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PostPosted: Wed Jun 18, 2008 4:21 pm 
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Joined: Thu May 22, 2008 11:28 am
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Location: San Diego, CA
Year and Trim: 04 Lesabre, Stock
98 Lesabre, Retired
So just for kicks I decided to determine the variance in the combustion chamber volumes on a set of 3k mile heads from an L26 (Series III n/a).

I used a variation of the old ATF in a burette method, but using a casting gel. Now, this is not going to give absolute combustion chamber volume (or allow me to directly compute compression ratio), because it does not account for the volume taken up by the sparkplug; however, it does allow a direct comparison between chambers.

I inverted the heads (with valves in) and leveled them with a wood block and a laboratory jackstand, then taped over the sparkplug holes:
Image

Then I cast a gel of measured density into each chamber, topped with thick acrylic plates to give a flat bottom at deck level:
Image

I weighed the castings, and used the density to determine volume.

I did this three times for each chamber, and averaged the results:

Cylinder, Volume (avg)

Cyl 1, 67.7 cc
Cyl 3, 66.2 cc
Cyl 5, 64.5 cc

Cyl 2, 68.9 cc
Cyl 4, 70.7 cc
Cyl 6, 69.8 cc

Wow, just wow.

That's a 9.2% difference between the largest (cyl 4) and the smallest (cyl 5) chamber. I might have expected maybe a 2% or 3% difference, and I still would have called that crappy. This is just an intolerable variation.

When you put that in compression ratio terms, that equates to a spread of nearly 0.9 compression units. In other words, while the nominal CR on the n/a Series II/III 3800 is 9.4:1, with these heads cyl 4 might be 9.0:1 and cyl 5 might be 9.9:1 (or a range from 8.5:1 to 9.4:1, or 9.4:1 to 10.3:1, you get the picture).

Ever wonder why some cars you just can't tune the knock out of? Or why some cars are factory freaks (for better or worse)? Variability. And this level of variability just rots :evil:

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PostPosted: Wed Jun 18, 2008 4:57 pm 
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Location: Central Illinois
Year and Trim: 1992 SSE
2009 G8 GT
That's a ton more difference than I would expect. That sucks.

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PostPosted: Wed Jun 18, 2008 5:03 pm 
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Location: Fayette City,PA
Year and Trim: 1990 Bonneville LE
Wow...that blows. I expected that they could hold the chamber volume within the same casting to a tighter tolerance.

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PostPosted: Wed Jun 18, 2008 5:08 pm 
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Location: Central Illinois
Year and Trim: 1992 SSE
2009 G8 GT
It's a good chance to take your largest casting and hog out all the chambers to match, though.

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WHITE WHINE - 1992 SSE Supercharged 236.26 ci (.040 Over) 15.090 at 90.2 MPH on old engine w/ slipping trans & melted O2 sensor - Gen 3 M62 and matching TB, Gen 2 Pully, Zillamotorsports Ported LIM, YT 1.72 Roller Rockers, SII FPR & Injectors, Hypertech Thermomaster chip w/ 160 Thermo, TransGo Shift Kit, Infinity/Pioneer Speakers and 10" Subs, 1300 watts, 140 amp Alternator, Ricepipe CAI w/ heatshield, Pilot Angel Eye Foglights, Clear Corners, '02 17" Chrome Bent 5's, Magnaflow F-Body Muffler and Hi-flo Cat, Ceramic Coated Ported Exhaust Manifolds, Fan Override
2009 G8 GT - Sport Red Metallic, loaded, SOLO Axlebacks, Rotofab Intake, Tuned, autodim mirror, removed intake manifold cover, HSV GTS triple gauge pod, two tone red-hot shifter and HSV SuperSport steering wheel, GXP rear sway bar and diffuser, 3.45 diff and various Camaro suspension bits, LED Taillights


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PostPosted: Wed Jun 18, 2008 5:17 pm 
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Joined: Mon May 19, 2008 2:14 pm
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Location: Orlando Fl
Year and Trim: '00 SSEi (proj), '99 Tahoe & '05 Bonne GXP
I have a set of S1 heads here. I would like to do what you did, and compare the differences between the S1 and S2/3. Though I suspect the numbers may be close to what you measured.

And one would think that the S3 heads would be closer in tolerance too.

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2000 SSEi - Resurrection in progress. Built L67 w/L32 fuel rail, ported heads, and cam. Camaro front brake system, GXP cluster, and much more in planning.

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