GoRideSno

Ok
Porsche provides two 1/2 " round openings to vent the crank case blow by gasses back to the intake for the S4 at 316HP.
This yields an area of .39"sq/316hp for a ratio of .0012 sq" of venting per 1 HP.
So .78"sq should vent 650hp provided it is venting to same pressure/vacuum i.e. back to intake and provided same % blow-by at 316 and 650hp. .78"sq is equal to a 1" ID tube.

It appears that a 1" id tube will be enough to vent the blow by gasses of a supercharged 928 (good because this is what I have designed and built for everyone)

What, however, is the cfm of blowby gasses that will need to be vented?




I could figure it if I knew the relation of exhaust cfm to intake cfm.
What is that relation?

gbyron ...anyone?


Andy K

SharkSkin

Isn't this going to vary with the condition of the rings on any given motor?

drnick

hey,

morning andy, nice pics in your other thread. in my quest for power ive taken a turn away from forced induction for the minute and im waiting to take receipt of a GTS engine - although i did think about putting the 8 valve heads on that and waiting for a twin screw solution...

Jeff McVicar

Andy, I've got an 83 I've been looking into supercharging systems offered for my year and people just dont seem to offer much for the earlier years. I have looked at pictures of yours (Kick a$$ stuff man) and from the looks of it that whipple blower might be a bolt on to mine? Any info along the lines of doing a system for an 83 would be a huge smile on my face since you have it all under your belt already (figuratively speaking of course hahahaha)
Thanks
Jeff

Lagavulin

Andy, this is strictly a guess-timation on how you may be able to get in the ballpark.

At 6500 RPM, our engine NA is pumping 500 CFM of air. Now using a measured leak-down percentage, multiply it against 500 CFM. Of course, those leak-down numbers are only accurate for 100 psi where the test was made which obviously is much lower than actual combustion pressures. Thus, I would guess (..again) that the actual blow-by numbers would be larger.

Ok, what does that come out to be? Let's do it the long way first, and let's assume all cylinders tested at 97%.

(500 CFM) / (8 cylinders) = 62.5 CFM/cyl

So at WOT, each cylinder is ingesting 62.5 CFM of air. Now for the amount of blow-by, which we are saying is 3% per cylinder:

62.5 CFM x .03 = 1.875 CFM/cyl

So each cylinder is 'leaking' 1.875 CFM. Now multiply by 8 cylinders and get:

(1.875 CFM/cyl) x (8 cylinders) = 15 CFM total for entire engine at WOT

Now that we have the basic idea, here's the quicker way:

(500 CFM) x (.03) = 15 CFM total

Now if you add 14.7 psi of boost which will double the CFM through the engine, we would get 30 CFM of blow-by at WOT.

Likewise, at 7.35 psi of boost, we would have 22.5 CFM of blow-by at WOT.

I have no idea if it really works this way, so take it with a grain of salt, but it may get you close enough to what you need to know.

Let's see what theoretical numbers we get with 2% and 1% leak-down:

500 CFM x .02 = 10 CFM
500 CFM x .01 = 05 CFM

fst951

Hi.

I also think that one important misunderstanding is that intake air equals exhaust air. I believe that when the gasoline is vaporized and then burned, and HEATED that your volume exhausted is increased. I know that combustion also createds mainly carbon dioxide and water as well which are hevier than the standard ingredients in air diatomic nitrogen and diatomic oxygen, water is lighter than both. Now at WOT the water is not settling out as a liquid and thus produces gas too. I suspect that you will find the output to be higher. The factory venting of crankcase pressure has more to do with the condition of the running engine with regards to leakdown and blow by than it does to the static calculations. I suspect that even in a properly built and good running engine that the calculations are not a linear progression either. 1cfm at idle isn't worth 7 at WOT. Maybe just look it up in an engine tuning and functions book. Corky Bell may have something to say about it, I don't have a copy any longer, so wait for reply from someone who does.

Old & New

Andy,

Measuring the pressure at the valve cover fittings may yield the information you seek.

Nathan Valles

Adding on to Lagavulin's answer, lets assume 30 CFM of blow by at max boost. Now lets add pressure and temperature to the equation. PV=NRT

First keeping NRT the quantity of constant, when the gasses blow past the rings the pressure drops in half from 29.4 psi (atm + 14.7) to 14.7 psi, then the volume must double to maintain the constant quantity of gas. That means your 30 cfm just became 60 cfm. (note that if you assume 5 psi in the crankcase then the 30 cfm only jump to 45 cfm)

Now assume temperature changes from 65 degrees F (524 degrees Rankin scale) to what? Exhaust gasses are very hot, but engine the engine is cooled by the cooling system. Lets assume exhaust gasses that get into the crankcase are cooled by the engine block to about 240 degrees F (699 degrees Rankin) for simplicity. Ratio of 699/524 = 1.33 so 1.33*60 cfm = 80 cfm. (If using 45 cfm then it jumps to 60 cfm)

Obviously this is all just a mental exercise at this point, but you have to admit it is an interesting one.

