I had Don and FBO systems curve a distributor for my setup, and I also bought his ignition. I have the car running and all, but I figured what the heck call Don at FBO and see what intial timing should be, after all I want to get the most out of my combo. I called him and he said to set the ignition at 24 degrees @ 950-1050 RPM(idle) and total should be 34 degrees @ 3000-3200rpm. Now total sounds right, but 24 intial sounds like a lot. I am not second guessing Don as he obvisouly knows what he is doing, and it shows by hom much his race car wins, just wanting to see what other people think about this. Like I said 24 sounded like a lot, But if Don says it needs that much I will try that tomorrow when I'm working on the car.
Ignition timing
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dgc333
Well-Known Member
The amount of intial timing an engine needs is a function of the cam and how much compression the engine has. Long duration cams need more timing as well as low compression.
24 degrees for an engine close to stock sounds like way to much but if you have a real radical cam maybe not.
24 degrees for an engine close to stock sounds like way to much but if you have a real radical cam maybe not.
It is a fairly radical cam, 248 degrees duration intake, and 250 degrees duration exhaust. Over .600 lift, so that makes sense. I figured Don knows what he is doing, I just never heard of that before. I guess I have been talking to a few to many chevy men. Thanks for your help.
Dan
Dan
moper
Well-Known Member
To be honest, any timing number he gave you, should be only a starting point for your own tuning. To me, 24° is too much, unless it's a very low compression engine and needs all the initial it can handle. I've never needed more than 20°, but every car, and location can be different.
340mopar
Sorgermotorsports
dgc333 said:
I must be missing something, the longer duration the cam the lower your compression number needs to be? So if I run 12:1 with a 255 @.050 duration it will run better with a 9:1 compression?
Or do you mean the longer the duration the cam the lower your cylinder pressure will be at cranking so you need more advance?
You must mean the latter because the cam will not change the compression ratio.
Mrmopartech
Well-Known Member
Do this,set the timing at 12 degrees BTDC,and see if you have either hard starting when its hot or pinging on accelaration.Are you going to be using the vacum advance at all,if he has done his job right you should not need to use it at all.If the car can handle all of what he says,use it,but from my years,your initial timing should land up at around 12-14 degress,Mrmopartech
dgc333
Well-Known Member
340mopar said:
What I meant to say is a engine with a long duration cam will need more intial timing to idle acceptably. And/or, an engine with low compression will need more intital timing idle acceptably.
Rick 5150 69
Well-Known Member
But you can effectively bleed off compression ( DCR ) by advancing cam 2 to 4 degrees via cam buttons if you are to high in Dynamic Compression Ratio.....!340mopar said:
moper
Well-Known Member
The only cam timing event that makes a difference to compression is the closing of the intake valve in relation to pistosn position in the stroke. That means by it being changed, the cylinder pressure is directly altered. The simple simple deal is centerlines set rpm ranges and shape of power curves, and lifts and ramp speeds depend on port flow characteristics. All of them interact with each other in both positive and negative ways. You cant change one to get something without giving something else up. What we call "long duration" means the openning and closing points of both valves is later in each's relevant part a stroke cycle. That means the intake cycle is tuned to use the inertia from higher rpms to fill the cylinder, and the exh is open longer for the same reason. That means at low rpm, the intake charge is no longer still moving into the cylinder when the piston starts to come up from bottom dead center off the intake stroke. The negative effect is reversion, or intake charge being pushed backwards up the port by the piston's upward motion. One way to "fix" low speed is to add power to the intake (to add inertia to the charge) by making the static compression ratio higher. That's why cam companies list a "minimum compression ratio" for each cam. And why the idle and emmissions gets bad when you dont have that higher static ratio. Another way to address the problem is to add initial timing. If the chamber design is huge (for the bore size), and there is little tumble and squish effect, and the fuel is fast burning pump gas, adding timing can help. If you go too far trying to add timing, you can get into problems because today's fuels burn so fast, the heat and pressure they create when firing goes away before the piston is in a position to really put the force on the crank. That's around 17-20° past TDC on the firing stroke. The mixture burn speed is critical to initial timing. But every engine, and every home town, can be different. That's why I say start with what he said, but try other things. I have gained a lot by limiting initial and slowing down the advancing in heavy cars with fast burn chambers. It's not "conventional", but it works. The power levels are where they should be, and the cars are drivable and snappy. It's all part of tuning the package.
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