picked up a 340 long block w/out specs

-

nixrsix

Member
Joined
Jul 3, 2019
Messages
20
Reaction score
20
Location
windward
I recently picked up a 69 340 long block from an auction. I couldn't get any info from the owner or even know who he/she is. All that was stated was a "69 Mopar 340 high performance 400hp supercharged race engine". It has roller rockers, electric water pump, and has the lower blower manifold(looks like it was never ran). I know the stock CR for these engines are 10.5:1. Should I just go ahead and throw on a 671/871 blower assuming the compression ratio is built accordingly, throw a regular intake manifold on it and run it without boost and enjoy it for it is?, or go through the hassle of pulling heads and calculating compression ratio? I'm guessing the correct route is the longest one and pull it all apart and check it all out myself, but just seeing what you folks think...

TIA
-N
 
Last edited:
So what's the best way to determine the cc space above tdc...measuring everything or using the fluid method?
 
Bolt on an intake and run it....might put a lot of smiles on your face.

When this stuff was new few of us knew anything about the details of these engines... We turned the key and drove!
 
Get it on an engine run stand with a starter and do a compression check. 400 Hp is a low ball number for a blower engine in my book, though, should be closer to 500, especially if it's a 6 or 8 71 series blower.
 
You probably got a good deal so why not take the time and find put what you have and double check everything?

If it were me that bad boy would be on a stand getting taken apart. Small effort IMO. One small detail not done during assembly could send everything through the pan....

Good luck,
JW
 
If I were to buy a car would I disassemble the engine just to see what's inside?

Hell no....

Sometimes we get so anal about this stuff...we really do.

In the oval track world few, if any, guys buy a motor and pull it apart. We buy the stuff to race... Most give it a once over, check the valves, might swap a carburetor and put the thing on the track.

GM crate engines are like that (you mostly know what your buying) but is true with "open" motors, too. Might give a guy some smiles if he knowsw the engine builder was but means very little in the grand scheme. Either it runs or it doesn't.

I got over all that hand wringing over small details long ago.

Just trying to make your lives a bit easier...
 
Last edited:
roller cam?
you need to lube lobes on a FT
so do the compression check
pull the intake and inspect (has intake been open to the dirt?)
you can degree the cam and see what it is
what do the valves springs looklike? duals? you could bull one and check and check retainers and keepers what quality
put a 4bbl intake on it and pre lube
fire it up
if it has never been fired up and we have no idea how long it's been sitting you do not want to start dry
you might get away with it on a junk yard take out but not here
 
While true, 340 blocks are a bit of a rarity now and need to be preserved as much as possible. To answer questions, I’d degree out the cam timing events with a dial indicator and a 360 degree tape on the balancer. A compression test to check cranking pressure and some fancy math from victory library or the like and you could get a reasonably close use of your actual compression and cam specs. But not knowing the history and if it had been run it usually isn’t a bad idea to check out the rod bearings because they do take a beating in a blower motor, especially if detonation has occurred. Even when everything is right, a blower motor will see detonation at some point in its run cycle, unless it has been fed a constant diet of expensive race fuel.
 
Yes, what wrmyrider says. Worst case scenario is it’s an alcohol engine with dykes rings and 11.5:1 compression and aluminum rods, but highly unlikely.
 
Last edited:
I recently picked up a 69 340 long block from an auction. I couldn't get any info from the owner or even know who he/she is. All that was stated was a "69 Mopar 340 high performance 400hp supercharged race engine". It has roller rockers, electric water pump, and has the lower blower manifold(looks like it was never ran). I know the stock CR for these engines are 10.5:1. Should I just go ahead and throw on a 671/871 blower assuming the compression ratio is built accordingly, throw a regular intake manifold on it and run it without boost and enjoy it for it is?, or go through the hassle of pulling heads and calculating compression ratio? I'm guessing the correct route is the longest one and pull it all apart and check it all out myself, but just seeing what you folks think...

