340 input (direction)

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joemoy29

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I purchased a rebuilt 340 5-31-72 four years ago, with rebuilt a833 attached to it. New summit dual plane intake, new water pump, new mini starter, new alt, hurts short shifter and new clutch flywheel. Paid 1400 total

It's been sitting for 4 years I always turn it over and check oil. Stored indoors. Just dropped it in my 70 dart swinger 1 owner. The car has full hotchkiss sway bars, Bilstien shocks, firm feel box, pst control arms, 1.01 torsion bars. And a bunch other stuff

I was told the 340 was built stock with a mild cam. Does anyone use 8.5 pistons on a rebuild (they were stock) is there any way to know what they used without pulling heads, should I change the cam so I know what is in there? Also I want the car to be 375-400 hp so I eventually will do heads. Also running dougs headers

Just looking for any input. Thanks in advance
Joseph Moy
 
For the same money I would certainly hope that whoever rebuilt it rebuilt it to 71specs. But to tell without pulling a head?? I don't know of anything that will guarantee any real accuracy.
 
And it would be totally ridiculous to do a rebuild using factory pistons correct?
 
In my opinion. Compression was reduced in 72 and to get a kit with those pistons would have taken some doing. I'm trying to pull this from memory so if not 100% accurate, forgive me. Federal laws say that if the rebuild is advertised as "OEM" for 72 it must comply with "OEM" 72 specs. That would have taken a little doing. If a kit was bought from Summit or such for a rebuild, the included pistons would have been 68-71 or some sort of flat top piston that wasn't sold as "OEM".
 
Ok thanks for the info. Unless he used the actual stock pistons which would be rediculous. I know the majority of people would say run it, learn it, then mod it. But should I get it running on stock crap or do heads and ignition and fitech all before starting off
 
If the engine was on the stand, you could roll it over to put a plug-hole as the high point,and measure how much oil it takes to fill the cylinder, when the piston is at TDC compression.Knowing the total fill volume and the displacement, you can calculate the Scr.While not dead-nuts-accurate, it will give you a pretty good idea.
But,if the engine is already installed,then, I guess it's too late now.

But don't despair; just do a compression test.
And find out the intake valve closing angle.
From these two it is possible to back-calculate the Scr. Again not dead-nuts accurate, but you can get a pretty good idea.
If you are using the factory balancer, you can use a properly installed timing tape, and a Leakdown Tester to find that ICA. If you also use these tools to find the intake opening angle, then you have a duration figure. From this you can get a pretty good idea of what to expect from the cam.
Alternatively, the engine's warmed up idle vacuum@ idle speed of,and with timing of, can be a pretty good indicator of the cam size; in general terms; as in small,medium, large,and flippin'huge. .
 
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It is already rebuilt and in the car. I would run it and worry about the rest later.
 
joemoy29 said:
Ok thanks for the info. Unless he used the actual stock pistons which would be rediculous. I know the majority of people would say run it, learn it, then mod it. But should I get it running on stock crap or do heads and ignition and fitech all before starting off
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That would depend on your bank account.
But if you really want 375/400 in a streeter, then headwork is a given. Well that or some form of supercharging.And then.... those 8/1 pistons (if they are in there) are looking pretty good
 
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Just pulled the valve covers and they are 73 heads so this is going to cause low power correct? Can I order a set of heads without knowing the pistons and compression.
 
Get piston to tdc and measure how far down in the hole and measure the bore. This will help to measure compression.
What type of pistons? Pics?
Someone that knows more about engines will chime in Im sure
 
You asked for input.............You bought this engine 4 years ago and you are just now taking stock of "how" it was rebuilt after it's in the car??? As to what pistons were used, not every build is a prostock motor.

I wish you good luck and hope it all works out for you.
 
JBurch said:
You asked for input.............You bought this engine 4 years ago and you are just now taking stock of "how" it was rebuilt after it's in the car??? As to what pistons were used, not every build is a prostock motor.

I wish you good luck and hope it all works out for you.
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Yeah it was sitting for a while. Was on a cart not a stand so figured it would be easier to work on it in the car. I am not one of those unrealistic 17yr old kids who thinks the 340 will do 450hp with a few bolt ons from jegs. Although the chrome accessories are good for at least 100hp.

