130lb cylinder pressure

tak419

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This is a 69 440 I built 15 yrs ago. Has been bored .040, has stock uncut 906 heads. KeithBlack hyperutectic pistons. I can't remember the cam but it is a Mopar purple 280/474 I believe. I may have been the 280/484, honestly can't remember. The head gasket is Felpro .039. All the cylinders measure roughly 130lbs pressure. That is tested engine warm, WOT. To me it seems very low. I bought the pistons thru Mancini and asked for a compression ratio about 9.5 using the Mopar steel .020 head gasket. A couple years after building the engine I changed the head gasket to the Felpro .039. I can't believe that would make the cylinder pressure so low. If when Mancini was figuring what pistons and they forgot to add the engine had been bored .040 would it make the compression ratio that much lower or did the sell me low compression pistons, and now along with the thicker head gasket my compression is very low ? I have also read the Cam can make the cylinder pressure read low, does the Purple shaft 474 do that ? The motor runs good except the low shaky vacuum at idle 9-10, idles decent tho, it's a six pack set up by the way. Any suggestions are appreciated. This is just eating at me and may turn into a winter tear down. Also, I never measured how far down the piston sits. Thinking I need to pull a head and investigate further.
 

beanhead

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Big chambers, pistons probably down in the hole farther than you think, the 280/484's intake closing [email protected]° and 15 years of wear...130 psi doesn't sound all that unreasonable. Elevation and cam installed position plays a role also.

Did you ever test when it was newer to have a baseline?

Just ordering a 9.5 to 1 piston doesn't necessarily mean you'll end up anywhere near that. Chrysler deck heights are notoriously inconsistent...

The fact that they're all close to the same reading is a good thing; that says it's still relatively healthy. You mentioned it runs good, so unless you're ready to re-build it right now, continue the beatings and enjoy it until you are ready. Then all that stuff can be figured out..
 

tak419

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Big chambers, pistons probably down in the hole farther than you think, the 280/484's intake closing [email protected]° and 15 years of wear...130 psi doesn't sound all that unreasonable. Elevation and cam installed position plays a role also.

Did you ever test when it was newer to have a baseline?

Just ordering a 9.5 to 1 piston doesn't necessarily mean you'll end up anywhere near that. Chrysler deck heights are notoriously inconsistent...

The fact that they're all close to the same reading is a good thing; that says it's still relatively healthy. You mentioned it runs good, so unless you're ready to re-build it right now, continue the beatings and enjoy it until you are ready. Then all that stuff can be figured out..
'
Was built 15 years ago but not many miles on it. No didn't do a baseline test. Wish now I had measured more when assembling it all. It runs good. Has power but I think it should have a lot more.
 

steve340

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What makes you think Mancini figured out anything. I bet they didn't.
An overbore will increase the compression ratio because if calculated the swept volume will increase.
If it runs good IMO leave it alone or open a can of worms.
 

beanhead

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'
Was built 15 years ago but not many miles on it. No didn't do a baseline test. Wish now I had measured more when assembling it all. It runs good. Has power but I think it should have a lot more.
Power "feel" can be tuning/traction/gearing/transmission related too....in fact you can pick up a lot of street pep by adding good tires, experimenting with the suspension, and tuning. A well-dialed ignition timing curve and carb setup can be the difference between a weenydog and whippet..
If you wanna tear into it, go for it....it's just kind of a crappy time to be building anything with parts delays and backorders being what they are.
Hell I waited 6 weeks just for the good valve cover gaskets I like...
 

Mr. Cranky

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'
Was built 15 years ago but not many miles on it. No didn't do a baseline test. Wish now I had measured more when assembling it all. It runs good. Has power but I think it should have a lot more.
If the compression numbers are even on all 8 cylinders, I'm thinking it's still in good health and didn't have much more if any psi when it was built....and unless you know what the 'compression height' of the pistons that are in it now it's assumed that they are low compression pistons. It's a given....unless you specify what you want, you're getting the low dose stuff. It's just the way it is.
 

diesel_lv

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Just for reference, my ford 4.6 4v built 8.5:1 for turbo, fresh build has 140psi on side of block and 130psi on other side of block. Due to dual overhead cams, 3 chains, tensioners etc. Runs great, no smoke. I've never tested my 10.5:1 Mopar so I can't say. But, if you didn't have the decks checked and milled to "0" deck height and just put in 9.5:1 pistons, I'd say 130psi sounds about right.
 

w.Hudson

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It sounds like the engine is in good health.

My first thought is where is the cam centerline ?

If cam timing is retarded it will lower cranking compression. Advancing it will raise cranking compression.

