I have been thinking and doing much research on the effects of cam timing on a turbocharged engine. Yesterday I was pondering EVC and IVO and a thought struck me that many of you probably have already had...

It seems that an andvanced camshaft generally speaking opens the exhaust sooner and allows the amplified (boost) pressure wave to escape early and help to increase the mass of air between the turbine and exhaust valve early in the cycle. The earlier closing intake helps trap cylinder pressure which seems to be more beneficial than allowing it to linger in the cylinder longer on the power stroke as a cam installed straight up would.

So on to my thought. It seems that the earlier opening intake valve would start to fill the cylinder with positive pressure while the piston is a bit farther down the bore. As the piston rapidly approaches zero velocity at TDC on the exhaust stroke the rod experiences massive stress as it tries to slow down all that mass. That mass only gets larger as piston speed goes up. But it seems if there is boost pressure exerting force on top of the piston earlier in the cycle this would reduce the tensional stress on the rod as it is slowing down.

Would this contribute to less stress on the rods or be negligible at best?

Maybe it depends on the point in the sine wave on the exhaust stroke where the piston/rod assembly is decelerating the fastest

 

The early charge force acting on the rod would be negligle compared to the force exerted on the rod during combustion. You can easily calculate this force by using you surface area of your piston ( pi * r^2 ) and the amount of boost it will see.

Example on a 302. 4" bore so 3.14156 * 2^2 = 12.56"^2 or 12.5sq-in Then figure in your boost pressure, say 10#. then you have 10#*12.5 so the rod will see 125lbForce... Not very much. It will see more than that by just the squeeze on the air charge at TDC.

 

I guess what I mean is will the early charge force negate some of the deceleration stress on the exhaust stroke where there is nothing to help decelerate the piston/rod assy except the charge force

 

Ahhh.... I see what you're going after. I would think that any charge left in the cyl as the piston is coming up would be much higher than your incoming air pressure anyways. I can't see opening it earlier would help decelerate the rod very much at all, might even give the escaping gasses a larger outlet and speed the piston up. Either way you would contaminate the incoming mixture so much that you would probably see simishing returns.

But if this whole exercise it to reduce stress on the con rod, thenI would think closing the EV sooner would help more by adding an air spring at TDC. It would increase the compressive forces on the rod, but decrease the tensile load.

 

And tensile load is what breaks connecting rods generally speaking

Food for thought. Thanks for the input

 

im no genius, but what about closing the EV sooner as suggested, but as the EV approaches the seat, keep it open lets say .005" so as to induce some compressive resistance as the piston comes toward TDC, but not actually build compression which as stated above would contaminate the incoming air charge. or another option could be similar to small honda single cylinder engines do to decrease starting compression. let the EV close early to slow the piston then give it a small bump open just before the intake valve opens to bleed all the residual pressure. my 2 cents

 

Decent article in Car Craft magazine that just hit the stands (June 2011) by Jeff Smith
called "The Truth about Camshafts and Turbocharging". Typically do not pay a lot of attention to the articles but in this case they had quotes by Kenny Duttweiler and Kurt Urban (and both of those guys have a lot of background with Turbos.

Kenny told me in 1995 to look at what was happening when you opened the Exhaust earlier on a boosted engine. Had a new camshaft ground that was "different" vs the normal stuff used at the time and that mod helped to win a National Championship in NSCA racing.

The other comment about using a pair of pressure gages (one at the Intake Plenum and one at the transition from the head to the turbo inlet was told to me by a guy named Danny Jones in 1978, (he was the Ford motor Company Indy Turbo Expert in the 60s.) Some good advide in that issue of the magazine.

Tom Vaught