no rust. I'm giving up a lot of ammenities though if I daily drive it rather than the Mazda (things like traction control, abs, heated seats, AIR BAGS, etc etc).

[robertisaar]

-wiring issues - I'm keeping this OBD I 7730

why are you running a 7730 when it should have came with a 16149396?

[jmgtp]

1991 Grand Prix came with 7730.

[robertisaar]

1991 Grand Prix came with 7730.

91GP should either be a 7727(if it was originally a 3.1) or a 9396(if it was originally a 3.4).

i'm confused...

[jmgtp]

Maybe its a 7727 and I'm confused. The car is an original dohc. The 9396 definitely sounds wrong though. It's the underhood ecm with the weatherpack connectors. Regardless, my point was I want to stick with the ecm is in there... I have all the burning hardware so it should be as easy as burning the right bin to the memcal.

[robertisaar]

so did the car came from the factory with a 3.4?

[SappySE107]

Yes and its a 9396 so no worries there. Best value is a 3500 with low miles. Get the TB adapter plate and external trigger, a used 3400 fuel rail and injectors, and your current wiring harness plus a pushrod injector wiring setup (plugs in, better wire routing). You could probably make what you have work as well however. It will take some tuning, but I have a 3400 swap chip that would probably get you pretty close and then fine tune from there.

I think you can use the 3.4 DOHC engine mounts on the bottom, as that is how I was hoping to do mine. The 3.4 mount setup is more stable than what the pushrod W body got anyway. I know the 4 bolts on theback of the block are there but I think it is missing 1 on the front that you would "need".

[jmgtp]

Does it matter what the 3500 comes out of? If I pull the trigger on this I don't think it will be until the spring. I don't think I have enough time to collect parts before winter and I'm sure the swap will be more than a weekend deal.

Also fairly concerned about these items and how they will function with the rest of the car:
-power steering - are custom lines necessary?
-alternator - I think the design that bolts into the newer motors has diff electrical connectors that may pose a problem?
-air conditioning... can I use my existing compressor? if so if I unbolt the compressor from the dohc and leave it in the bay could I just bolt it right up to a 3x00? The r12 in this thing still blows colder than any other car I've owned so I'd rather not have to evac it

There seem to be some good 3500 prices on ebay even with shipping - unfortuneately it seems none of them come with accessories (power steering pump, alternator, etc).

I keep thinking more and more about this swap and I'm sure I could build a reliable daily driver and keep my beloved grand prix!

[IsaacHayes]

Can't you get the 3.1mpfi or even 3100 W body front mounts that go to the Radiator Support to the heads? You could probably put a combination of mounts on there. With factory rubber it wouldn't vibrate much and more mounts would mean less movement. Best of both worlds.

Make sure the 3500 is not the VVT one. You want LX9. Alt should be fine, if not grab a 94 era 3100 alt it should work with the connector. PS lines you can just grab them from a W body 3100 probably. You'll probably need the gen3 PS that comes on 3x00s. AC probably, but the bottom bolt may not line up, but if you oval out where the bolts go through it might work. Barry did that with his 3100 AC compressor on his 3500 in his Beretta.

3500 swap would be nice. About the same power stock as the DOHC. Would make a great stock DD duty motor without mods even. Just make the exhaust good and you'll have a nice car. I'd use 2 front 3500 manifolds and then Y them together at the rear. The 3500 rear manifold might fit though in a W body.

[jmgtp]

Thanks for the reply. I'm not too concerned about the top 'dogbone' mounts. The car is drilled for the passenger side mount (dohc only uses a drivers side) and I'm sure I can get those to work without a prob. I already have a 2.5" stainless/madrel bends catback that I built this spring so that should be plenty big for the 3500. I knew I needed non vvt motor but now I know that is the LX9 - thanks for that bit of info.

I may jump on this sooner if my parents let me bum a spot in their garage. My house doesn't have one

A stock 3500 seems more than ample for a daily driver and should be a lot of fun with the 5 speed. I'll be taking a hit on the top end performance compared to the modded dohc but at the same time the 3500 will be A LOT more streetable, reliable and hopefully thrifty on fuel consumption. I bet even a 3100 with a 5spd would still be fun.

It seems like a lot of my questions have been answered and I'm sure more will come up. I realize a lot of it I'm going to have to figure out as I go too. The problem is that I'm not familiar with the pushrod accessories so I don't know what years/models have interchanging power steering pumps, alternators, etc.

