Madjak

In my quest to get more power, the next stage is to bolt on some forced induction. I understand running very big cams with a supercharger can cause issues when you have lots of overlap on the cams. My cams have an overlap of around 50 degrees -+ a bit due to the adjustable cam wheels.

At what point is it a massive issue and what problems does it cause? I assume it affects fuel consumption as a lot of fuel would be flushed out the exhaust. You'd also be loosing lots of air and so boost out the exhaust so how does the ECU account for this? Can you just upsize the supercharge a bit to compensate?

TNTUBA

iTrader: (1)

With that much overlap, you would possibly be better off selling your cams and buying stock ones when you put the blower on the car.

Alternative

Might you be mistaken on your cam specifications? That's very radical.

Madjak

They are big cams... 310+ duration. Overlap might be less but not by much. I think the intake cam is advanced 8 deg and the exhaust only 3-4. Might be reversed though.

What sort of overlap would be ok with FI?

Leafy

iTrader: (1)

310... holy moly.

guttedmiata

Our cams have 18* overlap set straight up and we are questioning if that is too much. FWIW

Madjak

That's the advertised duration @ 0.01". Actual duration is around 250 I believe @ 0.05"

They are a custom grind. I'll upload a video of the car at idle for anyone interested in listening to how it sounds. Lumpy is a bit of an understatement. Starting the car in cold weather is always fun, you get lots of pops and farts out each end.

guttedmiata

If that's seat duration, then yeah they're big, but not crazy big.

If that's measured at .050", then holy moly.

guttedmiata

^^^^^ Hah, dang. Beat me by like 2 seconds.

Madjak

With the duration what's the best figure to quote? The seat duration /advertised or the 0.05" lift duration?

My understanding is the ramp curve / angle affects the advertised duration but doesn't necessary equate to more 'cam' just a slower ramp. That overlap figure I gave you was calculated off the valve open / valve closed degrees so I don't think the 50 degrees overlap changes.

Alternative

Larger cams typically shift the powerband to the right, but your question is very dependent on what type of forced induction you are intending to use. Rotrex or Roots? Goals? Current Dyno?

Comparing seat numbers or 0.050"/1mm numbers are not the best way to compare cams but a lot of times that's all the MFR gives you to work with.

Alternative

Do you have a cam card?

guttedmiata

The difference between exhaust closing and intake opening, right?

Madjak

I don't have the CAM card as I purchased this long motor second hand already setup. I only have some of the specs.

Here is a link to my latest dyno (two days ago) - scroll to the end
https://www.miataturbo.net/engine-pe...ing-req-83892/

Madjak

I used an online calculator. It required intake VO / VC and exhaust VO / VC degrees. These aren't the actual cams but they are close. I'm pretty sure the intake and exhaust cams are the same profile unlike these specs:

Cam Lift:
Intake: 0.436
Exhaust: 0.412
Rocker Ratio:
Intake: 0.995
Exhaust: 1
Nett Valve Lift:
Intake: 0.426
Exhaust: 0.404
Advertised Duration @ 0.01
Intake: 322
Exhaust: 308
Duration @ .05'' VALVE LIFT
Intake: 261
Exhaust: 247
Timing @ .05'' VALVE LIFT
IVO: 30.5 BTDC
EVO: 51.5 BBDC
IVC: 50.5 ABDC
EVC: 15.5 ATDC
Suggested Centre Lines:
Intake: 100 ATDC
Exhaust: 108 BTDC
VALVE Lift @ TDC
Intake: 0.178
Exhaust: 0.131

Madjak

So duration should be (EVC: 15.5 ATDC) + (IVO: 30.5 BTDC) = 46 degrees

thats probably a little too high if my intake cam isn't as aggressive as those specs, though I don't think the difference in degrees between the cams is the same as that either. From memory I think I have 110 degrees difference.

I know I'm about 2-3 degrees from interference on both intake and exhaust cams... so doesn't that mean the valves are pretty much at max overlap possible? When I first got the engine and was trying to get it started, it had such a massive backfire out the intake that it skipped a tooth on the timing belt and locked the engine up on a valve. So lucky nothing got bent.

guttedmiata

Yeah, you need different cams.

Madjak

I've read up a bit more on overlap and FI... the Honda guys are running up to 20 degrees of overlap and making good power but any more and the loss of boost is greater than the gain from the cam.

If I'm over 40 degrees then the loss of boost would be massive considering that at 20 degrees the valves are only partially opening or closing.

This will probably delay my plans for a 400+ HP Rotrex build. I might enjoy the N/A power for a season or two.

Alternative

"Honda Guys" are running 20+ deg of overlap on stock cams, but those engines work a hell of a lot more efficient that a BP

The Innovator

This^^^^