Big block open chamber heads

[Michael H.]

Re: Another opinion

Steve,

I totally rebuilt the motor in my 66 425 HP car and left everything as it was, including the 11.0-1 pistons and closed chamber heads. If I'm going cross country and there's no racing fuel available on the way, I set the timing back two or three degrees and go. I just don't ever get on it. In 84, we traveled all the way from Illinois to the NCRS meet at the Hoover dam and back with zero problems and that was in temp's over 100 degrees for a good portion of the tour. At times, I even disconnect and plug the vacuum advance line if fuel is especially bad and I'm at higher altitudes like Colorado. (but don't tell Duke. He's correct but at altitude there were zero problems with temperature)

In town, I add about 30% racing fuel and have never heard it ping. Performance is like it should be and the exhaust note is crisp and sharp because of the comp ratio and cam. I wouldn't trade that part of all this for anything. I'd gladly pay the extra cost for fuel as the cost of new heads would far exceed that, even over time. I reciently drove the car to my new home in Florida (almost made it) and still no problems with fuel and pinging. (the spindle bearing is a different story)

That rebuild was about 21 years ago and the car still runs as good as it did when done in 83. It still has the standard soft valve seats but NOT the original valve springs and pushrods. Just another view

[Patrick H.]

Re: Another opinion

Mike,

I agree.

As another example, my 1970 Cutlass has high compression (10.5:1 nominal). Sure, it will ping and maybe knock on today's gas (92 or 93 octane), but ONLY at WOT. Driving around town, I never have any problems. In fact, since it's my wife's car she never notices anything at all.

I have yet to try a little high octane fuel in it, but I hope to in the next tank fillup. I'd just like to see if it changes the pinging at all.

With the changes Mike mentioned in his post, I would think you could use an engine with a nominal 11.0:1 compression ratio without a problem. Again, how many miles are you going to drive it? If it's going to be your 10,000 mile a year driver, that may be reason to change. But since most restored cars go on and off a trailer, maybe 10 miles driven to a show..... and for the small fuss of allowing it to keep the original parts and operate with them, it seems like a reasonoable trade off to me.

Patrick

[Steve Antonucci]

Re: Another opinion

Mike,

Did you use a solid lifter cam with original specifications, or ease-up
on that thumper cam? Why else would you change the valve springs?

Steve

[Ted S.]

Re: Big block open chamber heads

A key to determining what static compression ratio will run on pump gas is the dynamic compression ratio (DCR) calculation. Static compression ratios do not take into account the valve timing. Many of the factory high performance cams had such late intake closing events that they bleed off a significant part of the compression. Static compression ratio calculations assume the intake valve closes instantaneously at bottom dead center. In the case of the L78 cam the intake valve doesn't close until the piston is almost half way up the cylinder, therefore you don't have the full cylinder volume you only have a little more that 1/2 of it. If you do the calcs on many of the early HP engines they come in within the guidelines for running on pump gas. I have a 396 (punched to a 414) with iron heads that has a DCR of 7.96 and it runs just fine on 91 octane pump gas. The cranking compression is at 180PSI. I had a 327 in a '60 vette that had a DCR of 8.23 with iron heads and was having to bump the octane up to around 93.5 to get it to run without pinging. The cranking compression on it was just over 200 psi. The guidelines are a DCR of 8.0:1 for iron heads and 8.5:1 for aluminum heads. Some guys say you can push these a little with big blocks with .040 quench, removal of sharp edges on pistons and heads, etc. Just my $.02

[Michael H.]

Re: Another opinion

Steve,

I did use an original 3863143 solid lifter cam, the very same that would have come in the engine when new. I think the pinging problem would be worse with a shorter duration cam because the low/mid RPM ACTUAL cylinder pressures would be higher but the longer duration solid lifter cam seems to kill off some of that low RPM cylinder pressure which would be the same as reducing the physical ratio. I wanted the sound of that 143 cam and wasn't about to let it go.

I replaced the valve train components because, as far as I'm concerned, they account for more complete early BB engine failures than any other failure. The valve springs were the worst. The old single fat coil spring is very dangerous and should be avoided completely. The 2nd design was better but still with problems. The latest number, at least as of about the mid 80's, was finally a dependable spring. I only remember part of the number but it started with 62xxxxx instead of 38xxxxx. When a valve spring breaks, it allows the valve to drop into the cylinder at a very bad time in the cycle and when contact with the piston is made, it breaks off the head of the valve. The next cycle is even more interesting because now the piston is trying to occupy the same area that the valve head is taking up. Usually the only things left after that chain of events is the water pump and SOMETIMES one cylinder head.

