340 Performance Question

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Fenix13

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Hi guys I've got a potential buyer a 340 engine I have, and the member is asking what kind of performance numbers should be expected. I was expecting to make between 350-400 HP from the build.

Since it's never been ran, I wanted input from some of you folks more engine savvy than me, to make sure I don't over promise:

It's a 72 340 bored .04, with the stock 1.88 heads. Heads were cleaned, mag, valve job, surfaced.

Springs 100# a seat, 240# @ .460 lift.

It's got the following:

  • Keith Black Performance Pistons - KB243-040 Hypereutectic pistons
  • Mopar Perf. ''Purple Shaft'' Hydraulic Camshaft Kit: Camshaft, Hydraulic Flat Tappet, Duration 268/272, Lift .450/.455
  • 1.5 Rockers
  • Ede RPM Performer intake manifold
  • Summit Racing 850 CFM carb

Thanks for any insight guys.
 
325-340 HP,365-370 torque,(Just an personal guess, with non ported 1.88 castings.) The better piston & cam choices,should get close( with decent machine work) Depending on price,a decent combo....(Just my opine...)
Pocket porting,radius valve job / 2.02 " installed correctly" would make this one fun combination. Just giving options,for you to offer....
 
X2. In my opinion that 850 cfm carb is way to big for that combo.
 
well the stock 71 340 was closer to a true 310hp and most were not the true 10.3cr . That said with the better intake, bigger cam, 1.5 rockers?, and hopefully good machine work on the block and heads i would say 350 HP give or take 5 hp THAT'S WITH HEADERS!!
 
350-400..Not with that camshaft,lucky if it goes 310 h.p....
 
Your buyer should have brains enough to know that torque is what he should be interested more in. If all he wants is horsepower figures, find a smarter buyer.

That engine as is would probably make around 350-360 LB FT of torque. That's pretty good for a street engine. Plenty enough to melt tires.
 
I won't make any guesses but this is what Wallace said for my weight/mph for real hp figures in the 1/4, may give you an idea.....

motor spec.>

Stock 1971 340, 10:1CR?, 2.02's
Strip Dom intake, 850DP
1.75" race hdrs
DC .590 solid flat tappet

3550lbs all in, ran 12.4@108mph = 365hp fly N/A

This was a home re-built motor with fel-pro blues and no precision machine work decking or 3 angle valve jobs etc.
 
EL5DEMON340 said:
well the stock 71 340 was closer to a true 310hp and most were not the true 10.3cr . That said with the better intake, bigger cam, 1.5 rockers?, and hopefully good machine work on the block and heads i would say 350 HP give or take 5 hp THAT'S WITH HEADERS!!
Click to expand...

Recently I have noticed for the first time there is mention/hint in some of the posts that the stock Mopar rocker ratio is less than 1.5, since when? Where have I been all these years? I began with a 1965 273 with solid lifters and the factory adjustable rocker arms (back in the 1970's), where they or were they not a 1.5 ratio? I never thought to check them back then! Thank you.
 
goldduster318 said:
Carb is way too big for one...that thing needs about a 650 DP.

They built an engine basically just like this in Mopar Muscle but with the heavy TRW pistons and heads with 2.02 valves and it made right around 330hp in trim most similar to that.

http://www.hotrod.com/how-to/engine/mopp-0102-1970-340-high-performance-mill-dyno/
Click to expand...

Reading further down that article, they add a rpm manifold with a 800cfm Eddy carb and with headers its at 355hp. This is a stock 340 with stock cam 2.02 heads. OP engine does have the smaller 1.88 but bigger cam. The engine in that article makes over 390hp with a cam that has only a little more lift than the OP engine, I know the dyno they used in a lot of articles seemed to be a bit "happy". I know if I only got around 300hp from a 340 with a good Intake, and a 268 cam, with headers I'd be looking to see if I crossed a couple plug wires.
 
Might crack 300hp , depends on the dyno.
 
That hot rod article and the fantasy 391hp 268H cammed 340...

There have been a bunch of stock 2.02 J headed XE268H 340 mills I've been around and not one ever broke 335hp. Most were in the 325 range.
 
crackedback said:
That hot rod article and the fantasy 391hp 268H cammed 340...

