Vacuum Anomally - Roller Rockers

[DaveG]

I am putting this vacuum anomally out there for the 'brains trust' to review! Summary scenario:
I have installed roller rockers (Yella Terra YT6000). Prior to the installation I had a steady Idle vacuum around 50 kPa. My install has 3 plenum hoses (Brake booster, Clutch booster and Stepper). I did find I had lost one rocker pad which must now be in the sump somewhere, but the car runs and idles OK.

After installing the roller rockers the following are my test results:-
1. Zero 'lash' and no pre-load setting. Result = 46 kPa idle and of course a bit noisy.
2. Zero 'lash' and specified pre-load (1/2 to 3/4 turn with tappet adjuster which sets pre-load between 30-60 thou). Result = 54-55 kPa and quieter.

I had seemingly lost 8-9 kPa with the rollers. Double checked and re-sealed all hose, etc. The first pre-load setting I used around 70% turn and so I went back and reset again with about 50% turn. Result = The same 54-55 kPa.

I have started the retune and temporarily accepted the current readings, but have now noticed this vacuum anomally - see snapshot below:



You can see the engine is idleing normally under RPM Spark Correction (In Closed Loop), however when I apply a little throttle (increase TPS) above the set TPS threshold (0.8) RPM Spark Correction cuts out correctly. The rpm increases slightly as expected with throttle, but the vacuum IMPROVES by 2.5 kPa dropping to around 52.5 kPa!!
The vacuum should decrease (ie 100 kPa being zero vaccuum at atmosphere).

So what is going on? Any ideas? Clearly a good vaccuum is attainable, so maybe one or more lifters are playing up. I could try running the engine up to temp with the rocker covers off and adjusting each valve individually (A bit messy oil wise?).
It still does not explain the 'reverse vacuum' with a increasing throttle!
Any thoughts welcome.

PS. My cam is nothing special and close to the std TVR - Dur/ 282, ICA/ 112, Lift/ 0.47ins.

Edited by DaveG on Thursday 4th November 12:07

[blitzracing]

You cant regard low speed gas flow as a linear thing on performance cams- you get reversion of the airflow at the point the inlet and exhaust are both open at overlap when the inlet gas velocity is so low at idle it cant "ram" any reversion of air flow back past the open inlet valve. This is why the idle becomes so lumpy the more radical the cam. Im thinking at idle you have some reversion, but are loosing this effect as gas velocity increases so the vacuum increases?

[macdeb]

Can't help you, but my main concern would be where is that rocker pad?

[Boosted LS1]

Could it be something to do with the greater rocker ratio and valves opening more quickly? May the engine think it's got a bigger cam?

[DaveG]

Thanks for the comments. I have been back through my logs and I can see the reversion sometimes happens and seems subject to rate of applying the throttle, load conditions, etc. I kind of anticipated this, hence the cam details. So that aspect is ok.

What concerns me more is the variation in the vacuum. For the last 10 yrs I have been running at 50 - 51 kPa and suddenly with no pre-load I can get 44-45 kPA. Then with pre-load I lose 9-10 kPa (54-55 kPa)?
I am assuming all the lifters are wearing evenly (all replaced together in 2010 with cam) so maybe I have different valve lengths? The missing cap would simply be noisy - no vacuum loss. I have checked and rechecked the plenum-to-manifold seal/gasket, but as a last resort I will make up a custom paper gasket of the correct material and Hylomar it in place.
If that fails I can only think of once again doing individual rocker adjustment without the rocker covers or maybe try adjusting all cyinders by only 1/4 turn and see if that recovers vacuum and see how noisy it is?.
I would be interested in anyone who has had lifter problems (excessive wear, faulty springs?) and what the symptoms were.

Here is log with no pre-load and MAP = 44.5 kPa.
|https://thumbsnap.com/BmCKxWao[/url]

PS. Spoke with Yella Terra here in Oz recently and they confirmed my setup procedure was correct for the pre-load.

Edited by DaveG on Thursday 4th November 23:45

[url]

Edited by DaveG on Thursday 4th November 23:51

[SteveR1979]

DaveG said:

Thanks for the comments. I have been back through my logs and I can see the reversion sometimes happens and seems subject to rate of applying the throttle, load conditions, etc. I kind of anticipated this, hence the cam details. So that aspect is ok.

What concerns me more is the variation in the vacuum. For the last 10 yrs I have been running at 50 - 51 kPa and suddenly with no pre-load I can get 44-45 kPA. Then with pre-load I lose 9-10 kPa (54-55 kPa)?
I am assuming all the lifters are wearing evenly (all replaced together in 2010 with cam) so maybe I have different valve lengths? The missing cap would simply be noisy - no vacuum loss. I have checked and rechecked the plenum-to-manifold seal/gasket, but as a last resort I will make up a custom paper gasket of the correct material and Hylomar it in place.
If that fails I can only think of once again doing individual rocker adjustment without the rocker covers or maybe try adjusting all cyinders by only 1/4 turn and see if that recovers vacuum and see how noisy it is?.
I would be interested in anyone who has had lifter problems (excessive wear, faulty springs?) and what the symptoms were.

Here is log with no pre-load and MAP = 44.5 kPa.
|https://thumbsnap.com/BmCKxWao[/url]

PS. Spoke with Yella Terra here in Oz recently and they confirmed my setup procedure was correct for the pre-load.

Edited by DaveG on Thursday 4th November 23:45

[url]

Edited by DaveG on Thursday 4th November 23:51

Would you mind sharing your process for setting the pre-load?? I'll be fitting these to mine over Christmas, and would like to get an idea of whats involved.

Thanks!

[Boosted LS1]

I use a squashed measured paperclip that slips underneath the circlip in the lifter. I buy a shim kit from Real Steel and pick the best option.