Author Topic: Original 302 TRW 11:1 Piston Dome CC's  (Read 3952 times)

COPOZ/28

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Original 302 TRW 11:1 Piston Dome CC's
« on: March 04, 2020, 11:46:20 PM »
Has anyone gone through the labor intensive process of measuring the cc's of the original TRW forged '67-'69 302 piston dome cc's?  If my memory serves me (as well as it does as I plunge headlong towards  age 70!), the dome on these pistons displaced 10.0 cc's.  Is this correct?  Thanks.
Chuck B.
Ice cream run in our '69 Z/28 on Woodward Ave. in Royal Oak.
Daytona Yellow, NOR 05E, X33, M22, 4.88/9511CB.

firstgenaddict

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Re: Original 302 TRW 11:1 Piston Dome CC's
« Reply #1 on: March 06, 2020, 09:06:51 PM »
Correct way to check is to put the piston 1/2" down use clay to seal edges, then use a plexi plate with a hole to use a dropper to find the volume with the dome, THEN  igure the volume of a 1/2" tall by 4" diameter cylinder subtract the smaller from the larger = dome CC-- -
HOWEVER
You should be able to press the dome into some clay and then use a syringe to fill and and get a pretty good figure. (I've got a set of low mile OEM std bore GM pistons I could check tomorrow)
James
Collectin' Camaro's since "Only Rednecks drove them"
Current caretaker of 1971 LT1's - 11130 and 21783 Check out the Black 69 RS/Z28 45k mile Survivor and the Lemans Blue 69 Z 10D frame off...
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rmbuilder

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Re: Original 302 TRW 11:1 Piston Dome CC's
« Reply #2 on: March 12, 2020, 07:31:46 PM »
Chuck,

If you are referring to the original 4.00" bore,3.00" stroke, 11:1 CR engine, the dome volume on that is 10.84 cc If your heading toward 70 and only lost .84cc, you are doing fine.

Bob

COPOZ/28

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Re: Original 302 TRW 11:1 Piston Dome CC's
« Reply #3 on: March 12, 2020, 09:05:41 PM »
Thanks -- 10.84 cc's.  Yup, I am talking about the original 4' bore/3" stroke 11:1 pistons (made for Chevy by TRW).  Can I ask where this number originated -- did you go through the process of greasing the piston/bore and cc'ing the cylinder with the piston 0.500" down, or did it come from reference document?  Thanks.
Chuck B.
Ice cream run in our '69 Z/28 on Woodward Ave. in Royal Oak.
Daytona Yellow, NOR 05E, X33, M22, 4.88/9511CB.

COPOZ/28

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Re: Original 302 TRW 11:1 Piston Dome CC's
« Reply #4 on: March 25, 2020, 07:26:56 PM »
Thanks to Bob (rmbuilder) and Jerry M, I now know the 10.84 cc's number is the NHRA Stock Eliminator spec for the '69 Z28 302 piston dome volume.

In the meantime, I performed a true cc test on my '69 302 block/piston since my engine is currently apart.  To make the readings as accurate as possible, I applied pieces of scotch tape to locations on the piston skirt to keep the piston "square" in the bore.  I also wrapped a few layers of 3/4" masking tape around the empty ring lands to make the piston a fairly tight fit in the bore so it would not inadvertently slip down the bore during the cc'ing process.  Since my burette holds only 100 cc's, I then positioned the piston "down the hole" 0.400 inches (confirmed with a depth mic), rather than the more typical 0.500 inches just to be sure I wouldn't run my burette dry.  The dome height of this piston is just under 0.3 inches, so 0.400 inches down would work OK.  This was followed by an application of wheel bearing grease around the edge of the piston/bore interface to seal it off so no water would leak past.  All excess grease was then wiped clean.  Using a magnetic base dial indicator, I set it on the cleaned deck surface and zero'd it, then re-positioned the magnetic base to put the indicator's probe over the bore.  The block's deck was then leveled in both directions by putting wooden shims under various wheels of the engine stand and slightly rotating the block.  Once all this was done, I filled the burette with water and then took three separate readings, removing all the water and thoroughly blow drying the piston dome/bore with compressed air between each reading and re-filling the burette to the 100 cc mark for the next test.  The burette's exit is position very closely (an 1/8 inch or so) above the deck to keep the water exiting in a stream and not in droplets that would cause ripples in the water's surface.  As the water leaves the burette and the level in the bore slowly rises to meet the indicator's probe, the instant it makes contact, the burette valve is closed.  With practice, this point can be anticipated by watching the reflection of the probe getting closer to the rising water surface.  Very accurate volumes can be achieved.  This method was taught to me by Tim Connolly, Chrysler Engineering's one time "Guru" of cylinder head flow.  The other method is to use a flat (how flat is it really?) plastic or glass plate sealed to the deck with a film of (unknown thickness?!?) of grease with a small hole through the plate to let the water stream through.  With the Connolly probe method, when the water touches the probe, there is a major and obvious disturbance in the surface reflection.  The recorded three cc readings for my tests were 71.65, 71.65 and 71.70 cc's, for an average of 71.667 cc's.  For a theoretical true flat top piston having no dome, no valve notches at the same 0.400 inches "down the hole", and with my actual bore of 4.002", the volume of the space above the piston calculates to exactly 82.453 cc's.  The difference (82.453 minus 71.667) equals the volume of just the dome: 10.786 cc's in my case.  This figure makes sense as it comes in slightly below the max of 10.84 cc's allowed by NHRA for this engine.