TIA
-N

Pull the heads and calculate the compression ratio. You have no idea who built it, what parts were used, or what the engine is capable of. The stock specs mean absolutely nothing if the pistons and heads aren't stock. You don't have to tear it down to parts, but if you pull the heads and pan you can inspect what you've got so you know how to proceed with any additional modifications.

If I were to buy a car would I disassemble the engine just to see what's inside?

Hell no....

Sometimes we get so anal about this stuff...we really do.

In the oval track world few, if any, guys buy a motor and pull it apart. We buy the stuff to race... Most give it a once over, check the valves, might swap a carburetor and put the thing on the track.

GM crate engines are like that (you mostly know what your buying) but is true with "open" motors, too. Might give a guy some smiles if he knowsw the engine builder was but means very little in the grand scheme. Either it runs or it doesn't.

I got over all that hand wringing over small details long ago.

Just trying to make your lives a bit easier...

This isn't a crate engine with published specs, they don't make crate engine 340's. Sure, not every engine needs to be fully blueprinted to have fun with. But this is a completely unknown engine. Built by an unknown builder, with unknown parts and only a description of it being a "400 hp supercharged race engine". Well, a 400 hp supercharged small block is super mild. That description doesn't even sound right to me. It could be ANYTHING in there. Toss it in the car and fire it up and you could wipe out a brand new engine in no time flat.
 
You don't need to pull the heads to check the compression ratio and you don't need to screw with cranking compression.

Get the engine on the stand and roll the engine on the stand until the number one spark plug hole is straight up.

Get yourself a real graduated burette, not a turkey baster or one of those hardware store bull semen things. You have to move fast.

Get the piston to TDC and fill the burrette with with water and a touch of dish soap.

Fill the cylinder until the fluid reaches the spark plug hole and read the burrette. Then you can determine your CR.

You have to move quickly. Once you know the volume at TDC you can calculate the CR. If you are concerned about the fluid getting into the pan, you can use solvent, or denatured alcohol and it will dry up.

Again, you have to move quickly because you can't deal the piston with grease. I've done it many times, when some joker would come in and claim 11:1 CR and in truth he was 9.5:1 or worse.
 
Thanks, that's a great way to check! I might add that a bore scope would also be beneficial if you have access to one. What do the spark plugs look like? Has the engine been ran in the past?
 
You don't need to pull the heads to check the compression ratio and you don't need to screw with cranking compression.

Get the engine on the stand and roll the engine on the stand until the number one spark plug hole is straight up.

Get yourself a real graduated burette, not a turkey baster or one of those hardware store bull semen things. You have to move fast.

Get the piston to TDC and fill the burrette with with water and a touch of dish soap.

Fill the cylinder until the fluid reaches the spark plug hole and read the burrette. Then you can determine your CR.

You have to move quickly. Once you know the volume at TDC you can calculate the CR. If you are concerned about the fluid getting into the pan, you can use solvent, or denatured alcohol and it will dry up.

Again, you have to move quickly because you can't deal the piston with grease. I've done it many times, when some joker would come in and claim 11:1 CR and in truth he was 9.5:1 or worse.

That will get you the compression ratio, but it still doesn't tell you much about the engine components used. A borescope would help some more.

If it were just a stock rebuild I'd say fire it up, you know roughly what you've got. But the more modifications that have been done the more specific the tuning has to be and the wider the range of possibilities. For my time and money, pulling the heads is worth while. All it costs you is a set of head gaskets, and you can inspect the pistons, bores, get a look at the top ring, the valves, head chambers, etc. That's a lot more than just knowing the compression ratio and taking a look with a borescope. Same for pulling the pan, you get a look at the whole bottom end and all it costs is a gasket.

I wouldn't do all that if I knew the shop that built the engine, or the guy I bought it from. But on a completely unknown engine? Especially on a forced induction build. A few gaskets now will be a lot cheaper than replacing engine components if something goes wrong later because something was assumed.
 
Probably got 8.1 Badger pistons in it and that's how they decided to call it "a blower motor"...cause you're gonna need it. Jk. Lol

Let us know what you find out.
 
I'm a gambler.
I'd say use some good fuel, a carb and distributor you know is good to go. Fire in the hole.
 
