Camshaft Duration and Overlap

[thrash]

I suppose a better place to research this would be the vr4/evo forums, since all of their cars are twinks.. but I'll ask here anyway, since I know that most of the people on here don't bullshit much :)

Has anyone experimented with different profile cams on a twincam setup? Not necessarily a 4g63, or even a turbo, but just wondering if people have found good "general" setups? e.g. how does the torque/power curve vary when using shorter duration intake cams and longer duration exhaust cams, from using longer duration intake and shorter duration exhaust cams, from using same duration intake and exhaust cams?

I have been trying to find information on this, and there are many varying opinions.. a lot of the v8 people reckon same duration for intake and exhaust produce the most power.. but then again they are still using one cam per motor, and pushrods no less.. more important to them seems to be overlap. Some of the Nissan people reckon that longer duration intake will give better top end, but choppy idle quality.. and then i've read some workshops recommend shorter intake duration and longer exhaust duration for a street car, as this will apparently give decent idle and low rpm driveablility, while allowing the turbo to spool fairly quickly and also give a decent top end, but this is not the best for people who are chasing numbers alone..

Any thoughts? Or what I'd really like to know, any experiences?

[TOMSUN]

Any thoughts?

Yeah... Camshafts would have to be the most complicate mechanical component of a typical engine (rotary's not included.)

There are thousands of books written on them and cam profiles are the highest guarded secret of competion engines.

You got to be more specific. Eg. give us 2 cam specs for a specific engine to compare and we might be able to help. :beer

[iXNAY]

ive always heard with 4G63 DOHC that 272 cams for intake and exhaust are about as big as anyone wants to go cause u can still get a decent idle but get a good power-band and top end out of them.

[thrash]

didn't really want to go into specifics as I wanted to know what the more experienced people out there have.. err.. experienced.. lol

Two of the more memorable reviews I've read:

http://www.amsperformance.com/camtest.php

http://www.aliallage.com/evolutionm-pro ... am-review/

[TOMSUN]

Some basics that I learn at my Tafe course.
Teacher was David McCowage. Runs a dragster (not sure the class)
Course included dyno tuning, performance engine building tips, camshaft theory and flowbench testing, etc. it was a great course. I did it twice.

The Basics (but I'm sure you have already researched it)

Duration: Longer duration the longer the valve has left the valve seat.

@ .050" vs advertised.
On the cam card you might see a 278 duration. that is the "advertised" duration. 278 sounds great but in reality you should be looking at the @ .050" specs. which might be 230 deg. Why, well your not going to get any significant flow until the valve has move at least @ .050" off the seat.

Overlap: The amount that the intake and exhaust are open at the same time. Note: Boosted application tend to have less overlap, this it to reduce the tendancy of blow through.

Lobe Separation: The degrees seperating the centreline of the intake lobe and the exhaust lobe. Boosted application tend to have a bigger angle eg. 114 deg vs a naturally aspirated angle of eg 110 deg

So you can vary the overlap by changing the duration or the lobe separation. All cam specs are interrelated on each other.

Dialing in the cam (advance/retard)
Dialing in the cam can influence the performance also. If you advance the cam power can come on earlier and also help to spool the turbo earlier but the offset is typically a reduction in max. pear power later in the rev range. And the opposite, retard the cam and the power will typically come on later.

Lift: More lift typically = more flow. Note a cam will get to a stage where no matter how much lift it has it wont flow any more. Or the flow is insignificant and make the profile too aggresive and puts extra strain on the valve train.

Profile: Two cam have have the exact same specs on the cam card but are completely different. The rate of opening and closing a valve can vary hugely. A lot of the time that can depend on the type of lifters involved. Hydraulic/Solids/Hydraulic Roller/Solids Roller etc.

:beer

[thrash]

hey thanks for that tomsun, I did know most of that stuff, but it's good to have it collected in one place for reference :)

And on a side note, from my research it seems that lift is not important to turbo applications? this is apparently especially true for the 4g63. High lift and larger overlaps help NA motors make more power it seems.

[www.18u]

There is always a give and take situation with cams. You will suffer down low with high lift cams, and loose up high without them. the trick is to find the balance that does both. You will see a heap of people using a 264/272 combo in a 63, as it seems to give the best of both worlds. My old gsr runs this combo with a 2.3 stroker, a Gtpumps 20g and idles fine, and makes power almost all the way to redline.

Later cars using vvti or vtech yo :D are actually really using two camshaft profiles on the one cam. It will either be oil pressure or a solenoid controlling the oil feed that pushes the rocker arm, onto the second profile that has more lift.
There are other designs like a self advancing cam wheel, and even a combination of the two, but this was the original idea behind these setups.
I am sure the later generation of these sytems are much more complex, but this is how they can get away from having to find a balance as spoken above.

[thrash]

264 intake 272 exhaust? yeah,. i've heard that too.. then there are those CSG cams detailed in one of the links posted above that are 268 on intake and 266 on exhaust.. so smaller duration on exhaust, but apparently gives a much better mid range than the hks 272s.. I wonder how they'd perform against HKS 264s for mid range..

Ferrari apparently uses a "3 dimensional" cam profile, that varies along the length of each cam lobe.. so based on the rpm and perhaps other parameters, the cams are moved forward or backward to give a variable cam profile that switches seemlessly so you don't feel the little "kick" that vtec (yo) gives.

[enthuzed]

Very interesting reading guys. Love the no BS answers...

[redzone]

there's my no bullshit answer





LOL

[enthuzed]

rofl You're a nutter Dave!