350 HP Potential for CIS

[Ken Keith]

Let me speculate a little, and learn from the correction that others 
post.  I'm just trying to understand this stuff.

I think that since the turbo is a positive pressure device, it can fill a 
combustion chamber with air (and hence fuel) as long as the intake 
valve is open.  The only reason to open the exhaust valve is to let 
the spent gases out the rear.  When there is no overlap (both 
intake and exhaust valves open at the same time), these gases are 
not pushed out by the turbo, but rather only by the piston coming 
up and making the volume of the CC smaller.  If you do have 
overlap, with both the intake and exhaust valves open at the same 
time, then the incoming air/fuel charge will help push the spent 
exhaust out the open exhaust valve.

If you have higher intake pressure, from the turbo, the valve overlap 
doesn't have to be nearly as much (duration of overlap as long) as 
you would with lower intake pressure, since the spent gasses are 
zipping away at a higher rate out the exhaust valve.

So, the theory with not having an aggressive exhaust valve 
duration, is that with the increase boost pressure, the exhaust 
gases are being pushed out just fine, and any additional overlap (or 
lift, if suppose) will only result in some of your incoming air and fuel 
to be pushed out the open exhaust valve, like a blowoff valve that 
shoots it's load out the tailpipe.

So, the key appears to be that delicate balance of finding the 
correct cam for the setup you are running.  Since the stock cam is 
not optimized for power (it is a compromise for the majority of the 
car buying population), and it's not expecting anything besides 
stock boost levels, there is room for improvement in the cam 
department.

What I didn't understand when I read it was the sentence, 

"We came up with the asymetrical grind with a little more aggresive 
duration on the intake to reduce push out the exhaust valve."

Does that mean that the duration of the intake is more aggressive 
in that it last longer and there is increased overlap?  Or is the 
duration more aggressive in that it comes on earlier or later in the 
combustion cycle?

If I won't be using my Schrick 272 assymmetrical cam in my MC 
motor (gosh, someday I want a 2.5 liter MC-1 motor with an SJM 
spring and chip, and 3" open catback exhaust in my 4ksq), then I 
guess I should sell it.

Rod at TPC told me that the '93 Eurovan (8.5:1 c/r?) knock sensors 
are a different part number than the knock sensors for my '88 
5kcstq motor, so I'm concerned about knock sound frequency, and 
the sensors detecting it in a 2.5liter bottom end.  Does anyone 
know if these knock sensors will work with my MAC11(?) setup?

Thanks for reading, I look forward to gaining some knowledge here.

Later,

Ken

Todd Phenneger <tquattroguy at yahoo.com> wrote:
> 
>  Can you explain Why you dont want more push out the exhaust
> valve?  Wouldn't this decrease lag.  Are you thinking that above
> 4k RPM at full boost it will be to hard on the Turbo?  
> 
>     I'm pretty sure that Chad Clark ran a 272  (err, wait,  now I
> seem to remember it was a Piper 268 cam).   What is the duration
> on the Stock MC cam, do you remember?  Also, do you mean Elgin is
> also doing a 272 for you?    Will you try that in the MC for a bit
> and see what it does? 
> 
>    gary, what cam are you running?
> 
> thanks
> 
>     Todd
> 
> 
>   JShadzi at aol.com wrote: 
> In a message dated 9/22/00 8:55:35 AM Pacific Daylight Time, 
> tquattroguy at yahoo.com writes:
> 
> << Javad,
> I'm VERY curious as to the results. I know that a Shrick 272 had great 
> results in a couple turbo cars. Can you explain how you came up with the 
> 264/260 dimensions you did? I havent' put MUCH thought into this yet other 
> than I know I'm going to Elgin for it and I know I want a Cam. I had planned 
> on basically copying the Shrick cam but maybe I wont. 
> Anyhow, I'm Very curious to hear the results. l8r
> Todd >>
> 
> I talked to John and mentioned it was for a turbo car. We came up with 
> the asymetrical grind with a little more aggresive duration on the intake to 
> reduce push out the exhaust valve. This cam will slightyly soften the bottom 
> end, and may increase turbo lag a few hundred RPM, but hit 4000 RPM, and son, 
> hold on, that little I5 is gonna scream for the rev limiter!! =)
> I would not use a Shrick 272 cam in a turbo motor unless it was a race 
> car, you are going to be pushing boost out the exhaust valve up past 4000 
> RPM, a little aggressive for forced induction IMO. 
> Elgin is doing one of each for me, however, so I can let you know for 
> sure...
> Javad