Quote:
Originally Posted by donraye
I have a burning ( no pun intended) question for someone who is knowledgeable out there.
I am building a 418 and need to make a piston choice. The choice is all about copmpression ratio and what I can go to and still use 91 octane pump gas without any detonation problems.
Someone on another web forum just wrote that with aluminum heads you can go 1 whole step higher than is possible with iron heads and still not get detonation. My questions are:
1: Is that true?
2: Considering I have some monster 210cc aluminum fully ported and chambered CNCed heads that flow pretty big numbers, so what is the maximum SAFE compression I can go to for a 91 octane motor without running into any detonation or other problems.
I do not really want to "push" the edge, but I do want to get as much power as I can for a stereet/pump gas motor.
I had always heard 10 to 10.5:1 was it, but now I hear I may be able to go 11:1 or even more. I'm an old fart so the 10:1 may have been the rule before aluminum heads were even out there.
What's the story with this? What are some of you builders going to these days with no problems.
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Maximum static compression ratio is a complex choice that depends on many factors beyond just the octane rating of the fuel that the motor will use. In general, aluminum heads will allow some increase in static compression ratio with a given CAM and induction setup (this is because of the better cooling capabilities that most aluminum heads have as compared to OEM steel heads). The rule of thumb for this is 0.5 - 1 pt increase in static CR. What really matters in terms of detonation in a well built motor is the combination of ignition advance and dynamic cylinder pressure. Consider two otherwise identical motors with 10.5:1 static compression ratios. One has a CAM deisgned to produce lots of low-end torque with a modest amount of overlap and lift. The second motor has a CAM designed for peak HP at high RPM with an agressive amount of overalap and lift. Which motor is more likely to detonate? The first one because the limited overlap will produce much higher dynamic compression ratios at lower RPMs than the more radically CAM'ed motor. The best way to go about all of this is to first get a clear picture of what you want your motor to do (is it a streeter where lots of low end torque is more important or is it a race motor where peak HP is the primary goal?). Once you've done this, select the best cyclinder heads for your applications that you can afford (aftermarket OEM heads are a good choice here for lots of reasons), then call a good CAM grinder like Comp CAMs or Crane and tell them what you are doing, what heads you are using, what the other mods are planned for the motor, etc. and let them help you pick a good CAM for you application. When this is all done, ask them what the correct static CR for you application is and pick a good set of pistons to their spec. BTW, if you are building a performance motor and you can get 93 octane pump gas, its probably a good idea to build your plane about the higher octane rating. This will help the performance of your project a bit.
- Fred