Running 20 pounds of Boost

The objective is to run 20 psi on standard Internals on 98 octane fuel. The objective is to run 13 degrees at 21psi.

To keep temperatures low a A rich mixture such as 10:1 will be used.

Another 5 - 8 degrees can be ran is race gas is used with a air fuel ratio of 10:1.

The ignition used depends on how early the turbo makes boost.

Although u are running correct A/F ratio EGT can be high enough to melt pistons.

Leaning the the A/F ratio from 11 to 11.5 is about 5 horse power.

Detonation

We know that combustion temperatures are in the 3,000ºF to 4,000ºF range, but TIT and EGT "only" run around 1,600ºF, and CHTs down around 400ºF. How can this be? 4,000ºF is more than enough to melt steel, so how does the interior lining of the cylinder survive? Why don't we see hotter temperatures on our instruments? Why doesn't the aluminum piston melt down, when aluminum melts at less than 1,000ºF?

There is a "thermal boundary layer," on the order of a millimeter thin or so, that acts as a buffer to protect the steel cylinder walls and the surface of the aluminum piston. Think of it as the thermal equivalent of the aerodynamic boundary layer out on your wing. The metal and the molecules right next to it will be at roughly the CHT reading or a bit higher, the next layers will be hotter and hotter, until the layer next to the combustion event will be at the combustion temperatures. That very thin thermal boundary layer acts as a nice insulation barrier, limiting the rate at which heat can be transferred from the bulk combustion gases into the interior walls of the cylinder head, cylinder barrel, and piston.

The heat transfer is continuous, as the heat moves first through the boundary layer, and then the cylinder wall and is finally carried away by the cooling airflow around the fins on the cylinders. Each intake stroke brings in a cool new charge, which starts the process all over again. There is also a matter of time of exposure. The high-pressure part of the combustion event takes up only about 40 degrees or so of crankshaft rotation, and the very hottest part of that only about 20 degrees, so during the other 700 degrees of crank rotation, cooler temperatures prevail. Many pilots mistakenly focus on the temperature of the exhaust gas as measured by their familiar EGT probes. EGT shows only a number that represents a momentary flash of heat during a small portion of the combustion cycle (when the exhaust valve opens and exhaust gas flows across the EGT probe), and a rapidly dropping temperature at that.

This is NOT the major factor that determines how hot their exhaust valve is during operation. The events that happen a few degrees of crankshaft rotation earlier are much more significant because the temperatures are MUCH hotter than the piddling little 'ol 1500ºF measured by the EGT probe.

Once detonation becomes serious enough, it disrupts the previously well-organized thermal boundary layer and allows a greatly increased rate of heat transfer from the very hot bulk combustion gases (up around 4,000F) into the cylinder head and the piston. This last stage in the process is what starts the damage, and drives the CHTs up.

There are newly proposed "standards" that define "light," "medium," and "heavy" detonation. How those are arrived at is far too complex to go into here (which means "I don't know"), but suffice it to say that a little light detonation, even for hours at a time may not be harmful, and in fact, can be beneficial. It does a marvelous job of cleaning deposits off the top of pistons, for example!

The truth of the matter is, most of these engines can operate in the light detonation condition as shown in the graphics for several hundred hours with no detectable damage, PROVIDED the CHTs remain cool and you do not experience a runaway cylinder head temperature during the process.

The problem is how to detect it, and prevent it from becoming worse, because "light" can progress rather quickly into "medium" and worse. It is a "positive feedback" process, with a very negative result!

The mechanism that causes it to be self-feeding is that the shock waves from the light detonation tend to begin to "scrub" the thermal boundary layer inside the cylinder. As that happens, the rate of heat transfer increases from the bulk combustion gases into the cylinder. That starts the CHT rising. When the CHT rises, it tends to heat up the incoming charge of new air and fuel a bit faster than the previous crank rotation, and that increases the likelihood of there being more light detonation in the next combustion cycle, which increases the disruption of the thermal boundary layer even more, which heats up ... well, you get the picture. If the cylinder is not really well-cooled, with some cooling reserve, the whole process can snowball to hell in a hurry and you end up in deep detonation trouble.

That would be bad, because at some point, detonation is definitely harmful over the long haul. Braly has run his "Little Engine That Could" deep into heavy detonation for hours on end, and has put a lot of similar time on a poor old IO-470, and an IO-520 trying to destroy the engines. They still run pretty well (Well, sorta pretty well!), but you really wouldn't want those engines in your airplane.

Now, am I recommending detonation? Definitely not! But at the same time, it is not quite the fearsome monster we've all been led to believe. The approach to detonation is gradual, and even once it begins, it does not develop so rapidly that it cannot be caught and controlled. For the most part, some light detonation will not cause immediate failure. Even some short-term (a few seconds?) medium detonation probably won't cause an engine failure "right now," but it may well do some damage that will cause a failure some time in the future.

I think we can all agree it's better to just stay away from detonation entirely. Much better!

Detonation is a very serious problem at the Reno races. Those engines are run at manifold pressures up to double the normal limits (which are already quite high). Some are run at several hundred RPM higher than design limits, with all sorts of fancy devices to inject strange stuff into the process. At those settings, any failure or miscalculation can cause almost instant heavy detonation, and destroy an engine in seconds.

But in our world, it is very difficult to induce detonation in any engine without a supercharger. Even with supercharging, it's fairly easy to avoid it with a little knowledge.

Do's and Don'ts Based on Experience

Do not run more than 15PSI on 94 Octane and a max of 18PSI on 98 Octane. You cannot run 22PSI everyday unless you have very deep pockets and can buy at least two 45 gallon drums of VP C16 race fuel to use everyday. To tune the car properly you will need to take it to a rolling road/dynojet facility and find someone in your area who knows about tuning turbo cars. By using the dynojet AFR monitor, keep the AFR at 11.0:1 on 94 Octane and 11.5:1 on 98 Octane. This will let your car live.

the lower the octane fuel the faster it burns, the faster it burns the less timing you need to get a complete burn, when you have to much the burn finishes too early and causes detonation, when you add tolunene it slows the burn rate of the fuel, hence you can run more timing. timing is something that changes from engine to engine, one 4efte may be able to run 5deg more timing than another, so there isnt really a set amount of timing to put in each engine, this is where the dyno comes in handy when tuning for max power, on a good race fuel you will loose power from too much timing before it pings, but on crappy pump gas you will ping before it looses power, a exprienced dyno operator will be able to tell by looking at the graphs if the engine likes or dislikes the extra timing, usually i will add 2 deg at a time until the power stays the same then back it back down a little to keep it safe, ign timing also effects AFR, so you have to adjust both together, i generally get the AFR in the region where i want it then adjust the timing then re adjust the AFR etc, it is a process that can take a few hours just to get it right, but the end result is worth it, the customer is happy he has got the most out of his setup and it will be realiable, for some reason people think they will make 50hp more by putting in more timing, this isnt the case with the 4efte's going from 5deg too retareded to where it should be you will gain 5-10hp! you gain more power but tuning the AFR to where it should be, you can sometimes gain 30hp by leaning it out from say 10.5:1 to 11.3:1!! basically what im trying to say is make sure the AFR is correct and timing is on the safe side and you wont be hurting engines (this is the reason my 4efe is still going!! ) dont rev over 8000rpm and use the best fuel you can and you will have trouble free motering .