A thought here.....as I continue to digest your findings "reversion" has been on my mind. according to at least a couple of head porters "ideally" a head would flow less in the opposite direction of normal flow to combat reversion.
Looking at my own heads that had very high exhaust numbers I can't help but wonder what the reverse flow might look like on a bench.
Anyone have experience on this subject?
I agree Mike,
with the the smallish turbine wheels and housings that most of us T/R guys tend run
our back pressure numbers are normally in the 1.5 to 2.0 - 1 range when compared to manifold or
after innercooler boost pressure.
So with the the potential for the incoming boost to be overpowered backwards by a force
that is potentially twice as strong it for sure means that we need to pay attention with what
is happening at the crankshaft rotation degrees at and around TDC.
I also feel that there are some things happening in this time frame that we really do not understand
and you would need access to some some sophisticated test equipment to get a handle on in order to
understand what is happening with the intake and exhaust pressure pulses.....but since we know
that our back pressure is high and approximately 2-1 we can work with that.
Lets take for example one of our popular 224*/224* cams and assume that it is ground with
a 112* Lobe seperation angle. This will make the math easy to understand and this cam exists.
So with the the 224/224 112 lsa numbers if this cam is installed straight up you will have the
intake lobe lift opening and .050" lift at TDC crank rotation and the Exhaust lobe lift closing with .050" lift
at the same TDC.
We know that the LSA is 112* and if you cut the .050" total duration in half with 112* happening before and after
the max lift point which will also be used for the LSA we end up with the .050" numbers parked At TDC.
This is assuming a symmetrical and non Inverse radius ground lobe which most comp lobes that I have seen
are.....if they do grind a nonsym lobe it is really only a token amount of a degree or so and with the larger
diameter grinding wheels typically used it is difficult to put much more IR in a lobe than a token amount
so we will leave these out of this.
Back to our 224* cam which is parked at TDC with .050" lift..... we have to add rocker ratio since
this is what the valves will see so if we use a 1.6 RR our valves are now open .080" at tdc....
so it would be safe to say that we should concern ourselves with reverse flow properties of the
ports at .100" lift and under as this is the region we will be in.
When we are at the .050" lobe lift area on the cam the lobe lift is increasing or decreasing at approximately
.006" per every 2* of crank rotation....so if we back-up 10* crank rotation BTDC This would put our
exhaust lift at around .098" and the Intake at around .008" - .016" .........so we can see that the exhaust
valve with its much greater opening at this point will likely over power the intakes ability to flow into
the chamber.....and maybe past the intake valve and back up its port.....but depending on the dynamics
going on in the intake port maybe not.
And when we rotate the crank from 10* BTDC to TDC and then to 10* ATDC our valve events reverse
themselves....our Intake valve will be open around .098" with the exhaust at around .008"- .016" .....
and now we have the piston pulling away from TDC attempting to create a negative pressure in the
cylinder so now we can really get busy filling the cylinder.....at this point even though the exhaust is still cracked open a bit at .008" - .016" our intake charge with its heavier mass and velocity is likely able to over power the exhaust and begin filling the cylinder...IMO
So Back to our exhaust port with its simple shape it is very likely that it will flow pretty good backwards
and maybe this is something that we should be exploring.....Just one more reason I want my own
flow bench......If any one has good information on reverse flowing of the turbo regal stock location
ports feel free to share that here....
But for sure I think we can count on the exhaust port contaminating the cylinder volume from this
10* BTDC crank position thru TDC given our back pressure numbers.....and I think this has played out
in real world performance with what typically works for us with cam timing.
So if we have different wider LSA on the same cam the valve openings at TDC will be slightly less, as we
have moved everything away from TDC.....
and if there is shorter duration at .050" again Less valve opening At and near tdc.....
This valve overlap deal At and around TDC is likely the biggest reason we juggle the LSA and advance
the installed position of the cam is to help us manage the exhaust back pressure in the cylinder in this area of crank
rotation.......For every 2* of crank that we advance the cam we will gain an extra .005" - .0065" of
intake valve lift and we will lose a similar amount on the exhaust valve lift......At this area around TDC
Crank degrees
Every time I put a cam in after degreeing the cam where I want it I plot the lifter rise from
20* BTDC to tdc and then to 20* ATDC so I can see the entire swing of the exhaust valve closing
and the Intake valve opening.....just helps me to picture whats going on in the cylinder......
Now if only I had a flow bench and a TFX engine sensor package.......
Some of the traditional methods used to combat reversion flow of the exhaust that I have seen
are to ditch cut the back of the exhaust valve and a sharpish edge on the front of the intake valve...
neither of which would work us given the elevated temperatures our valves are running at...
So I am not sure what all options that leaves us to work with.....different valve seat angles?
but not at the expense of good flow numbers throughout the lift curve...
.A very fast closing exhaust lobe?
Once you get the cam lobe below the 050" lift
The ramps really start to ease up on the approach
And descent so the .006" per 2* crank rotation starts to diminish on the lash ramps and initial part of the opening ramps.
but for sure we want to do evrything we can to reduce cylinder and intake port contamination
with exhaust reversion, as jerryl suggested cam timing and compression are at the top of the list.
FBR
