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

[Kern Dog]

The 2.71 gear in an 8 1/4” axle is not a simple change like going to a different gear in an 8 3/4” axle.
This is a daily driver gear in a 3500 lb car. Torque is needed to get a heavier car moving. I don’t see how an engine will make more power after it breaks in. Better fuel economy and reduced oil consumption yes but power increase? I don’t see it.
A big block swap does mean a different transmission, drive shaft, exhaust from engine to H pipe, some slight changes to wiring along with almost everything attached to the engine. The P/S pump will interchange but the mounting brackets won’t. The distributor cap is the same, the distributor isn’t.
I agree though that even a 5.9 Magnum/360 with the longer stroke would be a lot easier to do. I built a 1990 roller cam 360 for a 67 Dart that runs like hell but it is a lighter car with a 4.10 gear.

 

[RemCharger]

The 2.71 gear in an 8 1/4” axle is not a simple change like going to a different gear in an 8 3/4” axle.
This is a daily driver gear in a 3500 lb car. Torque is needed to get a heavier car moving. I don’t see how an engine will make more power after it breaks in. Better fuel economy and reduced oil consumption yes but power increase? I don’t see it.
A big block swap does mean a different transmission, drive shaft, exhaust from engine to H pipe, some slight changes to wiring along with almost everything attached to the engine. The P/S pump will interchange but the mounting brackets won’t. The distributor cap is the same, the distributor isn’t.
I agree though that even a 5.9 Magnum/360 with the longer stroke would be a lot easier to do. I built a 1990 roller cam 360 for a 67 Dart that runs like hell but it is a lighter car with a 4.10 gear.

Post #9 is correct. How long it takes, and how much it changes, varies. But we generally don't use cast iron top rings anymore , and that has shortened things up.

 

[R413]

I don’t see how an engine will make more power after it breaks in. Better fuel economy and reduced oil consumption yes but power increase? I don’t see it.

It is absolutely true, happens all the time. Especially with an at home re-ring job.

Now better fuel economy and reduced oil consumption means the rings are seating and friction is reduced.

How can it not make more power after the rings seat.

 

[Dibbons]

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

Maybe 50 miles on engine (but mostly just idling during break-in, warm-ups before tuning carb and timing adjustments).
Made several trips to gas station (5 mile round trip) and one trip to Sam's club (about 15 mile round trip).

Ring Set Mahle 40787
top ring: cast iron with Plasma-Moly coating/facing
second ring: cast iron
oil ring: stainless with chrome facing

 

[66 Sat]

You need to break the engine in properly to seat the rings. Load the car up as heavy as possible, passengers, luggage whatever, and take it out for some spirited drives, up hill and down (if there are any hills where you live).
Accelerate strongly in the correct gear, don't lug the engine in too high a gear. Coming down the hills, or decelerating on the flat, leave it in a lower gear and compression brake.

 

[gtxrt]

very bad cam 14 degree spread between intake and exhaust plus on a 114 LSA even worse it's in a small block.

 

[Dibbons]

Update: Today began inspection for one or more wiped camshaft lobes. Pulled spark plugs (look OK) and took a quick video of both sides of the valve train.

 

[steve340]

If you only do short trips with no load the engine/rings will not break in correctly.

 

[Milo]

I thought 340 was for lighter A-body cars

 

[Dibbons]

Update: I tried using dial indicator on a couple of intake valves and turning the motor over by hand with a breaker bar. However, it seemed the hydraulic lifters were not pumped up or were bleeding down faster than I could record measurements.

In the end I just put a dial caliper depth gauge on top of the valve spring retainers and quickly measured the spring height closed and open. It was easy to measure accurately when the valve was closed but tricky when it was fully opened.

In the end, I only checked number six and eight intake valves because at first one appeared to be opening much less than the other. Four measurements closed: 4.44, 4.46, 4.54, 4.62 cm and four open: 3.52, 3.65, 3.69, 3.7 cm. Both valves showed the same range differences so I doubt we have a wiped cam lobe.

The Oregon (#2120) camshaft intake lift is advertised .438" so taking the longest (valve close) and shortest (valve open) measurements I come up with 4.62 - 3.52 cm. = 1.1 cm or .433"

Then again, taking the longest (valve closed) and least short measurements (valve open) I come up with 4.62 - 3.7 = .92 cm or .362". I assume this measurement was before/after the valve was fully open.

 

[Dibbons]

I thought 340 was for lighter A-body cars

It was offered mid-year for the 1971 Plymouth Road Runner (my parents special-ordered a '71 383 Road Runner that year before the 340 was available).

 

[Dibbons]

Update: Took the 340 out for its first real "road trip" today--a 90 mile round trip from La Paz, Mexico to the outskirts of Todos Santos, Mexico where I ate a bowl of menudo (tripe) for brunch at "La Garita Restaurante". Ran great cruising at 2,500 rpm or almost 70 mph (about 15.1 A/F ratio on the AEM gauge-but at idle closer to 14.0 A/F). The idle is still somewhat rough (900-1,000 RPM) for some reason (stalled one time in drive).

