1:8 Roller Rockers, what' my new duration from 1:7 rr???

[GTR LUST]

Hi

I will soon be buying a Corvette C5 Z06, with the 5.7 LS6.

I'm still don't have a full understand of camshaft numbers yet, but if someone could fill in the missing numbers below, I'm sure it would help put another piece of the puzzle in place for me.

I'm tempted to fit 1:8 roller rockers so I don't have to tear into the internals for a cam swap, and see they could offer an increase from around .550 lift to .580 , an additional 0.030" of lift at both the intake and exhaust valves.

The piston to valve clearance gets tight with a 1:85 on the 02 + LS6, so 1:8 is as high as I could go without fly cutting.

I'm hoping someone can fill in the blanks for me below so I can compair the rr and the extra power in comparision to camshaft swap gains??

LS6 cam 204/218 duration 0.551/0.547 lift, on 117.5 LSA
LS6 + 1:8 rr __?____? duration 0.580/______ lift, on __? LSA
Small cam 224/224 duration 0.551/ 0.551 lift, on 114 LSA
Medium cam 232/234 duration 0.595/ 0.598 lift, on 112 LSA
Big cam 236,240 duration 0.630/ 0.612 lift, on 111 LSA

Thanks in advance for the help

[stevieturbo]

duration will be exactly the same regardless of rockers used.

[Steve_D]

In addition to the valve to piston clearance have you considered if your head and springs can cope with the additioanl lift without the springs becoming coil bound and collets hitting the valve guides.

Steve

[garagewidow]

all you end up with apart from higher lift is slightly more overlap.

[Pumaracing]

stevieturbo said:

duration will be exactly the same regardless of rockers used.

Actually no it won't. Seat duration will be close to the same (although even that increases slightly due to the required valve clearance being taken up a bit sooner) but duration at any other lift point such as the 50 thou mark used by American cam makers will increase as the valve will get there quicker. The LSA is obviously the only thing that doesn't change at all.

Anyway to answer the OP. The valve lift at any point in the cam curve will increase by 580/550 = 5.5%. However what the engine sees is not valve lift but valve flow. To work out the extra power we need to know how much more valve flow will take place over the entire cam lift curve and that clearly depends on the actual flow profile of the cylinder head in question and needs a computer to calculate the answer.

Luckily one of the many engine analysis programs I've written over the years can take a cam lift curve and interpolate that against a valve flow curve to calculate the total area under the flow curve which will closely equate to the potential power available.

Running a generic flow curve against a generic stock cam profile for the same engine and then increasing lift everywhere by 5.5% I get an increase in total flow area of 3.0%. Given that you never quite get every bit of extra flow turning into extra power that gives me a potential power increase of perhaps 2.5%. The results on your specific engine will vary depending on how well the head flows but that's the ballpark.

That really isn't very much compared to putting a proper cam in the engine. A good fast road cam will normally produce about 7% to 8% more power than stock in a generic engine without losing much tractability - maybe a bit more if the standard cam is very mild. With something a bit hotter such as very fast road to mild rally you can normally squeeze that up to 10% to 12% with a slightly lumpy idle and some loss of torque below 2500 rpm. Looking at your supplied numbers the above is indeed about what I would expect over stock from the small and medium cam respectively. Anything bigger than that and you won't want to drive it on the road in heavy traffic.

Finally graphing out a generic cam profile at 550 thou lift and then increasing that by 5.5% everywhere I can see that the 50 thou duration figure only goes up by about 3 degrees or so. Again nothing like as much as putting in a proper cam.

Conclusion - don't waste your money unless the rockers are very cheap.

If I'd run that little exercise for a race engine customer it would have cost them about a hundred quid and saved them 10 times that in actually testing things for real. Where do I send my bank details to?

[Boosted LS1]

That bloke in Nigeria would like you to spec some race cams for him shortly.

[stevieturbo]

Pumaracing said:

stevieturbo said:

duration will be exactly the same regardless of rockers used.

Actually no it won't. Seat duration will be close to the same (although even that increases slightly due to the required valve clearance being taken up a bit sooner) but duration at any other lift point such as the 50 thou mark used by American cam makers will increase as the valve will get there quicker.

So duration ( or perhaps time spent ) at any given point will decrease, as the valve lift is accelerating faster, spending less time at any given point ?
It may get there quicker, but will spend less time there ? But it will add additional lift points instead.

Edited by stevieturbo on Wednesday 29th September 11:00

[ringram]

Same time at each point on the cam, its just that the lift will be greater. (Faster valve acceleration)
So 0.050 lift point will be obtained earlier in the duration of the lobe. Making duration at 0.050 slightly greater.

I agree with the above, a new cam is best and not much more £. Stock rockers work well in the LS. You will want springs as well though.
However bolting rockers on is a DIY job, so gains per pound compared with paying an installer are better.
Most LS2's put down 320rwhp, With rockers, intake and tune, I had 360rwhp. Ive not seen anyone with more than that for similar mods. That was only 6rwhp less than an LS2 with CAM and full exhaust from a well known installer (my exhaust was still stock)

But again, its the package. No one part will give you all you want.

Figure out what power you want and how you want it delivered and work back from there..

[rev-erend]

Puma - when you finally get round to writing that book I'm sure there will be plenty on here who will learn loads. Me included.

[andygtt]

Puma

Sounds like your the man to ask, how easy is it to get cams reclicated but in an exact mirror image of an existing profile?
Effectivelly I want to reverse the rotation of the cams but retain exactly the existing profiles, clearly I cant just reverse the cam rotation but need whole new ones made.

Its for a multi valve V12 project I am working on

[Pumaracing]

andygtt said:

Puma

Sounds like you're the man to ask. How easy is it to get cams replicated but in an exact mirror image of an existing profile?
Effectively I want to reverse the rotation of the cams but retain exactly the existing profiles, clearly I can't just reverse the cam rotation but need whole new ones made.

