Hey guys- I am rebuilding my 427 right now, .030 over with TRW l2268 pistons. Couple of DCR questions: I get a 7.49 when I run it. This seems a little low to me. I am using the seat to seat numbers as reported in the GM catalog: 307/298 duration on a 114 lobe centerline. I was putting it at a 110 centerline to advance the cam 4 degrees (I am assuming crane grinds them advanced).

The static compression works out to be 11.25:1 with the closed chamber heads (107cc). To get the 11.25 advertised compression ratio, I had to use a 30.8cc dome volume with a .042in quench.

First question: Does anyone know the proper dome volume for the l2268 pistons

Second: How does that DCR look?

Third: I see two different numbers for closed chamber, rect. port heads- 109cc, and 106.9cc. Was there really a difference, or was is just reported different throughout the years?

Thanks guys,
Matt

I believe you're getting an artificially low DCR reading by using the GM advertised duration numbers. I'm guessing they use the same SAE lift point of .006 as mine (same basic cam as yours) does. It being a solid lifter cam with lash required, I'm pretty sure you have to use a greater lift figure to measure the "actual" seat to seat duration. I have the Speed Pro version of the L-78/LS-6 solid cam. The cam card shows 309/295 deg duration at SAE J604d .006 lift and 242/242 deg dur at .050 lift. I think the advertised durations for hydraulic cams can be used because there's no lash to take away effective duration. Depending on who you talk to and what the recommended lash is for a particular solid lifter cam, somewhere around .018 to .020 lift should be used to measure the duration at, or at the point of 0 lash, which would be .024 int and .028 exh on yours. UD Harold says his advertised durations on solid cams are measured at .020 lift, so it's easy to plug in his numbers on the DCR calculator. But on others like the LS-6 cam you have to use a figure somewhere between the advertised duration and .050 lift duration to get an accurate DCR. Probably no way to get it without actually measuring it in an engine with a degree wheel and dial indicator. I would make a W.A. guess at around 280 degrees at .020 lift for the LS-6 cam. That would make your DCR higher.

I think Von is right on this. Looking through a couple catalogs, the 242º@.050" of your cam matches up with aftermarket cams in the 280 to 294º range. Without measuring it, there isn't any way to know what the actual seat to seat timing is. The lash is speced at .024"/.028". The correct intake tappet lift to measure at is .018", exhaust is .020". No telling where Chevy measured it.

That clears it up a little, in the GMPP catalog, they listed those duration numbers as "at zero lash point" which I assumed was the correct seat to seat number. After reading your webpage again a couple of times Pat, I think I get it. So even though we have "zero lash point" numbers-we need the duration at a point slightly past those, which will make the duration numbers come down a bit and DCR will go up...

So got that part- any comments on the chamber volumes or L2268 piston dome volume?

The Speed Pro catalog lists dome volume for a std bore L2268F piston at 36.3 cc. The overbore versions are a fraction of a cc less to keep comp ratio the same with the increased bore. If you'll give me the last 3 numbers of the casting number on your heads I'll look that up if it's a '69 or earlier head.

Thanks Von, that looks like a lot of piston volume for only 11.25:1 compression. I wonder if TRW assumes some huge quench for that number. I was using .042 quench and I was getting 11.25 with 30.8cc, so I get 11.79:1 with that volume. hmmmmmmm.

Here's the story with my heads, A couple years back when I had them redone, I just yanked them and had them done. I didn't even think to double check the casting numbers. I took a look the other day and I have a mismatch set:

3873858- 109cc (per mortec)
3904391- 107cc (per mortec)

They are both square port closed chamber, but mortec listed them as having different CC chambers... Is that the chamber volume you see as well?

That piston dome volume is probably correct. I have the 396 375 hp (11.0-1 CR) replacement pistons and they are about the same volume. My Chevy By The Numbers book lists the 858 head (1966) at 108.989 cc and the 391 head (1967) at 106.8 cc. Don't forget the head gasket volume and the deck height (distance piston is down in the bore from the block deck) also adds cc's.