Compression numbers, what does this tell you?

[Dino]

Jeff (71 SE3834V) offered to test the compression since I really don't know much sbout this engine.  The po claims the cam is an old purple cam, 284/474 I think....pistons are supposedly trw pistons and cr was in the 10:1 range.  That last part never made sense really.  I never had any issues with the engine itself but it's obvious the cam is not matched to my dei ng.  It seems it's happiest on the freeway turning higher rpm, but pretty sluggish in town.  28" tall tires and 3.23 gears of course do not help.

I've been dumping 93 octane in this thing forever, just lke the po, and now I'm wondering if it's even needed.  Not that it matters all that much.  I want the engine to last so if 93 is recommended that's what it'll get.

The cam will be replaced with a Crower 32240, that should have happened already, but plans changed so it's on hold for now.  If you've read my other thread here you'll  know I have other things to fix first.   

Even though a battery charger was hooked up during testing, the starter did seem to spin a little slower as we went, it was definitely the fastest on the first cylinder we tested, which also gave the highest reading.

So what do these numbers tell you guys?  If anything?

1. 132     2. 136
3. 140     4. 138
5. 140     6. 131
7.  145    8. 135

[71 SE3834V]

It tells me somebody (bodies) went through the trouble to pull the plugs on a big block with headers! 

[Dino]

LMFAO   

[71 SE3834V]

Ya know I think I'll have you pull my plugs when I do my initial comp test but I'll wait until I put the heat box back on and install the A/C lines and see if you can get to 'em from the top. 

[c00nhunterjoe]

Based on the specs of the 484 purple cam, and your static numbers, im saying 8.8:1

[Dino]

Ya know I think I'll have you pull my plugs when I do my initial comp test but I'll wait until I put the heat box back on and install the A/C lines and see if you can get to 'em from the top. 

Hey at least it'll be easier access without that hood on!

Based on the specs of the 484 purple cam, and your static numbers, im saying 8.8:1

That makes a whole lot more sense than having a cr above 10:1.    Thanks Joe 

So have I been wasting money on 93 octane or should I just keep using that?

[myk]

Lucky you.  The last time I had the engine compression tested I got 115 to 125 across the block...

[Dino]

Lucky you.  The last time I had the engine compression tested I got 115 to 125 across the block...

That's not too horrible is it? 

I think Ron estimated my cr in the 8.5 range once, how he does that without having those compression numbers I don't know, but the man is good!   

I can't wait to get that Crower cam in as I believe him when he says it'll wake the xar up.   

Unfortunately we have spent thousands on our cats already in the last 2 months and it's not over yet so I don't want to spend the money just yet.  I will fix the trans though so I can drive at least.  My wife wants me to do the cam swap now and get the street demon carb but it can wait, the money is needed elsewhere.

[myk]

Hang in there brother...

[Dino]

I will buddy, I will   

[c00nhunterjoe]

Fwiw, my 383 cranks 200 psi on a "509" cam and does not ping on 93 and 40 degrees of timing.

[Dino]

Damn!  That's impressive...how fast is that sucker anyway? 

When I get it back on the road I'll run a few tanks of 89, or 91 if I can find it, and take it from there.  Is pinging the only thing I'm looking for?  I don't want to risk damage just to save a few bucks....well a lot of bucks.  I fill up at least once a week but usually more.   

[skip68]

I had to run high octane or mine would ping.  I had the 509 cam.   The builder said I should be about 10 1/2 to 1 compression but I always had my doubts.  No matter what I did it would ping on regular gas.   

[c00nhunterjoe]

You should be fine with 89. I dont even run 87 in my pickup...lol. you can make some wide open throttle pulls then take some plug readings to look for any signs of detonation but i doubt you will, especially considering most guys run in the 34-38 timing range.
 I cant give you a definate comlression number on mine because i built it when i was 15 and honestly dont remember what pistons are in it nor the deck height measurements. I can say they are flat tops with no releifs, i did not mill the block, and i run 516 closed chamber heads. My "estimate" would be in the high 9s to 10:1. Some day i will pull a bead and measure it.
 Ive honestly never had a good day at the track with it. Shes not really fast, just alot of fun. Its primarily a street car. Everytime ive gone to the track with it, there has always been issues. Broke 2 automatics and went 13.21@ 105 with 2.76 gears. Then put the 4 speed in and couldnt keep axles under it. I got 1 pass down the track last year but the starter gave us a pro tree during a test n tune run so i had a .300 light, dumped the clutch at idle and bogged through the 60 (2.9 seconds)... got mad, yanked a few gears and let off. Following run scattered another rear. Lol. It runs FEALLY good for what it is and i hope to go up with it this fall and thrash it some more, and most likely break it again... but thats for another topic.

[firefighter3931]

The numbers tell me that the engine is fairly healthy with even distribution across all cylinders. 

As I suspected the engine has lower than stock compression and could use a new cam that is more appropriate to the static compression and intended driving style 


Ron

[c00nhunterjoe]

The numbers tell me that the engine is fairly healthy with even distribution across all cylinders. 

As I suspected the engine has lower than stock compression and could use a new cam that is more appropriate to the static compression and intended driving style 


Ron

Indeed, while compression is important, you can make low compression engines scream. You just have to plan and carefully choose your parts and total package.

[Dino]

Those are some nice numbers Joe but stop breaking those poor axles!     Maybe it's dana time?   

