Compression test results - Where to go now?

[toupee]

See if you can get a few pics of the cylinder walls for us to see. I'm wondering if there is any cross hatch visible on the walls ? 

Report back with your piston to deck measurements and if possible a combustion chamber volume.



Ron

Here are some more pictures of the pistons.

I spun the engine until #1 was at TDC.  Here's what I got.




Pics of #7 cylinder....



Pics of #3 cylinder



#5


#1




Straightedged the deck and didn't see any gaps



Straightedged the heads and there is a little gap... don't know if it's normal or not.  I would think the gasket would fill it.



OK, now for the measurements.  Don't know if all of these are needed but I figured more is better than less.

Bore = 4.25" = 10.795 cm
Piston to deck at TDC = 0"
Piston to deck at bottom of stroke = 8.55 cm = 3.366"
Total diameter of piston = 10.6 cm = 4.173"
Flat top of piston diameter = 10.2 cm = 4.016"
Width of countersunk area at edge of piston = .2 cm = .079"
Depth of countersunk area at edge of piston = .1 cm = .039"
Distance from edge of piston to cylinder bore = .1 cm = .039"
Small recessed area in each piston is 4 mm from edge of piston and 1 mm deep

I didn't know how to measure the heads since they aren't a perfect circle and the depth is graduated.  If they are stock 906 heads they should have an area of 84 cc's right?

[toupee]

Well, don't know if I'm even close or not but using my #'s I come up with a compression ratio of about 9.1:1. 

[firefighter3931]

Excellent pics 

The cylinder walls look fine and this is not a high milage engine. There is visible cross hatch in the bores so it has been rebuilt in the recent past....the carbon buildup is from a rich mixture. 

The pistons being at Zero deck is good news for your compression ratio.  Some of the earlier 68-69 hp 383's had positive deck pistons (+ .010) 


The "normal" chamber volume for a 906 casting is 88-90cc if they haven't been cut. Yours look like they have some warpage between the cylinders which will definately contribute to head gasket sealing issues. That should be corrected before you bolt it back together.

Take the heads in to a shop and have them look over the valves/seats etc... and get them to measure the chamber volume.


The calculations assuming an 88cc chamber are 9.02:1 static compression with the .040 Felpro permatorque head gasket which most engine rebuilders use.

With a stock style .020 steel shim head gasket it comes out to 9.42:1 which is what most HP engines came with even though they were commonly called 10:1 engines. 


So, you have a few options ;

(1) Mill the heads just enough to clean them up and use the felpro gasket which will yield a very small increase.

(2) Mill the heads just enough to clean up and use the steel shim gaskets which will bump it up .2-.3 of a point.

(3) Mill the heads .030 and use the felpro gasket for a 9.4:1 ratio


Of those three options i like #3 the best. The felpro 1009 head gasket is an excellent piece and as long as both surfaces are clean and straight it will seal up just about anything....even blower builds running 12-15lbs of boost. 




Ron

[toupee]

I was talking to my uncle today and he said he knew a local guy that did some head work to his 915 heads.  He guestimated about $400 for the 906's.
I'm not so sure what to do.  Aerohead racing has rebuilt 915's for $599.  They seem to be more desireable and it would bump up my compression.  I've also thought about just spending the extra $$$ and going with a set of stealth aluminum heads or eddy rpms.
I am going to go ahead and change the cam while I have the top end of the engine apart.  That way I will know exactly what's in there.
I think I might go with a comp extreme energy 268 cam and lifters.  From what I've gathered that's a good street cam and still makes enough vacuum for accessories to run.  I also have seen the lunati voodoo cams recommended on here, but their 268 seems like it is more radical than the comp one.  Anyone with any experience/advice?

[firefighter3931]

The Comp 268 is an OK cam for a mild build with iron heads but you can do better. 

The right cam depends on several factors ; weight of the car/stall speed/rear end gearing/intake & Carb/vacuum requirements for brakes etc...

How about a rundown on the whole deal 


Ron

[toupee]

The Comp 268 is an OK cam for a mild build with iron heads but you can do better. 

The right cam depends on several factors ; weight of the car/stall speed/rear end gearing/intake & Carb/vacuum requirements for brakes etc...

How about a rundown on the whole deal 


Ron

71' Plymouth Satellite. 3640 lbs according to some web site I found.

