Low compression 340 VS high compression...

[MOPAROFFICIAL]

Crevis volume = area above rings.

It looks like an 8.5 piston. I sold a set similar to them that were also a discontinued part number.
What camshaft?

 

[Locomotion]

Crevis volume = area above rings.

Agreed, and the generous bevel will add to the number as well. But then you can also factor in slight volume differences between spark plug brands & heat ranges.
Going by the photo, the measurement isn't particularly accurate anyway because its being cc'd .004 below deck. (I got a headache!)
So despite being almost "0" deck (block decked?), the notches must be bigger than the typical 5cc per piston + the generous bevel not found on forged pistons + the other odds & ends, results in the low compression.

 

[413]

Get piston top even with the deck.

Loosen the rod bolts. Push piston above deck, tap it down to even with the deck with a piece of wood.

Now do your CC check

 

[RustyRatRod]

I don't know how else to make sense of it. I had 30 ccs in the syringe and had 7 in it when I was done. 23 ccs were used. None leaked through the rings
If I'm wrong, I'd welcome the advice.

Sounds like you're right. Slap some bacon on a biscuit and run hell out of it.

 

[PRH]

“What I would do” is……..
Set the piston exactly .500” from the top of the block, and do a fill down volume check.
See if it confirms something close to that 23cc volume measurement.

Edit- to speed things along, I’d probably just do it at a lesser amount than .500”.
Maybe like .100” or so. Just farther enough away from the plate than TDC to make the filling easier around all the surface irregularities.
(Just calculate the volume for the specific fill down distance and subtract that from the measured volume)

 

[Kern Dog]

Crevis volume = area above rings.

It looks like an 8.5 piston. I sold a set similar to them that were also a discontinued part number.

"Crevice volume"....that is a good term to describe this.

What camshaft?

That is part of the issue. I have a new Comp flat tappet from 2010, a 280/480 and initially I figured it would be too big for a low compression engine. As I am learning here, these pistons result in a ratio even lower than I expected. The one thing nagging at me to use this cam is that I have a late 70s 360 in another car that was honed and reringed in 2002 and had low compression stock pistons, #308 heads and the Mopar Performance 280/474 cam and that one runs pretty strong for such a basic build. Given that the 360 has a .051 Fel Pro head gasket, it too must have 7.8 compression or thereabouts since they were rated just over 8.2 with a .020 steel gasket.

Get piston top even with the deck.

Loosen the rod bolts. Push piston above deck, tap it down to even with the deck with a piece of wood.

Now do your CC check

Thanks, but what would be the point of this? I was looking for a number that takes into account the volume of the chambers and "crevice volume" ?

Sounds like you're right. Slap some bacon on a biscuit and run hell out of it.

I appreciate the input from you guys. I learned a few things here.
With the other engines I've built for my own cars, I like to know the exact compression ratio. This engine will be sold sometime later so I just needed an estimate of the compression ratio to tell a future owner. I have very little experience with the 340, this is the only one that I have ever owned. I knew that there were differences in them depending on the model year, the balance factor, etc.

 

[MOPAROFFICIAL]

"Crevice volume"....that is a good term to describe this.



That is part of the issue. I have a new Comp flat tappet from 2010, a 280/480 and initially I figured it would be too big for a low compression engine. As I am learning here, these pistons result in a ratio even lower than I expected. The one thing nagging at me to use this cam is that I have a late 70s 360 in another car that was honed and reringed in 2002 and had low compression stock pistons, #308 heads and the Mopar Performance 280/474 cam and that one runs pretty strong for such a basic build. Given that the 360 has a .051 Fel Pro head gasket, it too must have 7.8 compression or thereabouts since they were rated just over 8.2 with a .020 steel gasket.



Thanks, but what would be the point of this? I was looking for a number that takes into account the volume of the chambers and "crevice volume" ?



I appreciate the input from you guys. I learned a few things here.
With the other engines I've built for my own cars, I like to know the exact compression ratio. This engine will be sold sometime later so I just needed an estimate of the compression ratio to tell a future owner. I have very little experience with the 340, this is the only one that I have ever owned. I knew that there were differences in them depending on the model year, the balance factor, etc.

Cranking compression is more important, to a point, than static. Called the dynamic or effective compression..it's what you see on a gauge while turning it over.. You can have a seemingly low compression but close the intake valve early enough to create a decent number /dynamic . For 91 octane anything under 140 psi is getting weak and best to aim is 150-165 psi on 91 pump gas, unless you have quench and or as high as you want with enough octane of course.

