340 valiant build advice needed.

[PetCemetaryValiant]

Hey yall. Im new this forum and classic mopars in general. I just recently bought my first classic american car, a 69 4 door valiant signet. I've owned it for about 2 months now, done some regular maintenance changing oils, plugs, battery and tires, and in that time i've found out some cool things about the car. I bought it thinking its just a 318 3 speed auto with some bad body paint. It was not that. It in fact has a 70 340 in it with a 727 gearbox connected to the 7 and 1/4 rear end. Power disc brakes in the front and fiberglass fenders. Im happy, but scared at the same time, because everyone says the engine is rare and not worth building, but selling. I would love to build this thing into a ripper, but im not sure if im competent enough for this engine. So my main question is does this engine need something special to work to its fullest potential? Im gonna post pics as soon as we have light here.

Build idea: A high rpm screaming cruiser. Reliable 7000rpm goal. Added power is a plus, but not the main focus, since its not a race car. Loud, but needs to pass inspection 98 decibels under 3000rpms.

What mods should i focus on, if i stay with the 340?
Is the 7 and 1/4 rear end good enough for the application?
Should i build the transmission at the same time as the motor?
What exhaust setups should i consider? thinking of going with and x pipe into mufflers and boom tubes (not a fan of the look).

Ps. Im not american so some maintenance parts might be tougher to get a hold of.

 

[Dartswinger70]

The build would be the same effort /cost as rebuilding a 318 . Pay attention to the book and the different torques specs etc specific to a 340 and don't lose any parts they are specific to the engine etc. Do it don't be apprehensive. id take it down inspect see if it needs machine work if not hone it re ring and bearing it etc. Oh and at a glance yes Id put a beefier rear in it . a Mopar 8 3/4 or Ford 8.8 does the heads have an x or j on them?

 

[65 Dartman]

Good afternoon and welcome to FABO. first thing I would do is confurm that you actually have a 340 in your Valiant. Get underneath on the driver’s side at look at the numbers on the side of the block. The last 3 or 5 numbers will say 340-a number. The 340 will confirm it’s a 340. To confirm the year look at the block on the driver’s side under the head. It will have some numbers and letter. Let us know what you find for both

If it’s a 340 it’s no different from a 273 or 318 to rebuild.

If it’s 340 ;or even a 318), that 7.25 won’t last long with a built V8

FWIW, my wife and I were in Heksinki snd Rovanemi a couple of years ago to see Santa

 

[junkyardhero]

Hey yall. Im new this forum and classic mopars in general. I just recently bought my first classic american car, a 69 4 door valiant signet. I've owned it for about 2 months now, done some regular maintenance changing oils, plugs, battery and tires, and in that time i've found out some cool things about the car. I bought it thinking its just a 318 3 speed auto with some bad body paint. It was not that. It in fact has a 70 340 in it with a 727 gearbox connected to the 7 and 1/4 rear end. Power disc brakes in the front and fiberglass fenders. Im happy, but scared at the same time, because everyone says the engine is rare and not worth building, but selling. I would love to build this thing into a ripper, but im not sure if im competent enough for this engine. So my main question is does this engine need something special to work to its fullest potential? Im gonna post pics as soon as we have light here.

Build idea: A high rpm screaming cruiser. Reliable 7000rpm goal. Added power is a plus, but not the main focus, since its not a race car. Loud, but needs to pass inspection 98 decibels under 3000rpms.

What mods should i focus on, if i stay with the 340?
Is the 7 and 1/4 rear end good enough for the application?
Should i build the transmission at the same time as the motor?
What exhaust setups should i consider? thinking of going with and x pipe into mufflers and boom tubes (not a fan of the look).

Ps. Im not american so some maintenance parts might be tougher to get a hold of.

you'll get lots of opinions and advice on building the motor, so no worries there.

i wouldn't hesitate to build out that motor.

the 7 1/4 rear end is entirely not suitable for the application. you'll need to upgrade.
the 727 transmission should have upgrades as a general principle, but you need not go hog wild. a shift kit with good internals built right will live well behind a hopped up small block. be ready to spend real money on a converter, that's where the magic is.

on exhaust, headers and and a 3" system with an x pipe will be more than sufficient for anything you're gonna run. in fact 2-1/2" is more than likely just fine.

most all of the maintenance bits are shared with the LA family 273, 318, 360 so common items shouldn't be a problem to get a hold of.

