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camshaftsbp

Camshaft Theory BP

We had never too much lift (https://youtu.be/NZEoIy0_rqc?t=1297) - DBilas

IVC, IC intake valve closes (at)

  • The significant value to estimate the target power rpm of the camshaft
  • Later IVC → More time to overcome the pressure differential, usage of the high velocity gas, favors HIGH RPM
  • Too late IVC → Cylinder pressure > Manifold pressure, loss of power (but efficiency gain through better expansion (reverse Flow Atkinson Cycle https://en.wikipedia.org/wiki/Atkinson_cycle)

OL, O/L overlap (OL duration = IVO + EVC)

  • Overlap can be used to evacuate the remaining intert gas out of the cylinder
  • Free Flow Exaust → Inertia of the exhaust gas helps via scavenging effect
  • Positive Pressure Intake → Inertia of the intake charge pushes the remaining inert gas out

If air or intake charge is pushed into the exhaust, NOx / HC emission increase. Also precious charge is wasted, resulting in higher BSFC.

EVO exhaust valve opens (at)

  • Too early → Gas goes through the exhaust valve and doesn't participate in piston downforce → power loss

Rotrex Specific

Consultation with Jeff David, who developed a Rotrex specific Camshaft for Honda K22

Preparation

In my experience it's a combination package. For the bolt on's like inlet manifold, TB, exhaust headers and exhaust system use what a good/excellent NA build would choose.

Excellent N/A Build Bolt-Ons:

  • Flattop / Skunk2
  • OEM / Skunk2 / 68mm Chinese
  • RacingBeat 4-2-1
  • High-Flow Catalytic Converter
  • 2.5“ / 3” Exhaust

The Rotrex will provide say 15psi on the inlet manifold. The bigger the exhaust system say going from 2.5“ to 3”, the less boost you will get as more blow through. But power most likely will be the same or more, as will fuel flow requirement due to the blow through.

Exhaust Backpressure

Blow Through / High Overlap

As it's important to trap the fuel/air on the valve overlap, higher lift and longer duration NA cams tend to loose power due to blow through. Specific fuel consumption goes crazy as well. On the K22 going from stock to Stage2 NA cams was +50% injector flow. Power was almost the same.

As you widen the lobe centre between the inlet and exhaust the engine becomes less prone to knock. This is a big factor if you have poor fuel and or a relatively high dynamic compression ratio on a boosted car. So wider lobe centers can make more power and reduce overlap.

My approach to the cam design was to try stock, other supercharger cams, then stage 2, model the results with software and then design my own. I ended up with wider lobe centers with less overlap than stock but lift is similar to stage 2 or stage 3 camshaft depending on the vendor. Also plenty of duration at 0.500”.

This is not easy to do while keeping acceleration, jerk and hertizan stress close to OEM.

So I would try the mildest inlet cam you can find, hopefully with more lift than stock. And a stage 2 exhaust cam as a starting point. Fit a venier exhaust cam gear to advance it's centre line. Position based on dyno tuning but 6 degs advanced is a starting point depending on the duration.


Camshaft specs don't give a clear understanding unless you can get the duration at 0.200“ . Please see the attached photo from 3 different exhaust camshafts. The advertised duration is 307,300,303 seeming similar. The lift @ 0.039” is 247,244,242, again similar. The duration at 0.200“ is 138, 137,122, vastly different and lift is 13.1, 12.6, 11.7. So you look at the graphs and the area that gives the air flow into the chamber is totally different. The overlap with the inlet is not much different as I have altered the lobe centre so the opening times are close.

So what I am saying is without a cam doctor and software all you can do is a best guess and dyno or measure acceleration.

To make an educated guess you need to know the duration (area) at 0.200” and none of the manufactures quote this. You can guess a bit from the lift.

You also need to think about driveability, longevity of valve train, cam chain, etc. probably from forums (not Facebook) might give a guide as to where to draw the line on how aggressive.

Maximum Lift

If you don't care about this then the Tomei 1499252108 inlet should be the best inlet.

The exhaust is harder to know. The Tomei might be a good choice as it is the matching exhaust cam, however from my Rotrex experience I would go much wilder. The only limitation is how smooth it is to drive at low engine speeds or light throttle. With the K Honda we have iVtec, so small and big lobes. I swap at 2800rpm on full throttle or 6000rpm on light throttle to the big cam so avoid this issue.

My general feeling is I would go with the wildest. The Rotrex is forcing the air in. To make power it's got to come out! The SC will create an engine feel with more torque and the big exhaust will help it breath from 6500 to 8000rpm.

Conservative….Mildest Tomei 1498256100 exhaust

Mid……………CatCam 3701411

Advance to 117 to 120 degs lobe centre based on dyno testing, Wildest best potential power CatCam 3701412

Even with relatively wild cams chosen, your lift is very low compared to my GP700 camshafts. The valve open area is also vastly smaller.

Quotes


.427 (10.8mm) cam lift intake, 272° .410 (10.4mm) cam lift ex, 266°

Clearances are bigger than OEM so actual lift is less of course. That's about the most duration I would run in an engine expected to idle. A VVT head might tolerate another 5-10° since you can dial out some overlap at idle and IVC doesn't really affect idle vacuum.

So these are really midrange cams. Even with IRTB's I would expect peak power to occur around 7700 and roll off to about 8400. Peak torque will still be in the 6100rpm range I think.

- emilio700 (https://www.miataturbo.net/engine-performance-56/2-0l-bp4w-182whp-63455/page5/#post835745)


The NB2 “VVT” stock intake cam has a very abrupt ramp which allows the valves to bounce violently past about 7800rpm.

- emilio700


k24madness Rotrex Setup (https://www.miataturbo.net/engine-performance-56/295whp-rotrex-c30-74-a-75473/)

I asked for grinds with around .400 lift and duration between 210-230 @.050

WebCam grind #264 intake and exhaust, 110LSA, 108in, 112ex.

.264in…………Stock in

243@.010…..248@.010

216@.050…..204@.050

108@.300…..062@.300

..264ex…………Stock ex

243@.010…..250@.010

216@.050…..210@.050

108@.300…..080@.300

Forget advertized when looking at cams, .080-.010 is lash spec so .010 is essentially seat time.

Less duration at .010 means more dynamic compression, less overlap. More duration at .050 means more flow. More duration at .300 means the valves are fully open when the piston is at maximum acceleration through the 90 degree sweep. This is a critical part of head flow.

- k24madness


The BP powerband shape is very sensitive to duration @.003. The few degree increase from a 4W to a 5A shifts peak torque a few hundred rpm. We found that an Integral stage 1 NB2 cam with a bit more lift but 10° more duration at .003 was no faster on track than the stock cams even though it made a bit more peak. If it is a 5 speed or driven on street with a stock longblock, we prefer stock cams for best driveability. Breathe on the longblock or do a full I/H/E +ECU and you can afford a little more duration as the low end improvements from the other stuff over up the losses from cams. For instance, the Tomei's we offer on a stock engine with just an ECU, are awful. Add I/H/E + ECU with the Tomei's, particularly with a 6 speed and you have a great balance between powerband and peak power.

- emilio700


camshaftsbp.txt · Last modified: 2022/01/03 09:41 by imakeyoufast