A friend of mine has a 66 chevelle with the original 375hp 396. He is also the original owner of the car so can verify that this is indeed an all original survivor. He'd like to drive the car a little more than he does, but it requires to have race gas mixed in because of the 11:1+ compression. (original iron closed chamber, rect. port heads, domed pistons)
He is not going to tear into the bottom end and change the pistons, but was considering putting on different heads for the time being to make it more pump gas friendly. He can always put the original heads back on at some future time and have it back to original.
Are there some open chamber rect. port heads he can put on there that would actually work with the closed chamber domed pistons? He wants to stick with gm heads, either iron or aluminum.

Any open chambered 781 or 049 would drop the compression, probably too much though. The big question would be the what is the Actuall compression now(static and dynamic)? VS the guestimate you posted. It may be better to get a cam that will "bleed off" some compression in the street driving RPM range, that way as long as he can keep his foot out of it, pump gas should be okay.

Milan

Any of the big port open chamber heads will work. It will lower the compression and power but it shouldn't detonate anymore.

I have a killer set of 049 heads I'll sell ya if your interested. Lots of work done to these heads, 2.25 intake 1/88 exhaust, Bowl blending, New Valves, New springs, Gasket matching, Let me know.

Oh and 122cc

What kind of timing is he running?

i'm unsure what timing he is running. Compression is whatever the factory compression ratio is for a '66 375hp 396 as this engine is untouched, factory stock. I don't think he'll do a cam swap either, but he might consider it if it was another solid flat tappet cam.
would the dome shape on the factory pistons designed for a closed chamber head clear the spark plug and valves alright on an open chamber head?

If he wants to stick with rect port heads, you can get service replacement iron open chamber rect port heads from Chevy for that engine. Casting numbers ending in 990 or 188. They have 118 cc open comb chambers so they would lower the compression ratio about .70. That should put it in the acceptable range for pump gas since his ACTUAL compression ratio (if the block hasn't been decked or heads cut) is probably around 10.5 now. They will work with closed chamber pistons.

In 1990 I bought a new 454 LS-6 crate engine, it came with closed chamber pistons & open chamber square port heads. It has a factory rating of 10.23 to 1 compression. Dave

I may be wrong on this, but I am pretty sure anything other than an aftermarket Open chamber head will have interference with the closed chamber pistons. The closed chamber piston dome is elongated as compared to an open chamber piston. Closed chambers were true bathtub shape and the pistons matched these. I would be inclined to check for clearance with a clay mock up before bolting on and trying to drive away.

I belive GRN is right; BTDT. The pistons kind of scraped on the edge of the CC, seems like it was on the exhaust side away from the plug, but that was about 25 years ago so I could be a bit foggy on the details.

Been there done that on my last 396 in my 66 , at the end of the day cheaper and easier and better results to change cam .... use Pat Kellys compression calculator to pick a cam with 8.0 or less dynamic compression .... and yes I believe Grn is correct. long time for me too since i did this those old 11.1 GM pistons are close with an open chamber head works good on the 360hp 10.25 motors .... check with others to ensure I 'm correct but I did the cam thing and no problems ... if not too concerned with performance talk to an engine builder and try to pick thicker head gaskets to reduce compression but optimum quench goes away I beleieve. Has he tried colder plugs and slowed down the advance curve and less vacuum advance .... FWIW

That 375/LS6 cam has alot of bleed down with such wide seat durations and 114 LSA. A cam big enough get the DCR down is going to be much too big for that motor.

Boy, it's too bad you can't use late model OC heads.

Iron Merlins maybe? Still wouldn't look stock.

In the last 7 years, we have put about 21K miles on our L78 with no problems. We run 92-94 octane with no additives.

Screw chnaging the heads...thats why I asked about the timing..its all in the timing..I'm sure my 454 makes way more cylinder pressure (210 lbs) than that original L78 and I get away with pump gas just fine.And as Gene said,the old L78/LS6 grind bleeds down cylinder pressure.
For all you know,after all these years,the advance bushing could be worn and its causing the timing to over advance.Get a dial back timing light and check all this..make sure it never goes over 38* total
DO the basics before changing the heads

Originally posted by Keith Tedford:
In the last 7 years, we have put about 21K miles on our L78 with no problems. We run 92-94 octane with no additives. EXACTLY!!

