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Cam Gear Problem - Dyno Graph Inside

TurbostyleR

By TurbostyleR,
in Engines & Forced Induction

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TurbostyleR Collaborator

TurbostyleR

Posted
Posted (edited)

Hi,

I'll be frank and say this not for an RB series motor, but rather an SR20DET...I have gotten it tuned recently to the current power level however I have a problem with the power band dropping off too much at high RPM...

The current specs for the head are as follows:

Solid lifter conversion - so the ramp rate of the cams are different to the hydrolic type cams..

hks step 3 264 inlet cam

hks step 3 272 exhaust cam

1mm oversized exhaust valves

HKS valve springs - coil bind is not an issue - its been checked and came up good

The turbo is a modified Apexi RX6 unit with a bigger compressor wheel, .80 A/R exhaust turbine with 67mm exhaust wheel...so it should not be dopping off boost or power top end wise...

Now the cam gears are currently set on 4 deg advanced on the inlet and 3 deg retard on the exhaust, i just took pretty much a stab in the dark with these settings...

The boost controller is an AVCR and I have no idea why the boost curve looks like that... :S

With this current power i ran a 13.1@125mph recently at willowbank with a 2.5 60 foot... :S

What would you guys recommend i change the cam gears to to get my power curve to keep on keepin' on...

Here is the dyno printout:

post-5459-1162101787.jpg

Edited by TurbostyleR
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The Alchemist Collaborator

The Alchemist

Posted
Posted (edited)

hey there mate, your boost control has too much sensitivity setting I'd say. Sort that out first as Nismoid suggested. The upper power drop off you see there seems to be fairly typical with the SR20, at least the ones I've seen/tuned on the dyno. Cam lobe center angle seems to be the key with any cam gear tuning.

The tighter the lobe center the better higher RPM flow & therfore torque will be.

Unless you time the cam gears to the crank so you know the lobe center angle to start with, any changes will be a try it and see whats happens affair.

Also can you post your dyno graph showing the torque please as this can be more helpful to see what is going on in the engine and its peak efficency point.

Hope this helps,

Mike

Edited by The Alchemist
TurbostyleR Collaborator

TurbostyleR

Posted
  • Author
Posted

Thanks Mike for your comment, much appreciated.

The boost controll issue is going to be taken care off with a bigger wastegate spring as the current one is too small and the avcr duty cycle is pretty much on the limit of where it should be.

I dont have any graphs showing torque unfortunately.

In regards to "tighter the lobe center the better higher RPM flow & therfore torque will be" What do you mean by tight, i dont understand all this lobe center stuff... :S

The Alchemist Collaborator

The Alchemist

Posted
Posted

need a diagram really to properly explain, if you do a search for Cams + "lobe center" on google you will find many sites showing a couple of cams set up with a circle 360 degrees around it and a centerline drawen down the middle of each cam lobe 2 the center of the circle. The angle between the lobe centers becomes an obvious thing.

The smaller (tighter) the angle is the more the cylinder head will tend to flow better at higher RPM. Its to do with the decrease in time allowed for air to pass through the head as RPM increases you see.

This increases the overlap (the time the inlet and exh valve are open for at the same time) which allows more air at higher RPM to pass thru the head. Complicated eh :) Anyway, small changes of a degree or 2 can have a substantial effect on the way the engine produces it torque and where it produces its torque. Using factory cam settings as per the timing marks with adj wheels on zero zero, the lobe centers tend to be too wide at say 114 degrees which is why you read about everyone adv the inlet and ret the exh which tightens the lobe centers and improves turbo response and overall torque. A good thing. The problem is that the other important cam setting is the overall relationship between the cams and the crank. Once you get the best lobe center angle you can move on to the adjusting the inl & exh cam together either advancing or retarding them. This moves the torque curve up the rpm or down the rpm range. All fine tuning but all worth it in the long run. Don't get to keen on the light and mid load tuning either cos this all changes when you change the cam timing and often has to be done again

Mike

TurbostyleR Collaborator

TurbostyleR

Posted
  • Author
Posted

Thanks Mike for the explanation I really appreciate it!

I dont think it was losing traction on the dyno, but its a good point, I will put it on another dyno and see if it does the same thing..

On a sidenote I took it to the track tonight as is and reeled off an [email protected] think that dyno reads a little low...

DVS JEZ Grand Master

DVS JEZ

Posted
Posted
Thanks Mike for the explanation I really appreciate it!

I dont think it was losing traction on the dyno, but its a good point, I will put it on another dyno and see if it does the same thing..

On a sidenote I took it to the track tonight as is and reeled off an [email protected] think that dyno reads a little low...

dude thats an impressive mph.

isnt that good for low 10's?

nice one.

Jeremy

cameron Mentor

cameron

Posted
Posted
Thanks Mike for the explanation I really appreciate it!

I dont think it was losing traction on the dyno, but its a good point, I will put it on another dyno and see if it does the same thing..

On a sidenote I took it to the track tonight as is and reeled off an [email protected] think that dyno reads a little low...

Wheel spin usally happens at max torque point which is well before it starts to loose power, but looking at the mph and the power they deffiently dont match.

Could be a tuning issue possibly a who in the ign map?, also try bringing the intake back to 0deg and leave the ex at -3.

If none of that works then you have a restriction somwere maybe start with basics, exhaust system?

altho in saying that it does hold boost well you just need to turn the F/B Speed down or even turn it off to fix the mountain like boost curve...

rob82 Mentor

rob82

Posted
Posted (edited)

By the look of that i would have said that your lobe centres are too tight. You've got to remember that in tightening your lobe centre's your reducing your power band. Also as cameron said you feedback speed is set to high - thats why your boost is oscillating.

Edited by rob82
TurbostyleR Collaborator

TurbostyleR

Posted
  • Author
Posted

awesome info guys thanks alot for that its really appreciated...

Im getting a bigger wastegate spring soon so the duty isnt up around the 60s for 1.3bar of boost...

the dyno does read low for my car, i dont know why but my car never reads right on it...on 300rwhp the car ran 124mph trap speeds, and now on 365rwhp im running 130mph trap speeds...

definately cant wait to fix the boost and power band up and go back to the track and crack a 10!

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