 

[Kendog 170]

Welcome to the site. Check the articles section for some rebuild guidance.
How to Rebuild a Small Block Part 1: Block Prep (Updated 1/30/2025)

 

[PetCemetaryValiant]

Thank you all for the warm welcome. I tried to take off the head covers to see what heads they were, can't see anything, but there was a letter next to the sparkplug holes E-. All i could find were some 440 edelbrock heads? Im gonna take the valiant on a lift after work, just to check the block number and driveshaft bushings. I know i do have the attached 340 intake manifold, but im not sure if it would even fit a 318. I am gonna try to find a better rear end, but they are either new and expensive or used and rare.

The build would be the same effort /cost as rebuilding a 318 . Pay attention to the book and the different torques specs etc specific to a 340 and don't lose any parts they are specific to the engine etc. Do it don't be apprehensive. id take it down inspect see if it needs machine work if not hone it re ring and bearing it etc. Oh and at a glance yes Id put a beefier rear in it . a Mopar 8 3/4 or Ford 8.8 does the heads have an x or j on them?

Good afternoon and welcome to FABO. first thing I would do is confurm that you actually have a 340 in your Valiant. Get underneath on the driver’s side at look at the numbers on the side of the block. The last 3 or 5 numbers will say 340-a number. The 340 will confirm it’s a 340. To confirm the year look at the block on the driver’s side under the head. It will have some numbers and letter. Let us know what you find for both

If it’s a 340 it’s no different from a 273 or 318 to rebuild.

If it’s 340 ;or even a 318), that 7.25 won’t last long with a built V8

FWIW, my wife and I were in Heksinki snd Rovanemi a couple of years ago to see Santa

 

[65 Dartman]

. I know i do have the attached 340 intake manifold, but im not sure if it would even fit a 318.

A 340 intake will fit a 273, 318 or 360

 

[273]

Reliable 340, 7000 rpm cruiser, is a bit of a reach, here’s a 440 hp 5,900 rpm 340 that you can rev out to mid high 6,000’s.

 

[TT5.9mag]

May I ask what the appeal of 7000 rpm is if the engine doesn’t make power up there? An engine designed to operate at 7000 likely isn’t going to be happy in cruiser status, and the amount of effort to make one live up there (oiling and valvetrain modifications) isn’t worth it for cruiser duty. If I was charged with building your engine I would have you sit down and seriously consider adjusting your goals.

 

[Dartswinger70]

I do believe the early 340 was advertised as able to rev at "6000 rpm all day long" for $2800 bucks or something like that...there was a 69 and 70 Swinger 340 ad that stated such. I wonder if the later 340 still claimed to rev high...another thing to point out is throttle response and how quickly the engine revs to that point etc. but my point is build it to stock specs it should still be a ripper...

 

[furrystump]

As stated before, the rearend will not survive with spirited driving and you want to avoid the temptation of “let’s see how bad I can blow the rearend up” You can hurt the 727 pretty bad and if you don’t address that you can REALLY hurt yourself. I have no real idea if it would be a strictly better financial decision to sell the 340 (the value in your country could be much higher than here) and look for a cheaper 360 to build. (Less desirable, cheaper). Any LA small block will cost a similar amount to build. Now an LA small block at 7000 rpm does sound nice. It is a mistake to even have that be a consideration, until you are FORCED into that position. First define what your performance goals for the car are. If it is a fun car, street driven, unless you choose to paint yourself into a 7000 rpm corner, there is no need to turn it to 7000 rpm. You can build a small CI short stroke motor and spin it to the moon, but you would probably be happier with a 4” stroke 400+ CI small block. Which would be less needy with more of that “fun” torque that street driven cars really like. You should build the motor to a performance goal. The formula you use will pretty much determine the operating rpm range. It is almost always better to choose a formula that meets your goals at a lower rpm. Pretty sure every engine builder if given the the option to reduce operating rpm while still reaching the performance goal, they all would take that deal. It does sound nice pretty good…..

