Why won't these engines rev

[QuattroFever]

Okay, so i've pondered long enough and I'm after some technical opinions on why these engines can't rev on hydraulic lifters.

so many opinions on hydraulics being naff and solids being hard to maintain.

is it that the oil supply to the head is not sufficient or partly due to the valve gear being too heavy.

everybody seems to recommend a dry sump setup, which is great for removing sloshing oil in the block and oil starvation at high cornering G's, but what does this actually do for the head?

This is what leads me to beleive that maybe the oil supply is not enough, if people are suggesting an external pump of sorts.

I want to touch the 8k rpm mark without going into solid lifter territory. only 500-600 more rpm's but seems it's forbidden to our I5's.

[terrybullwon]

i am going to go 8200 on hydro lifters but i have the pro mo racing diamond coated oil pump and every part is brand new of the engine.

[Error404]

what do Audi run on the latest generation V8s as they rev to 8k

[QuattroFever]

you think it'll hold?

or you just gonna go for it and see what happens.

[terrybullwon]

pretty sure it will be okay, but its one of those if it breaks i will just replace it with the next best thing. I already have half a dry sump system so i dont know how long i will drive with the coated pump but it seems to work very well.

[QuattroFever]

did you use any of the fancy lightweight lifters or just the stock ones?

[terrybullwon]

yeah they are from some RS4 engine or something i cant remember, all the oil ways in the head were opened up and the head had alot of work done on it.

The engine was 100% lightened and balanced and revs very very well your welcome to come and see it.

[JP]

OK there is various things with the I5's and other VAG engines.

First of all the weight of the lifter is such that under the consistent pounding of high rpm it cannot sustain full expansion resulting in the lifter potentially collapsing.
Secondly the spring rating is such you can potentially suffer from valve float where the strength of the spring is not great enough to close the valve quick enough. Fit stronger springs to prevent valve float and you now compound problem number 1.

Putting those two points aside we get onto the oiling sytem of the engine. Corner hard enough and the oil pickup will struggle to pick oil up from the wet sump at times unless running a very high oil level. The end result being that the cylinderhead which is at the end of the pressure system will not have enough pressure to be able to properly expand the lifters and your cylinderhead will sound tap-ety. Fitting a dry sump setup will counteract this issue as now you are picking oil up from a reservoir so irrespective of how hard you corner you always have a permanent oil feed solving the tap-ety lifter issue during hard cornering.

The next problem is with high rpm is that you need greater bearing clearances to dissipate the extra oil, the problem with this is if the oil is not under enough pressure because of the greater bearing clearances the oil is no longer supporting the non contact bearing surface. The more boost you run the greater the downforce of the rod on the crank journal and the harder a pounding the bearings take if not enough oil pressure (this is why we have sputter bearings which are much harder than stock). Run a thicker grade oil to increase oil pressure at high rpm or have a pump that creates more pressure through greater flow per rpm and you help prevent bearing collapse at high rpm.

And this is where the greatest problem arises from the stock design of the Audi oil pump. The design is flawed at high rpm due to the way the gears mesh. As you increase the speed of the pump you create an issue of oil frothing where small air bubbles get put into the oil from the high velocity of the oil pump gears. Now we all know what happens when your hydraulic brake system is not properly bled and still has air in it. Air is much easier to compress than the hydraulic fluid the same with the oil. So as your oil becomes frothy at high rpm it can no longer support the bearings correctly causing hotspots to occur on the crank journals where there is a lack of lubrication due to air content allowing the bearings to momentarily make contact at high rpm, eventually leading to big end bearing failure. The frothing of the oil is also the main cause of lifter failure as once again you no longer have the same ability to support the internals of the lifter due to the air content in the oil causing them to collapse, The smaller and lighter the lifter the less severe this effect which is why you can get away with revving 1.8t's harder on hydraulic lifters.

You can coat the gears to aid oil pick up to increase pressure you can run a thicker grade of oil to increase pressure but ultimately you will still have the frothing which will be your end cause to failure at long sustained high rpm usage without a dry sump setup.

hth JP


edit: I still don't understand why someone wants to rev above 7500-7600rpm unless an out and out track car with too big a turbo on it to work low down in which case you'd dry sump it anyway.

[Rusty]

... I still don't understand why someone wants to rev above 7500-7600rpm ...

Because it's

[QuattroFever]

perfect, thought you might pop along
I see what the diamond coated pump gears are for now.
good write up.

Throttle response is better higher up the rpm range and more speed in each gear without paying mr quaife your wife.

plus, gotta be able to stay with these stroker boys somehow

[QuattroFever]

Because it's

because it sounds

[JP]

perfect, thought you might pop along
I see what the diamond coated pump gears are for now.
good write up.

Throttle response is better higher up the rpm range and more speed in each gear without paying mr quaife your wife.

plus, gotta be able to stay with these stroker boys somehow

Yes but remember if your rpm drop is between 1000-2000rpm depending which gear you are in you only need a useable rpm band of 2000rpm that is 5500-7500rpm Most silly size turbo's are on full chat by 5500rpm so therefore no need to go higher . Although I can't deny the noise factor though :mischeif:

[QuattroFever]

If it's got more air to flow might as well use it if you can. After all, you've got the lag may as well reap the benefits.

[Jamo]

Vtec revvers! never understood the obession with high revs, high revving cars where the powerband are high up are such a chore to drive IMO and not enjoyable.

Suppose if it's for a track it's understandable but the road, nah

[Le S2 Audi]

All Above my head...

But a very interesting read....

& I have learnt something new today...