I recently had my LS5 454 freshened with semi-high dome pistons and a mild cam. Everything else is stock with the exception of the DUI HEI distributor.
I am having an issue with detonation going up a moderate hill and pouring the coals to it. Running a static 12 degrees of advance with a static and mechanical advance of 32 deg. The Quadrajet was rebuilt by Shawn Murphy in CA. Don't see an issue there. The only other thing that I can think of is that the car runs consistently runs hot. roughly 210-215 degrees. Will the extra heat cause the detonation? Help, I am dumbfounded. Oh! compression ration is supposed to be 9.47 to 1. A very reputable builder did the machine work.
Thanks for any follow through.
Hello George, just a couple of thoughts to try to help.
Your first sentence, domed pistons and a mild cam, could set the stage for a detonation problem. The Dynamic Compression Ratio of the motor could be high enough to exceed the anti-detonation qualities of the fuel you're using. The generally-accepted limit for pump gas is 8.50:1 DCR with iron heads and about 9.0:1 DCR with aluminum heads. Equating this to cylinder pressure, Crane Cams engineers have stated that a figure of about 165 psi would be the limit for pump gas. They make no concessions for aluminum heads or a tight squish.
Here's an article that I wrote for another forum that may help you understand.
First thing I would do is to perform a compression test of the motor.
1. Warm the motor to operating temperature.
2. Remove all spark plugs to prevent unnecessary laboring of the starter.
3. Disable the ignition system at the coil to prevent a fire.
4. Use wire or rope/string to hold the primary throttle blades wide open so the motor can breathe.
5. Allow each cylinder to make at least 5 "chuffs" to that the compression gauge will register the max pressure of the cylinder.
Write 'em down as you go. If the compression exceeds 165PSI, then either retard the cam that's in there now or change to another cam that closes the intake valve a little later and re-test.
If cranking pressure is under 165 psi, then the static compression ratio and cam timing are OK with each other for operation on pump gas and we will have to look elsewhere for the problem.
My next stop would be to verify the harmonic damper for showing the correct TDC mark. If that mark is incorrect, then everything else that depends on that mark will be incorrect. Here's an article I wrote that may help you....
There is another possibility, excessive squish/quench. Most builders will suggest the anti-detonation qualities of a 0.035" to 0.045" squish. This figure is set when the planning of the motor takes place, not as an afterthought. If everything else is marginal and the squish is too wide, then detonation is assured.
As far as the operating temperature, it may be contributing to the detonation, but it is not the main cause. Millions of autos and trucks are out there operating with temperatures exceeding what you are seeing with your motor. Ideally, in my opinion, a rodder should engineer the package to run at about 195/200. These temps will help to vaporize acids that collect in the motor oil as a result of the internal combustion process. The auto makers pay one heck of a lot of money to hire engineers who will spec out all the parts for a motor to operate at max efficiency for a long time. So, I have to ask you, when's the last time you saw an original equipment motor from the factory that used a 160 thermostat??????????
All of the fellows reading this should make themselves familiar with reading spark plugs. It should be the first "go-to" when you are either chasing a problem or you just want to know that the motor is maximized for the task at hand. In order to read plugs properly, you need to make a max power run, then chop the ignition, go to neutral and coast to a stop, where you will pull the plugs and read them. Idling back to the pits or around the corner to your house will re-color the plugs to indicate the tuning at less than full throttle and will be worthless information to you.
The first indicator of detonation will be seen up on the porcelain of the plug, where it will require a 10X loupe to see what's going on. When detonation first begins, the sledgehammer strike on the piston crown jams the piston down in the bore and squirts engine oil from the top ring all across the chamber. The only place you will see this is on the porcelain of the spark plugs. As things get worse, the black oily specks that you first see will be replaced by silver specks. These are little pieces of the piston crown......the piston is melting. Get yourself a plug loupe like this.....
For the rest of you fellows, it should be evident that it is vitally important to know the exact static compression ratio of the motor so you can match up other components. Finding SCR requires 5 values....
1. cylinder volume in cc's
2. combustion chamber volume in cc's
3. head gasket volume in cc's
4. piston deck height volume in cc's
5. piston crown volume in cc's
I will be most pleased to help any of you fellows to determine the exact static compression ratio of your build, with dished pistons, flat-tops or pop-ups. All you have to do is ask and provide me with the 5 values.