What's the REAL Compression Ratio for Stock 69 440 HP??

[Jack Luckhowec]

Mopar engine experts, I disassembled a virgin 1969 440 HP from my GTX. Measured everything and did all the calculations. I am getting a real CR of 9.5:1.
Here are my calcs:
- Cylinder Head Volume (cc) 90
- Head Gasket Thickness (in) 0.02
- Bore 4.320
- Stroke 3.75
- Piston Height in Bore 0.047

Static Compression Ratio 9.48

Question to the group, when rebuilding back to stock, should I be targeting 9.5:1 compression or something higher?? I plan to use stock intake and exhaust, and use a resto purple shaft cam.

Thanks,
Jack

 

[moparsaver]

 

[furious70]

Do you want to be able to use 87 octane or ok with using 93?

 

[zyzzyx]

Don't forget the hardened valve seats!!!!

 

[wyrmrider]

use the kb quench dome pistons
you will have thicker head gaskets
measure piston to deck at all 4 corners unless oops you have it apart already
chances are good block's decks are not = or square

I would not use a copy of the 68 smog cam - the one with long closee points the intake giving exhaust dilution and rge intak killing low end witg the dcr killing late close

 

[Don Frelier]

I think 9.5 : 1 is the upper end of the spectrum with iron heads, so no I wouldn't go any higher.

 

[Nate S]

I think 9.5 : 1 is the upper end of the spectrum with iron heads, so no I wouldn't go any higher.

Yep, 9.5-9.75 is what they really were. The factory claims were a bit high. 9.5 is also about the limit of what you can do on pump 93. I just built a 426w with 9.25, factory magnum cam, -975 heads and it’ll knock on 89 sometimes but never on 93. (With proper 38 degrees all-in timing). You can back off the timing a bit but then you’re giving up power so why did you go up on compression in the first place?

 

[66Satellite47]

The published data was not a true representation of a normal production engine. To the OP, your 90cc chamber is a little large compared to my experience with virgin 906 heads, mine were 86-88cc and your deck height a very slightly further down the hole than the '68-70 blocks I measured (.038-42) but again sure in the ball park. Your calculations look basically correct, I use 4.38 for the head gasket bore and get
9.37 CR.
I'd decide on static CR based on what gas the OP plans on running and cam design used. And I would sure look at a more modern cam design as wyrmrider suggests.

 

[PRHeads]

The KB quench dome pistons, and reworking the chambers to make the quench effective is really the right choice for that type of build.

 

[wyrmrider]

Thanks PHR with the quench dome pistons and iron heads
9:1 with 256 @006 with 460 lift gives you the most torque
9.5 with a 262 @.006 or
9.7 264 @.006 cam
10:1 with a 268 Lunati Voodoo
more compression you need a bigger cam
bigger cam with 9.5 you start to loose low end and have to start thinking of converter and gears
that stock cam is really big and would like a real 10.5 or 11:1
so pick the rpm range you want to run then pick the cam and pistons

 

[Nate S]

I think you’re a bit off with the stock cam. It’s 268/284. Yes when measured to .006 it’s like 300 but it’s just the last tiny bit of ramp. .050 is 215/224 and it runs like other cams with those specs.

 

[wyrmrider]

rebuild back to stock?
first the stock cam may be 268 at .008 but its way over [email protected] and the exhaust is over 300 and your dynamic compression will reflect that. It only runs like a [email protected] cam when you get it reved up. Has very low lift for a 215 cam The intake closes even later at .002 and you do not start building compression till the valve is closed.
The intake opens early but the big problem is the exhaust closes really really late which gives a lot of overlap- this is for the EGR effect of dilution of your intake charge
It's not 1968-72 anymore
to duplicate 68-72 gas would be expensive no Cheveron Custom Supreme at the pump
Is this a matching number deal?
if not find some 67 915 heads and work with them, D Dished pistons for 9.5:1 or for
sure use the kb pistons specially designed to work with your open chamber heads
tight quench and the right cam can reduce your egt by 500 degrees with gasoline and 800 with natural gas or propane- I have lots of dyno time on those builds and helped KB design the piston with then chief engineer John Erb.

you are going to need seats and i recommend the premium dura bond sinterted metal ones
you are going to need guides and I'd use conversion guides and use 11/32 stems for a lighter valve which your valve train will love-(unless you are doing marine or a motorhome) even with a lazy stock cam or an old school cam.
If you are going to use a low lift cam think Engle, Crower, Racer Brown
you do not need roller tip rockers
If your intake seats are receeded you can go to an oversize valve
the stock stem 2.14 or long stem chevy valves in 11/32
either way you want the valve job to open up the seats and at least hand blend the bottoms- no need for a larger exhaust but ask PRH unless you are not doing seats then the 1.81 exhaust works to give you a new seat
OMG been doing these for 60 years
cut your guides for viton seals

 

[BSB67]

........To the OP, your 90cc chamber is a little large compared to my experience with virgin 906 heads, mine were 86-88cc and your deck height a very slightly further down the hole than the '68-70 blocks I measured (.038-42) but again sure in the ball park.

Agree with this.

I think most factory heads were 86 or 87 cc. I don't think measuring a head today will reflect what it was in 1968. At the end of the day we're splittin hairs 9.5 - 9.7 is where most probably were.

Your planned compression ratio should reflect what parts you plan to use, you're elevation, and gas. In my opinion, you will experiance detonation with 9.5:1, iron head, no quench, stock cam running around in NJ.

 

[wyrmrider]

with thicker head gasketss six packs came out to 9.5 with the block and heads just cleaned up
hp slightly less
the valve reliefs in the six pack pistons offsets the compression height
ps use today's light pistons not the old heavy TRW/ Sealed Power slugs
you can get a better ring pack too
It'll rev like a 327 chevy--oops 340

 

[Nate S]

Do you have any dyno data showing a comparison between the magnum cam and other similar ones? Going back to the Hot Rod 440 and 383 articles, after they did a bunch of stuff to raise the rpm range of the engine they put in a notably larger cam and got another 50 hp. So I wonder, with a cam working in the same rpm range as the magnum how much more would be gained?
The “modern cam” and “smog cam” arguments are interesting and compelling, do you have dyno sheets showing the difference?

I’d happily upgrade if everything stayed in the same rpm range, just like to see the data first.

 

[Jack Luckhowec]

Thanks everyone; lots to think about, but one thing is for sure; I will not be targeting 9,5:1 CR. Likely target 9.2 - 9.3 so I can reliably cruise on 92 pump gas.

 

[Kern Dog]

Mopar engine experts, I disassembled a virgin 1969 440 HP from my GTX. Measured everything and did all the calculations. I am getting a real CR of 9.5:1.

You are on the right track.
I have heard for years that the published numbers are wrong but this Summer I saw it for myself for the first time.
The 1970 383 2 barrel in my Charger is rated to 8.7 to 1.

F A L S E.

Mine calculated to an even 8 to 1. This is after checking head chamber volume and coming up with 90ccs. Pistons .076 in the hole. If the heads were at 80 ccs, the number grows to 8.68, pretty close to the published number but there are no 80 cc heads.
I figured that if I just took the engine apart, honed the cylinders, put the stock pistons back in with a .039 Fel Pro head gasket, I'd then be around 7.7 to 1.
Pathetic.

 

[RemCharger]

Your 383-2bbl didnt have dished pistons?

 

[Kern Dog]

Your 383-2bbl didnt have dished pistons?

Nope:

 

[furious70]

Someone had been into that 2bbl engine then