 

[Newbomb Turk]

How about the .028 Mr gasket head gaskets? you'll lose 4.79 cc and gain some compression, or pay and have the heads milled .020 to avoid intake face correcting/manifold fitment issues.

I had two of those fail. One wouldn’t seal when I turned the water on. I won’t use them any more. Luckily I caught them both on the dyno or it would have been far more expensive to unscrew.

 

[Newbomb Turk]

You can get a pretty thin gasket from Cometic. I would do a 1/2 down fill and redo the math.

 

[Illahe]

Care to elaborate?



I always use the compression ratio calculator posted on the Summit Racing site.

View attachment 1716480422

I have two sets of head gaskets, both are .051 from the box. The 23 ccs that I measured could be off a cc either side of 23 but that makes so little difference here from what I can tell.

I'm running Silvolite 1267 pistons which look similar, and they are approximately .014 above deck. Possibly 9.6:1 with the heads being decked considerably.

You must have a considerable dished piston to be that low...

UEM Silvolite 1267.030 MOPAR 340 LA Engines Flat Top Cast Pistons Set Of 8

 

[crackedback]

Put it together, install the 280/474 cam at 102-104 and let it eat.

Run a thinner head gasket too

Just my 2 cents

 

[Kern Dog]

I don't have precise tools to check the compression height of the piston but I can try and see what I get.
The flat smooth areas where the orange arrows are pointing are all within .002 of each other but I admit, my tools are not NASA precision.

The center was .004 below deck but the top and bottom edges varied. Could that be due to piston rock? I don't remember exactly but I think the top was at almost zero deck ( .002 ) and the bottom was at .005. I wonder if that could just be crude manufacturing/casting ?

 

[RustyRatRod]

"Crevice volume"....that is a good term to describe this.



That is part of the issue. I have a new Comp flat tappet from 2010, a 280/480 and initially I figured it would be too big for a low compression engine. As I am learning here, these pistons result in a ratio even lower than I expected. The one thing nagging at me to use this cam is that I have a late 70s 360 in another car that was honed and reringed in 2002 and had low compression stock pistons, #308 heads and the Mopar Performance 280/474 cam and that one runs pretty strong for such a basic build. Given that the 360 has a .051 Fel Pro head gasket, it too must have 7.8 compression or thereabouts since they were rated just over 8.2 with a .020 steel gasket.



Thanks, but what would be the point of this? I was looking for a number that takes into account the volume of the chambers and "crevice volume" ?



I appreciate the input from you guys. I learned a few things here.
With the other engines I've built for my own cars, I like to know the exact compression ratio. This engine will be sold sometime later so I just needed an estimate of the compression ratio to tell a future owner. I have very little experience with the 340, this is the only one that I have ever owned. I knew that there were differences in them depending on the model year, the balance factor, etc.

It rounds to 8:1. So 8:1 it is. lol

 

[RustyRatRod]

Put it together, install the 280/474 cam at 102-104 and let it eat.

Run a thinner head gasket too

Just my 2 cents

I agree. It'll run good.

 

[Icetech]

Cheap gas baby!!! this is the way! Wouldn't a low LSA cam help bring the cyl pressure up?

 

[Illahe]

I don't have precise tools to check the compression height of the piston but I can try and see what I get.
The flat smooth areas where the orange arrows are pointing are all within .002 of each other but I admit, my tools are not NASA precision.

View attachment 1716480618

The center was .004 below deck but the top and bottom edges varied. Could that be due to piston rock? I don't remember exactly but I think the top was at almost zero deck ( .002 ) and the bottom was at .005. I wonder if that could just be crude manufacturing/casting ?

Find some closed chamber heads, if your going to stick with those pistons...

 

[Kern Dog]

I'm just trying to stuff this one together with what I have here. I don't mind buying head gaskets but I'm not buying new pistons or different heads.
Besides the 5.2/5.9 Magnum, what factory heads had closed chambers?

 

[crackedback]

I'm just trying to stuff this one together with what I have here. I don't mind buying head gaskets but I'm not buying new pistons or different heads.
Besides the 5.2/5.9 Magnum, what factory heads had closed chambers?

This is similar to what we did in the late 70's early 80's. You end up with a bunch of parts and have to make them work.