Man you guys must be either rich or lucky.

No way I'd take an auction bought engine and just toss a blower on it and see what happens. I mean, what's the worst that could happen right? Oh yeah, blow every piston out of the hole with massive detonation because you have no idea what it's set up for or how to tune it. Or waste a few hundred bucks and all the install time buying a regular carb intake and running it only to find out it really was set up for a supercharger and it's a complete dog because the compression is 7.5:1 and it has a blower cam.

I mean seriously. This engine is on a stand. It shouldn't take more than a couple of hours to pull the heads, inspect everything, CC the chambers, and button it back up. If that's too much time and effort, just imagine how much fun it'll be to have to yank it back out and tear it down because something broke?
 
Depending on when it was built, (thinking back to the pro fairground fad of the eighties) there is a good possibility that there is an antiquated cam grind and low compression piston that gives a guaranteed 6:1 dynamic compression ratio if you try to run it in a naturally aspirated configuration. If it has a distributor, it would be worth seeing if it is locked rotor or mechanical advance and what the advance rate is set for. Whole different set of tuning challenges when running an iron lung.
 
Man you guys must be either rich or lucky.

No way I'd take an auction bought engine and just toss a blower on it and see what happens. I mean, what's the worst that could happen right? Oh yeah, blow every piston out of the hole with massive detonation because you have no idea what it's set up for or how to tune it. Or waste a few hundred bucks and all the install time buying a regular carb intake and running it only to find out it really was set up for a supercharger and it's a complete dog because the compression is 7.5:1 and it has a blower cam.

I mean seriously. This engine is on a stand. It shouldn't take more than a couple of hours to pull the heads, inspect everything, CC the chambers, and button it back up. If that's too much time and effort, just imagine how much fun it'll be to have to yank it back out and tear it down because something broke?
agreed!
 
Man you guys must be either rich or lucky.

No way I'd take an auction bought engine and just toss a blower on it and see what happens. I mean, what's the worst that could happen right? Oh yeah, blow every piston out of the hole with massive detonation because you have no idea what it's set up for or how to tune it. Or waste a few hundred bucks and all the install time buying a regular carb intake and running it only to find out it really was set up for a supercharger and it's a complete dog because the compression is 7.5:1 and it has a blower cam.

I mean seriously. This engine is on a stand. It shouldn't take more than a couple of hours to pull the heads, inspect everything, CC the chambers, and button it back up. If that's too much time and effort, just imagine how much fun it'll be to have to yank it back out and tear it down because something broke?

That sums it up.... On the stand right in front of you..... I have seen the dark side of a purchase like this and the problem with it was easily avoidable but everyone was in a hurry to go an extra step and check behind who put the motor together. But I won't lie in that some people have a horseshoe up their @#$ and pull it off. I just ain't one of them....

JW
 
I would go ahead and get it apart simply to know what's there.
 
That will get you the compression ratio, but it still doesn't tell you much about the engine components used. A borescope would help some more.

If it were just a stock rebuild I'd say fire it up, you know roughly what you've got. But the more modifications that have been done the more specific the tuning has to be and the wider the range of possibilities. For my time and money, pulling the heads is worth while. All it costs you is a set of head gaskets, and you can inspect the pistons, bores, get a look at the top ring, the valves, head chambers, etc. That's a lot more than just knowing the compression ratio and taking a look with a borescope. Same for pulling the pan, you get a look at the whole bottom end and all it costs is a gasket.

I wouldn't do all that if I knew the shop that built the engine, or the guy I bought it from. But on a completely unknown engine? Especially on a forced induction build. A few gaskets now will be a lot cheaper than replacing engine components if something goes wrong later because something was assumed.



I agree. If it were me, I'd take it apart. But that's a quick way to do a check of CR. I've seen guys damn near cry when you make them watch and they find out they got hosed on CR, and that's one reason why the engine was an underperformer. One I'm thinking of also shook like a dog pooping peach seeds. Looks like someone drilled a hole in the front CW and called it balanced.
 
-
Back
Top