Hope this helps others to give this a try on their head's chambers some day.  A 100 cc burette (non-ISO lab quality) isn't all that expensive.
Chuck B.
Ice cream run in our '69 Z/28 on Woodward Ave. in Royal Oak.
Daytona Yellow, NOR 05E, X33, M22, 4.88/9511CB.

ZLP955

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Re: Original 302 TRW 11:1 Piston Dome CC's
« Reply #5 on: March 26, 2020, 09:44:31 AM »
Thanks Chuck!
Tim in Australia.
1969 04A Van Nuys Z/28. Cortez Silver, Dark Blue interior, VE3, Z21, Z23, D55/U17, D80, flat hood.
Sold at Clippinger Chevrolet in Covina, CA.
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rmbuilder

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Re: Original 302 TRW 11:1 Piston Dome CC's
« Reply #6 on: March 30, 2020, 09:01:49 PM »
Chuck,
We built a number of engines from 1968-1972 to both OEM/NHRA 11:1 CR (3946876) and Trans-Am (TRW L-2110A-020) spec. Initially we used the piston in the bore method and found it to be very time consuming with a significant potential for error. We found the solution via a friend of my machinist who made molds of the piston crown that accurately replicated the +/- volumes. We would then measure the crown volumes in the same method as the combustion chamber. This worked well as we always prepped the crowns by putting a radius on all fine edges of the dome and valve reliefs. As a result there was a slight reduction in dome material volume (compression ratio) which we could account for, and adjust, by flat milling the cylinder heads to compensate. In Trans –Am we did not have a spec combustion chamber volume, consequently those modifications were legal.
We did have the manufacturer’s spec, however in 1970 we moved to aftermarket pistons which required us to match the pistons and the chamber to meet the rules for TA, making spec volumes less relevant.


firstgenaddict

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Re: Original 302 TRW 11:1 Piston Dome CC's
« Reply #7 on: March 31, 2020, 03:37:43 PM »
This worked well as we always prepped the crowns by putting a radius on all fine edges of the dome and valve reliefs.

Everyone take note...
this is done to limit the areas in which a "hotspot" could occur in order to mitigate detonation.
For those who do not realize - Detonation is two OR MORE flame fronts colliding inside the combustion chamber.
James
Collectin' Camaro's since "Only Rednecks drove them"
Current caretaker of 1971 LT1's - 11130 and 21783 Check out the Black 69 RS/Z28 45k mile Survivor and the Lemans Blue 69 Z 10D frame off...
https://plus.google.com/photos/112392262205377424364/albums?banner=pwa

motorman

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Re: Original 302 TRW 11:1 Piston Dome CC's
« Reply #8 on: April 26, 2020, 04:39:46 PM »
the TRW piston catalog give all the CC numbers
new Camaros owned 68 and 69 Z-28. new Corvettes owned 59,62,63,64,65,66,97,99 02,05 and 08. retired race engine builder, former NASCAR tech inspector

 

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