True Cam Measurement (measurement Point)

[pnblight]

Hi Guys,

Just trying to remember where true cam measurement is taken from, Memory is telling me at 50 thou (is the more excepted measuring point). I reason i'm asking is i going to measure up the true cam timing and actual overlap of my rb26, so i can reset on rebuild and have a bit more meaning fully information on where the cams are at the momnet.

pete

[proengines]

Pete,

.050" timing is what most cam manufacturers use as it's where the lift becomes a little meaningful. SAE timing is at .006" lift from memory. It doesnt really matter what you use as long as it's repeatable, RB's are a bit of a prick to measure lift, I made up an extension for the dial gauge that bends in under the cam and down again, you just need to be careful that you get the gauge parallel to the valve axis each time.

[Sydneykid]

Hi Guys,

            Just trying to remember where true cam measurement is taken from, Memory is telling me at 50 thou (is the more excepted measuring point). I reason i'm asking is i going to measure up the true cam timing and actual overlap of my rb26, so i can reset on rebuild and have a bit more meaning fully information on where the cams are at the momnet.

pete

<{POST_SNAPBACK}>

Hmm, why are you going to this amount of trouble? If you want to put the cams back in the same place, just mark them. You are going to change the timing on the dyno anyway, so why does it matter where they are now?

cheers

[pnblight]

Cheers guys, just to clarify why i'm doing this is

1/ to get a better (meaning full) estimate of what the tomei cams are ie most other brands of cams from US are measured at the 50 thou lift point which gives you no idea what referrence is to a Japan cam (so i can compare apples with apples)

2/ Sydneykid the other reason is if you know where the cams are in relation to crank (not just setting on cam wheels) you can reinstall/ and reproduct the same setting on a different engine, as you are removing the variables out of the systems, plus you will find setting that work well and can go to them staight up and if you require dyno tune from there. Which can save a lot of time on the Dyno (which equals saved money and wear and tear) because anyone who has tuned exhaust cam timing will tell you it takes alot of time. I should also have the ability to go to a motor and dial in a cam timing i know will work.

pete

[ylwgtr2]

my HKS cam sheet tells me to do it a 1mm lift.....

[datsqik]

I would imagine with the higher pressures in the inlet and exhaust the point of meaningfull lift would be smaller in a turbo engine compared to n/a but as previously said as long as you only want a base line doing it at 50thou would be ok.Companies like tomei list factory cam timing in lobe centre points for in/ex and their duration so you could work out the opening and closing points they use.They also use this for their own cams but i would imagine this is done at "advertised duration" aswell.Using lobe centre points and lobe seperation can be another way of keeping data.This data would also need to be compared to things like boost pressure and the size of the turbo and exhaust a/r as an engine that runs factory turbos at 1.5 bar making say 340rwhp will react differently to cam adjustments on the same engine running a larger turbo with a big a/r exhaust housing running say .7bar and making the same rwhp.This is where keeping a track of exhaust back pressure can also come in handy.Some people worth talking to about turbo camshaft timing/design would be phil morris from motec,clive from clives cams and ivan tighe as these guy have much experience with camshaft design for turbo engines.

[Warpspeed]

Fifty though is probably your best bet with GTR solid lifters for several reasons.

It is the historical figure that most cam grinders call the "advertised duration" and it is almost universally used as a yardstick when comparing different cams. The reason being that it is always going to be well beyond any known starting and ending ramp built into the cam.

Hydraulic cams don't require an initial slow ramp to take up tappet clearance, but solid lifter cams do. Hydraulic cams can easily be measured "seat to seat" at five or six thou lift, because the cam lifts instantly off the base circle. But measuring at five to six thou lift on a mechanical cam is meaningless because the tappet clearance would not even have closed up.

Fifty thou is also good because at that lift the follower will be moving pretty fast, and there will be a lot of movement per degree of rotation. That is good for accuracy. Trying to set a mechanical cam at five thou will be difficult because the lift per degree will be extremely slow at that point, and setting it will not be so accurate.

[pnblight]

Pete,

.050" timing is what most cam manufacturers use as it's where the lift becomes a little meaningful. SAE timing is at .006" lift from memory. It doesnt really matter what you use as long as it's repeatable, RB's are a bit of a prick to measure lift, I made up an extension for the dial gauge that bends in under the cam and down again, you just need to be careful that you get the gauge parallel to the valve axis each time.

<{POST_SNAPBACK}>

have to agree

was not very easy to measure but i got i done in the end

pete