2manyhobyz

iTrader: (4)

"Maximum intake valve duration is limited by reversion into the intake tract as the intake velocity loses out to pressure building from the piston moving up the sleeve, and charge contamination by the exhaust back pressure on the other side of the stroke. So optimal camshaft design would vary wildly by turbo sizing, intended operating range, operating pressures, and whether VVT was available."
Thanks TravisR, those are a lot of variables to contend with. Would you please venture an educated guess on an intake cam with an engine build with a 2000 1.8, 2560R at 14 psi, 8.8:1, operating range of 3 to 7500, mild P&P, dual plenum design intake manifold (@ 2.25 liters) with runners about 200-250mm.
It sounds like I could use the stock exh cam and adjust it to minimize overlap. Upgrades would be +1mm intake valves and possibly shim under bucket lifters.
Thanks for your help

TravisR

iTrader: (9)

I dunno, 220 or 228 @ 1mm? Something around 10mm lift? You don't want to go above 228 really. Stock exhaust cam should be fine. Rotate LCA on intake until its happy, stock exhaust cam should probably be in ok position as spec'd from factory.

aaronc7

iTrader: (8)

NB exhaust cams seem to be pretty well suited for the job... I'm looking into it at the moment... I think I may just give it a shot if I can find one

Boost Joose

Does anyone know the lobe seperation angle, intake openeing point, and exhust closing point of the 99-00 intake and exhaust cams?

blaen99

iTrader: (6)

Holy ----. Give more details on those cams - 1.6 bignose with cams for the 323 from Kelford, right?

JasonC SBB

In case this hasn't been pointed out, in a twin cam motor these 2 parameters are separately adjustable with cam gears, no matter what all the other specs are (inlet and exhaust duration).

JasonC SBB

On my VVT motor with 8.6:1 pistons, I can tell you that 15 psi wanted more intake retard up top then at 10 psi, probably due to increased TIP (turbine inlet pressure), wanting less overlap.

I measured TIP here:
https://www.miataturbo.net/showthrea...&highlight=TIP

JasonC SBB

Later closing intake always shifts peak torque and peak power RPMs higher (leading to higher peak power). If doing this, it will also help to optimize the intake manifold to said higher RPMs too.

Longer duration allows higher lifts; beyond a certain amount of lift, ports often don't flow any more, unless they are ported to take advantage of said lift. This typically has less impact on overall flow than porting that improves flow at lower lifts, where the valve spends more time anyway (than at max lift).

bobbiemartin

iTrader: (2)

I have been trying to find this for some time now. The NASA Spec Miata rules list the stock cams, their website is here: http://www.nasaproracing.com/rules/spec_miata_rules.pdf

Now, these numbers are somewhat different to other posted numbers, so who do you believe. Also, I have pretty well determined that stock Miata cams have asymmetric lobes, that is the closing ramp is a few degrees longer than the opening, so max lift is not the center line of opening & closing.

To determine the LCA, I would use the duration at .050” and I think you would be fine. I have been using the calculators here: http://www.wallaceracing.com/Calculators.htm

I get LCA of about 100 degrees. I would think you would want something wider, maybe 110 or so, but I can't really say for sure. The NASA numbers also give an overlap of 66 degrees, which seems like a massive amount, again I would think 35 would be more in order, but what do I know?

I did find some aftermarket cams (Schrick) that time at 20-60/60-20. That works out to an LCA of 110 and overlap of 40 degrees. What I don't know is are those timing numbers seat to seat, lash point or maybe .010, .020 lift? It doesn’t say, so make your own assumptions. The lift is pretty close to stock cams.

So it makes me wonder; how would the stock cams work timed at full lift at 110? I believe that would be advancing both cams 10 degrees?

Boost Joose

Omg camshaft calculations suckkkkkkkkkkkkkk

JasonC SBB

The old obsession with LCA is a holdover from the days of single cam V8's. It's far less important for a twin cam motor. LCA is easy to measure (angle between points) and is proportional to overlap.

The numbers which are most important, in approximate decreasing order of importance are:

- intake closing point
- overlap
- intake duration
- exhaust opening point

Ignoring ramp opening and closing rates, and lift, here are the 4 numbes that completely describe cam timing:

- IVO
- IVC
- EVO
- EVC

The earlier numbers I listed, and other specs, such as LCA, duration, etc, can be derived from the above 4. Many other combinations of 4 specs, will allow you to derive all the others.

Now when you have a twin cam motor with adjustable gears, the only variables you can't change with the gears, are intake and exhaust durations. Thus the latter specs become the 2 most important ones to *describe* a set of cams. (Again ignoring lift and opening and closing rates). This doesn't mean that plopping longer duration cams without adjustment is better than adjusting the cams you already have. Adjusting cams allows you to change overlap and intake closing point, the 2 most important parameters. This is why VVT is more significant for widening your spread of torque, than upgrading cams in a non VVT motor.