Cam timing with turbocharger Q

[351 celi]

This question is mainly directed at Does10s. Would it be worth the effort to advance the cam one tooth on a '73 stock 250 cam when using a small turbocharger, currently at 7 psi, with plans to go to 10psi? I am assuming my '73 is equipped with the emissions-retarded version of the timing gear, can you verify this? Is advancing the cam done to close the exhaust valves sooner to reduce the likelyhood of reversion? Will you please elaborate and also share your thoughts on the possible gain on my meager set-up: stock longblock with turbo, intercooler, 16 degrees initial timing with 10 mechanical advance, and possible water injection if needed due to future detonation?

Thanks! -Joe

 

[rbohm]

This question is mainly directed at Does10s. Would it be worth the effort to advance the cam one tooth on a '73 stock 250 cam when using a small turbocharger, currently at 7 psi, with plans to go to 10psi? I am assuming my '73 is equipped with the emissions-retarded version of the timing gear, can you verify this? Is advancing the cam done to close the exhaust valves sooner to reduce the likelyhood of reversion? Will you please elaborate and also share your thoughts on the possible gain on my meager set-up: stock longblock with turbo, intercooler, 16 degrees initial timing with 10 mechanical advance, and possible water injection if needed due to future detonation?

Thanks! -Joe

8) joe, you do not want to advance the cam one full tooth as you will not get the engine to run right, if at all. on a V8 one tooth on the cam gear is a 16 degree change, i forget what it is with a six. if you do decide to advance the cam, you want to go a maximum of 8-10 degrees with the later cams as ford retarded their cams 8 degrees to meet emissions.

 

[Does10s]

I did this exact thing on Kelly's first engine! I don't remember the exact numbers so if you try it PLEASE do your own math!

But I did use a retarded timing set and jumped the cam sprocket a tooth. After degreeing it, I ended up with 2 degrees advanced camshaft.
I also had 3 timing sets on hand to see if I could get it close to where I wanted it. I seem to remember spending a good part of one day just degreeing it!

Generally adv. the cam will move the peak hp and torque to a lower RPM. In our case as mostly a drag car, we want to get the car moving as fast as we can. So low end torque is where we want to be. The only time our engine sees 6000rpm is right before the stripe.

I believe your '73 should have the retarded timing set. But who knows if it's ever been changed???
The real important issue with turbos is valve overlap. If the duration of the intake and exhuast events are such that both valves are open at the same time (overlap) then you'll get a thing called "intake reversion". This is because the turbo creates "backpressure" (I hate to use that word...a restriction is more precise!) in the header. This pressure will exceed the intake pressure when the intake side is in vacuum, or atmosphere or just coming up on boost.
So for the sake of argument, if there is 2lbs. of "backpressure", and you just floored the throttle (atmosphere)(zero on the boost gauge), the 2lbs. of "backpressure" will want to go back down into the cyl. and reduce the amount of intake charge that enters. Once the turbo spools up and goes above our 2lbs. of boost example, it works as normal.

Hope that helps!
Later,
Will

 

[351 celi]

Thanks for the reply, gives me something to tinker with this winter, not like I do not still have much to do! I'll do my own calculations