There are plenty of engine that don't have the exact compression ratio that was desired that ran fricken hard. It's how you align all the pieces to function. Some is not optimal, but you aren't starting out with an optimal base either.

Really, what can you do if the compression ration doesn't come in where you want with your current parts list? Mill heads, thinner gasket... the piston isn't going anywhere.

Install it like I mentioned, put about 16* initial on it to start, and get the rest of the tune up in order. It'll run just fine.

Pick your parts, pay your money.

 

[Illahe]

I'm just trying to stuff this one together with what I have here. I don't mind buying head gaskets but I'm not buying new pistons or different heads.
Besides the 5.2/5.9 Magnum, what factory heads had closed chambers?

I'm just trying to stuff this one together with what I have here. I don't mind buying head gaskets but I'm not buying new pistons or different heads.
Besides the 5.2/5.9 Magnum, what factory heads had closed chambers?

Magnums. AMC lifters. Indio Machine in Ca is very good with them. These are original New Zealand EQ, and they were the best out of the box head I’ve tried. Competition Products.

To get the most out of you combination, a call to Schneider Cams in So Cal will reap far more results than an off the shelf grind.

I believe my low compression (9.6:1) 340 is in at 103. She’ll go 13.2 @ 6500’ DA with 1.88 valves.


I run 308 castings on my 360 with 11.5:1 and 1.94 intakes 242/242@.05. They’re okay, but they’re not magic. Far happier with the EQ headed set up.

 

[Kern Dog]

I'm just going to move ahead and finish as planned. I have a Weiand 4 barrel for it. Nothing wild, just a little taller than a ThermoQuad intake. Advancing the cam wasn't something I had planned to do but I can see how it would help crutch the low compression and resultant soft low speed response. I can tune the distributor to limit advance, run it at 16-18 degrees of initial and use a basic 1 5/8" header. It ought to run pretty good for the next guy.

 

[MidnightSwinger]

I'm just going to move ahead and finish as planned. I have a Weiand 4 barrel for it. Nothing wild, just a little taller than a ThermoQuad intake. Advancing the cam wasn't something I had planned to do but I can see how it would help crutch the low compression and resultant soft low speed response. I can tune the distributor to limit advance, run it at 16-18 degrees of initial and use a basic 1 5/8" header. It ought to run pretty good for the next guy.

Hell yeah, send it!

 

[Illahe]

I'm just going to move ahead and finish as planned. I have a Weiand 4 barrel for it. Nothing wild, just a little taller than a ThermoQuad intake. Advancing the cam wasn't something I had planned to do but I can see how it would help crutch the low compression and resultant soft low speed response. I can tune the distributor to limit advance, run it at 16-18 degrees of initial and use a basic 1 5/8" header. It ought to run pretty good for the next guy.

I'm just going to move ahead and finish as planned. I have a Weiand 4 barrel for it. Nothing wild, just a little taller than a ThermoQuad intake. Advancing the cam wasn't something I had planned to do but I can see how it would help crutch the low compression and resultant soft low speed response. I can tune the distributor to limit advance, run it at 16-18 degrees of initial and use a basic 1 5/8" header. It ought to run pretty good for the next guy.

Understood. Best of luck with your project.

 

[RustyRatRod]

I'm just going to move ahead and finish as planned. I have a Weiand 4 barrel for it. Nothing wild, just a little taller than a ThermoQuad intake. Advancing the cam wasn't something I had planned to do but I can see how it would help crutch the low compression and resultant soft low speed response. I can tune the distributor to limit advance, run it at 16-18 degrees of initial and use a basic 1 5/8" header. It ought to run pretty good for the next guy.

I don't think you're "crutching" anything. You're simply giving the engine what it needs to run well.

 

[Kern Dog]

Thanks again to those that offered advice.
Sometimes there are people that offer help then are pissed when you didn’t follow their advice. I meant no offense there.
I’ll probably finish this and install it in a car out back to break in the cam. I’ll put a few road miles on it to make sure all is well before I pull it to sell.
Cheers

 

[mopowers]

Thanks again to those that offered advice.
Sometimes there are people that offer help then are pissed when you didn’t follow their advice. I meant no offense there.
I’ll probably finish this and install it in a car out back to break in the cam. I’ll put a few road miles on it to make sure all is well before I pull it to sell.
Cheers

Why not keep it if it's a good runner? It'd probably worth more to you than someone else. Plus, you've got so many project cars - you'd use it eventually, wouldn't you?