The big question left:
What vehicles came with the LX9 and of those vehicles do they all have the same motor mount locations? DOHC mounts bolting up is the biggie. This really just means the front and rear mount as those are the two major ones. Ben mentioned above that there is 1 bolt that won't line up. I dont think he mentioned which but I'd bet it is that PITA one on the front mount.

Thanks guys.
Still looking for any info/experience you guys can offer so keep it coming.

[LeftVentricle]

2004-2006 Chevrolet Malibu, the "new" body style, not the "Classic."

[jmgtp]

Well guys... against my better judgement I think I really may be going through with this swap. It looks like I can't hog the garage though so the actual swap won't take place until the spring when the garage becomes available again. That gives me a lot of time to find an engine at the right price and begin to collect the odds and ends.

[jmgtp]

Hello all,

I've been looking at LX9 3500s quite a bit lately! I am 99% that I am going to go through with a swap and make a reliable daily driver.

Given that it is going to be just that, a daily driver, I'm not overly concerned with HP figures. I don't want it to be a slouch and I am going to be really sad to see the high rever DOHC go, but the goal of this swap is to be practical, simple, efficient and dependable.

That said, I think a 3400 would be a simpler swap for a few reasons:
-already has cable driven TB
-I believe the fuel rail and maybe even injectors are useable (think 3500 is returnless?)
-correct ICM
-does it have the correct crank sensor already?
-higher production # - could this mean cheaper replacement parts in the future?

Beyond the cost of the engine I'm looking at the odds/ends, accessories, etc that will be added on cost in either swap.

These are all obstacles I would have to overcome on the 3500. I have by no means ruled out the 3500 though. Can someone can tell me why the 3500 would be more: efficient/reliable... and okay, how much more powerful, than the 3400 (2.5" exhaust is only planned mod).

I believe out of the box the 3500 is about 20 hp more than the 3400. Torque?
It is obviously a newer motor. Is there anything that makes it more reliable? Better bottom end, valvetrain, etc? The grand prix is a 5speed 284 - what kind of MPG could I reasonably expect.

I appreciate all the support/responses!
Thanks and keep it coming in.

[pocket-rocket]

You could do a 3400 with a 3500 top end...

[IsaacHayes]

The TB issue is not a big deal. I've used a 3400 TB and now have a 65mm Olds Intrigue TB on mine. While I don't have a 3500 swap, I have a 3500 plenum. The car is my only car so I had to get the stuff swapped on and done right quickly. The TB was the least of my issues (main issues was rockers but that is unrelated to a 3500 swap).

Yes you'll need 3100/3400 fuel rail, but many people online have spares from several 3x00 swaps or junked motors. If time is not a rush, no problem to get that stuff. Same deal with the ICM. Hell you can even use 2.8/3.1MPFI ICM no problem (same thing)

3500 does need the external crank sensor setup as the internal block one is not a 7x reluctor. You don't need to worry about bolting on the 24x crank trigger since you are not OBDII. Your ECM should accept an A1 bin which many people have made 3400 tunes for and even 3500 tunes. Or you could tune your DOHC bin as well.

Replacement parts are not really an issue. My 3100 has a boat load of miles on it. If it were needing a rebuild, I'd go for a newer motor as it would be cheaper. The other parts other than the motor itself are shared with lots of 3x00's. There are metal gaskets out now for the 3500 so that is not an issue either.

3500 is a lot more efficient. The heads are awesome. And a lot of time cheaper to buy than a 3400 in the yards since there isn't much demand for them since they are so new. The crank in most of them is steel, but even the old 3400 cast cranks hold up to 16psi of boost/etc so reliability is not an issue other than the lower intake gaskets but that's solved with a metal gasket. Just throw that on (even if you do a 3400 do this) before dropping in the motor and you're good. With a decent tune no reason not to get great mileage. You should get better mileage than the DOHC. Not sure what rev's you turn with that trans, but the 3500 has plenty of power to pull you at low RPM.

Hell my 3100 turns 2100 RPM at 75mph even up most hills we have here without unlocking the torque converter.

So... crank sensor, TB adapter, fuel rails and I think that's about it as far as major stuff. I think the little bit of extra work is worth it for the extra power and torque. It would make a very stout DD. It's not a whole lot of extra work over the 3400. Now a 3900 swap would take a lot more over a 3400. The LX9 3500, not much..


EDIT: 3400 with 3500 top end = cost more than just 3500 swap, and you loose compression and .1 displacement.........