 

[1967coronet]

It was offered mid-year for the 1971 Plymouth Road Runner (my parents special-ordered a '71 383 Road Runner that year before the 340 was available).

340 , bench seat, column auto, 3:23 white interior, black ralley dash.

 

[polyjohn]

Wow you are a lucky guy Mr, Dibbons indeed.
One of Mopars finest B-body's and wall-to-wall sunshine.

Yep the '72' 340 is just a big 318 after 1970.
I did this conversion in a 1970 Dart for a Moparmate.

He ran it down the 1320 and it ran consistant 15.20's.
To wake a cast-crank 340 up you need more comp and more cam.

But the biggest improvement will be some 3.23 gears methinks.
Your beautiful Mopar needs a 727 and a 8-3/4.

I am lucky too as my 71 GTX has a 440 six-pack.
You will be getting twice as much gas-miles as me, maybe three times more!

 

[Dibbons]

Yep the '72' 340 is just a big 318 after 1970.

The 340 I have was purchased used in Sacramento, California. It was hodgepodge machine from the start with parts and pieces from various years:

It had an orange '68 intake manifold and a Carter AVS from a 440 (low performance). I replaced it with a '73 intake manifold (w/electric choke well) and a '72 Thermo-Quad for a 340 automatic (6139S).

It had some rust inside which made the performance purple shaft camshaft useless (probably was the 292/509 version) So I went with a new Oregon Cam grind #2120. I replaced the rusty non-adjustable rocker arms with the rebuilt/bushed 273 adjustables. I used Lunati Micro-Trol hydraulic lifters.

I purchased pushrods from Smith Brothers.

It had a forged crankshaft but without the holes in the throws like a 340 crank should have. I suspected it was a 318 crank so I purchased/installed a real forged 340 crankshaft that we turned (.020 under) with all new bearings.

The cylinder heads had the 1.88 intake valves. I replaced both intake/exhaust valves with new stainless steel versions when we did the valve job (with same size valves).

I re-used the cast .040 over flat top pistons (see foto in post #5) with what appear to have been the high compression versions (used new set of Mahle piston rings). Used a Felpro head gasket and stock head bolts. I re-used the factory 340 connecting rods (full-floating) which were re-conditioned (large end) and we installed new ARP rod bolts.

Oil pan that came with the 340 had a rear sump! I replaced that with a Kevko baffled 5 quart oil pan. New oil pump (not high volume) with pinned style oil pump drive. Then installed Direct Connection chrome valve covers. Fuel pump is new from Autozone.

The 340 came with 2 identical 340 log-style passenger side exhaust manifolds, so I used the factory 318 exhaust manifolds for now. (I have a pair of '68 high-performance 340 exhaust manifolds I will install later with TTI head pipes).

This hodgepodge 340 came with a factory '68-'69 340 un-silenced air cleaner (black crinkle finish) which I have currently installed. I have a correct factory 1972 orange (trap-door) 340 air cleaner I will install later.

In summary, I believe what I have is a high-compression 340 with 1.88 intake valve cylinder heads, a copy of a 1968 factory 340 manual grind hydraulic camshaft with adjustable rocker arms. Carb is the '72 340 Thermo-Quad and intake manifold is the factory '72-'73 version using the factory combo divorced/electric choke. The ignition is a billet Petronix distributor (which is awaiting a new vacuum advance) so this week I'm running a single-point conventional distributor I borrowed from my 318 barracuda until the Petronix part arrives in the mail.

 

[Dibbons]

Update: Did some more tuning with the idle mixture screws. I brought the A/F at idle down from circa 14.0 to more like 12.5 and it picked up an inch of vacuum (was unsteady at 14 inches and now steady at 15 inches). Idle speed still about 1,000 rpm which it needs because it drops to 800 rpm in drive.

 

[polyjohn]

Nice rebuild there Mr. Dibbons.
I've seen some very bad 340 builds over the years too.

Pre 71 heads would have been 2.02 intakes and that's where the hurt is most.
Yep the better exhaust system will certainly give more power as well

Good luck in all you do in La Paz, sounds wonderful there indeed.
Best wishes from cold wet England UK

 

[HawkRod]

Update: Did some more tuning with the idle mixture screws. I brought the A/F at idle down from circa 14.0 to more like 12.5 and it picked up an inch of vacuum (was unsteady at 14 inches and now steady at 15 inches). Idle speed still about 1,000 rpm which it needs because it drops to 800 rpm in drive.

I would check the distributor and timing regarding performance. No, not just initial and total, but the CURVE that is built into the distributor. Many are very sluggish, and this leads to a car that feels like sluggish.

Rough rule of thumb: Your distributor should have about 11* of centrifugal timing (that's 22* at the crank). Centrifugal advance should start coming in at about 1000 RPM and then move up fairly quickly, and be all in by about 2800 RPM.

You'd be surprised how much just working the advance curve will do for the performance feel of a car. If you haven't checked it, you should do so. If you are unsure, you can send it to someone who knows old distributors like @HALIFAXHOPS