340 4 barrel and 340 6 barrel: were they really UNDERrated?

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Kern Dog

Kern Dog

Build your car to handle.
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Mopar Muscle magazine August 2017:

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While looking through this, I ran across this article:

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The text of the article stated that with this test engine, they claimed to use the stock cam, stock compression ratio but roller tipped rocker arms…

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They show 320 HP compared to the factory rating of 275 on the 4 barrel.
Really? The 68-70 440 Magnum had 100 cubes on the 340 and has been found to actually register between 335-350 Hp despite the 375 HP rating.
Taking it further, they swapped on a 6 barrel induction and scored 356 HP.
Again, really ??

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I ask this because it seems exaggerated to me.
I’m building a 1990 360 with compression in the mid 9s, #308 heads, 1.6 ratio rockers and a Hughes roller cam with .544 lift, approximately .100” more lift than a 340 cam. I’ll be running a Holley 750 and 1 5/8” headers but I can’t imagine that I’d be at 376 HP with my combination. Their claims seem too high.

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I've always thought it was funny that the 370HP 350 Chevy was built about the same as the early 340s. Only 10 more cubes and 370HP. It did have a solid lifter cam and .5 point more compression, but that's not all that different.
 
Problem is so far as I've seen anyone whose dyno a stock 340 there never a 100% stock.

To me it's not hard to believe that a 100% stock 340 would dyno more than it's 275 hp rating. especially in dyno trim, usually means, no accessories, headers, dyno carb max tune probably gonna be mid 300's. I could see the 275 hp actually being it's net rating or pretty close to it.
 
RustyRatRod said:
I've always thought it was funny that the 370HP 350 Chevy was built about the same as the early 340s. Only 10 more cubes and 370HP. It did have a solid lifter cam and .5 point more compression, but that's not all that different.
Click to expand...
Richard did a dyno comparison of the high hp Chev's 302 vs 327 vs 350 and all made about 355 hp with headers.

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If they used the stone stock cams in the Ford engines, they were handicapped by retarded camshaft timing. They didn't specify.
 
Don't know about being underrated. My 340 Cuda with cam and bolt ons would lose by a car length to my best friend's 383 Super Bee with headers every time. Both had 323 gears. Looking back, I think the Torker 340 manifold was a mismatch to the cam.

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Old articles out there claimed chrysler would periodically pull '68 - '70 340 off assembly line and put them on engine dyno as part of quality control. Its was claimed that they would usually see somewhere between 300 - 320 hp on the dyno.

Keep in mind the chrome exhaust tips had internal diameter around 1 5/8 inch or so, driver side head pipe had giant pinch to clear torsion bar, crush bent exhaust pipes etc. So once installed in car I would imagine a significant power loss compare to what was seen on engine dyno.
 
I just had a .030 over 340 on my dyno.

Basically stock except from a criminally small MP SFT cam with .508 lift.

To help with that I used 1.6 rockers which took net lift to a paultry .542 lift.

It is a measured 10.48:1 with a Strip Dominator.

That’s it. 2.02 valves. A clunky 5/64 ring pack. And no tuning.

It made right at 400 hp at 5600 rpm. Like I said, with almost no tuning.

And the distributor was locked out. You can add 15-20 hp to max power with the correct timing curve.

NHRA thought the 340 because they factored the hell out of it.

They are horribly underrated.
 
A good tune on a stock 340 would wake them right up. The FSM had the initial timing at 5 or 7 degrees or something like that? Probably where the ‘under rating’ was.
 
NHRA horsepower factors don't go off of thought they go off of runs below the index, runs below the trigger point, and bi-annual average under the index for the combination.

And those NHRA stock eliminator engines are FAR from stock. They have had solid lifters and adjustable rockers for years. Electric water pump, fuel pump, tiny Nippon Denso type alternators. Then any valve spring. Stainless steel step headers. Then Roller rockers came in, fuel cell, aftermarket racing seats, plenty of friction reducing drivetrain parts, etc etc. the list can go on and on for those with a big budget. Lots of technology into the engine and entire car. Ever listen to one idle, is that a stock camshaft? Hell no.

A 72-73 340 Duster can run in G/SA which has a 12.00 index, and several cars can run 10.90 how stock is that? It isn't!

These guys are within the rules, they pass tear downs. The 350/255 horsepower Camaro and Nova cars are in the 10's also.

440+6 Cudas, 396 Camaro, they run high nines. Far from stock engine/vehicle guys.

Many of you know this, most do not.

@GTX JOHN could add to this but today he and Jon are wrangling around 4 cars and setting up at the Vegas 4 wide nationals.
 
This guy on youtube tested a supposed stock rebuild 340 with manifolds and it came out pretty good... I believe he said all stock.. i have a hard time listening too him

(380tq/350hp) so you don't need to click it

 
Icetech said:
This guy on youtube tested a supposed stock rebuild 340 with manifolds and it came out pretty good... I believe he said all stock.. i have a hard time listening too him

(380tq/350hp) so you don't need to click it

Click to expand...

1972?

*pushes up glasses*

well... actually... that's not representative of a true 340. besides that fact that the 1968 manual transmission cam was notably engineered specifically for the motor and by april of 1970 the pistons got heavier and compression--

*gets thrown into a trash can*
 
I bet a modern build has a blueprinted and verified compression ratio.

I bet a 1970-ish production engine does not.

I bet that could explain a LOT.
 
Newbomb Turk said:
I just had a .030 over 340 on my dyno.

Basically stock except from a criminally small MP SFT cam with .508 lift.

To help with that I used 1.6 rockers which took net lift to a paultry .542 lift.

It is a measured 10.48:1 with a Strip Dominator.

That’s it. 2.02 valves. A clunky 5/64 ring pack. And no tuning.

It made right at 400 hp at 5600 rpm. Like I said, with almost no tuning.

And the distributor was locked out. You can add 15-20 hp to max power with the correct timing curve.

NHRA thought the 340 because they factored the hell out of it.

They are horribly underrated.
Click to expand...
Tell your wife you need another knock in the head. A HARD one.
 
junkyardhero said:
1972?

*pushes up glasses*

well... actually... that's not representative of a true 340. besides that fact that the 1968 manual transmission cam was notably engineered specifically for the motor and by april of 1970 the pistons got heavier and compression--

*gets thrown into a trash can*
Click to expand...
It couldda been. About the first half of 72 still had the forged crank and good compression.
 
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