I have a 1968 Corvette with its 427 engine. I rebuilt the engine back in the early to mid 1990's. When I built this engine I wanted to make power the old fashioned way. Compression!
My mechanic and I fought over this but I ended up using Closed Chamber heads with pistons that have a high dome on them. My compression is 12.25-1, this is considered "too High" by many for use on the highways of Virginia. When I do a compression test I get numbers in the 240 plus pound range.

I used the closed chamber heads because they are more resistant to pinging or knocking according to the books that I read before the rebuild. I spent a lot of money having the Iron heads ported and polished. I could not afford the aluminum heads back then so I used Iron heads. I would switch to aluminum today if I could find a "Closed Chamber" head, it seems that everybody making heads only make "Open Chamber" heads anymore

I was terrified to hurt this new motor so I mixed lead into the gas to make the necessary 106 octane that they say this engine needs. I had no problems using the lead as long as you had a respirator, gloves, eye protection and a rubber apron to mix it up. I was able to get the octane rating I needed and all was okay.

Being a private pilot and having access to "Low Lead 100" designed for aircraft engines. This stuff worked great for a few thousand miles until I started getting lead deposits on my spark plugs. I spoke a man from Shell and he told me something interesting. It turns out that aviation gas has 8 times the amount of lead that was used in automobile gas in the 1960's. Lesson learned, DON'T USE AIRPLANE Gasoline

Now I am using a SNOW PERFORMANCE methanol/water injection system. It squirts the fluid below the base of the carburetor and mixes it up with the fuel going in. It gives me the equivalent of 116 octane fuel while running this simple Windshield washer fluid. This is the best way to prevent detonation and pinging. It reduces the temperature in the cylinder during compression and slows down the burn to a much more manageable level.

I liked it so much that I ran it on my 1994 Cadillac STS Seville just so I did not have to pay for premium all the time while getting 11 mpg in town.

Living with compression is not hard to do if you prepare for it. The higher compression makes your engine more efficient. It also makes a lot of power without using a hair dryer or super charger.

I can drive this '68 Corvette into the gas station and put 87 octane in my car without any problems. I do have two Windshield washer reservoirs in my car, one up front and one behind the passenger seat that hold 1/2 gallon of plain old windshield washer fluid used by the Snow performance system

Many years ago when GM was racing Corvettes at Le Mans they had a serious problem of broken piston lands which sidelined many of the cars. The following year they used a Bosch Control system that had the ability to identify which cylinder was pinging or knocking and to retard the timing on that particular cylinder a little (~5 degrees per rotation) at a time to suppress the pinging or detonation. It would only retard the specific cylinder while the other 7 were left alone unless they needed help.
I don't know if GM is doing this on Street Corvettes yet but they should be if they are not. I found a guy who makes this type of control system but mostly for turbo or super charged Mustangs. I am trying to persuade him to make one for my 1968 Corvette. This would be the best way to solve the detonation as it doesn't immediately retard all of the cylinders when a knock is detected.

 

The ECM wont increase timing without end, it has timing modifier limits hardcoded in, so I don't see any gain to be had there.

You would need an aftermarket tune to add more timing than it currently puts in.

 

Understood. But if I put in 93 octane, the system *will* add timing beyond what it gives running 91 octane, right?

 

I dont know all the modifiers for the LT1 tune, I would doubt that it would have much more built in from the factory than what gas was back in the 90s. So if the guys out west only had 91 back then, then I doubt it would have additional timing to give a 93 octane fueled engine.

 

Well with 12.25 to one, if your saying it works, then it must work because that sucker would be pinging

 

Not exactly the way it works ...

Again - I'm not the expert on LT1/LT4 PCM programming, but think of it this way - the Powertrain Control Module (PCM) has a big table of timing values for the engine - it reads sensor data that tell it RPM, throttle position, engine temp, intake air temp and it also includes an overall "volumetric efficiency" number (although I'm not sure that VE table impacts timing). Those inputs tell the PCM what "cell" of the table to use for the timing value (and conversely - what duty cycle to fire the injectors at). That is the timing value - unless the knock sensors are telling the PCM that they are seeing knocking - at which point the PCM will pull timing back. Nothing you do will add more timing in beyond what that table is telling the distributor to set. The only way to get more timing is to literally "edit" the tables that are programmed into the PCM from the factory. And that is exactly what "tuners" do - they edit the data in the PCM.


And YES - water / methanol injection (sometimes called chemical intercooling) is a truly wonderful way to combat pinging - and it does absolute wonders on a forced induction motor - but (and this is a HUGE but) - if you are expecting to get methanol injected, and have programmed the PCM to extract the power that can now be had since knocking is now dramatically less of a concern, and the methanol tank runs dry, or the injector clogs, or a pump gets tired and doesn't produce the pressure expected - or worse yet - fails - major engine damage can be done frighteningly fast (as in faster than a driver can react)!!!! As the Instruction Books sometimes say - "For Advanced Users Only" !

 

Hmm I see... the engine control is always operating within values set by the table. For some reason I thought that computer control (ECM) would push the timing forward to the edge of detonation and keep it there, constantly adjusting to keep optimal power and efficiency... especially during acceleration. So I thought since increased octane would inhibit detonation even more, the ECM could push even more timing, and give more power. Maybe higher octane can only be effectively used by engines with higher compression ratios than my LT1.

 

No. The ECM doesn't "Add" timing. The timing curve is programmed...it removes timing when it detects knock/pinging.

Also, to your other question, you can't just keep adding timing and gaining power (even w/200 octane). Even if you could resist all detonation, at some point, the timing is so far advanced that the explosion is pushing against the piston that is trying to rise, more than after it has begun descending.

I've had two engines that I experienced this on; an AMC 401 and a poorly built Chev 350. With both of these engines, I could advance the timing to the point where it would lose power and start to kick/buck...and no audible pinging. The Chev was an aluminum headed engine w/8:1 compression and a 234/244 cam...at high elevation -no cylinder pressure was why it wouldn't ping. IDK what the deal was w/the AMC.


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Remember - the last C4 was made in 1996, so it's more than 20 years old... Computer technology has changed a "bit" in the last 20 years. Today the PCM's are faster, have more memory and the sensor technology is also significantly better.

So - if a Powertrain Engineer who was assigned to program the PCM wanted to - he could push the envelope a bit "harder" today. Let's say that at a 2,000 RPM highway cruise, the optimum engine timing is 47 degrees BTDC with 93 octane gas and 70 degree intake air. The programmer could program in 49 degrees, and rely on the knock sensors to pull back just a touch of timing is light pinging is detected. If the gas in the tank is actually a 93.7 octane instead of the 93 octane, or if the intake air was 20 degrees - it might just net a slightly better fuel economy number, and that IS a big deal these days. Supposedly an American Automaker that competes heavily with GM did exactly this when they did the PCM programming on one of their newer trucks - they programmed in a very lean very high advance set of values right at the point where the EPA drive cycle is conducted. Go a few MPH faster (higher RPM) and you have different table values, and far better drivability but significantly worse mileage that the EPA rating....

Most of the time the factory is at least somewhat conservative in how they program the PCM - they have to make sure the car makes it through the warranty period. This is typically QUITE true in the Open Loop mode - since so little of the EPA drive cycle occurs in Open loop conditions. So - they run the mixture richer, and they pull back on the timing bit to make sure that if an injector flow diminishes a bit, the motor doesn't go lean at WOT. This is typically where the aftermarket tuners make their gains - they typically run a bit closer to the edge than the factory is willing to ... But these days - with more pressure to get better HP and fuel mileage numbers - most of the factory programming doesn't leave too too much on the table....