360 piston id and compression test results

[tanis4457]

I have a good running 1971 360 in my dart that i dont know anything about and recently did a compression test. With the engine warm all the cylinders were roughly 140-150 psi. I stuck a bore scope in the spark plug opening to see what kind of pistons i have, i saw a flat top piston with no valve reliefs and the number 30 etched in the center. Is this a .30 over cast piston by chance? Is my compression ratio roughly 8.1 with the 140-150 psi hot test? My vacuum gauge recently died but i remember the car idling at roughly 16-18inches to give an idea of cam. Im thinking of a cam swap in the future with some better flowing heads (magnum) for this la 360 with the intent to build a stout magnum short block down the road and transfer the heads over. I know ill have to change springs when going from flat to roller cam.

 

[70aarcuda]

I have a good running 1971 360 in my dart that i dont know anything about and recently did a compression test. With the engine warm all the cylinders were roughly 140-150 psi. I stuck a bore scope in the spark plug opening to see what kind of pistons i have, i saw a flat top piston with no valve reliefs and the number 30 etched in the center. Is this a .30 over cast piston by chance? Is my compression ratio roughly 8.1 with the 140-150 psi hot test? My vacuum gauge recently died but i remember the car idling at roughly 16-18inches to give an idea of cam. Im thinking of a cam swap in the future with some better flowing heads (magnum) for this la 360 with the intent to build a stout magnum short block down the road and transfer the heads over. I know ill have to change springs when going from flat to roller cam.

Piston 2380p is a flat top with no valve reliefs

 

[krazykuda]

The only way to know your compression for your parts is to measure them... Can't go by psi numbers as cam grind can affect the pressure...

How to Check Compression

 

[Divenut]

The only way to know your compression for your parts is to measure them... Can't go by psi numbers as cam grind can affect the pressure...

X2 I've had cams with lots of overlap that gave low psi numbers.

 

[krazykuda]

X2 I've had cams with lots of overlap that gave low psi numbers.

Exactly, dynamic pressure will affect psi readings...

 

[RustyRatRod]

If it is .030 over, then it has received a drop in compression ratio from stock. The 71 360 was the only LA 360 that had a higher compression ratio from the factory, due to a longer compression distance. Those pistons have never been reproduced, so they had to be replaced with the 72 and up pistons, which lowered the compression ratio. As with all Chrysler engines of that era, actual compression was lower than stated. Generally, 360s of that era are under 8:1.

 

[tanis4457]

thanks for the info everyone, guess when I remove the heads this fall, I'll do a volumetric test of the cylinder and figure what I'm working with. Then I'll be able to make a good cam decision.

 

[nm9stheham]

That is a higher number than I would expect for stock, even with the very mild cam indicated by the vacuum level. So it sounds like after market pistons or decking or head milling occured. Make my guess 8.8:1.SCR. Waht do we win for the best guess? LOL

 

[AJ/FormS]

Ok I'll jump in
At 8.8 and with a 54* ICA that would make 7.6DCR@149psi.
Are we there yet?

This would be no cam at all,lol a 256/110 in at 106 with maybe 44* or more overlap. The 126* of compression, and the 140/150 psi, means it would already be fairly stout, a hard combo to beat without spending a goodly amount of money. This will run well with city gears and factory stall.

Heres another; 262/270/108 in at 103; same 54* ICA.This might pull 16",meh, probly not.
Or 268/276/104,in at 100; same 54*ICA. Course this is not gonna pull 16/18 inches vacuum

 

[tanis4457]

Well i did just install a set of dougs headers and swapped the 2.76 gears for 3.23 (was told by previous owner it had 2.94s, he also told me it was a 273 boy was happy when i found out i had a 360) anyways with the headers and 3.23s this thing definitely picked up a bit. Going to be putting a holley sniper efi on it soon that i took off a different motor, i have the carb pretty well dialed in with a wideband 02 but just looking to improve the motor as much as i can while making it streetable.

 

[tanis4457]

That is a higher number than I would expect for stock, even with the very mild cam indicated by the vacuum level. So it sounds like after market pistons or decking or head milling occured. Make my guess 8.8:1.SCR. Waht do we win for the best guess? LOL

Mmm maybe ill have to come up with a prize for later on when i figure it out...to be continued

 

[RustyRatRod]

If the heads are in good shape, those numbers are right in line, actually. My stone stock 351M Ford has 160 PSI across the board and its pistons are .200 in the hole, WITH a dish, but it does have new heads on it. With everything working right, a low compression engine can have decent compression numbers.

 

[tanis4457]

If the heads are in good shape, those numbers are right in line, actually. My stone stock 351M Ford has 160 PSI across the board and its pistons are .200 in the hole, WITH a dish, but it does have new heads on it. With everything working right, a low compression engine can have decent compression numbers.