68' 383 block. Bore and such everyone knows from the above posts

727 with a cheetah auto/manual valve body

8 3/4" rear end.  I believe it has 3.23 gears.

275/60 15 rear tires

Low stall.. factory I'm guessing.

906 heads NOW, but I am open to suggestions!

Hedman street headers - 1 3/4 primaries

Eddy DP4B intake - Like the car this intake is older than me!    Open to change

I currently have two carbs sitting around - a 600 Eddy performer and a 670 Holley street avenger, will go bigger if need be. 

Power steering

Power brakes

A/C
.......................................................................................................................................................................

I have a budget of roughly $2000.  What this really means is this is the amount of money that I have set aside to goof off with and if I go beyond this I will then be in my "family" money and my wife will become less than pleased with me 

I was planning on buying a mini starter as I hate the large factory one and considering an aluminum water pump.  Though those things aren't really gonna affect the build that much.

Just looking to make as mean of a street cruiser as I can without havening to tear apart the bottom end of the motor. 

[firefighter3931]

Ok, that's a pretty good description & breakdown of the combination in question. 

With that budget i would concentate on the heads and do a cam swap. The 906's flow well enough for a hot street 383 build so just get them milled .030 to bump up the compression a bit. Have the valvejob inspected and corrected if necessary.

The 262* Voodoo split pattern grind would work well in that engine and really wake things up under the hood. Lots of manifold vaccum for power brakes and easy tuning.  You'll want to upgrade the valvesprings while the heads are off. A double roller timing set unless there's allready one in there.

The DP4B is an excellent dual plane and the 670 avenger is perfect....no need to change either of them.

I would like to see a little more stall and gear in the car to get it moving....a PTC 11in street converter and a set of 3.91 gears would really help it launch.

Tuning the ignition is allways worth the effort and it costs very little.

All this should easily be do-able within the budget and make a noticable increase in torque and horsepower.



Ron

[toupee]

Well,

I ordered my 262 voodoo cam with lifters today.    
Say Ron, I noticed lunati suggests roller rockers with the cam I ordered.  Won't make that big a difference if I don't have them will it?

Talked to the local machine shop about my heads.  Here's what I got-

$35 dollars per head to check them for cracks, etc. = $70

To mill the heads = $40 per head for the chamber side and $40 per head for the intake side.... ran into a problem though.  They don't think they can do the intake side at the local shop so the nearest place they THOUGHT might be able to do it is 60 miles. 
Anyways...
Milling = $160

Valves = $160

Valve job/cleaning = $110

So, that means I'm sitting at $500 without valve springs.  Seems like an awful lot to me.   

[firefighter3931]

Well,

I ordered my 262 voodoo cam with lifters today.    
Say Ron, I noticed lunati suggests roller rockers with the cam I ordered.  Won't make that big a difference if I don't have them will it?

No need for roller rockers...the stock pieces are fine. 

Talked to the local machine shop about my heads. Here's what I got-

$35 dollars per head to check them for cracks, etc. = $70

To mill the heads = $40 per head for the chamber side and $40 per head for the intake side.... ran into a problem though. They don't think they can do the intake side at the local shop so the nearest place they THOUGHT might be able to do it is 60 miles.
Anyways...
Milling = $160

Valves = $160

Valve job/cleaning = $110

So, that means I'm sitting at $500 without valve springs. Seems like an awful lot to me.

Here's what i would do with the heads :

Hot tank and magnaflux + pressure test. If there are no problems have them check the guides and valves. If it needs new guides/valves/springs and some milling to raise the compression...this is where it gets expensive. 

If the guides are good and it only needs a valvejob & springs then it's worth doing. Taking .030 off the deck surface doesn't require the intake face to be milled so you can eliminate that machining expense.  Don't assume that the valves need replacing because they might not need to be. 

Basicly, the best course of action will be to get a good estimate before deciding. It might end up being more economical to go with a set of Stealth heads but you won't know that until you see the estimate.

Get pricing on a 3 angle valvejob when you talk to them next.



Ron

[jdscofield]

just curious.  referring back to your comment about being trusting pertaining to the previous owner telling you it had a fresh rebuild.  Did you buy it from a guy named dave in south bend Indiana?  I was like you, trusting.  My fresh rebuild had a blown head gasket, and a blown rear main seal, LOL.  looked rebuilt.  fresh paint on the block except under the motor mounts.  I didn't catch it because I picked it up and hauled it out of state.