Save the hassles and expenses.. put the 94 in it and drive it... graemlins/thumbsup.gif

that old 396 should be running just fine on pump gas.......get the timming under control and drive it.......so many times i have ran 11 to 1 or better compression on pump gas..........and the factory cam is big and sloppy and it bleeds plenty of cylinder pressure off...........there really is no need for race gas..........do the fine tunning to the motor and drive it......

less even, 33-35 total.

I would try to use what you got first before you spend money on heads. are you running headers? what carb? you might want to try the following. we once ran the same motor with small tube headers. pulled the intake and ran a lifter valley heat shield and blocked the manifold heat. car had a 750 dp(maybe a little fat?). ran the car with NO vacuum advance hooked up. also had a erson solid cam .542 lift. we used sunoco ultra 94 and never had any ping. sold the motor to a guy who put it in a chevelle and ran high 11's. maybe just a few things you might want to try before you spend $ you don't have to. mike

On our 4346 carb we run #73 jets in the primaries and #76 in the secondaries. Keeping engine temperature down helps as well. Try a 180* thermostat.

i agree. graemlins/clonk.gif

Mr 4 speed, My 375-396 '67 Chevelle had 210 lbs. of compression on the weakest cylinders. It had 215-218 on the better ones. With a ZL-1 cam it dropped to 155-165 range but ran much better. I never tried it on low octane gas as it wouldn't shut off unless you put it in gear and loaded it against the brakes with the 102 octane of the day with the stock cam.

Originally posted by Keith Tedford:
In the last 7 years, we have put about 21K miles on our L78 with no problems. We run 92-94 octane with no additives. I tend to agree with Keith and Mr 4 Speed...I would try a thicker composition type head gasket (I'm assuming it still has the original shim type), a little less timing, and a good tune on the carb. Maybe a power valve that richens up a little sooner. I bet it would run fine on 93 unleaded.

Have you looked into Klotzlube lead additive? I'm not sure if it's lead or not, but considering it raises octane a whole number for what little is added, I believe it's TEL.

Try Kemco Lead supreme 130 real lead gas treatment. It is real Tetra Ehtyl lead..

BTW, why won't a set of squared off by the exhaust valve, open chambered smog heads work??

Or is it b/c the closed chamber piston's dome is flat too tall all the way a cross??

I think they are around 113 or so cc's if they are usable

pdq67

The closed chamber heads of the day and the matching pistons had a more elongated shape, the dome typically went all across the top of the piston. Open chamber domes are wider while closed chamber domes are longer.

Some other things besides the total timing that can reduce the tendency to detonate are: richen the mixture by upping the jet and squirter sizes, and try to get the engine to run cooler. Try to eliminate carbon from the combustion chambers by using a top engine cleaner, and try a colder spark plug. A vacuum advance limiter or adjustable vac advance may help too.

I'm running a 69 L-78. I left the crank and pistons alone (replaced rings and bearings), Went with rebuilt 049 heads and left the original Rect port intake. Funny looking mis-match but seems to make for a decent power band.

Also changed the cam to a Crane hydraulic 286/296 advertised duration, plus Harland Sharpe roller rockers.
Holley 750 single pump. Runs nice with no pings but can't seem to control the burn eyes of a rich burn :eek:
Try to run sunoco Utlra 94 whenevr possiable

If it's truly an untouched L78, you should be fine on pump gas, with a can of octane boost at the most.

I rebuilt a stone stock L-72, which is nearly identical to an L-78, barely milled the heads just to clean them up, used stock factory pistons, etc...I cc'ed the entire assembly during the Certified Stock tech at the Pure Stock Drags, and it came in at a whopping 10.2:1. I usually put a splash of race gas in with a tank of premium, but had to be careful how much *good* fuel I added or performance would suffer...

While we are on the subject. I just purchased a 68 with the L78 and it denotates at shutdown. I'm assuming an octane booster with better gas will fix that. My question is, the original car has stock heads, should I have hardended inserts installed in the exhaust valves?

An L-78 is supposed to be an 11 to 1 CR. motor whereas the other 396 motors were all 10.25 to 1 except the little 265hp, two barrel 396 motor at 9 to 1 CR.

I would figure dropping off about a 1/4 of a point in compression unless the motor is blue-printed and so an 11 to 1 becomes 10.75 and a 10.25 to 1 becomes a 10 to 1 CR. motor which is just right at the max of an iron headed pump gas motor..

As always, jmho..

pdq67