340 Swap (from 318) Feels Like Same Performance as Before to Me

[Dibbons]

Just normal driving around town (and up to 70 mph briefly) the newly built 340 "seat of the pants" impression feels no different to me than the factory 318. Maybe after break-in with full throttle I will feel the additional 45 horsepower. Vehicle is a '72 Satellite Sebring Plus with factory 904 Torqueflite and 8 1/4 differential with 2.71 ring and pinion (converted to sure-grip).

Factory ratings are:
318 230 horsepower @ 4,400 rpm & 340 ft/lbs torque @ 2,400 rpm
340 275 horsepower @ 5,000 rpm @ 340 ft/lbs torque @ 3,200 rpm

Photo #1 shows complete 340 as purchased and photo #2 shows as rebuilt and now installed.

 

[pnora]

Did you degree the cam?

 

[Dibbons]

Did you degree the cam?

Yes I did. Oregon Camshaft grind #2120 is a hydraulic flat tappet with close to '68 340 manual transmission specs.

Advanced the camshaft four degrees measured:

Intake center line 91.5 + 130 = 221.5 / 2 = 110.75
Exhaust center line 72 + 163.5 = 235.5 / 2 = 117.75

Cam lobe separation (LSA) 110.75 + 117.75 = 228.5 / 2 = 114.25

 

[Rebelrouser]

Just normal driving around town (and up to 70 mph briefly) the newly built 340 "seat of the pants" impression feels no different to me than the factory 318. Maybe after break-in with full throttle I will feel the additional 45 horsepower. Vehicle is a '72 Satellite Sebring Plus with factory 904 Torqueflite and 8 1/4 differential with 2.71 ring and pinion (converted to sure-grip).

Factory ratings are:
318 230 horsepower @ 4,400 rpm & 340 ft/lbs torque @ 2,400 rpm
340 275 horsepower @ 5,000 rpm @ 340 ft/lbs torque @ 3,200 rpm

Photo #1 shows complete 340 as purchased and photo #2 shows as rebuilt and now installed.

View attachment 1993584

View attachment 1993587

View attachment 1993588

More specs are needed on the engines. And you did not mention what year 340 the later ones were low compression.
And it evidently has an aftermarket camshaft, do a cranking compression test if it has much less than 140lbs it may have too much duration for the static compression in the engine.

The 2.71 gears maybe lugging the 340 a little especially if the cranking compression is low.

 

[Dibbons]

The 2.71 gears maybe lugging the 340 a little especially if the cranking compression is low.

1,3,5,7 = 160, 160, 160, 160
2,4,5,8 = 170, 170, 160, 120

I did a leak down test on low cylinder #8 (120 reading) by connecting air hose to spark plug hole @ 100 psi and I could not detect any air escaping from tail pipe, carb intake, nor valve cover breather.

I squirted a bunch of oil in the #8 cylinder (120 reading) and repeated the compression test and got 180! I believe one of the cylinders was previously sleeved (it may have been cylinder #8, I'm still looking in my notes to find out location of sleeve).

At this point the motor has about 2 hours run time. I don't see any smoke whatsoever emitted from the tail pipes and nothing coming from the valve cover breather either. Not sure about compression but re-used the cast .040 over pistons with four valve reliefs that sit a little above the deck (see photo). The motor fires up instantaneously on a cold start and stops just as quickly which indicated to me either good or high compression (or both). However, the idle is not as smooth as I would expect. Still not sure if I didn't wipe a camshaft lobe during break-in period.

 

[Dibbons]

Update: I haven't noticed any pinging during normal driving. I need to recheck timing (would like to see 35 degrees total with vacuum advance disconnected), adjust carb idle screws some more (1972 factory 340 Thermo-quad), and remove valve covers and check for valve lift (to rule out wiped camshaft lobe possibility).

 

[Dibbons]

Update: Here is a thread I posted on FABO regarding my suspicions of a sleeve in cylinder #8. What I did not mention that when I honed all of the cylinders with a 400 grit ball hone, the #8 cylinder ended up with a different looking finish (different color and different reflective property):

Looks like cylinder #8 has a sleeve?

 

[autoxcuda]

2.71 gears
both motors have same short stroke.
Just driving around town.

Go to a freeway on-ramp and floor it. And/or manually shift it at 5,000 rpm.
Should feel a difference.

