3.9 L Mirada CMX

Joe12459

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I just picked up a 98 Dakota low mileage 3.9 Magnum drivetrain to put in my son's 81 CMX. I plan on using all of the electronics, etc. I'm thinking about a turbo setup or supercharger. Has anyone here boosted a 3.9 Magnum, and if so, what parts did you use? There don't seem to be a lot of performance parts for these engines.
 

AJ/FormS

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Isn't the 3.9 engine the one with the funky offset crank journals?
Or maybe that was the Buick.............lol
Getting old sux.
Ok I checked; It's the rod journals that are offset.
You'll have to do some research allrighty. It doesn't look strong enough to me, but Happy hotrodding.On a more positive note, it looks to have about the same power as that old 318,. and wants to rev well past 5500; that's a plus. And check out the torque peak at 3200,sweet. Get a 2600/2800TC, and 3.23s,and put a carb on it and just DRIVE it lol. EFI would be nice, but that all ways takes a really long time to get running right, and the turbo may have blown it up twice during that time,lol.
Now a carbureted 5.9 with an A500, and a turbo,whoa, that would be killer. The 5.9 can take 400hp easy enough, and can do it on modest pressure. The A500 means you could run 3.55s around town, and the turbo means you can run the stock TC. The 3.55s after overdrive will be 2.45s pretty much what you might have currently,so you're still good for same 20ish Mpgs
Now, the turbo is a dial-a-power deal. You can run zero boost, and run stock 230ish HP, or 4psi for maybe 300hp, or 8psi for maybe 400 and so on until the engine blows up. And if you've never had a ride in a 400hp CMX, it's probably a good thing, cuz it's gonna be scary.
I suppose you could boost the 3.9, but I'd keep a spare on hand,maybe two. She'll need something like 9/10 psi to make 400hp,lol. IDK if those offset pins are gonna hold together. Course, I guess you could ease into it a couple of psi atta time. By 3psi, you'll be making 318 power.By 6psi, 360 power. By 9, 440power.By 12, Hyup....HEMI POWER.......if it lives................lol.

Course with 9.2factory compression, ratio you probably won't be able to run much over 5psi.............if you get my drift.

s://www.allpar.com/mopar/39.html
power-39.gif

s-l225.jpg
 
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Joe12459

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Isn't the 3.9 engine the one with the funky offset crank journals?
Or maybe that was the Buick.............lol
Getting old sux.
Ok I checked; It's the rod journals that are offset.
You'll have to do some research allrighty. It doesn't look strong enough to me,
View attachment 27438
I believe it has offset crank journals. I don't really know a lot about the 3.9, so it will be somewhat of a learning curve.
 

Joe12459

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Isn't the 3.9 engine the one with the funky offset crank journals?
Or maybe that was the Buick.............lol
Getting old sux.
Ok I checked; It's the rod journals that are offset.
You'll have to do some research allrighty. It doesn't look strong enough to me,
Happy hotrodding
On a more positive note, it looks to have about the same power as that old 318

s://www.allpar.com/mopar/39.html

View attachment 27438
OK, rod journals makes sense. I don't want to run a ton of boost, just enough to wake it up a little. I just can't seem to find any performance parts made for it, so I think a small turbo might make be my best bet.
 

Mr Volare Imperial

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Or....
Look up some Turbo Slant Six videos... the small 170ci are the high rev engines and love to be blown... just 4 main bearings, low friction, lower compression, shorter stroke than the 225s. Add your own EFI and engine management systems to the degree you want from the projects like at the Slant Six Forum site, and go turblowing up some stout Slant Sixes The Slant 6 would even be factory original in origin
;)
 

AJ/FormS

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OK, rod journals makes sense. I don't want to run a ton of boost, just enough to wake it up a little. I just can't seem to find any performance parts made for it, so I think a small turbo might make be my best bet.
Well that might be a problem. You're probably gonna be limited to 4 or 5 psi cuz of detonation issues. So that will get you maybe 100hp. But your gonna spend a wheelbarrow of cash to get it, and mountains of time in fabricating. And before you know it maybe two years has gone by.
What's a crate 300 Magnum go for in your neighborhood? That's already 125hp over the 3.9. And it's a bolt in deal, and drive it over to the muffler shop, a week later your racing. Well maybe not racing if you got 2.45s back there,lol. Hang on a sec, step back a lil
..........................375 footpounds!
That's 65% more than the stock 3.9; and I'm guessing more than the 4 or 5 psi could do.
 
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Mr Volare Imperial

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Anda a one, anda two, all together now...

