VTC Clarification/questions following dyno

[John Dixon]

Finally got my car on the dyno again last week and it's thrown up some questions regarding the VTC system so here goes:

First initial dyno run I did was on a different dyno and hardly had any time before release bearing siezed so just going to ignore that one for now.

Last run was a proper mapping session with plenty of time to look at the results and they look very weird for my engine spec. To cut a long story short, the torque is all in the wrong place - makes >300ft-ibs from 2000rpm to 4500rpm then falls off a cliff very quickly! With my spec, it should just be picking up about then

It's kinda making me wonder if the VTC system is not capable of coping with these cams or if the cams have been installed wrongly.

My understanding of the VTC operation is:

1) Solenoids OFF (ie not grounded) - cams retarded by 20 CRANKSHAFT degrees from position 22) Solenoids ON - cams in advanced position.

With this understanding, I asked the engine builder to install the inlets 20deg retarded from the quoted full-lift position so when advanced they would be as-ground.

The other thing is that from the 'butt-dyno' point of view I would say that the feel of the power delivery has changed since I first ran it up and it's led me to wonder if perhaps the VTC gear is not man enough for the job and has broken or slipped back in some way.

On the plus side, it's still fun to drive and is making ~370bhp so not too bad - again though, is less than the initial dyno run suggested so again can't help but wonder if something has shifted.

Everything else looks good. AFR is right, timing optimised so def not a mapping issue. Even ran with open intakes to see if the (modified) air box was a restriction but it made no difference either.

Any thoughts/comments appreciated.

[kingkilburn]

I could be wrong but I think the cams are in a normal position before VTC comes on and then it advances then to be more aggressive.

Shouldn't you install the cams straight up as if there was no VTC so when it kicks in you get more aggressive valve timing?

[Steve Lloyd]

I don't know the history of your build, do you mind filling me in on what cams you are running and what year ECU you are tuning. Also, any new valve springs being used??

Thanks

[John Dixon]

Cams are 30/70 70/30 regrinds with 10.5mm lift. ECU is aftermarket so I know what it's doing in terms of switching OK. Valve springs again are aftermarket doubles, the whole engine is built to be safe to 9k rpm.

[tempestas]

When VTC is on, it advances the intake cams, which makes more low end torque. Just imagine shifting the whole HP/TQ curve to the left. VTC is normally off. It's off at idle, most likely crusing as well. High load under 4,600rpm (you step on the gas) it kicks in and creates better low end torque. At 4,600 it shuts back off to allow the high RPM power.

I would think of it as more.... normal timing vs 20 deg advance for low end torque. NOT normal vs 20 deg retarded for high end HP.

So if they were truely set 20 deg retarded, it just changes intended purpose, from adding low end torque when on - to now it needs to be on for normal use and off for high end power... only problem is 20 degrees is alot. Most people play with 3-5 degrees retard. 10 would be a whole lot. 20 degrees, while in theory retarded should make more top end HP.... it may be too much and just cause problems. You have no valve overlap at all. Something like the intake vavles are not opening until 10+ degrees AFTER the exhaust valves close. Thats bad.

I'd rethink the spec. If anything.... 5 deg retard for install.... or try stock first and go from there. And even 5 degree retard may be too much... if you arn't retarding the exhaust cams as well.

[John Dixon]

My point is though, if installed 20 degrees retarded then with VTC ON they should be on grind at 30/70 70/30 which should give a power curve somewhere from 4.5-8k Currently I can't get anything much past 4.5k with the VTC in either position.

[tempestas]

My point is though, if installed 20 degrees retarded then with VTC ON they should be on grind at 30/70 70/30 which should give a power curve somewhere from 4.5-8k Currently I can't get anything much past 4.5k with the VTC in either position.

where is your HP peaking? Torque is going to drop off in high RPM no matter what. Even an engine making maximum HP at 9,000 rpm will have torque flat or dropping above 5k-6k.

If a/f is good and your not making good power... its either restriction in intake or exhuast (sounds unlikely), ignition advance, or cam timing.

[John Dixon]

I'd come to the same conclusion which is why I'm looking at cam timing. Exhaust is a 4" system fed by 4-1 manifolds with 44mm primaries, intake is 48mm ITB's so neither of those is the likely culprit. Ign advance was optimised on the dyno for the current engine so nothing more to be gained there either.Peak HP is 7.5 but it doesn't reach a natural peak, it's completely flat from 5k to 7.5k. You can just see looking at the curve that something is not right.

[Aus450zx]

can you post up the graph?

I forget from your build but was much headwork done?

[John Dixon]

Yeah, will try and get a photo of it or something and stick it up.Head work was quite extensive. The 2 round ports were opened up into one big oval and the throat area re-worked then the valves back-cut.

[Orphan]

I hope you figure it out John, your build is really impressive and I hope you get the rewards for your effort and investment. I'd love to see that thing making 450+ or as close to 100hp/L as possible if not above.

[Chrispy300]

Cams are 30/70 70/30 regrinds with 10.5mm lift. ECU is aftermarket so I know what it's doing in terms of switching OK. Valve springs again are aftermarket doubles, the whole engine is built to be safe to 9k rpm.

For the uneducated, what does 30/70 70/30 mean in regards to cam sizing? I've always known it as degrees duration and mm lift

[XJared]

The way I read the valve timing activation with the stock ECM, the solenoid is ON, advancing the cam, and allowing increased overlap from just off idle to 4600rpm when the engine is at operating temperature (158-230F). Any other time the solenoid is OFF and the cams are straight up. The OEM setup is for the camshafts to be setup at their 0 centerlines and allow the solenoid/oil pressure to advance them from there.

[kingkilburn]

That is exactly what I thought.

[Aus450zx]

The way I read the valve timing activation with the stock ECM, the solenoid is ON, advancing the cam, and allowing increased overlap from just off idle to 4600rpm when the engine is at operating temperature (158-230F). Any other time the solenoid is OFF and the cams are straight up. The OEM setup is for the camshafts to be setup at their 0 centerlines and allow the solenoid/oil pressure to advance them from there.

so with VCT activated the idle should be worse?

[Chrispy300]

That's how it works with the VG30 (disconnect, worse idle), and it appears to be the same system.

[Mettler]

I was under the impression it advances at 1500rpm and retards again at 4600.

Remember that 20 degrees valve timing based on crank revolutions is only 10 degrees advance/retard at the lobe since it's turning half as fast as the crank. Perhaps your cam guy moved the profile by 20 degrees.

[John Dixon]

I was wondering if he moved it too far too. He's a pretty experienced guy but it's possible he did. I'm just going to have to get the covers off and measure it all to see what's going on.It's a pain with the VTC being as it is because you can't time them in using equal lift on overlap with this profile as you def want the 20deg retard vs the 60 degree overlap that the on-grind gives. Means using a degree disk on the crank and that's always a pain with the engine in-situ.

[John Dixon]

For the uneducated, what does 30/70 70/30 mean in regards to cam sizing? I've always known it as degrees duration and mm lift

It refers to the opening / closing positions of the 2 cams.EVO 70deg BBDC, EVC 30deg ATDCIVO 30 deg BTDC, ICV 70deg ABDCThis is then 280deg duration with 60deg overlap