Cam duration vs lift

[sotiros86]

Hey guys, i need a bit of help with determining how an engine will respond to a different set of cams.
Im currently rebuilding a bigport with aftermarket quad throttle bodies (sk/oer setup), valve springs, shaved and ported head, thinner head gasket, aftermarket cams and cam gears, and all the other usual crap.

My question is regarding the cams that are currently installed in the engine. The cams are HKS 272 degree with 8.35mm of lift, however, i also have a set of Toda 288 with only 7.9mm of lift.
I have previously driven the engine with the 272s installed, and it absolutely hauled ass, and was super responsive, however, ive had the 288s sitting at home for a while and have been contemplating fitting them.

By changing to 288s, im aware that it will shift the power band higher up in the rev range, however, how will the reduced lift effect the power and responsiveness of the engine?

The car is currently daily driven, which makes me think that sticking with the 272s is probably a better idea, however, im extremely interested in hearing peoples opinions on the issue.


Cheers,
Andrew

[Rice86]

Lift

The camshaft "lift" is the resultant net rise of the valve from its seat. The further the valve rises from its seat the more airflow can be realised, which is generally more beneficial. Greater lift has some limitations. Firstly, the lift is limited by the increased proximity of the valve head to the piston crown and secondly greater effort is required to move the valve's springs to higher state of compression. Increased lift can also be limited by lobe clearance in the cylinder head construction, so higher lobes may not necessarily clear the framework of the cylinder head casing. Higher valve lift can have the same effect as increased duration where valve overlap is less desirable.

Higher lift allows accurate timing of airflow; although even by allowing a larger volume of air to pass in the relatively larger opening, the brevity of the typical duration with a higher lift cam results in less airflow than with a cam with lower lift but more duration, all else being equal. On forced induction motors this higher lift could yield better results than longer duration, particularly on the intake side. Notably though, higher lift has more potential problems than increased duration, in particular as valve train rpm rises which can result in more inefficient running or loss of torque.

Cams that have too high a resultant valve lift, and at high rpm, can result in what is called "valve bounce", where the valve spring tension is insufficient to keep the valve following the cam at its apex. This could also be as a result of a very steep rise of the lobe and short duration, where the valve is effectively shot off the end of the cam rather than have the valve follow the cams’ profile. This is typically what happens on a motor over rev. This is an occasion where the engine rpm exceeds the engine maximum design speed. The valve train is typically the limiting factor in determining the maximum rpm the engine can maintain either for a prolonged period or temporarily. Sometimes an over rev can cause engine failure where the valve stems become bent as a result of colliding with the piston crowns.


lol from wiki

[sotiros86]

Haha yeah, i had a look at the wiki before posting this

What im more interested about though is how the decreased lift, yet increased duration will effect the engine?
As the increased duration of the 288s will mean that the valve is held open for longer, thus potentially allowing more air to flow through.

I understand how the difference between lift and duration will effect an engine when the cams are completely different, however, these two different profiles are fairly similar and has got me a little confused.....

[xero]

with those cams there will be negligible difference in power but the 288s will have a higher limit.

what you will notice is the below 4000rpm performance. at the moment the 272s are probably just ok for daily driving but fucking sweet as above 3500rpm, with the 288s you will find its a lot more sluggish getting away from lights, will need to drop back a gear or 2 to over take and go up hills. in short it will just make the car bit more of a pig drive around to uni and shit.

BUT! get the car on to the track with the 288s installed and i reckon you wont want to go back to the 272s... until you go drive it to work the next day and it just wants to be a ####.

i say keep the 272's get some cam gears and get them dialled in nicely and enjoy the fine balance they will bring your engine.

[timbo]

It would be great to see comparative dyno runs with both cams!

The 288's would be good as you wouldn't need to replace springs and buckets with that lift, but seeing as you are going with new springs etc you can safely use more cam lift. Wait till the "turbo" nerds chime in, I'm as interested as you are.

[meadan]

I have some 292 7.9mm cams in my smallport at the moment.
With the throttle bodies and current exhaust it isnt much of a pig down low. But from the idle quality and general angry nature I cant say I would enjoy driving it everyday.
It is definately a whole world different to a stock 16v or 20v though, not very smooth at all, but lots of fun up high.

[fantapants]

also be carfeul that you are using a standard duration... i THINK, though and often wrong that you are better of ignoring the advertised duration, what you need to know if is the duration at i "think" 50thou???

I know of at least one lad with fairly wild advertised cams, that at 50thou are only a smidge bigger than standard...

[sotiros86]

Haha yeah, Adam, the general drive ability of the car is still a bit of a concern for me atm, as the Trueno is still daily driven (for the time being anyway), which was why i was thinking that sticking with the 272s would make it a little bit more comfortable. However, if the 288s would net a much more use able rev range on the track, then i would be willing to sacrifice the comfort of the car (hell, ive sacrificed everything else to make the car more fun to drive lol)

Adan, out of curiosity, when does your engine stop making power?

The other thing i was thinking, is how would running a 288 on the intake and a 272 on the exhaust effect the engines response and where it makes power? Im unfamiliar with how running different cam profiles on the intake and exhaust effect the performance of the engine, and as to whether or not there is any real advantage to doing this (i know of people doing it with smaller profiles i.e. 256 and 264, but nothing larger)

Cheers

[timbo]

This is true. Most local cam manufacturers give both, not sure about Japanese brands.




Seems to be done quite often. I remember looking at old TRD N2 and rally specs and some cars ran different durations. Might be worth looking into.

[meadan]

I havent had the car on a dyno yet, so a proper tune will probably make it feel slightly different.

But from my butt dyno, its definately still making power at rev limiter (7800 for now, id say the cams will definately still be making power after this once its tuned)