Its for a multi valve V12 project I am working on

Measuring a profile, reversing it and grinding that on a new blank isn't any great problem. Actually most cam lobes are symmetrical anyway apart from the opening ramp for tappet clearance. It's also just as easy to design a totally new, and probably better, profile. I generally design my own profiles these days rather than rely on whatever someone has in their catalogue. Back in the good old days grinding a new cam profile meant laboriously making a 4 times scale master lobe for the hydraulic copy cam grinder to follow. That was always the major PITA.

Nowadays with CNC cam grinders and computerised cam design it's basically a matter of specifying the lift, duration and LCA you want, tapping that in at the keyboard and Bob's your aunty's husband. Obviously one needs to know about things like acceptable cam velocity and acceleration limits, how to take the follower diameter into account as that determines the maximum valve opening rate before the cam lobe runs off the edge of the follower, and a couple of other things which ain't no great sweat. Finally, on some engines, the direction of rotation can affect the edge profile of the lobe to ensure the cam follower rotates properly but as I don't know what engine you're tinkering with I don't yet know if that's an issue. I could tell you a funny story about some supposedly clever people who tried to design their own cams and got that last bit wrong and wiped out all the cams and cam followers several times before someone spotted the error for them. Anyhoo....

Speccing valve springs for a given cam acceleration, lift, valve train mass and rpm limit needs some rather advanced maths but I wrote the computer program for that yonks ago so again no biggy.

However if you're thinking of reversing the direction of rotation of an engine you'll need to work out the new firing order which means the cam lobes are going to be in different places so the cam blanks will need to be redesigned properly too. That's the head-scratchy bit if there's lots of pistons whizzing about. Making the blanks and getting the metallurgy correct is the expensive bit. Grinding the profiles is no problem at all.

Sounds like someone needs to be paying me obscene amounts of money in the near future. Yummy

[Pumaracing]

rev-erend said:

Puma - when you finally get round to writing that book I'm sure there will be plenty on here who will learn loads. Me included.

Given my powerfully pronounced proclivity for procrastination I wonder if I'll ever get round to writing that. However I might be doing some technical writing with David Vizard in due course, or at least he's looking for someone to take part of the load off him as age and health issues take their toll. He's just had to undergo yet more spinal surgery for a long existing back problem.

[GTR LUST]

WOW, now that's the kind of answers I've been looking for. Thanks for taking the time to explain this as well as you have, I appreciate it, and will happily buy you a pint!

So it sounds like the rockers would make about 10hp, a small to mid cam 30hp, and huge lumpy idle cam could gain around 50hp.

So, I'll take a stab at estimating the LS6 with 1:8rr would like something like this:

LS6 cam 204/218 duration 0.551/0.547 lift, on 117.5 LSA
LS6 + 1:8 rr 210/220 duration 0.580/0.548 lift, on 116.5 LSA
Small cam 224/224 duration 0.551/ 0.551 lift, on 114 LSA
Medium cam 232/234 duration 0.595/ 0.598 lift, on 112 LSA
Big cam 236,240 duration 0.630/ 0.612 lift, on 111 LSA

This may be compleatly wrong, but is roughly what I expected, and is based on being around 1 third the gain of a small cam....

I may bite the bullet and cam it (undoubtedly the better option), but how good are the rockers at giving the extra power comming out of bends and turns??? That's when I really want the extra power...

Edited by GTR LUST on Wednesday 29th September 21:10

Edited by GTR LUST on Wednesday 29th September 21:32

[ringram]

LSA wont change as mentioned.
If you want more torque at a certain RPM cam will help.
Peak tq RPM in the LS doesnt change much as its directly related to the intake tract and tuned lengths.
The Comp 216/220 XER cam does well and beats most larger cams up to around 5000rpm IIRC.

Are you running 2.5" exhaust with full length headers? 1.75" ideally, though 1.625" would also be ok
If not then no point camming it as you will be exhaust constrained. A cylinder head swap will add power with no downsides. I think AFR shows a straight head swap on a stock engine giving 30rwhp. Head swap will raise the entire torque curve, cam will raise and tilt the curve depending on duration etc. Cam lift itself is also good as it will lift the curve but not change the shape of it. (aka higher ratio rockers)

So you want a small duration high lift cam, maybe on a tighter LSA to gain from cylinder scavanging.
But not too tight else you will fail emissions

If it were me Id do heads and cam with decent exhaust and intake. Most stock LS1's struggle to do more than 320rwhp, heads and cam I was at 385rwhp, but I used cheapo as cast Dart Pro1 heads. A better cam and heads and I recon 400rwhp is doable with an emissions compliant cam. But you will need to get the exhaust and intake side sorted as well.

As per above figure out what you want power wise and work backwards.

[ringram]

LS6 heads are pretty good in stock form, so you could do cam and exhaust and see where that brings you.
Why do you think you will lose valve clearance? Duration is the primary determinant of that, not lift.
Personally Id look at the Comp LSR 219/227-113 cam. It runs over 0.600 lift though so you will want the 26918 comp springs. But as the LS6 heads run hollow and sodium filled valves you should have a nice light valve train.
I wouldnt be surprised if you managed 380rwhp with that and a proper exhaust.

[stevieturbo]

Also ask yourself....what do you need from the engine ?

torque, revs ? How do you drive ? Is it auto or manual ?

And what is its current spec ? Has any tuning work been carried out ? If not, perhaps some very basic mods and a remap might be worth while.

I'm sure either Richard above or Monkfish could sort something.

[ringram]

Good point. A remap on stock should pick up some nice power anyway. Just depends what you "need"
Though I try not to get involved in tuning to much. Its too costly with time.