Ron I'm literally dreaming of that new cam these days.      Any idea why this engine has lower than stock compression?  I'm happy with the reliability of the engine and am very happy to hear it seems to be a healthy engine, but am a bit curious as to why the compression is what it is.  I'm sure the car will run great after the cam swap.  It runs great now as long as I keep the rpm up but it's a bnit of a dog around town.  Well relatively speaking I suppose, it's not a 4 banger of course, it just misses that oomph until you hit 3000 rpm.

The po is a straight up guy but he didn't know much about the engine except that it came out of a 71 squad car.  I did find a dual snorkel when I got the car with a Chrysler TNT tin on it.  The pistons are supposedly TRW's but he sure was wrong on the cr!

So the plan would be to get that Crower cam, the Johnson lifters, new timing chain, and I'll probablky retire the old TQ and get a Street Demon although I'm not sure of the cfm yet.  I suppose they all have that crisp throttle response due to the small primaries thoguh and that's what matters to me most.  I want this to be my last carb until I can get sequential efi...after PA school.   

[myk]

Was your motor ever rebuilt?  According to Challenger 340, the pistons that would provide the proper compression are almost nonexistent.  So if your Pistons were ever replaced then that might be the reason.  I know that's what happened to my block, and why my 440 feels more like a 281.

Where the hell was this information 20 years ago?  

[c00nhunterjoe]

The information was always there and any reputable machine shop could have easily known that yhe compression would not be as advertised when they were building the engine. Even to this day, the advertised numbers on the standard replacement parts are incorrect. They have always used the nhra inflated numbers for some reason.

And the dana went together last year.... lol

[myk]

I meant the information for me, us, the end user.  Like I said in Drache's thread, if it ain't their car they're not too worried about it...

[c00nhunterjoe]

In their eyes, the end user doesnt need to be informed or know any better. If noone on this forum would have told you that your engine was actually 8:1 and not 10:1, could you honestly tell me that you would have ever 2nd guessed what you were origonally told? I would say no. The typical "joe schmo" only needs to know that his old smoking wore out 440 goes into the engine shop, and comes out "new again". You can hang any numbers on it you like. How many times have you heard the phrase "my engine builder says it will make 550 hp" and we all know those numbers are USUALLY over inflated.
   As you, the end user, who we will use a baseline and say "mildly educated" on engine building; you can roughly calculate what your compression ratio would be using the data available on those junk pistons. Simple math: rod length, piston height, head gasket thickness, head volume are the basic variables. You should already know 90% of the answers. To learn more on the pistons, read the sticky about the piston measurements and also browse summits online catalog and read the actual piston data vs their advertised ratios.

[1974dodgecharger]

GOOD READ, DINO!!

I myself questioned the same thing as you. My engine builder told me 10.5 to 1, but when he broke the engine in he used 89 octane!!!  When I met him he said he dynode the engine with 89 octane because I stopped him and said, 'whoaaa...89 shouldn't it be 91 or higher?' he said, 'NOPE'.  Being shocked and having 20k miles on the engine now I had to researched further and with help on this board and some compression testing on one side.  I determined it was under 10.5 to 1 with a pressure of 130 to 140 on one side and yes my battery kept turning weaker and weaker and that was for 4 cylinders testing.  To be exactly what the C.R. is hard to determine My guess was 9 to 1 or high 8s to 1 based on my heads and gaskets I used and pistons I have.  How he determined the the C.R. that I don't know he should of measured the piston stroke, but maybe he did and then the heads changed the ratio.

One thing he did get right was 500HP/515TQ...we determined drivetrain loss at 20% due to old transmission and old rear end.  I got 383HP to wheels with 393TQ

and yes I don't use 91 octane either, but hey its HOT here in AZ. 

[Dino]

I'm revisiting this thread as the engine will be pulled from the car in the near future.  The a/c should be discharged next week and I will commence on tear down immediately.

So the original plan was to remove the purple cam, install a Crower 32240 cam, and spruce up the exterior a bit so it looks nice and shiny for a change.  http://www.crower.com/mopar-350-440-b-compu-pro-hydraulic-cam-260-hdp.html

With all this I'm reading on low compression and loss of power and all that, would it be wise to get different pistons like the Speedpro 2355's to bump compression back up and wake up the smogger or is there more to it than swapping pistons?  I do not have a budget to do machine work to the engine and it's not like I will be doing much more than cruising it, but I would be lying if I said I don't like the tire to spin when I drop the hammer.  I can only do that from a stand still right now.    

The previous owner claimed the engine had TRW pistons with above 10:1 compression so seeing the compression is way lower, I seriously doubt it has those pistons.  I can't fault him though, he was told this by the owner he got it from.  After a few decades I think it's time for the bs to end and KNOW what this engine is made of.

Advice welcome and appreciated!   

According to the link I posted, the cam seems happiest with a compression around 9.5:1, that's about stock for a 440 hp isn't it?  I am ditching the headers for hp manifolds if that matters at all.

[WHITE AND RED 69]

You can always get the heads milled down to bump the compression up a bit. Or swap in a set of aluminum heads like the 75cc edelbrock e-street heads. I always hear that those are perfect for lower compression motors and should give it a bump up in compression and power.  

What heads are currently on the motor?

[Dino]

Yeah I thought about that but that's a cost I can't justify for my use.  I just found that since my engine is a '71 HP, a static CR of 8.75 is actually normal.  The engine may thus have 346 heads instead of 906 heads so if that's the case, maybe I should find a set of 906's and stock-ish pistons?

I will very likely be more than happy just swapping the cam, but if a set of cheap heads really wakes this thing up then why not?  Mileage is already a joke anyway...

These are for sale locally at this moment:  -906 Big block heads 2.14/1.81 valves bolt on and go! $400.00
                                                              -A set of 906 heads- $250

There's a third set but no specs or price.