My question is I got the fresh motor so cheap I am not looking to rebuild the whole thing. Just looking for direction on how to optimize what I already have

Thanks for the input
 
Measure the combustion chamber volume. Then take other measurements I suggested. Those will help you to ballpark your compression ratio. With heads pulled.get a look at the.pistons and you can also remove the casting flash and junk in the intake and exhaust ports. Be careful though to not hit a water jacket.
Im doing this all myself right now as well. Amd.learning a lot. Im damn near 50 amd always worked on my own wheels. Im now starting to understand how the seperate pieces work together.
Just dont be in a rush. Enjoy! I am
 
I would not expect the '73 heads' combustion chamber volume to be significantly different than the earlier heads. So that would not effect your CR by itself.

Using the original type pistons would not be at all ridiculous. It would be fine. The later 340 pistons dropped CR some. Is there a reason that you mentioned 8.5 CR pistons earlier?

As AJ states, use a compression gauge and measure cranking compression to get some idea of the internals. Checking the ICA (intake closing angle) would give a more complete picture.

BTW a bore scope or other tiny camera may help; at least you can see if there are 2 or 4 eyebrows in the piston tops.

As suggested, run it as is to see if the engine is basically sound. Then work up the mods later.

A lot of what is optimum or not depends on how you plan to use the engine/car. Are you going purely drag racing? Grocery shopping? Road race? Running mooshine? Please expand.
 
Decision point.............run it, drive it as it is right now and not worry about "what" is inside of it, enjoy the car, that was the premise of buying the engine to begin with; right? OR tear into it to learn what it's made of and change what you don't like; just remember there's a 100 guy's out here who can't wait to spend your money for you.

My vote is to just drive it.
 
Not to be pessimistic..... but I would bet, based on what you paid for it (a lot less than what a properly rebuilt 340 (or any engine for that matter) and non od 833 is worth), that you have a backyard rebuilt 340 with a re ring and gasket kit. Likely it had a dingle ball hone run through it and original pistons and cam re installed.

At this point, see how it runs, if it uses any oil and how the oil pressure is at a hot idle. If all is good run it as long as you are happy with the performance. If you want more sell it as a running 340 and build a healthy 408. You will be miles ahead dollar per horsepower and can still look like a stock 340.
 
BTW
>You can marry low-C with a small cam, to get a high Dcr and a get a pretty good streeter out of the wedding, especially in a lightweight chassis.
>You can marry high-C with a big cam, to get a great dragracer.
>You can marry medium-C with a medium cam ,to get a dynomite streeter; or with a bigger cam and a hi-stall and race gears, and get a pretty good drag-racer..
>But what you should not do,is to try to marry lowC with a big cam, for street use; the hi stall and too-big-for-street gears,after the initial excitement wears off,can be a major PITA. Besides, after you calculate the costs of what it takes to get those installed, you would probably have been bucks ahead,just fixing the compression first.
In my personal vehicles,over the past nearly 50 years, I have had all kinds of compressions and various cams, and IMO, High-C and a medium cam wins every time. And the bigger the small-block,the better. Or, more accurately, low-C sucks with anything but the OEM cam,the oem TC and reasonable street gears from 2.94 to 3.55.
IMO, for street fun, if you can't get the performance you are after with 3.55s and up to about a 2800/3000TC (if an automatic), then your engine is too small,or your chassis is too heavy.
First gear,on the street, for the most part, is just a tire burner. Second gear is where it's at. The 1.91 x 3.55=6.78 is a pretty good ratio. This will get you 65=about 5500. With a 220ish cam,and a 340, the power-peak will be about 5200, so you are right in the ballpark.
At the other end, the torque peak will be near 3900@46mph. So a footstomp with a DP carb should light up any street-tires you can fit in the stock 70 Swinger tubs.If it can't, the compression is probably low,the tune is off, or you need to ditch the excess baggage in the rear seat,lol. If you wanna show off with your all your chubby buddys on board, you will need more power, like BB power.
With a220ish cam, the torque starts early,so by 35mph you should be set, for second gear. This will be about 2950rpm.
You can run more gear. But if it lights up the tires at 2950, what would be the point?
You can run less gear, with a bigger engine, and if it lights up the tires at 35mph,bingo.
You can run a smaller engine with a bit more gear,and if it lights up the tires at 35,bingo.If it doesn't, the first go-to is gears.
As far as sizing goes, the 318 is a medium sized small-block, the 273 is the baby, and the 360 is the big-dog. The 340 is kindof orphaned between medium and big-dog. With a clutch, this is no big deal,even if the compression is a little down, unless it is also married to hi-way gears.:( I'm thinking this is why 340 cars got 3.55s with sticks,and 3.23sfor autos; Chrysler new a little something about matching stuff.
And just so you know,and have something to compare to,a typical stout 360 with a 6.78 second gear, can spin 295s from 2800 to redline, and beyond, certainly to beyond the speed-limit anywhere in Canada; 6000=71, 6500=77, 7000=83. I like my 6.78 second gear; it's like Dial-a-Spin.
 
