I am rebuilding my 396 and could use some (more) help.

The engine is in a 67 camaro in front of a wide ratio muncie and a 3.07:1 gear. The car is a weekend cruiser and I have no intention of getting into racing so I want to keep this motor on pump gas. The engine always seemed to lack performance, so one day I got a bug and pulled it apart. I found the compression ratio to be 7.9:1! I think this gives a DCR of 6.5:1.

Engine
396 2 bolt main
0.090" over 4.186" bore diameter
6.135" standard rods
3.766" crank stroke per literature

Cam
Competition Cam Magnum 270 hydraulic
270/270 duration (224/224 @ 0.050)
0.510/0.510 lift.
Lobe Separation Angle 110
Intake Lobe Centerline 106

Heads
GM 820 Open Chamber Heads
117.6, 118.6, 119.5, 119.4, 117, 118.8, 118, 119.6 cc'ed

Pistons
TRW 375 hp replacement
Domed, 36.6 cc displaced per Summit Racing
Deck Height 0.020" per Summit Racing
Gasket is standard Fel-pro 0.040"


Right now my quench is 0.060". From what I understand, 0.040" - 0.045" is optimal.

With these specs I come up with the following for compression ratio's:

SCR/DCR
9.96:1/8.19:1 - 117 cc chamber
9.87:1/8.11:1 - 118 cc chamber
9.78:1/8.04:1 - 119 cc chamber


I think I should be okay at this ratio with premium gas but I am wondering if the quench is too much. Dropping the quench with a thinner head gasket to .045" gives me the following compression ratios:

SCR/DCR
10.33:1/8.48:1 - 117 cc chamber
10.22:1/8.40:1 - 118 cc chamber
10.13:1/8.32:1 - 119 cc chamber


So here are my questions.

1. The variation among my chambers seem wide. Is this normal for a cast head?

2. Do I need to even them up better and if so, how do you recommend I do it?

3. Or is this really splitting hairs with my setup?

4. Will I have a problem with detonation if I leave the quench at 0.060"?

5. If I go to a thinner head gasket, I'm guessing I will need to shave some metal off the domes to lower the compression ratio?

I'm not looking for a race engine so I feel like I may be over complicating my rebuild. I could sure use some advice. Thanks.

Dave Jurek

anyone????

What's wrong with going with a set of 350hp/396 pistons?

Shouldn't they be more like 10.25 to 1 CR. vs 11 to 1 CR. for the 375hp/396 pistons??

Then you can run the numbers to see if both your static and dynamic compression ratio's don't drop down to more usable numbers with the thinner .022" shim headgaskets you are talking about to get about a .038" quench which should really be fine, imho.....

I figure anything from about 7.5 to maybe a schosh under 8.0 DCR is right in there too for being able to running pump gasoline nowadays...

How am I doing guy's???

pdq67

Scooby, I'm running a similar configuration currently in my '69 camaro. The differences are pistons, heads, cam. Here's why I say they're similar. I've got the TRW 2240 pistons +.090 with 063 heads or 100.9 cc heads. The 2240s have a 21 cc dome vs the 2242s have a 38.3 cc dome (or a 17.3 cc difference). Likewise our heads have about a 17 cc difference. I am running an XE274 and am able to run on 91 octane pump gas just fine. You may want to swap to something like the XE274 to get a later intake closing event to help out on the DCR. Hope this helps. Ted

Where did 11:1 come from?

I think PDQ67 is thinking of the 106.9cc heads with the L78 pistons. 11:1 was the advertised compression for the L78 vs 10.25:1 was for the 350hp version. Naturally by using the open chamber heads your static compression ratio is lower.

Dave,
Which pistons do you have in the engine now ??

See this thread for some background information. It has links to the pistons I pulled out of the engine.

http://www.camaros.net/forum/ultimatebb.php?ubb=get_topic;f=9;t=011511;p=1

Dave

In other words Chevy never made an L-78 with open chambered heads and really tall domed pistons???

Sorry, my bad..

pdq67

Dave

The difference in cc's in the combustion chamber can be attributed to both manufacturing process (casting and valve seat machining with the tolerances the factory spec'd out) and also if the valve seats have been repaired since it was built. Sometimes machinists will cut deaper to get a good seat. I bet if the runners are stock and you cc'd them youd see a difference from runner to runner also.

Unless your building it for heads up racing I'd just check to make sure the seats are in good shape and run them like they are. No need to spend money on a couple of cc's. I'd spend it somewhere else that would give you more bang for the buck.

Todd

Will those pistons work with closed chamber heads?

If so, you might invest in a 'blueprinted'set of small cc heads and a cam that closes the intake down at 50-53 ABDC. XE262 maybe

Even with a stouter DCR, the cam timing is a bit long for a 396, stick and shallow gears.

You need to verify the fit of the pistons to the heads at the ends for clearance before changing anything. Open chamber pistons typically have a relief at the sides of the dome, closed chamber pitons the dome goes all the way across. On a closed chamber head the comb. chamber is actually a little bit longer than the open chamber,not much though. I would check for clearance with clay before I start cutting or substituting parts. The high dome TRW's may need a little machining to fit without interference in an open chamber head. You could look into the KB Hypers, I am fairly sure these are made to fit both types, open and closed.

The trw 2242's are the 375 hp piston. They are a closed chamber piston with an advertised compression ratio of 11-1 with the small cc closed chamber head (215 , 063). They have a .335 dome. I have these exact pistons 396 + .030 with a open chamber head 119 cc and have no problems what so ever and I figure my compression ratio is very close to your first set of numbers with the quench at .060. It runs fine on pump gas. I would not go with the 2240's as that would lower the compression. With the thicker gasket you are fine, trying to run the thinner gasket you probably will be okay, just the DCR it getting on the high side a little bit.
Check everything for clearance using the thinner gasket It should be okay but check it. If it pings on pump gas ( I don't think it will ) switch to the thicker gasket all you are out is a couple gaskets and a little time to switch them.

66chevelle, thanks. I think your right. I'm over engineering this thing.

yes, these pistons will work. Yes they will fit. the heads are open chamber heads. The pistons are closed chamber pistons.

Ryan, thanks. Glad to hear someone has the same setup working on pump gas.

I'll be putting together the short block this weekend. I'm sure learning a lOT but at this point, I'm ready to get it back running.

Dave

Am I figuring this right, you'll be at 9.3:1 static? 7.72 dynamic with the cam phased in +4 advanced? ( I'd be sure to get it +4 or a freckle more).

I don't think so? SCR is the same no matter what the cam is timed in at, right? I'm calculating close to 10:1. Not sure how you got the DCR but using Pat's calculator I get close to 8.2:1.

Dave

That cam will work great in this engine, I previously had your identical setup except with 4:10s and it had good power. I spun a rod bearing, tore it down, and found the low domes to go along with 119cc chambers.I am also running the L2242 in a.060 over 396, with 118cc merlin ovals. I run 0 deck and an .041 gasket, and increased the cam to 230 @.050, 292 adv, and the power increase was great. Traction is now a major problem,I can't really twist it up until I'm in 3rd gear. I would think that going to a slightly shorter gear like a 3:31-3:42 would help a lot.