Misunderstood Mopar tech 400, Thermoquad etc.

[The Professor]

I was thinking today about buying a mid/late 70’s Mopar to add to my collection and that got me to looking at the 400. I’ve read a few articles on it and think it’s got the makings of a really good engine.

I think it’s deeply misunderstood by a lot of Mopar folks and completely unknown to GM and Ford troglodytes. Most Mopar loyalists think of it as a “smog” engine and as such they discount it.

Looking at the 400’s specs like the bore/ stroke, rod ratio etc, it’s obvious to me it really can be quite the monster. It was burdened with a horrible cam from the get go hence its smogger reputation. It replaced the much loved 383 in a period where many thought that it did so because it was a weaker motor created to meet smog standards.

The 400 needs a real cam and some tweaked heads but the bore and stroke make for a quick revving engine. It has a 4.34 bore which is bigger than the 440 but its stroke is shorter at 3.38 vs 3.75. So it has the makings of a revy lil beasty.

The 400 was saddled with a small cam, a misunderstood carb (thermoquad) and a restricted exhaust, all things that are easily fixable. The heads are meh/ok but they can be woken up or better Mopar/aftermarket heads can be used.

Want a stroker? Put a 440 crank in it and you have a 451! Bore it 30 over and you are pushing 460 cubes of sweet Mopar justice! Add some 440 source stealth heads and a cam and your at 500 horsepower easy.

I use Hughes cams only, they are a Mopar only outfit and they understand that Mopar uses a 906 lifter that allows for a more aggressive ramp rate. Most cams are designed off of a Chevy grind so you are cheating your Mopar of free HP/TQ by not using a Hughes cam.

Anyway my opinion and 50 cents might get you a cup of coffee but there ya have it.

As always, Get Real….Get Mopar

 

[Geoff 2]

The short stroke 383/400 could have been a real screamer......if Chry had put some decent flowing heads on it.

 

[DanielSlocum]

The main problem w the stock 400 is the pitifully low compression ratio. Was advertised at 8.2,but doing the math reveals its actually only 7.7 .
Bringing it up to at least 9.0 really wakes it up. By the way,most stock advertised compression ratios are false and actually .5 to 1.0 ratio less.

 

[HawkRod]

Yes, awhile back the 400's were "discovered" and are recognized now to be a great building block (pun intended) for an excellent engine.

I have a 383 Road Runner. I took out the original, untouched numbers matching 383 to protect it and replaced it with a stroked 400 displacing 500 cubic inches, roller cam, . Back in the car, it still looks (mostly) like a stock 383 but it has torque galore. Awesome motor!

 

[Timmayy]

One of the things I remember hearing in my youth was that the 400 had thin walls and an overbore would cause over heating problems. Is this true or just an old wives tale?

 

[YY1]

I don't think so.

"Thinwall" casting was an engineering "improvement" but IIRC did not affect over boring.

If that were true, the 451 recipe wouldn't work.


The other issue most had was a pitiful final drive gear.

2.4x was pretty common as was 2.76.

I had a 77 400-2 from a van, and even with the 2 barrel, when I put it in my 70 Super Bee as a stand in engine, it screamed with the 3.55 gears.

The "problem" with 400 builds is finding pistons to get it to 9.x :1 that don't cost an arm and a leg.

BTW- if you don't over bore, and just use a stock 440 crank, IIRC you get a "444".

 

[bee71]

I had a 400 in my 76 Cordoba which was never going to be used as a street racer. It was very dependable especially when twice the water pump let go and sent the fan into the radiator. Both times I nursed it home and the engine was glowing when I opened the hood. Put it back together both times and it started right up. Nine years and over 200,000 miles I had to give it up due to MI winters and salt.