There is no....
Replacement...

For'rrr di'iss'ss-
Placement!

Buh duhh dee duhh buh Duh Duhh!
 

AJ/FormS

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Here's what the factory 3.9 looks like
240/112 cam, ICA of 48*
Static compression ratio of 9.2:1.
Effective stroke is 2.89 inches.
Your dynamic compression ratio is 8.15:1 .
Your dynamic cranking pressure is 163.78 PSI.
V/P (Volume to Pressure Index) is 103...................................................103
Notice that the pressure is right around the magic 165psi, about the maximum you can run on pump gas. Also notice the meager VP of 103, better than a slanty, but not by much.Remember, the big 225 is 3.2 liters (EDIT; 3.7 liters). and the 3.9 is just 238.8 cubes.

Here we have same 3.9, with 5psi boost
Static compression ratio of 9.2:1.
Effective stroke is 2.89 inches.
Your dynamic compression ratio is 8.15:1 .
Your dynamic cranking pressure is 163.78 PSI.
Your effective boost compression ratio, reflecting static c.r., cam timing, altitude, and boost of 5 PSI is 10.92 :1.
V/P (Volume to Pressure Index) is 103
Notice the effective compression ratio is quite high,at 10.92. You may not be able to keep this engine out of detonation at low rpm. You will have to delay the onset of boost to maybe 3000rpm, which with 2.45s, and a 2.74 low, will be about 35mph. With a 2.45 and 2.73s it would be about same,lol. And who wants to wait for that?not me!

Here is the 5.9Crate 300hp
250* cam,ICA of 51*(really small cam)
Static compression ratio of 9:1.(I guessed)
Effective stroke is 3.08 inches.
Your dynamic compression ratio is 7.88:1 .
Your dynamic cranking pressure is 156.77 PSI.
V/P (Volume to Pressure Index) is 146...................................................146
Notice the VP of 146; this is 146/103=plus 42% more than the stock3.9 and will feel like it too, from idle to in the range of 2000 to 3000, when the torque is gonna blast that CMK into twist-the-chassis territory. At a Dcr of just 7.99/157psi this will probably run on 89 or less octane gas.

And just in case you ask; here's a stock 318LA non-Magnum
Static compression ratio of 8:1.(old LA)
Effective stroke is 2.89 inches.
Your dynamic compression ratio is 7.10:1 .
Your dynamic cranking pressure is 136.77 PSI.
V/P (Volume to Pressure Index) is 114....................................................114
Yes, this burns the worst gas you can imagine, and still manages a VP of 114
which is 114/103=plus 10.7% over the 3.9. And I think those old LAs were 180hp.

Ok one more, a 5.2Magnum
Static compression ratio of 9.2:1.(I guessed again)
Effective stroke is 2.89 inches.
Your dynamic compression ratio is 8.15:1 .
Your dynamic cranking pressure is 163.78 PSI.
V/P (Volume to Pressure Index) is 136...................................................136
I think these are rated 230 hp
Notice that the VP is up higher again to 136/103= plus 32% now over the stock3.9. And to 146/136= within 93% of the Crate5.9. Remember, this number diminishes with rpm, and is probably most noticeable below 2000rpm
Read about VP here;
V/P Index Calculation
So anyway; Happy HotRodding
 
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Joe12459

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Thanks for all of the info! I actually chose the 3.9 drivetrain because it is unusual. My son wants a modern fuel injected engine and electronic transmission, but didn't want to do another Gen 3 Hemi car. He wants to have something different, but not too custom when it comes to parts availability and reliability. If he just wanted more power, I would build him a stout small block. I did actually consider turbocharging his 170 /6, but add fuel injection, engine management and transmission upgrades/overdrive, the cost becomes prohibitive. I got this drivetrain for a song, and I want to try to keep costs to a minimum. After reading all of the specs you've provided, I don't think boosting it will fit that bill. I'm not looking to make a ton of power, I just want to get whatever I can out of this engine without sacrificing reliability or breaking the bank.
 