 

[PetCemetaryValiant]

May I ask what the appeal of 7000 rpm is if the engine doesn’t make power up there? An engine designed to operate at 7000 likely isn’t going to be happy in cruiser status, and the amount of effort to make one live up there (oiling and valvetrain modifications) isn’t worth it for cruiser duty. If I was charged with building your engine I would have you sit down and seriously consider adjusting your goals.

As stated before, the rearend will not survive with spirited driving and you want to avoid the temptation of “let’s see how bad I can blow the rearend up” You can hurt the 727 pretty bad and if you don’t address that you can REALLY hurt yourself. I have no real idea if it would be a strictly better financial decision to sell the 340 (the value in your country could be much higher than here) and look for a cheaper 360 to build. (Less desirable, cheaper). Any LA small block will cost a similar amount to build. Now an LA small block at 7000 rpm does sound nice. It is a mistake to even have that be a consideration, until you are FORCED into that position. First define what your performance goals for the car are. If it is a fun car, street driven, unless you choose to paint yourself into a 7000 rpm corner, there is no need to turn it to 7000 rpm. You can build a small CI short stroke motor and spin it to the moon, but you would probably be happier with a 4” stroke 400+ CI small block. Which would be less needy with more of that “fun” torque that street driven cars really like. You should build the motor to a performance goal. The formula you use will pretty much determine the operating rpm range. It is almost always better to choose a formula that meets your goals at a lower rpm. Pretty sure every engine builder if given the the option to reduce operating rpm while still reaching the performance goal, they all would take that deal. It does sound nice pretty good…..

Good afternoon and welcome to FABO. first thing I would do is confurm that you actually have a 340 in your Valiant. Get underneath on the driver’s side at look at the numbers on the side of the block. The last 3 or 5 numbers will say 340-a number. The 340 will confirm it’s a 340. To confirm the year look at the block on the driver’s side under the head. It will have some numbers and letter. Let us know what you find for both

If it’s a 340 it’s no different from a 273 or 318 to rebuild.

If it’s 340 ;or even a 318), that 7.25 won’t last long with a built V8

FWIW, my wife and I were in Heksinki snd Rovanemi a couple of years ago to see Santa

Update: I got under the car today to clear the numbers on the block. It is not 340 sorry, its a 360. i also opened the headcovers to see what head numbers there were, but this thing is so disgusting inside i cant see anything im gonna get some engine flush tomorow, since im gonna weld some new exhausts to it anyway, because it sounds like its not getting fuel on the driver side and we wanna make sure its not just an exhaust leak first.

Honestly i just pulled the 7000rpm number out my ***, if i could exlpain it better i would say something that has very good throttle response and a snappy driving style. Example would be something like craig909's f100 on youtube.

If anyone could recommend a carb i should get, id appreciate it. I have some edelbrock carb on it rn (600 i was told by the seller). Are all mopar intakes spread bore or could i fit something like a holley 4160 on the factory 340 intake?

Again i do apologize for the false information i gave about my engine.

 

[crackedback]

engine flush... oh no... how many times do you want to change the oil to get all the trash out and have that liquified junk sucked into the oil pump?

You build that 360 to spin up to 6K making power and you can put a hurt on plenty of cars.

750cfm carb on a good running 360 works well.

 

[PetCemetaryValiant]

engine flush... oh no... how many times do you want to change the oil to get all the trash out and have that liquified junk sucked into the oil pump?

You build that 360 to spin up to 6K making power and you can put a hurt on plenty of cars.

750cfm carb on a good running 360 works well.

Is there a better way to get all the gunk out of the heads? Again im new to these classic cars, so if you guys have any tips like at all. Please feel free to post them.
I was thinking a 750 might be too big, but thats good to hear. Is there any specific model i should look at?