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AJ/FormS said:
NO! and NO!
Click to expand...
double click,Sorry
But since I'm wasting space anyway; here it comes.
Have you driven this car yet?
I mean just cuz your wife has a big belly doesn't mean she's pregnant!
Ok thanks for the info. Unless he used the actual stock pistons which would be rediculous. I know the majority of people would say run it, learn it, then mod it. But should I get it running on stock crap or do heads and ignition and fitech all before starting off
Click to expand...

IMO you need to focus.
A) if the engine is on the stand, do some homework. Find the fill-volume and the ICA
B) if the engine is already in the car, quit worrying and drive it.

It's all in the combo. Yes, you can have a lot of fun with a low-compression carbureted 340-4speed, with the factory heads! The fastest way to a good time,on the street, is with TM;Torque Multiplication. Every gear size difference is nearly one cam size, with no accompanying compression loss.
For instance; 3.91s/3.23= plus 21% TM. That is huge,and it is everywhere in the rev range;as in bottom-end not traded away for top end.
So, if you are not that interested in hiway running.TM is a practical way to have fun on the street. Just don't over-do it, cuz then first gear ends up useless.

If you don't understand this, perhaps this will help;
say your current combo torque-peaks at 3600rpm, making 340ftlbs (233hp).
And say you had those 3.23 gears, and say 24 inch tall tires. Then at 3600rpm, your engine would be putting down 340x2.66x3.23= 2921ftlbs@30mph.
But say you swapped those 3.23s out for 3.91s. Now still at 3600, your engine will be putting down 340x2.66x3.91=3536 ftlbs, but now at 24.7mph!
So not only do you gain the 21%TM, but you also get it 21% earlier in the road-speed range!! This is like supercharging it.
You cannot duplicate this with a camshaft or compression or anything except cubic inches, or supercharging.And it would take in the neighborhood of 21% more cubes, or 21% more supercharged power, to match this result, all other things being equal.
So, again, it's all in the combo.
The trade off is hiway cruise rpm. It also jumps 21%, so from 65=2940, to 65=3559. And the only way around that is 21% overdrive,lol; So another gear .
And, friend, that is why Mustang 5.0s have more gears. TM.

And BTW,
don't look to fuel injection for more power, that ain't happening, at least not significant power. But EFI will get you driveability and perhaps economy. But gains over a sharp carb-tune will be minimal.
Don't look to big heads for power in first gear. You already have more power in first gear than the tires can handle. And in second, 65mph will be around 5300 with 3.55s, and 28 inch tires; so depending on the combo,very little gain to be had here. The gains will be after the tires stop spinning,and once the car is settled down for the long haul from 65 to maybe 106 mph, IF correctly geared.
So what I am saying is: if you only have so much many to spend, then, you need to spend it where it will give you the most excitement for your buck, for your intended useage. Excitement in this "hobby" can quickly consume an awful lot of money, if you spend it in the wrong place first.
 
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I know compression will help make power .
Yet I have had low cr motors that run better then I expected and, many times people have less compression then they think.
Most 340/360 heads are said to be between 68 and 71 pc's.
The block could have been decked and heads killed?
Or it might have been ringed and bearings ?
Either way if it was put together right it
Should run well !!
Truly even a real 300 go motor in an a body will create tubs of fun , especially with low gears and a 4 speed !
Enjoy !
When you are ready to do heads contact iq52 he will make them flow well !!
Best of luck !!
 
And while I am a big fan of hi-compression and tight Q;
It is not the final word.
In fact from worst case factory engineered 8/1 cr to best case on pump gas cr, the difference is said to be in the range of 4 or 5 %. On a 300hp engine that might be about one cam size in terms of gross power.
But what they don't tell you is the effect Hi CR has on low speed operation.
So if your current combo, is a happy combo, then just drive it.
 
Thanks guys it makes sense to make sure it is healthy before touching it. Then if it is a dog I can make the decision to change parts on it or put something else in
 
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