20 replies to this topic

[Bubblebobble]

Ive just got a 998 cooper head , with valves but no rockers . Does anyone know what rockers i need ? Do i need the special 's' ones form minispares or is  a set of standard 1000cc ones ok ?  

[6joshh6]

A set of standard rockers will be fine, ideally a 998 would have around 1.3 ratio rockers, anything over that will make the engine more peaky and lose horsepower lower in the range, exactly what you don't want in a car for the road. Standard ones produce between 1.2-1.25 depending on which type you use, which isn't quite optimum for performance but is more than adequate. Given the cost of after market rockers, I'd probably spend the money elsewhere on other things such as a performance camshaft if you don't have one already as you'd gain more performance for the money.

[Bubblebobble]

Thanks 6joshh6 . I looked on minispares and they sell a cooper 998 rocker but the whole lot would probably be over £100. I think they were 1.3 ratio so not much in it .  A secondhand set it is then , and  some lightening work to them .

Edited by Bubblebobble, 24 July 2014 - 05:03 PM.

[blacktulip]

i think the cam choice plays a factor in the rocker choice, as you are using a cooper 998 head (i assume a 12g295?) you will most certainly need an uprated cam to get the best from the head. some cams work well with high lift rockers, others dont i.e the MD266. the S rockers you have been looking at are very good. they will give you close to the 1.3 ratio that works best with a 998. if you go with these you should also think about getting the thicker rocker shaft. that way you will get a more accurate valve lift without a risk of flex in the shaft.

[Bubblebobble]

Is the cooper shaft thicker ? Im looking at sets on ebay as i dont really want to spend £100 plus on rockers just yet .  I think i need to get some forgerd or cast ones rather than pressed if i want to lighten them ? 

 

 Im not looking to build a balls out motor at the moment , just some usefull increases . Also am i right in thinking the swiftune sw5 cam is like the old cooper one ? 

 

Thanks so far for the answers.

Edited by Bubblebobble, 24 July 2014 - 05:00 PM.

[blacktulip]

Is the cooper shaft thicker ? Im looking at sets on ebay as i dont really want to spend £100 plus on rockers just yet .  I think i need to get some forgerd or cast ones rather than pressed if i want to lighten them ? 

 

 Im not looking to build a balls out motor at the moment , just some usefull increases . Also am i right in thinking the swiftune sw5 cam is like the old cooper one ? 

 

Thanks so far for the answers.

not sure about the SW5, have you seen the book called "how to power tune an A series engine" by David vizard? lots of free increases to be had from reading it. what sort of engine spec are you building? im about to build a nice 998 in the near future too.

[Bubblebobble]

No , im going to get a copy .Ive been reading Trickeys book which is quite good although obviously dated somewhat . Im thinking of just fitting the head and maybe twin su's to start , stuff without removing the engine . A cam would be further down the line if it happens . 

[timmy850]

If it is a 12G295 head in standard shape it will have big combustion chambers on the head and your compression ratio will go right down. 998 coopers had raised pistons to counter this. You'd have to skim it a fair amount to get your CR ratio back to what it was with your standard head...

You have to keep lots of factors in mind before changing engine bits, as they all need to mesh together perfectly to make more power

[gazza82]

Forged rockers won't be cheap ... Even on fleabay.

Sintered should be ok or just use the pressed ones and have them welded along the join

[IainNeon91]

I'm running the 1275 pressed steel rockers on my 12g295 (the pressed steel rockers with the oval pads) these have a slightly higher lift than the 998 pressed steel rockers (round pads) and quite a bit more lift than the later A+ sintered rockers. I have the 12g295 on my 998 which is also running a 266 cam, I'm yet to have the engine rolling road'ed but it already seems a lot more pokey! Further down the line I'm think of getting the 1.3 roller rockers to give that final bit of lift, just to maximise the lift of my cam.

[Captain Mainwaring]

Thanks 6joshh6 . I looked on minispares and they sell a cooper 998 rocker but the whole lot would probably be over £100. I think they were 1.3 ratio so not much in it .  A secondhand set it is then , and  some lightening work to them .

 

I wouldn't waste your time lightening them unless you've converted it to desmodromic valve gear.

[Bubblebobble]

I'm running the 1275 pressed steel rockers on my 12g295 (the pressed steel rockers with the oval pads) these have a slightly higher lift than the 998 pressed steel rockers (round pads) and quite a bit more lift than the later A+ sintered rockers. I have the 12g295 on my 998 which is also running a 266 cam, I'm yet to have the engine rolling road'ed but it already seems a lot more pokey! Further down the line I'm think of getting the 1.3 roller rockers to give that final bit of lift, just to maximise the lift of my cam.

Did you have to skim your head down , as mentioned above ?  

[Bubblebobble]

 

Thanks 6joshh6 . I looked on minispares and they sell a cooper 998 rocker but the whole lot would probably be over £100. I think they were 1.3 ratio so not much in it .  A secondhand set it is then , and  some lightening work to them .

 

I wouldn't waste your time lightening them unless you've converted it to desmodromic valve gear.

 

I thought some benefit would come of lightening them , as mentioned in books .  And its free....

[Captain Mainwaring]

 

 

Thanks 6joshh6 . I looked on minispares and they sell a cooper 998 rocker but the whole lot would probably be over £100. I think they were 1.3 ratio so not much in it .  A secondhand set it is then , and  some lightening work to them .

 
I wouldn't waste your time lightening them unless you've converted it to desmodromic valve gear.
 

I thought some benefit would come of lightening them , as mentioned in books .  And its free....
 

 
All things that move have a polar moment of inertia . now, this has both velocity and direction - it's a vector, so things like pistons undergo much more loading as a result of acceleration and deceleration thus the need to lighten them.
 
Nahhh take your rocker arm - actually it's balanced anyway, though there will be a little bit of friction on the rocker shaft - but what it is operating ( a vlave spring) takes up much more energy than moving the arm in the first place - though the spring unloads its energy back into the camshaft as the valve shuts.
 
in short a wasted exercise on a standard engine running at normal speeds.

I can only add that those who recommend lightening rockers should back it up with calcs

Edited by Captain Mainwaring, 26 July 2014 - 09:21 AM.

[Bubblebobble]

 

 

 

Thanks 6joshh6 . I looked on minispares and they sell a cooper 998 rocker but the whole lot would probably be over £100. I think they were 1.3 ratio so not much in it .  A secondhand set it is then , and  some lightening work to them .

 
I wouldn't waste your time lightening them unless you've converted it to desmodromic valve gear.
 

I thought some benefit would come of lightening them , as mentioned in books .  And its free....
 

 
All things that move have a polar moment of inertia . now, this has both velocity and direction - it's a vector, so things like pistons undergo much more loading as a result of acceleration and deceleration thus the need to lighten them.
 
Nahhh take your rocker arm - actually it's balanced anyway, though there will be a little bit of friction on the rocker shaft - but what it is operating ( a vlave spring) takes up much more energy than moving the arm in the first place - though the spring unloads its energy back into the camshaft as the valve shuts.
 
in short a wasted exercise on a standard engine running at normal speeds.

I can only add that those who recommend lightening rockers should back it up with calcs

 

Surley if you lighten them its taking less energy to move them .. .... which is good ?