Explorer 302 or 289?

Av8r37

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Dec 19, 2020
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Rochester Hills Michigan
Hey guys, I have recently acquired a 1964.5 mustang! I am currently in process of rebuilding it and am balancing my options for motors. What I have is my roller without an engine
and a totaled 1966 with a stock 289. My plan is to swap over all the driveline and interior to my solid roller. I am just building a toy car and not planning on ever taking it to the drag strip or even the track. However, I still would like it not to be sluggish! What I don't know is whether I should pull a local 302 out of an explorer or mountaineer at a parts yard. I know they have the gt40p heads and intakes. Should I just chuck in a cam and put an explorer 302 in the car? Would it be better to take the gt40p heads and put them on my 289? I bought the 66 from Copart and have got the 289 to run seemingly well but don't know its full history. Is pulling gt40p heads or the whole engine even a good idea or should I keep it stock? What do you guys think? Also, from browsing forums I have learned most exhaust headers don't fit the gt40p heads because of the spark plug positioning, I heard 90-degree sparkplug boots can alleviate this issue, is that true? Also what HP numbers am I looking at with a performance cammed 289 with gt40p heads vs a cammed Explorer 302? Anyway,I would appreciate any feedback on how I should proceed with my build without emptying my bank account! Thanks!
 
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65ShelbyClone

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"Better" is subjective and all you're really said is that you don't want the car to be sluggish. It's not going to be sluggish with either engine in good running order. Nothing was said about transmission type, gear ratios, etc.

There is going to be a lot of nuances to using the 5.0. You'll need a distributor with a steel gear, a 50oz flywheel and damper, a decent intake that won't choke the heads and cam, and headers to work around the heads. On that topic, an Explorer 5.0 is not guaranteed to have GT40P heads; the early production ('96 to maybe halfway through '97) ones had regular GT40s. The roller 5.0 block won't have a place to mount the z-bar if you use a clutch and will need an adapter bracket. You'll also need springs for whatever cam you get. That's just off the top of my head.
 

02 281 GT

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In regards to the Explorer swap, I would only caution that it’s not 2005 anymore. A lot of Mustang guys still tout the Explorer swap without realizing that any Explorer/Mountaineer you come across in a yard will likely have a tired, 200K+ engine in it. I wouldn’t drop one into my car without at least giving it a thorough look over and compression check, if not disassembling completely to have it checked by a machine shop.
 
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MustangIIMatt

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In regards to the Explorer swap, I would only caution that it’s not 2005 anymore. A lot of Mustang guys still tout the Explorer swap without realizing that any Explorer/Mountaineer you come across in a yard will likely have a tired, 200K+ engine in it. I wouldn’t drop one into my car without at least giving it a thorough look over and compression check, if not disassembling completely to have it checked by a machine shop.
There's also the other downside to using an Explorer donor...

Soccer moms are notorious for not maintaining their vehicles.
 

7991LXnSHO

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There's also the other downside to using an Explorer donor...

Soccer moms are notorious for not maintaining their vehicles.
I had no idea how true this is until we went to replace the bandwagon. No wonder insurance did not believe me as to it’s pre-accident condition without pictures and receipts and a recent inspection report.
If I knew the 289 was good with your goals, I’d transfer it over, making sure it had a 4bbl intake or looked stock. You can be driving it while working up something peppier if it is not fast enough.
 
Apr 19, 2021
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Denver, CO
I have a 1970 mustang with a 302 and a c4. It definitely has bad valve stem seals, but even after start up the car smokes a bit so I thinking blow by? I plan on pulling the engine to do some work on it or swap in another engine.

I also have a 351w that came out of an 82 econoline. It had an internal water leak. However I ran it for a bit with no water and it seemed to run well, my biggest concern on building this is figuring out were the internal leak is.

My other option is I have a 97 explorer with a 5.0. It is a relatively low mileage rig that I'm thinking of stealing the engine/accessories and rear end from.

What would you do?
3 engine options to choose from.

I'm thinking if I can verify the 351w is good, that is the choice (no replacement for displacement). I got a flat machined surface to check for warp, I'm assuming it a bad headgasket because one of the exhaust manifolds was cracked between #3 and #4. The reason I haven't pulled the heads yet is because I have been contemplating a way to test for leaks with the head installed to verify the problem, if anyone has a good trick or opinion let me know.
 

65ShelbyClone

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You haven't said what your end goals are.

It sounds like your 302 needs at least some diagnostic work if not an overhaul.

