71 340 Compression Test

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I had to run that little SB 350 at +10deg advanced on timing w/GM-HEI ignition but it worked really great.

Think I am going to have the little 340 rebuilt while I have it out for the paint guy. I like the warm/fuzzy feeling of knowing what I have from the start. Engine heads are 73 model indicating 1.88" intake valves, block and intake say 1971.
Would be awesome if the block has a forged crank but not holding my breath. Engine builder says he tries to stay between 400-450 hp on cast cranks. 400 is about where I wanted to be.
If I were to decide to put a forged 3.31" stroke forged crank in it, any of you guys know where a guy might pick one up?
Also thinking of trying to stay around 9.5-10:1 on Scr in hopes of being able to run the 91-93 octane fuel? May not be possible? Going to have the shop cut in 2.02" intakes on the heads. Any advice as to brand for valves "Manley" etc?
 
Just remember that you change the damper and flexplate/flywheel and or TC if you go from a cast 340 crank to a forged 340 crank. The cast crank has balance weights on the damper and flexplate or TC (or flywheel), and is known as externally balanced crank (which is in reality is actually just partially externally balanced).

I see such cranks advertised on places like CL. Caveat emptor....

As for running the higher SCR on iron, that may be fine depending on the cam. Drag eneinges run 11-12-13 all the time but with big cams. The key is your DCR. What do you use this car/engine for? Will you be changing the cam?
 
nm9stheham said:
Just remember that you change the damper and flexplate/flywheel and or TC if you go from a cast 340 crank to a forged 340 crank. The cast crank has balance weights on the damper and flexplate or TC (or flywheel), and is known as externally balanced crank (which is in reality is actually just partially externally balanced).

I see such cranks advertised on places like CL. Caveat emptor....

As for running the higher SCR on iron, that may be fine depending on the cam. Drag eneinges run 11-12-13 all the time but with big cams. The key is your DCR. What do you use this car/engine for? Will you be changing the cam?
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I will mostly drive it around to car shows or maybe a Saturday night cruise. We have a club down here that do 1-2 Saturday night classic cruises each month. Have some friends running those pesky Camero's and making fun of the little 340.
If push comes to shove I would like to shut them up. One has a 383 striker in it. Said it made about 500 hp on paper but no actual Dino tests.
Some things look god on paper but just don't hold up in the real world. I am going to change out th cam and go with something like a 280H/.480"-.480". I think that is about two steps up from the factory cam at about 268H?

Thanks for the info on the forged crank and balancing. What is the damper that you are referring to? Is it the weight on the front of the crankshaft?
 
If you are gonna spend money on it and are thinking making power, a quick swap to a 3.58 stroke, (or more of course), is cheap way to more off-the-line torque. The extra stroke will allow you to run a bit more cam too, before loosing Dcr, and a bit less TC. Between the two, you might be happy without any extra head-porting.
An acquaintance of mine did that to his 340, and shaved the piston-crowns to fit X-heads with "pretty good quench",(his words). That motor was plenty strong......
I highly recommend to select/buy a cam after your Scr has been measured.
And streeters need the fastest cam lobes you can find, and are best selected at under 230*. Even 230s in short-stroke engines usually need gears and a bit of a TC
You can't really have a cruiser and a "shut-em-up" sbm-car at the same time, if staying NA (normally aspirated). If you insist on it, you will likely be disappointed.
To shut down a 383 in a similarly weighted car, you are starting from a 44 cid handicap. You are down 11.5% on cubes. To make it up, you are gonna have to build nearly a race-engine. If he is at 1.2 hp per cid, then he has 460 hp, and you are gonna have to make 1.35 hp/cid. Which, while doable, is not cruise friendly. You are gonna have to make a decision, and it might not be cheap.Having a 460hp/340 is just the beginning.
After that comes; the rest of the powertrain,engine support-works,the suspension, steering and brakes, frame connectors,traction aids, sticky tires, and safety equipment. Up here this could be $16000CanPlus. Maybe closer to 20/25 if you have to pay labors.And that doesn't cover maintenance, breakages, and rebuilds as weak areas come to light.That's a lotta coin for bragging rights. :(
You may remember 340 cars as being fast, but you have to also remember the times.
 
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AJ/FormS said:
If you are gonna spend money on it and are thinking making power, a quick swap to a 3.58 stroke, (or more of course), is cheap way to more off-the-line torque. The extra stroke will allow you to run a bit more cam too, before loosing Dcr, and a bit less TC. Between the two, you might be happy without any extra head-porting.
An acquaintance of mine did that to his 340, and shaved the piston-crowns to fit X-heads with "pretty good quench",(his words). That motor was plenty strong......
I highly recommend to select/buy a cam after your Scr has been measured.
And streeters need the fastest cam lobes you can find, and are best selected at under 230*. Even 230s in short-stroke engines usually need gears and a bit of a TC
You can't really have a cruiser and a "shut-em-up" sbm-car at the same time, if staying NA (normally aspirated). If you insist on it, you will likely be disappointed.
To shut down a 383 in a similarly weighted car, you are starting from a 44 cid handicap. You are down 11.5% on cubes. To make it up, you are gonna have to build nearly a race-engine. If he is at 1.2 hp per cid, then he has 460 hp, and you are gonna have to make 1.35 hp/cid. Which, while doable, is not cruise friendly. You are gonna have to make a decision, and it might not be cheap.Having a 460hp/340 is just the beginning.
After that comes; the rest of the powertrain,engine support-works,the suspension, steering and brakes, frame connectors,traction aids, sticky tires, and safety equipment. Up here this could be $16000CanPlus. Maybe closer to 20/25 if you have to pay labors.And that doesn't cover maintenance, breakages, and rebuilds as weak areas come to light.That's a lotta coin for bragging rights. :(
You may remember 340 cars as being fast, but you have to also remember the times.
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Sounds like you have the experience and appreciate passing it on. Guess I have a decision to make as you say. Wonder if it would be possible to run the forged crank setup, 9.5:1, 280H, big valve heads, 770 Holley, headers with 125 shot of nitrous, if needed. If I could get around 450 hp on pump gas + the nitrous shot. Thought about an extra small tank with some C12?
 
