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Discussion Starter · #1 ·
I currently have the HKS 272i and 264e cams.

I was thinking of going for the 272e cams, but don't fully understand the benefits and disadvantages of doing this.

Can I have some advice please?
 
G

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Discussion Starter · #2 ·
Blade,

You will definitely benefit from the 272 exhaust cams especially if you are running a bigger turbo. I can't explain it properly though as I am not an engineer :) But you will get more exhaust flow out of your engine. Idle quality is still ok.

A set of adjustable cam gears will be a great help too.

Regards
 
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Discussion Starter · #3 ·
Thanks TurboAWD - I already have the HKS adjustable cam wheels, so will go get the 272 cam. Should be able to sell the other one easy enough!
 
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Discussion Starter · #4 ·
Thanks TurboAWD - I already have the HKS adjustable cam wheels, so will go get the 272 cam. Should be able to sell the other one easy enough!
 
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Discussion Starter · #5 ·
Blade,

I dunno how much you know about valve timings etc so i'll start basic and apologise for boring you:-

The period of a cam is the number of degrees rotation of the crank that the valve is held open for. In a classic 4 stroke (otto cycle (sounds like a kraut motorbike :))) the period would be 180 degrees which is the duration of the exhaust stroke. However due to inertial effects i.e gas doesn't want to start moving if its not already moving and on the flip side if you got gas moving fast it wants to stay moving fast, you actually want much longer openings than this. 290 isn't uncommon for fast road 2 valve/cyl engines. 4 valve/cyl have less due to increased curtain area (flow area around circumferance of valve).

To improve cylinder emptying you therefore need to open the exhaust valve towards the end of the power stroke whilst the piston is still moving down to start the gases moving. You also want to keep the exhaust valve open after the piston has gone back up on the exhaust stroke and then started to move down again on the induction stroke. Before this point the inlet valve will already have opened whilst the piston is still moving up in the exhaust stroke. this is known as overlap.

To much overlap is bad for bottom end power and emmissions as you get cross contamination of the inlet charge with the exhaust gases with low gas speeds in the ports. However you have the advantage of having vernier pulleys on both the inlet and exhaust cams so can change the amount of overlap this way. This will also allow you to change the cam timing relative to the crank as well.

Turbo cars tend to run less overlap than non-turbo as the exhaust gas pressure may be substantially higher than inlet (especially with small turbo's). Hence as you have put on a bigger turbo you may well get away with more overlap.

Another area to consider is valve lift. Where does max flow occur on your cylinder head? It may not be worth getting a high lift cam if the flow doesn't increase (as point of interest I did alot of testing on a 8v manta head - the standard GM cam actually lifted the valves higher than was required to achieve max flow). Lifting a valve further than its needed causes unneccessary increased valve train wear.

Also valve accleration. Turbos like a very quick opening of the exhaust valve to give a nice pulse of gas into the turbo. This will bring the turbo on line quicker. The down side to this is that very high lift rates cause alot of wear, although the roller rockers on the evo will minimise this.

A word of warning. Big cams are not nice to drive on the road. Did you notice any loss of bottom/mid when you first had the HKS cams? Having said that with your new turbo it may work nicely. It will tend to shift the volumetric efficiency curve higher up the rev range (breathe better at high revs).
 
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