Stealth heads and compression testing

[J.Bond]

I needed to supply Hughe's engines, a compression test, before ordering my new cam. My compression numbers are 135 psi across the board. They seem some what low. The question here is, the Source 440 Stealth head uses a non stock, long spark plug compared to factory shorty's. The compression testing tool only screws `in about one quarter of the thread travel of the long plug. Am I getting low numbers because I have added that open space, that is usually taken up by the spark plug thread body?

[BSB67]

Good observation.  The answer is yes.  The amount different, however, I don't know.  If you give me a reasonable estimate on the volume difference I could probably get you something close.  I suspect that it won't be a lot, like 140, but I'm totally guessing now.

[heyoldguy]

My off hand estimate is you might gain 2.3% or go up to 138PSI with the full depth of the spark plug in.

[jlatessa]

Maybe a silly question, but are your throttle blades wide open when you are cranking?

Joe

[J.Bond]

Sorry for the late reply, received the five piece, trunk lip and tail light chrome trim for the 67 for Christmas, so, That and, do a service on the wife's car and remove the rear bumper and inner sail panels out of the 66 , and rebuild the 67 tail light assembly, I'm back.
Some info first, 67 383, cut .030, TRW 10 to 1 flat tops, stock rods and crank, c/w windage tray. Direct Connection,P4286677 ,455 Lift,272 Dur, 112 Center line and 48 degree overlap, installed straight up. Had 906's, have changed them out to a pair of 440, Stealth heads, with new 10 degree locks. A street dominator single plane with a Holley 780 vacume sec. This car is an automatic with no modifications done to the trans, or convertor. A323 rear gear with 275/60/15's

I have been concerned about piston to deck height, and wanted to post a pic, when the motor was last apart. I did not get a measurement, and this is what is on my plate.

[J.Bond]

Maybe a silly question, but are your throttle blades wide open when you are cranking?

Joe

Yes, and after charging up the battery, 140 psi is the new number.

[J.Bond]

Will the cam overlap or centerline have any effect on compression test results??


Some clarification, above cam P4286677, 1500-6000 rpm and a single plane intake, connected to a auto without a convertor, is a dog off the line. I want to change it out to the Hughe's 2942BL-7  and install it advanced at 5 degrees, and install a dual plane intake. Something a little more like idle-5500 RPM...........I forgot to mention above, stock HP manifolds as well.

[BSB67]

Hmmmm.  Not sure where to begin.

1) Your compression ratio is probably 9:1, assuming you have the 2315s, 84 cc heads, and a 0.040" head gasket.  If not the 2315s, or a thicker gasket, the compression ratio is even lower.

2) If they are 2315s, they should be about 0.015 to 0.020" below the deck, assuming little or no deck milling.

3)  The 383 is torque challenged, adding a bigger cam with tighter LSA may, or may not change the low end power.

4) Building more cylinder pressure, and/or a looser converter will sharpen up the low speed response.

5) Only the intake valve closing point will effect cylinder pressure.

6) I'm sure that you have a converter now.

7) The Huges cam is better in a lot of ways over the MP .455, but I'm not sure the difference will be that big in your application.  

Personally there are several things I would do before I changed cams, as follows, in this order:

1) Put a dual plane on it, maybe even the factory iron intake.
2) advance the .455 cam to 106°-107° ICL
3) remove the heads and mill/head gasket to 0.040" quench, and at least 10:1 CR.

After all of that, then I would consider changing the cam if you are still not satisfied.  If you are compelled to change the converter, as most everyone on this forum will suggest, I would make that #4.

FWIW, I've been in the 12s at 109 mph with that MP cam, stock converter, 3.23 gears, and exhaust manifolds with 2 1/4 " exhaust pipes.  It was however, a 440.

[Mebsuta]

Personally there are several things I would do before I changed cams, as follows, in this order:

1) Put a dual plane on it, maybe even the factory iron intake.
2) advance the .455 cam to 106°-107° ICL

For low end, I might also try a 600 cfm Carter or Edelbrock AFB, or a Holley 600.  I tried a 750 on 383 RR for a little bit and went right back to the 600.   

Do the stealth heads, with bigger valves and greater port volume, favor high rpm rather than low end torque? 

[J.Bond]

Still having a hard time getting my brain around this issue. I know it was 1985 when I built this motor, and I built it with two motors at the time. The original 2 barrel 67 motor, and, I had a 70 RR 383 at the same time, I used the 67 block, the 70 rotating mass and 906's. When I ordered the pistons, I ordered pistons for a 67, here is where the question comes in, would a TRW 2315, be different for the 1967/516 closed chamber head, as apposed to the open chamber 906/68 head?

Dual plane intake is on the list. As for a convertor, any recommendations would be appreciated.

As for advancing the cam, offset cam dowel pin??? Single bolt cam.

And, this was assembled with standard Fellpro head gaskets, .040, I believe. Can I run with a thinner gasket, .020?

Carbs, I started with a 625 Carter, switched up to the 780 Holley and the car woke up dramatically. Again , open to suggestions.

At this point I'm thinking about pulling the heads, to check some measurements and confirm piston part number.

[BSB67]

here is where the question comes in, would a TRW 2315, be different for the 1967/516 closed chamber head, as apposed to the open chamber 906/68 head?

As for advancing the cam, offset cam dowel pin??? Single bolt cam.

And, this was assembled with standard Fellpro head gaskets, .040, I believe. Can I run with a thinner gasket, .020?

Carbs, I started with a 625 Carter, switched up to the 780 Holley and the car woke up dramatically. Again , open to suggestions.

At this point I'm thinking about pulling the heads, to check some measurements and confirm piston part number.

No.  The 2315 has a 1.920" compression height.  It does not care what year the block is, or heads you put on it.  The 67 piston will be lower in the block than the 68 HP piston, or the 2315s.

Yes, sort of.  Offset dowel bushing.  You will need to drill out the pin hole in the cam gear to install the bushing.  then you will need to measure the cam (degree) to determine its location.

FelPro makes a 0.055" head gasket that usually comes in the gasket kit.

I probably would not put a carb change on the top of your list of things to do.

Your cylinder pressure test tells us a lot (i.e. not very high) but you will need to pull the heads to know what you really have.  One might argue, if you are not planning to do anything to change your compression ratio/cylinder pressure, why pull the head.  Part numbers help, a measurement is better.