Newbomb Turk
Well-Known Member
Good question and I'm not sure I have an answer. If we are talking about pump gas stuff then about the highest I'll go for compression is 12 or 12.2:1. It may come it a skosh higher than that but on this particular build I'm kinda stuck there. Maybe on the W-2 engine I'll shoot for 12.5 or even 12.8:1. I don't have the gonads to try 13 unless I'm 100% sure I can make it work. 13:1 is a lot to ask of pump gas.
At any rate, I go as high as the fuel and combination will allow then back off the timing to keep it from detonating.
I think the big thing is not actually the total timing, but the shape of the timing curve that matters.
As compression ratio goes up (making the assumption that you cam timing is correct for your compression ratio) you need less initial timing. Then once you figure out where peak torque is, you have to get the curve to fit that.
As an example, you might need 20 initial at 1000 RPM but peak torque isn't until 5300 and at peak torque it wants 25. That means you have 4300 RPM to gain 5 degrees of timing. That's about 1 degree of timing per 860 RPM.
Then if you need say 34 total at 6800 you have 2500 RPM to find 9 degrees of timing. That's about 278 RPM per degree. Can you see the issue?
And most likely it won't want 34. It may only want 30 at 6800. What if it want's only 28? Or 27? That's what you do on the dyno.
So how low would I go? I don't know. If I could run 26 I would. The last I knew what Pro Stock was doing they we only at 26-28 total and that was at 10k.
As an aside, my mentor says the way he tunes nitrous is to set the timing at 4-5 degrees total and put the hose to it until it runs with the timing that far down. He told me he thinks people don't retard the timing far enough using NOS.
To that end (whether NA or power adder) we shouldn't be setting timing based on anything other than making sure the burn is 90-94% done by 12-18 degrees after TDC. The later you can light the plug and still get the burn done at the right time you will make more power.
Steve Morris posted a video yesterday about a PST engine he had on the pump. On his last pull he added 2 degrees of timing and he gained 20 horsepower IIRC.
The up is if you look at the curve you can see that 2 degrees more around peak torque hurt it. He didn't seem to care because it was probably low enough at the track it would never see that RPM under load but he is abusing it on the dyno if he's pulling it that low.
For a street/strip car or even a foot brake car you get too much timing around peak torque and you will kill power by what I consider a lot. Like 20-30 foot pounds.
You are playing the game of trying to get the most power over the widest RPM range. And that's not all that easy if you are a street/strip car and you don't want to retard timing at peak power to stop tip in detonation.