GoRideSno

Thanks everyone that replied.

Dave , Yes.

Congrats and best of luck drnick.

Jeff,
I am working on a 16V setup. When I can find the time to complete it it will go pretty quick. I have the basic design and some parts made, just need time.
O&N it seems that one would have to completely seal off the crank case venting to do that?


As fst951 has stated, intake cfm is probably not equal to exhaust cfm. Given that, the realtion of cfm blow-by gasses to cfm exhaust is more important than to cfm of intake.
If we had a figure such as intake cfm = X exhaust cfm we could simply throw that into Lag's formula and get a rough estimate.

I'm looking at several oil separators that will remove 99.9% of oil mist. The vary in CFM. The gases could then be plumbed back into the intake with no added risk of detonation. Or maybe even plumbed into the smog pump like Porken has done. Why? to help create a vacuum in the crankcase.


Andy

Jim bailey - 928 International

the 16 valve engines vent the crankcase directly into the oil separator (oil filler) they have no provision for venting the cam housings . A louvered baffle plate was added in later years to help keep oil from being splashed up into the separator . The 32 valve engines were fitted with two breathers on the right side camshaft cover and a small breather hose near the top of the oil filler spout (the spout no longer is a separator, no mesh screen ) . The GTS which has more issues with windage and oil mist added two more cam cover breathers to the left camshaft cover . All 928 engines have 8 oil drains from the top of the head to the block each about 3/4 inch by 3/4 inch . The blow by gasses originate from the area below the pistons and with the 32 valve most must migrate up to the camhousing cover to be vented (the small hose to the top of the oil fill vents some) .

GoRideSno

Jim,
My reference to the 1/2" openings is in regards to the y fitting that feeds the hoses from the rear cam cover fitting and oil breather into the rubber boot below the MAF. This is the final venting of the crank case so all flow is restricted to these openings.

The oil filler/ crank case venting I have designed has a 1"'id tube coming from inside the crankcase then the cam cover openings allow additional venting but the final opening of all venting merged is 1" (but could be made biger easily). It is more similar to the 16V setup than 32V. At the end it will allo the addition of an oil separator that will filter oil mist and particals down to .01 microns. Virtually no oil will get past this.



All I really need to know is the CFM of exhaust per HP for a gasoline engine and I can get a close enough figure.

Andy

Jeff McVicar

You are absolutely right. intake volume does not equal exaust volume..... especially in forced air and nitros systems. Andy seems to be the man on the Blowers and I have nitros on almost everything I ride. I know when I'm building a nitros motor (bike) 16 valve in line 4 I can go with a mild intake cam but the exaust cam has to be radical and the cam timing has to be 4 degrees advanced at a minimum to get the valves opened before the end of the comperssion stroke to facilitate the massive amount of expanding gasses. The intake ports just get a clean up ..but the exaust side is a full hog out with the 2 1/4" exaust (very short) just to keep the gasses movin away from the horse power.
To give an example of venting/ compression on the motor on a naturally asperated pass to a nitros pass. I can run naturally asperated for 7- 10 runs and have NO evedence of oil in the clear 5/8" plastic tube to my catch reservoir. Start out with 2 qts of oil and end the day with 2. 12:1 compression with 98% good on a leakdown on all 4 cylinders @ 140hp. All in 9.30 seconds

On a Nitros day Same motor. The catch can will have 2 tbls of oil in it in 7.9 seconds. The compression gets up to about 20:1 @ 311hp. ( thats at 1.168 litres gang haha) and The 2 qrts of oil I started out with at the begining of the day will be 4-5 thats right 4-5 at the end of 7-10 passes. Those expanding gasses will forse the fuel/ air past those rings at a much faster rate under nitros/ turbo, blown. And like one of the guys said ....ring condition will play a large part of it if they are worn.
So summin it up it goes into the intake in tablespoons and comes out the exaust like basketballs.....weld the cranks fellas
Couple of pics in case I seem full of $hit hahaha

GoRideSno

Jeff,
You are a wild man....I'm scared just looking at that bike.

Thanks for the comparison of oil blow by with the runs.


I guess the Figure isn't really that important with my application because I seem to only have 1 choice. The oil separator with 1" ID pipe size fittings flows 450cfm so that should be way more than plenty. here is the page for those interested part # is 9826K85.


Andy K

Jeff McVicar

Thanks Andy I'll go check it out now. Yeh that bike is a treat my quickest 0-60mph time is .97 hahaha That 1" id tube sounds like plenty where are you venting from? the covers? Is there one or two of them. Sorry if you said already I may have missed it.

Nice idea on that oil seperater.... you arent worried about the 150 degree max?

GoRideSno

Jeff,
A single 1" tube, it vents from where the oil filler neck used to be. There is a big opening into the crank case underneath the oil fill neck.

The 150degree max is the only thing that's holding me back now. Need to check on that. My guess is that it's probably ok though. The blow by passes through 4 feet of tubing before it reaches the separator so it cools some before reaching it.

Andy