It sounds like the 351W needs head gaskets at minimum, an overhaul at most, and a new block at worst.

The Explorer 5.0 is, IMO, the best option regarding ease of installation and power potential. It already has GT40P heads (unless it's an early '97 in which it could have regular GT40s which have easier header fitment) and a roller cam. It will also accept your 302's front accessories. It's capable of 275+rwhp without even pulling the heads. You'll need a 50oz damper and flexplate for it.

The early '80s 351W is going to be a wheezy SOB without some massaging.
 
Apr 19, 2021
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Denver, CO
Thanks for your opinion!

I'm not sure I agree that the Explorer swap would be easier. Along with a flexplate, I would need an electric fuel pump, a way to adapt my power steering to the mustang steering box (if I did the swap it would be nice to use the late model accessories). I think I would need an ac delete pulley as well. Also would I need anything to adapt the alternator?

But just because it is difficult doesn't mean it is not worth it.

For the 351w I would reuse the mustang flexplate, I would need a distributor, new seals, headgasket, The timing set seems a little sloppy so I was going to replace that as well.... I guess maybe it is a lot of work too, but it seems more straight forward than the explorer upgrade route. How to verify that I don't have a cracked block is the big hang up for me. A cracked head doesn't seem that big a deal because I could find a cheap set of junkyard heads and they are coming off anyway.

My goals:

short term-make my mustang a reliable driver, I'm planning a long trip at the end of June so the quicker the timeline the better.
long term-I want to eventually turbo the mustang so I was planning on gapping the piston rings of whatever engine I use (if I went with the explorer motor I would likely skip this to save time).
 

65ShelbyClone

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Sep 9, 2000
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Antelope Valley, SoCal
Thanks for your opinion!

I'm not sure I agree that the Explorer swap would be easier. Along with a flexplate, I would need an electric fuel pump, a way to adapt my power steering to the mustang steering box (if I did the swap it would be nice to use the late model accessories). I think I would need an ac delete pulley as well. Also would I need anything to adapt the alternator?

But just because it is difficult doesn't mean it is not worth it.

It's not difficult. Everything on the front of your 302 will fit on the front of that 5.0. You don't need adapters for anything. I know you said you want the accessories off the Explorer engine, but an Explorer setup is IMO the least desirable serpentine conversion. It would be simpler and easier to get serpentine pulleys for the accessories you already have.
 
Apr 19, 2021
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Denver, CO
UPDATE: I replaced the valve springs on the 302 in my mustang last weekend. All but one cylinder were totally wasted, I think this car sat for a long time before I started working on it.

Unfortunately the car still smokes.

I tried unplugging the vacuum modulator line to the carb because I heard this could suck trans fluid in, but this did not help the smoke.

Next I did what I should have in the first place and did a dry compression test on every cylinder, it was bad but a consistent 105 psi on all cylinders.

My symptoms are bluish smoke not so much when idling but on/off throttle.

The other problem could be that I used the wrong valve stem seals, I used fel-pro SS 13364-1 for the intake and SS 13807-1 for the exhaust valves. I can guarantee I used the wrong exhaust valves, the ones advanced auto parts listed were way off (compared to the ones that came off the one good cylinder) so I used similar seals that I had laying around for my 351W. Attached is a picture of one of the umbrella seals I used for the intake. My understanding is that they should have a snug fit on the valve and not interfere with travel limits, but other than that what else should I have looked for in regards to bad fit?

That is a lot of info, if anyone has any tips please let me know.

Gonna remove the 351w heads tomorrow to check for a blown head gasket (separate engine I am thinking of building for the mustang).
 

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7991LXnSHO

wanna catch the space herp
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Valve stem seals only help do much with worn valve stems and guides. Without looking back, I cannot remember if you did compression and leak down tests.
 
Apr 19, 2021
5
1
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31
Denver, CO
Valve stem seals only help do much with worn valve stems and guides. Without looking back, I cannot remember if you did compression and leak down tests.
I did a dry compression test all the cylinders were pretty close to 105 psi. What would be a symptom or way to test bad valve stem/guide clearance?
 

7991LXnSHO

wanna catch the space herp
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Guides leaking tend to smoke 1. at startup, especially parked on an incline. 2. When you let off the accelerator and vacuum pulls oil past them into the combustion chamber. Without measuring parts, I suppose you can move a piston to tdc, remove one spring at a time and check for valve play. A bore scope should also show excessive oily carbon buildup on the back of the valves.
105 psi dry sounds low to me. Did you squirt oil in and retest wet cylinder pressures?