Try advancing it 4 degrees and see how it performs. It will also gain some low end power.

Just my 2 cents
 

w.Hudson

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If a degree wheel and dial indicators are not available, you can do it another way.

After you get the timing cover off, you can do a compression test to get a baseline number.
Then after you advance the cam timing, do another compression test. If it was moved in the right direction, cranking compression will increase.

Most timing chain sets have different key ways to do this or this might be a good time to replace the timing chain and gears with a quality set.

This certainly is easier and quicker than a rebuild and can get you the results that are wanted.

It is worth the time to try.
 

BSB67

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Well, chalk it up to a learning experience.

Options:
1) Put a smaller cam in it and install it advanced,
2) Number one plus different cylinder heads (or maybe just clyinder heads)

Number one probably won't actually add more power, but will put it in a place more desirable to you and it will idle better.
 

PRHeads

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If it runs good IMO leave it alone or open a can of worms.

First thing..... try a different gauge.

If it’s still low and you really can’t live with it, like Russ says...... put a smaller cam in it.

If that still doesn’t end up being satisfactory....... open up that can of worms...... and your wallet.
 

w.Hudson

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Engine Masters did an episode on camshaft timing.

Actually they used a low compression 440 from a motor home in their test.

You should be able to watch it online. It is worth seeing.
 

Cojohnso1

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Big chambers, pistons probably down in the hole farther than you think, the 280/484's intake closing [email protected]° and 15 years of wear...130 psi doesn't sound all that unreasonable. Elevation and cam installed position plays a role also.

Did you ever test when it was newer to have a baseline?

Just ordering a 9.5 to 1 piston doesn't necessarily mean you'll end up anywhere near that. Chrysler deck heights are notoriously inconsistent...

The fact that they're all close to the same reading is a good thing; that says it's still relatively healthy. You mentioned it runs good, so unless you're ready to re-build it right now, continue the beatings and enjoy it until you are ready. Then all that stuff can be figured out..
I agree. Compression tests are only showing static cylinder pressure. And I have learned not to get hung up on that number. I raised my compression on my 451 from pump gas 10.5:1 to E-85 gas 12.7:1 by going with a smaller closed chamber B1 aluminum head. But my static cylinder pressure only went up about 10 lbs. But I was happier to see was that it was consistent to all 8 cylinders. (From 130 to 140.) Even though I was expecting more? Performance appears to be what was expected. Another words. It's not a direct ratio. In fact, the math for that I found to be subjective at best? I know the engineering crowd will beg to differ. But just reporting what my gage and time slips show. Car went from high 10 sec. To low 10 sec. Just as many E.T. and HP calculators predicted. (There were other changes too. Like lighter rods and small cam adjustment. But compression was the biggest change.) In my world? On a 3000 lb A-body? An increase of about 90-100 HP equated to a little more than 1/2 sec 1/4 mi ET gain. (Same tire/gear) I of course was hoping for more? But I'm never as fast as I hoped. Lol.
 
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Johnpat

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The old saying applies here( If It Ain't Broke Don't
Fix It)
 

tak419

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Engine Masters did an episode on camshaft timing.

Actually they used a low compression 440 from a motor home in their test.

You should be able to watch it online. It is worth seeing.
I'll see if I can find it. It sounds like some interesting material.
 

tak419

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I agree. Compression tests are only showing static cylinder pressure. And I have learned not to get hung up on that number. I raised my compression on my 451 from pump gas 10.5:1 to E-85 gas 12.7:1 by going with a smaller closed chamber B1 aluminum head. But my static cylinder pressure only went up about 10 lbs. But I was happier to see was that it was consistent to all 8 cylinders. (From 130 to 140.) Even though I was expecting more? Performance appears to be what was expected. Another words. It's not a direct ratio. In fact, the math for that I found to be subjective at best? I know the engineering crowd will beg to differ. But just reporting what my gage and time slips show. Car went from high 10 sec. To low 10 sec. Just as many E.T. and HP calculators predicted. (There were other changes too. Like lighter rods and small cam adjustment. But compression was the biggest change.) In my world? On a 3000 lb A-body? An increase of about 90-100 HP equated to a little more than 1/2 sec 1/4 mi ET gain. (Same tire/gear) I of course was hoping for more? But I'm never as fast as I hoped. Lol.
Yeah I think I'm getting hung up on the 130 number. Looking at the KB piston selection and comparing it to the CC of the 906 head my car should be in the mid 9's for compression. I have the car running good, probably going to try and put the 130 out f my head.
 

JackpotJim

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Find a set of 75cc chamber Edelbrock E Street heads. In my opinion, they're just what a lower compression 440 needs.
 
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