 

[AJ/FormS]

I guarantee you that you will not like a street engine with power at 7000 rpm.
With 3.23 gears,
1) you can be speeding at the top FIRST gear. and
2) top gear is calculated to be ~180 mph, a speed your 340 Duster can never achieve., and
3) if yur not careful, it will be a dog off the line

For an Iron-headed 7000rpm engine, you will likely need 4.30 gears or thereabouts, and then, cruising 65= 3500 or more, so now, you need an overdrive. and altho 4.30gears will make her less of a dog off the line, having run this combo myself, I can't tell you how stupid this was.
And not only that, how far can you drive on the 66 liters of gas in the tank?

IMO
you need to rethink that.
I did and ended up with a real sweetheart.
Just put the Power-peak down around 5200, install some nice Hi-flowing alloy heads on her, pump the pressure up close to 200psi, and install some valve springs that can hit 7000, on occasion without pumping up. You'll be many many smiles happier.

 

[junkyardhero]

I do believe the early 340 was advertised as able to rev at "6000 rpm all day long" for $2800 bucks or something like that...there was a 69 and 70 Swinger 340 ad that stated such. I wonder if the later 340 still claimed to rev high...another thing to point out is throttle response and how quickly the engine revs to that point etc. but my point is build it to stock specs it should still be a ripper...

 

[Mattax]

But when you read the magazine reports from the time it's pretty clear that's the shift point or where it goes to shifted. Peak is a bit below 6,000 (but of course depending on gearing we often want the shift to occur just past peak).

First thing I recommend is drive it and tune it, then decide what direction to take it.
You can find a lot of throttle response and HP just by tuning an engine - especially one that has been hot rodded.
Other things, like whether it has a good (useful) valve job would have to take it apart.

Photos will help us identify what you got. For example intakes are pretty easy to pick out just by shape and design.

It's almost certainly going to have 360 heads on it. Whether it was opened up for the 2.02 valves will be a question but I don't think it matters unless/until your planning a build and the cam can use that extra potention flow.
The casting numbers can be hard to read even on a clean engine as they run partially underneath the valvetrain.

 

[PetCemetaryValiant]

But when you read the magazine reports from the time it's pretty clear that's the shift point or where it goes to shifted. Peak is a bit below 6,000 (but of course depending on gearing we often want the shift to occur just past peak).

First thing I recommend is drive it and tune it, then decide what direction to take it.
You can find a lot of throttle response and HP just by tuning an engine - especially one that has been hot rodded.
Other things, like whether it has a good (useful) valve job would have to take it apart.

Photos will help us identify what you got. For example intakes are pretty easy to pick out just by shape and design.

It's almost certainly going to have 360 heads on it. Whether it was opened up for the 2.02 valves will be a question but I don't think it matters unless/until your planning a build and the cam can use that extra potention flow.
The casting numbers can be hard to read even on a clean engine as they run partially underneath the valvetrain.
View attachment 1716445471

Yeah we did try to tune the timing and carburetor yesterday, but my dad and his friend said that one of the rockerarms might be stuck and not opening. Im gonna weld some new exhaust pipes on it tonight just to see if that takes care of the uneven firing sound. Heres some pics. Ugly as can be, but thats what i like.

 

[Dart Frog]

A stuck rocker arm probably not. I would do a proper compression test on all cylinders to find out where you're at. Procedure is on many posts. To get where you want you need quality machine work and quality parts. A strong running 360 will will quickly show you the weak points in the rest of the car. Have fun.

 

[Mattax]

Yeah we did try to tune the timing and carburetor yesterday

To what?
The factory specs are not going to be that helpful because the engine is modified and we don't know a lot about it. The more we can learn about it the easier it will be to suggest a baseline. Performance tuning is a process different than doing a factory tune up on a factory setup.

I'd start with a notebook. part of it can be a log book.
For timing, need a tachometer, a timing light and timing tape or a dialback timing light.
Measure timing from idle rpm to as high as you feel comfortable. Write down the rpm and timing (with vac advance disconnected and the vac line plugged). Then we can talk.
A compression test is a good idea too.

Heres some pics.

LOL. None of us have X-ray vision. At minimum remove the air cleaner so we can see the carb and intake.

I see headers. If the heads are later 360 heads, there are air ports under the exhaust port that have to be plugged.

We can also make suggestions that may prevent some non-engine failures down road.