You are in the wrong box again.lol
Think 325 hp plus 125nitrous.
Nice smooth 15/20mpg cruiser-engine, and 450 to bust everything behind the engine with. Keep your savings account topped up;Nitrous has a bit of a learning curve.
The 480* is too big, especially if it is a slow lober. 268 to 276 is more street friendly. But again, if you pick the cam before the SCr is in, you may have to spend more money adjusting the compression for the cam. A cruiser cam will often be less than 268, unless you just can't live without the lope. The factory 340 cam was 268/276/114 with what today would be considered very low lift.And it had just 44* of overlap; perfect for a cruiser with manifolds, but not so good for headers.But possibly good for nitrous.The factory 360 2bbl cam will absolutely shred tires,off-the-line, with a little compression.
And I repeat, get the fastest lobes you can find. These are sometimes called fast-rate cams, or hi-intensity. Some manufactures are able to put a lot of lift on these, others not so much.Get your monies worth. Nitrous cams are usually different from NA cams, although some can serve double-duty.

But you know what, all this talk is just talk.
If your engine is otherwise healthy, it is already a great cruiser engine, AND it is a pretty good candidate for nitrous. You could be a 383 slayer,real quick.
Right until the bottle runs out.................You probably want the extra-large size,lol
 
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1badfish67 said:
Some things look god on paper but just don't hold up in the real world. I am going to change out th cam and go with something like a 280H/.480"-.480". I think that is about two steps up from the factory cam at about 268H?

Thanks for the info on the forged crank and balancing. What is the damper that you are referring to? Is it the weight on the front of the crankshaft?
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Yes, the damper is the crankshaft vibration damper behind the crank pulley. The forged one is inherently balanced, and the 340 cast crank one is purposely imbalanced; that imbalance is essentially a balance weight for the crankshaft that is external to the crankcase.

I am wholly with AJ on the cam, going wider duration with low lift is about 4 decades old cam design and more suited for strip than street. Going with smaller duration and as high a lift as you can get will keep the cruise manners better. All wider RPM, good-cruise-manners cams have been this way for a long time.

I personally would go with a mild turbo or stroker over nitrous.... just my preference. The engine will be more broadly usable with either IMHO.
 
I will get with my engine guy on the cam selection and fast lobes. I gotta be honest in that I like a little lope to an engine but still street able.
You guys might be able to shed some light on the actual hp of a 1973 340? The data that I have seen shows to be about 230 hp with the small valve heads? I have heard that the x-head 68-69 340's were advertised at 275 but we're more like 300-325? Is it anyone's opinion that the 73 may be under rated as well?
This car has a A833 4spd (not sure if fine or course spline) I do know that it has a 489 rear end. These has the heavy duty axles in them, right? Also has SureGrip but no idea of gear ratio.
Sounds like I need to get block re-done, decide comp ratio of Pistons, have heads re-worked with larger valves then let engine guy check the CR and valve clearance with heads on to make sure. While he is working on engine I can verify gearing?
Any idea how many ponies those factory manifolds are good for? At least initially for the car shows it might be nice to look original on the outside.
Appreciate the good information guys.
 
for 340-4bbls
All the heads were pretty much the same, as were the A/T-cams. The power differences were in the pistons, and the carbs, and the tune.But more than anything was the rating system. The 72 engines were pretty much the same as the 71s, with just carbs and timing. The 71s were rated 275 and the 72s were rated 245,IIRC. Same engine but with retarded timing and new rating.
In 73 the compression was dropped and the engine was downrated to 240, go figure.
I'm going from memory so corrections welcome

The 71 TQs were factory rated275, but NHRA wasn't fooled and rated them 290
The 6 bbls were rated up to or factored 325hp.
the long blocks on both were the same.

The factory 4bbl intakes are all pretty much the same
The exhaust manifolds are a mess, but headers are a must to make the cams work properly.
 
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If you look closely at the numbers you can see that the TQ was worth 15hp over the AVS, and the 6-pac was worth 50HP over the AVS. That tells you that the little AVS was a total choke. It also tells you that the 6 pack rating was also nuts. Come-on 50 hp! I don't think so. And neither did anybody else, think that. Except the NHRA.
If you look at the 72 rating. It appears the the compression drop from 10whatever to 8whatever, was only 5 hp. Come-on! that too is nuts. So the point is that while these engines were powerful little buggers in stock form, with modern technology, they can annihilate those other brands. And don't believe every hp rating you read.
 
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