Overlap on Cams vs Supercharging - Miata Turbo Forum - Boost cars, acquire cats.

Welcome to Miataturbo.net   Members
 


Supercharger Discussion For all you misguided souls.
Sponsored by:
Sponsored by:

Reply
 
 
 
LinkBack Thread Tools Search this Thread
Old 05-28-2015, 04:03 AM   #1
ʎpunq qoq
Thread Starter
 
Join Date: Jan 2014
Location: Western Australia
Posts: 502
Total Cats: 100
Default Overlap on Cams vs Supercharging

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?
Madjak is offline   Reply With Quote
Old 05-28-2015, 06:55 AM   #2
Senior Member
iTrader: (1)
 
Join Date: Jan 2011
Location: Chattanooga, Tn
Posts: 1,038
Total Cats: 157
Default

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.
TNTUBA is offline   Reply With Quote
Old 05-28-2015, 08:16 PM   #3
Junior Member
 
Join Date: Jan 2015
Posts: 272
Total Cats: -22
Default

Might you be mistaken on your cam specifications? That's very radical.
Alternative is offline   Reply With Quote
Old 05-28-2015, 09:17 PM   #4
ʎpunq qoq
Thread Starter
 
Join Date: Jan 2014
Location: Western Australia
Posts: 502
Total Cats: 100
Default

Quote:
Originally Posted by Alternative View Post
Might you be mistaken on your cam specifications? That's very radical.
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?
Madjak is offline   Reply With Quote
Old 05-28-2015, 10:09 PM   #5
Elite Member
iTrader: (1)
 
Leafy's Avatar
 
Join Date: Jun 2012
Location: NH
Posts: 9,093
Total Cats: 90
Default

310... holy moly.
Leafy is offline   Reply With Quote
Old 05-28-2015, 10:14 PM   #6
Junior Member
 
guttedmiata's Avatar
 
Join Date: Sep 2012
Posts: 438
Total Cats: -3
Default

Our cams have 18* overlap set straight up and we are questioning if that is too much. FWIW
guttedmiata is offline   Reply With Quote
Old 05-28-2015, 10:14 PM   #7
ʎpunq qoq
Thread Starter
 
Join Date: Jan 2014
Location: Western Australia
Posts: 502
Total Cats: 100
Default

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.
Madjak is offline   Reply With Quote
Old 05-28-2015, 10:16 PM   #8
Junior Member
 
guttedmiata's Avatar
 
Join Date: Sep 2012
Posts: 438
Total Cats: -3
Default

Quote:
Originally Posted by Leafy View Post
310... holy moly.
If that's seat duration, then yeah they're big, but not crazy big.

If that's measured at .050", then holy moly.
guttedmiata is offline   Reply With Quote
Old 05-28-2015, 10:17 PM   #9
Junior Member
 
guttedmiata's Avatar
 
Join Date: Sep 2012
Posts: 438
Total Cats: -3
Default

^^^^^ Hah, dang. Beat me by like 2 seconds.
guttedmiata is offline   Reply With Quote
Old 05-28-2015, 10:21 PM   #10
ʎpunq qoq
Thread Starter
 
Join Date: Jan 2014
Location: Western Australia
Posts: 502
Total Cats: 100
Default

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.
Madjak is offline   Reply With Quote
Old 05-28-2015, 10:30 PM   #11
Junior Member
 
Join Date: Jan 2015
Posts: 272
Total Cats: -22
Default

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 is offline   Reply With Quote
Old 05-28-2015, 10:31 PM   #12
Junior Member
 
Join Date: Jan 2015
Posts: 272
Total Cats: -22
Default

Do you have a cam card?
Alternative is offline   Reply With Quote
Old 05-28-2015, 10:39 PM   #13
Junior Member
 
guttedmiata's Avatar
 
Join Date: Sep 2012
Posts: 438
Total Cats: -3
Default

Quote:
Originally Posted by Madjak View Post
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.
The difference between exhaust closing and intake opening, right?
guttedmiata is offline   Reply With Quote
Old 05-28-2015, 10:40 PM   #14
ʎpunq qoq
Thread Starter
 
Join Date: Jan 2014
Location: Western Australia
Posts: 502
Total Cats: 100
Default

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 is offline   Reply With Quote
Old 05-28-2015, 10:45 PM   #15
ʎpunq qoq
Thread Starter
 
Join Date: Jan 2014
Location: Western Australia
Posts: 502
Total Cats: 100
Default

Quote:
Originally Posted by guttedmiata View Post
The difference between exhaust closing and intake opening, right?
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 is offline   Reply With Quote
Old 05-28-2015, 10:57 PM   #16
ʎpunq qoq
Thread Starter
 
Join Date: Jan 2014
Location: Western Australia
Posts: 502
Total Cats: 100
Default

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.
Madjak is offline   Reply With Quote
Old 05-28-2015, 11:21 PM   #17
Junior Member
 
guttedmiata's Avatar
 
Join Date: Sep 2012
Posts: 438
Total Cats: -3
Default

Quote:
Originally Posted by Madjak View Post
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
Yeah, you need different cams.
guttedmiata is offline   Reply With Quote
Old 05-28-2015, 11:40 PM   #18
ʎpunq qoq
Thread Starter
 
Join Date: Jan 2014
Location: Western Australia
Posts: 502
Total Cats: 100
Default

Quote:
Originally Posted by guttedmiata View Post
Yeah, you need different cams.
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.
Madjak is offline   Reply With Quote
Old 05-29-2015, 07:15 PM   #19
Junior Member
 
Join Date: Jan 2015
Posts: 272
Total Cats: -22
Default

"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
Alternative is offline   Reply With Quote
Old 05-29-2015, 08:27 PM   #20
Newb
 
Join Date: May 2015
Posts: 9
Total Cats: -1
Default

Quote:
Originally Posted by TNTUBA View Post
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.
This^^^^
The Innovator is offline   Reply With Quote
 
 
Reply

Related Topics
Thread Thread Starter Forum Replies Last Post
Project Gemini - Turbo Civic on the Cheap Full_Tilt_Boogie Build Threads 59 Yesterday 09:00 PM
MP62 maintenance Pist0n Supercharger Discussion 3 10-10-2015 05:23 PM
New M62 Miata owner Pist0n Meet and Greet 4 10-01-2015 09:18 PM
another noob kr307 Meet and Greet 1 09-26-2015 11:33 AM
High-lift cams, SUB, new valve job = lifter tick? Der_Idiot Engine Performance 14 09-16-2015 10:30 PM


Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off



All times are GMT -4. The time now is 11:05 AM.