There have been a bunch of stock 2.02 J headed XE268H 340 mills I've been around and not one ever broke 335hp. Most were in the 325 range.
Click to expand...

I know I left some hp on the table with my build back then with the (TRW replacement pistons we used)...heads were untouched, just valves re-lapped and all, no porting but we re-balanced it and put it together well as in the bores sealed well, spring heights all the rest, but you gotta admit track hp is gotta be closer than that dyno, so they made 30 more hp with a cam with .130 less lift?. Our car hooked as you can see by the ET/mph so no silly mph to bump the hp no's up.

We were repeatedly told that our 850 was too big by other racers in our pits and as in the posts here suggest....so to shut them all up I put a 750DP on the car and we jetted it up a bit as well to make it a fair test and we went 2/10ths slower,...in fact, I persuaded one of the guys that ran a 215ci V8 to try my 850 over his 600 carb and what do you think that did....went 2/10ths quicker, ok it was a manual and he did rev it but come on an 850 on 3500cc...yeah the throttle response was crap but, so big carbs don't work on small motors then!
 
crackedback said:
That hot rod article and the fantasy 391hp 268H cammed 340...

There have been a bunch of stock 2.02 J headed XE268H 340 mills I've been around and not one ever broke 335hp. Most were in the 325 range.
Click to expand...

What are the specs on this cam?
 
67autocross said:
What are the specs on this cam?
Click to expand...
Must be smaller than the 268 cams used in Chevy 350's and ford 351w, my buddy's 69, 351w made 346hp with a comp 268 cam, rpm intake, 750 Holley, with the stock heads light bowl work 1.94 valves. Those heads even with the bigger valve and bowl blend probably flow less then a 1.88 mopar head, not to mention the flat intake port along with the flat intake manifold runners.. The carb is about even with the top of the valve covers LOL
 
OP cam? If so pretty good amount of duration at .050, and a 110 lsa, should have a lumpy idle in in his 346ci SB!!
Summit Racing Part Number:DCC-4452761
Cam Style: Hydraulic flat tappet
Basic Operating RPM Range: 1,500-5,800
Intake Duration at 050 inch Lift: 228
Exhaust Duration at 050 inch Lift: 231
Duration at 050 inch Lift: 228 int./231 exh.
Advertised Intake Duration: 268
Advertised Exhaust Duration: 272
Advertised Duration: 268 int./272 exh.
Intake Valve Lift with Factory Rocker Arm Ratio: 0.450 in.
Exhaust Valve Lift with Factory Rocker Arm Ratio: 0.455 in.
Valve Lift with Factory Rocker Arm Ratio: 0.450 int./0.455 exh.
Lobe Separation (degrees): 110

Much hotter than the comp 268 and others that have about 218 @.050
 
EL5DEMON340 said:
Must be smaller than the 268 cams used in Chevy 350's and ford 351w, my buddy's 69, 351w made 346hp with a comp 268 cam, rpm intake, 750 Holley, with the stock heads light bowl work 1.94 valves. Those heads even with the bigger valve and bowl blend probably flow less then a 1.88 mopar head, not to mention the flat intake port along with the flat intake manifold runners.. The carb is about even with the top of the valve covers LOL
Click to expand...


I was wondering about the specs on the 268H cam that hot rod used in the 340...sorry
 
Thanks for all of the info guys!

It is a thermoquad carb, and I messed up it's an 800 CFM.

Just want to make sure I give the best info I can on the motor, when we were sizing it up my mechanic and I must have been a little more optimistic.

Thanks again guys!
 
Well then the TQ isn't to large being a spreadbore, it's nice and flexible.
 
crackedback said:
That hot rod article and the fantasy 391hp 268H cammed 340...

There have been a bunch of stock 2.02 J headed XE268H 340 mills I've been around and not one ever broke 335hp. Most were in the 325 range.
Click to expand...

I followed that build to a T. HP ended up being 351. TQ was 391
 
..that brings to mind the 383 vs chevy stroker video.They reported the
383 mopar had a cam and intake change and went from stock 335 hp
to about 450 or something....hmm.
 
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