How to get 400 hp out of 1971 340 .30 over

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Jays71 demon

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I have 1971dodge demon 340 that is .30 over flat top kyb pistons ,comp cams 268/280 ,750 double pumper, edelbrock rpm air gap port matched and headers 3.73 gears and with automatic. My engine builder said it might make 400hp but he said it's hard to get that out of 340 unless you stroke it. Why is this so hard.
 
That's what I thought. I mean Steve Dulcich can get that out of 318 wtf . I thought it would make over 400 hp . I guess I will find out when I take it out when weather gets better. Anybody have a guess
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Jays71 demon said:
My engine builder said it might make 400hp but he said it's hard to get that out of 340 unless you stroke it. Why is this so hard.
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It's not. He's just taking the easy way out, that will make him some more money. Sounds like an LS guy- find a different builder.
 
To get 400 HP out of a 347" (340 +.030") requires that it makes 1.153 HP per cubic inch. However if you had 416" engine you only need to make .96 HP per cubic inch to get 400 HP.

That said it all boils down to the amount of CFM your heads flow as to what kind of HP you can extract out of a given cubic inch.

Tom
 
My builder got 400HP for me with a stock stroke 340, Edelbrock heads and a 650 carb.
This was just to run and break in the engine with really no tuning for max power.

1709257843494.jpeg
 
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I’m right at 400 out of my 340 .030 over with lightly ported 2.02 J heads, 10.2-1, air gap, a similar cam, Comp magnum roller rockers, tti headers, 750 Holley DP. It’s not hard, no crazy parts necessary.

New engine builder ☑️
 
Jays71 demon said:
I have 1971dodge demon 340 that is .30 over flat top kyb pistons ,comp cams 268/280 ,750 double pumper, edelbrock rpm air gap port matched and headers 3.73 gears and with automatic. My engine builder said it might make 400hp but he said it's hard to get that out of 340 unless you stroke it. Why is this so hard.
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For certain! Find another engine builder.

It’s NOT hard. Not at all, not one bit. He’s full of sh#%!!!

Edelbrock has there “Power Package” using there parts. The 330 used in there testing using all of there parts made 417hp. I went to the Edelbrock web site and copied this below. All you need to do is have a 10.5-1 - 340 short block ready to go and install there parts and get headers for the car.

Here are a few links to Summit Racings web site for the power package and two cams. First cam is the expensive RPM cam, second cam is the crazy expensive “Rolling Thunder” cam. But it will at least give you an idea of the camshaft size needed to make the 400+hp (RPM CAM) and/or the roller will make more power without increasing the lower RPM range to do so.

Expensive but worth it in the whole package and the fact it’s a zero brain effort build.

Power Package
DODGE Edelbrock 2049 Edelbrock Total Power Package 417 HP Small Block Mopar Top-End Engine Kits | Summit Racing

RPM cam
DODGE Edelbrock 7177 Edelbrock Performer RPM Cam and Lifter Kits | Summit Racing

Rolling Thunder cam
DODGE Edelbrock 2213 Edelbrock Rollin' Thunder Hydraulic Roller Camshafts | Summit Racing


IMG_2044.jpeg
 
Is there a reason that you want or need 400hp?
I mean 325>350 is already a ton-O-fun on the street,
and
with only 3.73 gears, you'll need some bottom end torque, which will go away with a bigger cam....... requiring a higher stall ....................
What you didn't mention is your convertor's stall speed, which with a 340 and 3.73s, will likely need to be over 3000.

As for a bigger cam, each 6 or 7 degree bigger cam, moves the power up about 200 rpm, and steals cylinder pressure off the bottom to get it. Your 268 cam is probably already peaking at or near 5000 rpm.......... which with 3.73s
is 40 mph in First, and 67 mph in Second.
Is that what you want?
Or are you like me, I want the power from;
take-off/zero mph, to ~40/50 mph, not very often after that. If that, then
you will need one of several things;
1) a higher stall
2) more Torque-Multiplication, either in the back or in the transmission
3) more cylinder pressure
4) a lighter car
5) a longer stroke
6) or some combination of those.
7) or ..... forced induction.

The purpose of ALL OF THESE is to increase the ratio of; torque at take-off to pounds of car. Leave the spare tire in the garage ............................
 
2,02 intake and a little bowl blending and the next size up cam will do it with room to spare
 
Here's the dyno of that 340, stock on the left fully modded on the right, modded gains from 3000-6200 rpms (everywhere dyno'd) and a 50 lbs-ft gain right at 3000 rpm, by the looks of it probably some gains under 3000 rpm.