AJ/FormS

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And a 225 slanty
Static compression ratio of 8.0:1.
Effective stroke is 3.70 inches.
Your dynamic compression ratio is 7.28:1 .
Your dynamic cranking pressure is 141.35 PSI.
V/P (Volume to Pressure Index) is 88...............................................................88
Notice 88/103= 85.4% as strong on the bottom,as the 3.9

And same slanty at 9.2
Static compression ratio of 9.2:1.
Effective stroke is 3.62 inches.
Your dynamic compression ratio is 8.20:1 .
Your dynamic cranking pressure is 165.09 PSI.
V/P (Volume to Pressure Index) is 101.............................................................101
Now you're at about same strong from idle to 2500

The 3.9 is unusual
Check out the AllPar graph. There is a reason this is a truck engine. It has a very early reasonably strong torque peak, yet the power peak is at 4800. Fitted to a 904 trans, she will want the 1-2 shift to be at about 5800. With a 998/999/A500 it will want to be wound up a lil higher, maybe 6000/6100. With the aforementioned 3.91s it will make a great little CMX. Certainly on a par with a teener of that vintage.
And now that we know a little bit more about your goals and capabilities, boosting it seems if nothing else ,interesting.
If I had my heart set on doing that,Here are 2 scenarios I'd favor.
In the first; I'd do it with A big remote mount turbo at the back, to at least double the horsepower. I'd run the A500 and 4.10s. The A500 has a fairly nice stock TC, and the 4.10s x 2.74=11.23 starter gear. This will help the 3.9 jump into the (delayed boost rpm because of the hi-compression),PDQ. The hi-cylinder pressure of the 3.9 should cover the hole of no boost before say 2800 quite nicely. And the A500 overdrive, will get you a nice hiway gear of 2.83
In the second, I would drop the compression ratio to a tic under 8, and add a lil turbo up front to really pump up the torque, so it behaves like a 440, AND reduce the rear gear to something like 3.23s. This will let you run a non-overdrive trans.
Of course .......I favor the latter.
Happy HotRodding
 

Mr Volare Imperial

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And a 225 slanty
Static compression ratio of 8.0:1.
Effective stroke is 3.70 inches.
Your dynamic compression ratio is 7.28:1 .
Your dynamic cranking pressure is 141.35 PSI.
V/P (Volume to Pressure Index) is 88...............................................................88
Notice 88/103= 85.4% as strong on the bottom,as the 3.9

And same slanty at 9.2
Static compression ratio of 9.2:1.
Effective stroke is 3.62 inches.
Your dynamic compression ratio is 8.20:1 .
Your dynamic cranking pressure is 165.09 PSI.
V/P (Volume to Pressure Index) is 101.............................................................101
Now you're at about same strong from idle to 2500

The 3.9 is unusual
Check out the AllPar graph. There is a reason this is a truck engine. It has a very early reasonably strong torque peak, yet the power peak is at 4800. Fitted to a 904 trans, she will want the 1-2 shift to be at about 5800. With a 998/999/A500 it will want to be wound up a lil higher, maybe 6000/6100. With the aforementioned 3.91s it will make a great little CMX. Certainly on a par with a teener of that vintage.
And now that we know a little bit more about your goals and capabilities, boosting it seems if nothing else ,interesting.
If I had my heart set on doing that,Here are 2 scenarios I'd favor.
In the first; I'd do it with A big remote mount turbo at the back, to at least double the horsepower. I'd run the A500 and 4.10s. The A500 has a fairly nice stock TC, and the 4.10s x 2.74=11.23 starter gear. This will help the 3.9 jump into the (delayed boost rpm because of the hi-compression),PDQ. The hi-cylinder pressure of the 3.9 should cover the hole of no boost before say 2800 quite nicely. And the A500 overdrive, will get you a nice hiway gear of 2.83
In the second, I would drop the compression ratio to a tic under 8, and add a lil turbo up front to really pump up the torque, so it behaves like a 440, AND reduce the rear gear to something like 3.23s. This will let you run a non-overdrive trans.
Of course .......I favor the latter.
Happy HotRodding
Those are some handy mathematics, thanks.
One point, tho, did I read 3.2L = 225CID? I think the 225 should be listed as 3.7L, and the 170 is 2.7L. I think the 194 slant 6 may have been at 3.2L, if that changes the #s.
 

Joe12459

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And a 225 slanty
Static compression ratio of 8.0:1.
Effective stroke is 3.70 inches.
Your dynamic compression ratio is 7.28:1 .
Your dynamic cranking pressure is 141.35 PSI.
V/P (Volume to Pressure Index) is 88...............................................................88
Notice 88/103= 85.4% as strong on the bottom,as the 3.9

And same slanty at 9.2
Static compression ratio of 9.2:1.
Effective stroke is 3.62 inches.
Your dynamic compression ratio is 8.20:1 .
Your dynamic cranking pressure is 165.09 PSI.
V/P (Volume to Pressure Index) is 101.............................................................101
Now you're at about same strong from idle to 2500