The early pushrods are terrible also but if you have a 1970, you probably already have the newer style replacements. Most of these problems were taken care of by the beginning of the 67 model but I'd still replace all jst to be safe. I probably now have 40,000 miles on the 66 engine rebuild and have never had a problem with anything. Good luck with your project.

Michael

[Mike Cobine]

Re: Another opinion - My experience

If you detonate and break a ring or ring landing, your original block can become a sleeve candidate very quickly.

And detonation is like an impact hammer. Each bang may not be that much, but over time, it will break things.

And since it is inside, you won't see when it does to stop and prevent further damage.

Walk around a Super Chevy swap meet where there are lots of drag racing parts. You'll find lots of 7 piston sets and so on. Granted much is on the extreme, but that extreme in a quarter mile can translate into a few thousand of regular driving. And a few car shows or cruises add up in a hurry. I think you'll find that cars get more miles than you think. Think about all those at Nashville last year that ran between Bowling Green and Nashville.

I had a 350 that broke a ring probably on the way to Chicago in Georgia. When I pulled the engine out about 2000 miles later, one cylinder had a groove down it that was a half inch wide and about 1/8th inch deep.

I only think the ring broke then, because I was pulling a trailer and the engine shuddered and heaved and I prayed that it got my very pregnant wife and me to Chicago without stopping regardless of the cost to the engine. Immediately after, it began blowing oil out the breather and using oil at the rate of a big block.

That block became a core on an exchange.

[Mike Cobine]

Re: Another opinion - Very true!

Usually the only things left after that chain of events is the water pump and SOMETIMES one cylinder head.

How very true! This link is a picture of a valve head breaking off (spring didn't break) but same results, same amount of time.

In the upper picture, the upper right side you see a dark spot in the grass by the guardrail. That is approximately where the valve broke. You can see that literally within a few feet the engine was scrap!

I have 7 pistons, 7 rods, an intake, a water pump, and one head. And a very accurate way to measure how thick the cylinder wall really is.

If the link doesn't work, go to
and down to
BANG! What happens to a 1966 Corvette when a valve drops

[Ted S.]

Pump gas and DCR Info

Steve, Here's a link that may help. This one has a downloadable DCR calculator. There's several other articles out there if your interested in running on pump gas.




DCR Article

[Michael H.]

Re: VERY interesting article, thanks! *NM*

[Steve Antonucci]

Re: Another opinion - Very true!

Mike,

Your experience is exactly what I am trying to avoid!
I really won't know it happened until it is too late.
And poof! There goes all of my research, money & time
in original dated castings and other expensive engine
components. This is the reason for my interest in going
to open chambers.

Steve

[Mike Cobine]

Re: Another opinion - Very true!

Steve,

keep your headers tight.

My problem was that we had a vibration (transmission)that was shaking the headers loose. They should have been safety wired, but weren't, since they were a header bolt that was supposed to stay tight without wiring. As such, on the race before, they had loosened and coming back into the paddock, had no doubt sucked cold air in the exhaust ports. At least we believe that is what happened.

The cold air on very hot valves most likely cracked the No. 8 exhaust valve because a few laps in the next race, the head of the valve dropped, broke and canted the piston, which then took out the No. 8 cylinder rear wall.

The pictures are primarily steam from the water pressurized by the 220 psi cylinder pressure going into the crankcase and water jackets and coming out of the block overflow catch tank. It also contained oil, so after most of the steam evaporated, then the oil on the headers caught fire.

Imagine an oil fire fueled with a stream of air at 70 mph.

[Ted S.]

Pump Gas / DCR Article

Here's another article on the topic. In this Jim Hand discusses decking the block, which is a no no in our hobby. The alternative is using the steel shim gaskets like the General did to set the quench appropriately. Again just trying to help folks out before they swap out their numbers matching components. Hope this helps.




Pump Gas / DCR

[Ted S.]

Raising Octane on the Road

One other point, is if you're out on the road and need to raise the octane, an easy way to do it is to stop by the local hardware or paint store and buy a gallon or so of Toluene. A 10% mixture (9 gallons of 91 and 1 gallon of Toluene) will raise the octane from 91 to 93.3 or a 20% mixture will raise the octane to 95.6. Toluene is 114 octane and I was finding it at a local paint store for about $7/gallon. If you look at the blend of most race fuel compared to pump gas you'll find the major difference is the % of Toluene. Again just trying to help.

[Craig S.]

Re: Big block open chamber heads

Mark - the original 454 LS6 11:1 GM closed chamber pistons clear the 118cc iron and/or 077 aluminum heads fine. I have a pair of 077's I bought new in 95 (last dealer parts locator set in the country at the time) on a crate LS6 engine I built up....Craig

[Mark #28455]

Build it to run on 93 octane! *NM*