 

[Geoff 2]

Rebuilt engines [ rings etc ] can take some mileage to bed down....& make a few more hp. After a few hundred miles, you will likely notice the subtle difference. Whether or not it is the increase you are looking for remains to be seen.
The 114 LSA cam is NOT helping & is giving away free hp.

 

[1 Wild R/T]

1,3,5,7 = 160, 160, 160, 160
2,4,5,8 = 170, 170, 160, 120

I did a leak down test on low cylinder #8 (120 reading) by connecting air hose to spark plug hole @ 100 psi and I could not detect any air escaping from tail pipe, carb intake, nor valve cover breather.

I squirted a bunch of oil in the #8 cylinder (120 reading) and repeated the compression test and got 180! I believe one of the cylinders was previously sleeved (it may have been cylinder #8, I'm still looking in my notes to find out location of sleeve).

At this point the motor has about 2 hours run time. I don't see any smoke whatsoever emitted from the tail pipes and nothing coming from the valve cover breather either. Not sure about compression but re-used the cast .040 over pistons with four valve reliefs that sit a little above the deck (see photo). The motor fires up instantaneously on a cold start and stops just as quickly which indicated to me either good or high compression (or both). However, the idle is not as smooth as I would expect. Still not sure if I didn't wipe a camshaft lobe during break-in period.

View attachment 1993600

Blow up the picture, it's clear as day...

 

[RemCharger]

Factory ratings are:
318 230 horsepower @ 4,400 rpm & 340 ft/lbs torque @ 2,400 rpm
340 275 horsepower @ 5,000 rpm @ 340 ft/lbs torque @ 3,200 rpm

Would be interesting to see the 340 tested at the same rpm 4400/ 2400 respectively .and then hook that up to the 2.71.

Go for about 50mph in first gear. See if it pulling.

 

[1 Wild R/T]

This is a 72-73 piston

This is a 68-71 Piston.. Notice the chamfer around the top of this piston? That is because these pistons sit slightly higher than the deck... The chamfer assures the piston doesn't make contact with the head gasket...

He has the early style pistons... They look like they were recovered from the Titanic but they are early pistons..
FWIW 340's are only 22 cubic inches bigger than a 318... And that cam isn't exactly a torque grind... 340's like RPM's 2.71 gears are not your friend when running a motor that likes to spin....

 

[Runcharger]

Yes: Around town below 3k it will feel doggy with 2.76 gears. Above 3k it should wake up if its healthy. All my 340s ran 3.91 in A bodies with 4 speeds, thats where they really shine.
360s were known to be doggy when new but only because they were low compression. Too late now but IMO the longer stroke of a 360 would work better with your freeway gears, probably stock convertor and heavier B-Body.

 

[Bighouse]

With those 2.71’s it should really perk up right around the time your starting to go faster than you feel safe going.

 

[71Beeper]

I agree with the rest of the group.....those 2:71's are not doing you any favours.
Plus your B-body is heavier than the A-bodies you have.
3:55's or 3:73's will wake it up real good.

For shits and giggles find a set of small dia 14" tires and try them on the rear....

 

[Dibbons]

The last two road trips (318) were 3,000 miles or so (Mexico to USA and back) which is where the 2.71's help out a little with the mpg.

 

[1 Wild R/T]

The last two road trips were 3,000 miles or so (Mexico to USA and back) which is where the 2.71's help out a little with the mpg.

Well than honestly the 318 or a 360 is a better match for the car.... 340's make horsepower, not a lot of torque....

 

[bigbluedodge]

Intake center line 91.5 + 130 = 221.5 / 2 = 110.75
Exhaust center line 72 + 163.5 = 235.5 / 2 = 117.75

I'm not familiar with small blocks but are these numbers actually advancing the cam? Maybe I'm not thinking right but 110 past tdc seem retarded.

 

[R413]

How many miles on it so far?
What kind of rings did you use?

2.7 gears will hold it back at low rpm.
Jump on a 10 speed that's in 10th gear and take off from a stop. No torque, the gears killed it. A stronger rider will not have much better results.

 

[#41]

2.71 is the net multiplier of your 45 extra HP. I'm not saying that you might not need to hunt around and tune the new engine to get the most of it, but a simple swap to 3.23 or bigger will change your viewpoint significantly.