andy-gts

last sentence correction..."All the time giving you the extra 15 hp the dry sump guys get from dry sumping?"

GregBBRD

Maybe. Thought about this...hard to keep the crankcase separate from the heads and still have drains...they are difficult to make independant.

I'm going to guess that if we can keep the exhaust cam from being submerged...which means that the springs, retainers, and lifters are also completely submerged...that we will see more than a 15 horsepower change. That has to take a bunch of power to "slug" those parts through solid oil.

We also have to be loosing power, as the oil gets air mixed with it... I'd think that the bearings would drop through that combination pretty quickly and actually be touching the crankshaft....

GregBBRD

Here's a little bit more data, that I've been "sitting on" for years.

Mark Anderson's dry sumped race engine has always, regardless of what oil pan we have on it, "sucked the dry sump tank down" once we ran the engine above 5,000 rpms. The oil level will drop down about 1.5 inches, as I recall.

I always thought that this was a function of the oil pan not working very well, at higher rpms. It would now seem that this is a function of the heads "packing" with oil.

123quattro

Maybe add another suction state and T it to the heads?

Greg,

Why not run a one of these per head to evacuate the oil? If you completely skipped the factory returns for the heads and created a dedicated circuit with pumps, that could also alleviate dropping the return oil onto the cranktrain.



http://www.weldonracing.com/product/..._Oil_Pump.html

svp928

I recall Mark also had to speed up the pump to alleviate smoking out of the exhaust at high rpms, so this, coupled with the draw-down in the tank probably means he needs another scavenge section in the pan to pull oil down those damn passages. Last I saw, his pump was only 2 sections. Most SBC drysumps are at least 4, with one section per pair of cyls....and sometimes another for each head..

GregBBRD

Exactly. From what I'm seeing, I think a stage per head would be minimum. Even then, I'm guessing that part of the time, the returns would still be all "plugged".

GregBBRD

Looked very carefull, at that. That seems like a really nice pump. It just seemed so complex, trying to find a nice place for the pumps, running power to them, mounting relays, plumbing, etc.

hernanca

Introducing vacuum, supposedly, can have it's own set of drawbacks - if you go too far, and introduce too much vacuum for a given motor, then this can somehow starve the engine of oil in needed places.

If another statement made earlier is true - that the 928 motor is more pressurized than most - then I would proceed cautiously in starting to introduce vacuum.

svp928

I tried to get Greg to hook a shop-vac up to it, but he was somewhat resistant....
Chicken.

txhokie4life

Just wanted to say how much I appreciate these open discussions.

I might not be able to ad anything, but your findings and discussions do not fall on deaf ears.

Mike

BC

So essentially this effects all of us in some way, as a lesser amount of this hot mess is happening when we are cruising at 3500rpm and repeatedly running to redline.

Correct?


And as always many listen, few post, and we all appreciate the clear concise info that Greg Brown posts - posts that many times contain more info in a few sentences than could be gotten from years of reading the "internets"

But don't let it go to your head Greg. You are still a special kind of *******.

(I hope that joke went over well. Otherwise I may not be invited back to to sunny beautiful Anaheim, CA)

hairywithit

has any one tried to "open up" the outlet port size (or what ever is the dominant restriction in the outlet system)

Above 6000 rpm the pressure relief system may be overloading, forcing more oil than designed into the head. Seems that ideally the relief system should create a relatively constant flow rate to the engine but its not. Also, extend the oil return passages to prevent windage packing.

dprantl

What about reducing the volume and/or pressure of the oil pump at higher RPM? Does that much oil really need to be going in there?

Dan
'91 928GT S/C 475hp/460lb.ft

Hilton

Seems like the simplest (heh) solution is to dry sump, and use a 4+ stage pump so you've got suction on each head, and the sump, as a minimum. Greg Gray's mentioned a few 5+ stage pumps - IIRC he's planning on a 6-stage one for his own build.

The trouble is for a street car, locating the oil reservoir.

Presumably, if the head-packing issue can be avoided, then the oil reservoir needn't be as huge as is current wisdom for a dry-sumped 928?

Thinking about it - this reminds me of Cheburator's (Alex's) engine failure - dry sumped engine, ran out of oil and spun a bearing in his very custom GTS race engine. His thread here. A few posts in that thread said he needed a much larger oil reservoir.. this thread would explain why, I'm guessing.

Thanks Greg for the information and stimulating some civilised discussion!

Lizard928

I am curious as to what the thoughts are then regarding the work that Louie Ott did with the video camera on the covers. I believe that this shows evidence to the contrary.

However from your description you have all 4 breather port on the cam cover in use, but no breather from the filler neck.
These 4 ports are then connected to the airbox. This at WOT is going to generate a venturi on these 4 hoses creating a vacuum inside the crankcase. This will only aggravate the problem as all the air is going to be forced up the drainback holes and out the valve covers.

Consider trying what I suggested in my earlier post. Use a large (1.5") tube connected to the oil filler neck port. Run this into a 3" ID canister with a couple copper scrubbers in it. This will have a very slow airflow speed which will then help the oil to drop out of the air. The copper scrubbers remove the rest of the suspended oil. This way there is zero vacuum and airflow up and through the heads.
If you do with to vent the heads, route the exit of the for mentioned canister to the airbox, and vent the cam covers to atmosphere.
The venturi effect in the airbox will help to generate a suction bringing air through the cam covers and down into the sump, therefor drawing the oil back into the pan.
I would also recommend putting a vacuum limiting valve on MA drysump in the same fashion to help generate a high velocity air stream through the heads into the main crankcase. Thus helping to pull the oil out of the heads and back down.

Now then there is one question that remains, do both heads fill, or just 1. If just one then it is possible that the throw from the crank is having an effect and the addition of a guard to those ports could help.
While that still could be the case, it is possible that there is too much oil to the heads, and the use of the 944S2 valve is a good idea, or another restrictor in the oil path to the heads.
Or simply do both.

On my street car, I capped off the cam breathers, and only vent from the base of the oil fill neck with a 1" hose. I see zero ejection at high RPMS unless I am on track duty, and then I would need a larger canister with a separate drainback.