How does exhaust cam affect performance?

[jevansio]

I'm currently running a HKS 264 intake cam & jspec exhaust.

 

I'd like to know what effect putting a 264 ex back in will have (power / spool) and why that effect takes place. I'm having trouble understanding why a bigger ex cam will increase airflow through the engine (in my head I see the piston shoving the exhaust gas out regardless of the cam profile - albeit coming out in a shorter time in a mild cam)

 

I'd also like to see some T67 spool graphs to see how my spool compares to other t67s (I will sort a graph out - but currently making 1.4 BAR by 3.5k)

 

The questions are geared up really to know whether to replace the jspec, as if it's the reason it's spoolig well & I'm happy with the power then I see no reason to change.

Edited April 7, 2010 by jevansio (see edit history)

[jevansio]

I'd also like to know if I wished for a slightly higher rpm limit (say 7600) is a spring upgrade all that is necessary? Bottom end is currently built, and is it wise to run a jspec cam upto 7600 or is it more the springs that need attention & the cams will take it asl long as everything else is upto the job. Will a jspec become restrictive @ 7600?

Edited April 7, 2010 by jevansio (see edit history)

[Tricky-Ricky]

I used to run my Jspec exhaust cam quite advanced, which form my reading on the subject helped to some extent in keeping my EGTs down.

From my limited experience of cams and cam timing,

 

In theory if you're shoving more airflow through the engine via big turbo, you should be able to run slightly more lift and overlap on the cams to make the best of this, but this also tends to move the peak power and torque in particular higher up the RPM range.

 

I would be inclined to change the exhaust to 264, and possibly go back to 256 on the intake, i have heard good things about a staggered set up, i would have thought that the std exhaust cam is stifling things and spool up.

 

The theory with FI is that you don't need much overlap and scavenging as an N/A or you would end up loosing intake charge out the exhaust port, whereas the N/A needs the draw through effect that overlap has to increase intake charge.

 

Basic but it gives the idea, there is a lot more to it, but then I'm sure you knew that anyway

[jevansio]

Cheers bud, as I've already got the 264 intake I'll prolly just keep it but I guess going back to the 264 exhuast does seem sensible, I mean it's a tried & tested setup isn't it

[sdistc]

A mate of mine tested out 264 exhaust cam, then added an intake cam of 264. Exhaust cam made much more difference.

Niko's RZ T04Z with exhaust cam upgrade

Niko's RZ T04Z with exhaust & intake cam upgrade

Edited April 7, 2010 by sdistc (see edit history)

[jevansio]

Many thanks sdistic,

 

Do you think the ex seemed to make a difference in spool is correct? Don't HKS themselves say the 264's give an increase in power at a 200 rpm loss in spool, just sort of makes me wonder about the results. Oh & he's running more boost on the 2nd ex run which would account for the increase on power through-out the run.

 

You sure the ex made that much difference? When he dropped the intake in he made more power on less boost?

[sdistc]

You're right, the intake did give more power - but remember that the results are skewed a little because the dyno runs with the intake ALREADY had the exhaust cam installed So if you have another look, the spool gain from the exhaust cam was nullified but there was more top end.

The exhaust cam seemed, both on the graph and on the road, to really open up the midrange power.