My goal now with this motor is to see what can be accomplished with a lower compression motor with the usual bolt on parts, I'm curious to find what kind of power lies in something like this and to show other board members what can be accomplished without spending money on a new shortblock. Back to the old adage, run what you got.

 

[RustyRatRod]

My goal now with this motor is to see what can be accomplished with a lower compression motor with the usual bolt on parts, I'm curious to find what kind of power lies in something like this and to show other board members what can be accomplished without spending money on a new shortblock. Back to the old adage, run what you got.

That's a good attitude. A lot of people think you have to have 9:1 or better to make power. Simply isn't true.

Also there are some who are against advancing camshaft timing to help cylinder pressure on a lower compression engine. Again, there's nothing wrong with that. Some call it a band aid, I call it using what you have.

This is why I always use a degree wheel to degree a cam along with a compression gauge. If I degree the cam where the card says and the compression pressure is still what I think is low, I keep going 2* at a time until I like it. Has worked well every time I did it that way.

Here's the thing. If all you want is a snappy street driver, you're lookin for performance around the 660 foot mark. I detest 1/8 mile, but that's the truth.

Most street races......or just playin around don't last much longer, so you want all the power soon as you can get it.

Of course, if you're racing for points on the 1320, you want what that cam can give you on the other end, too. It's all a compromise, and if you can live with that, then it's all good.

Just keep this in mind.....and most people either don't know it or forget it. Advancing a camshaft will give it GOBS more bottom end at a VERY small expense to the top. Retarding a cam however will remove GOBS of bottom end and give a much smaller amount on top in comparison.

The reason for that is because of the changes in cylinder pressure from advancing versus retarding.

 

[tanis4457]

Got the motor out and torn down over the past 2 weeks. All the pistons ended up being roughly .1 to .09 in the hole. The camshaft had a 911477 stamped into the end

trying to find the specs for it. The j heads appeared to be stock with double springs for each valve. Not sure what my compression was with the pistons but i could boil the tires from a stop in first gear just putting the throttle to the floor. I had my tbi efi system dialed in real good and picked up lots of performance from that.

Trying to decide what to do at the moment, ive been thinking of getting some closed chambered heads and some different pistons but these trw forged pistons appear to be pretty reliable. I definitely want more horsepower and torque. With a 904 and 3.23s it was fun but I'm looking for more probably just 380hp/400lbs of torque, so that might mean a piston change. Thoughts anyone?

 

[stixx]

Higher compression and cylinder heads with better flow are the standard recipe to gain power.
So either do some porting on your given J heads (can be done by yourself, find 318willrun's threads
on how to do this here on FABO) or go aftermarket (--> Edelbrock and the like).

 

[nm9stheham]

So what are you doing with this engine/car? The gears speak to higher CR and torque in some fashion. Even with the flat tops pistons, with your dimensions given for the piston top in the hole, give 8.1:1 Static CR with a .028" head gasket and stock J head chambers. But, really, you need to cc your chambers. I suspect the heads have been milled for you to get the cranking compression numbers you showed. And the 'tire boiling' backs that all up. Or it's a pretty short duration cam.

More HP is simply in head flow as said. Edelbrock Performers have the flow and the smaller chamber puts you up just under 9:1 SCR.

 

[tanis4457]

So what are you doing with this engine/car? The gears speak to higher CR and torque in some fashion. Even with the flat tops pistons, with your dimensions given for the piston top in the hole, give 8.1:1 Static CR with a .028" head gasket and stock J head chambers. But, really, you need to cc your chambers. I suspect the heads have been milled for you to get the cranking compression numbers you showed. And the 'tire boiling' backs that all up. Or it's a pretty short duration cam.

More HP is simply in head flow as said. Edelbrock Performers have the flow and the smaller chamber puts you up just under 9:1 SCR.

Looking to do autocross events and some open track racing. Already have the suspension, brakes, tires setup for corner carving. I'll cc the heads and see where they are at and see what i was working with.

 

[nm9stheham]

OK, well for that, you will want a broad torque curve, not emphasis on max HP. I've rallied for a long time and it is the same thing..... you'll never know what RPM's points you will need to be out of each turn as you change tracks and courses (or stages on a rally), and so a broad torque curve does you the best.

As high a CR as you can get will help extend that torque band down as low as you can get. If you want to keep those pistons, then I'd mill the present heads as much as you dare, even if you have to mill the intake side too. But seriously, cc the heads and see where you are if you are going to keep the stock ones.

And if you go with those Magnum heads and the 62 cc chambers, all of that head flow as you will get OOTB, from Indy RHS's or Edelbrocks, will be all usable for you. Those heads will get you to around 9.2:1 SCR on the present pistons.... not bad.

 

[tanis4457]

OK, well for that, you will want a broad torque curve, not emphasis on max HP. I've rallied for a long time and it is the same thing..... you'll never know what RPM's points you will need to be out of each turn as you change tracks and courses (or stages on a rally), and so a broad torque curve does you the best.