The 3.9 is unusual
Check out the AllPar graph. There is a reason this is a truck engine. It has a very early reasonably strong torque peak, yet the power peak is at 4800. Fitted to a 904 trans, she will want the 1-2 shift to be at about 5800. With a 998/999/A500 it will want to be wound up a lil higher, maybe 6000/6100. With the aforementioned 3.91s it will make a great little CMX. Certainly on a par with a teener of that vintage.
And now that we know a little bit more about your goals and capabilities, boosting it seems if nothing else ,interesting.
If I had my heart set on doing that,Here are 2 scenarios I'd favor.
In the first; I'd do it with A big remote mount turbo at the back, to at least double the horsepower. I'd run the A500 and 4.10s. The A500 has a fairly nice stock TC, and the 4.10s x 2.74=11.23 starter gear. This will help the 3.9 jump into the (delayed boost rpm because of the hi-compression),PDQ. The hi-cylinder pressure of the 3.9 should cover the hole of no boost before say 2800 quite nicely. And the A500 overdrive, will get you a nice hiway gear of 2.83
In the second, I would drop the compression ratio to a tic under 8, and add a lil turbo up front to really pump up the torque, so it behaves like a 440, AND reduce the rear gear to something like 3.23s. This will let you run a non-overdrive trans.
Of course .......I favor the latter.
Happy HotRodding
Hmm, both ideas sound doable on a reasonable budget. I plan on running the tranny from the Dak as well, so I'll probably run a numerically high rear gear, whether I end up boosting it or not. I have to keep in mind, the car belongs to my 17 year old son, so I'm not sure how much power I really want it to make!
 

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Hmm Interesting! Joe and AJ are awesome members and know theyre talking about. when I first joined ffjbo joe had his car against moparmikes in a show off. Both cars are impecible. The choice of which was the nicest came to a coin toss for most voters. These guys know the deal. Take there advice, you wont be disappointed. If I were building my 17y/o a car, Id leave the 3.9 stock. Its a big enough project stock to swap everything over. Id put fresh bearings and rings in it, valve seals, gaskets trans seals and wire it up. It will be a great car for him as is. Maybe by the time he is 25 you can get a turbo motor together for him to bring his first son home from hospital. But for now, Id just leave it stock and freshen it up before you drop it in. It will be a perfect ride for a young man. Great torque, good power, reliable and different.
Good luck
 

Joe12459

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Hmm Interesting! Joe and AJ are awesome members and know theyre talking about. when I first joined ffjbo joe had his car against moparmikes in a show off. Both cars are impecible. The choice of which was the nicest came to a coin toss for most voters. These guys know the deal. Take there advice, you wont be disappointed. If I were building my 17y/o a car, Id leave the 3.9 stock. Its a big enough project stock to swap everything over. Id put fresh bearings and rings in it, valve seals, gaskets trans seals and wire it up. It will be a great car for him as is. Maybe by the time he is 25 you can get a turbo motor together for him to bring his first son home from hospital. But for now, Id just leave it stock and freshen it up before you drop it in. It will be a perfect ride for a young man. Great torque, good power, reliable and different.
Good luck
Yeah, that's kind of where I'm going with this project. He's driven if for a while with the slant six, now it's time for something different, but still manageable and reliable for him. I was really looking toward the future when I started the thread, just trying to see what others have done with this drivetrain. Step one will just be getting it all together and running. Step 2 will probably be rear gears. Then we'll see where we go from there.
 

AJ/FormS

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Hmm, both ideas sound doable on a reasonable budget. I plan on running the tranny from the Dak as well, so I'll probably run a numerically high rear gear, whether I end up boosting it or not. I have to keep in mind, the car belongs to my 17 year old son, so I'm not sure how much power I really want it to make!
Did you say tranny? No way did you say tranny! Tranny's a bad word around here. I can't believe you said tranny. The last time I said tranny, I got schooled. I didn't even know that a tranny isn't always a tranny. To me tranny was just a quick way to type transmission, and with my one finger tranny, whoops I mean typing, I take all the short cuts I can.Boy I haven't seen tranny in a long time.
So what tranny was in that Dak. Doh! I'm not supposed to say Dak either. If Dak isn't short for Dakota, what else could Dak mean? I mean I've been calling Daks Dak since way Dak,whoops I mean back. Tell me Dak is Dak, and Daks Dat. Please
 

4speedjim

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Did you make a deal with Gator for his cmx? Sounded like a good car. I just couldn't afford it, and didn't want to insult him with what I could pay.
 
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