Phase 1: Stock 340 baseline, Chrysler electronic ignitionPhase 2: Replace small production AVS with Edelbrock 800 CFM carbPhase 3: Replace factory HP manifolds with Hooker 151/48-inch headersPhase 4: Replace factory iron 340 HP intake with Edelbrock Performer RPM Air GapPhase 5: Replace SSI springs and retainers with Engle #604LS retainers and #993 springsPhase 6: Replace stock 340 HP cam with Comp Xtreme Energy 268

https://www.motortrend.com/how-to/mopp-0102-1970-340-high-performance-mill-dyno/
Horsepower
RPMPhase 1Phase 2Phase 3Phase 4Phase 5Phase 6
3000185.2195.5199.6206.4206.2214.7
3200195.4206.1213.9220.7220.8229.3
3400208.9224.6233.0240.6240.5251.5
3600224.8238.7252.7260.8259.9275.7
3800236.4253.7266.6277.0276.9298.1
4000249.5267.6280.0291.8291.8311.7
4200259.3278.9292.9306.0306.2327.1
4400265.0287.1304.3321.3321.3344.6
4600268.8297.2313.3335.2334.7355.9
4800274.7306.8321.1343.1342.5366.7
5000278.5312.7329.1352.6350.4377.0
5100280.2316.9330.6354.5352.4381.3
5200281.3316.9330.8353.6353.6384.3
5400273.7314.9329.3350.8355.4386.1
5500273.5313.9330.4350.7355.9387.4
5600269.3314.5330.2350.9353.5389.4
5700312.7329.3348.6353.3391.6
5800308.8326.2347.6351.4387.2
6000329.2353.4383.7
6200314.8373.7

Torque
RPMPhase 1Phase 2Phase 3Phase 4Phase 5Phase 6
3000324.2342.3349.5361.4361.0375.8
3200320.7338.3351.1362.3362.3376.4
3400322.7347.0359.9371.6371.5388.4
3600328.0348.2368.7380.4379.1402.2
3800326.8350.7368.4382.9382.7412.0
4000327.6351.3367.6383.1383.1409.3
4200324.3348.8366.3382.7382.9409.0
4400316.4342.7363.2383.5383.6411.4
4600306.9339.3357.7382.7382.1406.3
4800300.6335.7351.4375.4374.8401.3
5000292.6328.5345.7370.4368.1396.0
5200284.1320.1334.1357.2357.1388.1
5400266.2306.3320.3341.2345.6375.5
5600252.6295.0309.7329.1331.7365.2
5800279.6295.4314.8318.2350.6
6000288.1309.3335.8
6200266.7316.6
 
100% correct!
1709234895513.jpeg



AJ/FormS said:
Is there a reason that you want or need 400hp?
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Mo-Power is better…. LOL
AJ/FormS said:
I mean 325>350 is already a ton-O-fun on the street,
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Ye that’s a little bit of a spirited driver.
AJ/FormS said:
and with only 3.73 gears, you'll need some bottom end torque, which will go away with a bigger cam....... requiring a higher stall ....................
What you didn't mention is your convertor's stall speed, which with a 340 and 3.73s, will likely need to be over 3000.
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Would this and…
AJ/FormS said:
is 40 mph in First, and 67 mph in Second.
Is that what you want?
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And this also be a tire size dependent issue?

How do you calculate 40mph & 67mph w/3.73 gear?
At what RPM & tire size. The OP didn’t give the tire size and you didn’t give the RPM @ which this occurs.

I feel this is important. Not that your intending to be misleading.
But without a tire size and RPM…. There is a big difference between a stock tire diameter & 28 inch tire diameter.
At what and how do you calculate converter slip?
 
125+hp from bolt-ons? The 340 must have been poorly designed or seriously under rated.
 
1MeanA said:
125+hp from bolt-ons? The 340 must have been poorly designed or seriously under rated.
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I don't get that why wouldn't it? 340 comes with decent heads and cr the two main things you require in a good shortblock, everything else to get there is basically a bolt on.

We basically know you can get over 2hp per cfm, on a healthy street car it's more like 1.7-1.9 hp:cfm say 1.8 hp:cfm as an easy obtainable goal = 396 hp.
 
There was a big issue with the Motortrend article/test as was pointed out by others in older threads. They gained 35 hp by bolting on a carb. This means there was an issue with the old carb most likely. Thus, the baseline hp should have been around 300 or perhaps the whole test is questionable.
 
1MeanA said:
125+hp from bolt-ons? The 340 must have been poorly designed or seriously under rated.
Click to expand...
LOL, well, the cylinder heads being as they were for the day did t flow very well and the intake was OK and just fine for the daily driver duties it was designed to do but, WOW! Imagine what a LD-340 on top would have done with a better head.

Alas….. the factory has to do things on the cheap and what’s on hand. Bummer huh?!?!
 
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