As high a CR as you can get will help extend that torque band down as low as you can get. If you want to keep those pistons, then I'd mill the present heads as much as you dare, even if you have to mill the intake side too. But seriously, cc the heads and see where you are if you are going to keep the stock ones.

And if you go with those Magnum heads and the 62 cc chambers, all of that head flow as you will get OOTB, from Indy RHS's or Edelbrocks, will be all usable for you. Those heads will get you to around 9.2:1 SCR on the present pistons.... not bad.

Ive been thinking of getting the la indy heads from imm and was planning on switching to a 1.6 roller rocker. Havent figured out what type of cam to shoot for yet, was planning on getting my cam reground and was going to speak to a cam regrinder first. Don't know if im dead set on keeping the pistons, when i drop my engine off I'll be talking to the machine shop about a few options with some of the spare parts that i have.

 

[tanis4457]

OK, well for that, you will want a broad torque curve, not emphasis on max HP. I've rallied for a long time and it is the same thing..... you'll never know what RPM's points you will need to be out of each turn as you change tracks and courses (or stages on a rally), and so a broad torque curve does you the best.

As high a CR as you can get will help extend that torque band down as low as you can get. If you want to keep those pistons, then I'd mill the present heads as much as you dare, even if you have to mill the intake side too. But seriously, cc the heads and see where you are if you are going to keep the stock ones.

And if you go with those Magnum heads and the 62 cc chambers, all of that head flow as you will get OOTB, from Indy RHS's or Edelbrocks, will be all usable for you. Those heads will get you to around 9.2:1 SCR on the present pistons.... not bad.

Cc'd my j heads worked out to about 73 cc. My head gasket was .049 thick with a diameter of 4.19.

 

[nm9stheham]

Good.. that is pretty much the standard size and the head gaskets sound like standard Felpro 8553PT's. Works out right at 8.0:1 SCR. Getting the cranking compression numbers you did would have taken a pretty small cam. (And who knows.... your gauge may be reading a bit high. )

Those heads would put you up in the low 9 SCR range as said with Mr Gasket 1121G head gaskets. IMHO, I'd be thinking about new pistons to get the SCR up a bit more for the low RPM torque in your planned use. (Plus just freshen things up so you can lean on the engine without the bottom end going away soon.) 1.6 rocker ratio is good; helps the lift to duration ratio which helps low end torque too.

If the budget allows, lighter SCAT rods and some hypereutectic pistons will lighten up the rotating assembly. Frees up some torque to go to the wheels and the shifts will be faster.

Is this a manual trans car?

 

[tanis4457]

Good.. that is pretty much the standard size and the head gaskets sound like standard Felpro 8553PT's. Works out right at 8.0:1 SCR. Getting the cranking compression numbers you did would have taken a pretty small cam. (And who knows.... your gauge may be reading a bit high. )

Those heads would put you up in the low 9 SCR range as said with Mr Gasket 1121G head gaskets. IMHO, I'd be thinking about new pistons to get the SCR up a bit more for the low RPM torque in your planned use. (Plus just freshen things up so you can lean on the engine without the bottom end going away soon.) 1.6 rocker ratio is good; helps the lift to duration ratio which helps low end torque too.

If the budget allows, lighter SCAT rods and some hypereutectic pistons will lighten up the rotating assembly. Frees up some torque to go to the wheels and the shifts will be faster.

Is this a manual trans car?

No, 904 auto. Yeah ive been thinking some new pistons are probably in order based on what I'd like to do, been thinking some hypers. Going to balance the assembly for a neutral balancer with my b&m Flexplate and also for neutral converter unsure of stall speed yet waiting on that once my cam specs are chosen. Plan on 1.6 rockers as well, i think with a few new parts and the rest freshened up it will be a nice stout combo. Unsure of how much to build the heads up though, stick with 1.88 intake or go to 2.02?? Thinking of staying 1.88, bowl blend mild port work, beehive springs worth the jump??

 

[nm9stheham]

OK, on the trans.... a manual sure would be nice for what you plan; using the reverse torque of engine braking via a manual sure is useful for helping weight transfer and setting the rear out entering turns. But that is down the road. And I asked about the trans mainly to assess any crank weight savings benefits from lighter rods. But with the heavy rotating mass of an autotrans' internals, that would not do much to change the rods.

Ummmm.... I'd stick with the stock damper. Neutral balance on one end and external assisted balance on the other is doable I suppose, but it may have some complications. And you won't neutral balance either end of that crank without added weight to the crank, which tends to be pricey. Money can be better spent elsewhere IMHO.

Head flow is ALWAYS good. Price up the new valves first and then look into the cost of new heads like RHS or Edelbrocks and compare. And the Magnum valvetrain has 1.6 rockers....