Rebuilding My 930

[snowman]

Your clearly are NOT married. Or if you are you will soon be unmarried.

[350HP930]

Already unmarried, but thats what allowed me to finally get started on my lingering project.

But to return to the subject of my 930, I have the result of my intake flow tests.

I took one before measurement and have calculated a flow improvement of almost 40%. That makes sense since the new ports are about 45% larger than the old ones.

My current calculated flows of >255 CFM @ 25" vacuum and 0.45" valve lift are also very respectable.

Compared to other numbers I have seen thats about as good as it gets for heads using 49mm intake valves.

The CFM numbers may vary depending on the equipment used and what is used to guide the flow into the port but the main purpose of this whole experiment was to determine that all the head flows were evenly matched.

I am happy to report that all the heads except one are within 1% of each other, and one is only off about 1.5% off from the rest.

I will be looking to see what is different about my oddball so I can try to correct it.

As usual I will keep everyone updated on my progress.

[350HP930]

Tonight I broke out the internal calipers and went in search of the differences between the worst and best flowing heads.

I was pleased to discover that all major dimensions and curvature profiles were the same but I was perplexed as to why the flows would be so different.

Upon further visual inspection I noticed a very subtle difference of the short side radius where it meets the valve seat.

It appears that there is a minor difference in the castings resulting in almost a millimeter difference in the location of the port and a corresponding increase in the length of the short side radius. I guess its enough to cause the measured 1.5% flow difference.

I have attached a picture below to show the difference and I will be spending some time in the shop tomorrow trying to match them up.

[350HP930]

Its hard to see it but if you look carefully you can see it.

[snowman]

That just goes to show where the most important area to improve is, ie the short side radius. Hog as much as you can off this side and you will get even better flow, only limitation is running into something, like a valve seat or in some cases water jackets (not a problem common with Porsches). Just a smigen of a sharp edge on this surface can hurt flow by as much as 40%!!!!!

By the way that is one BEAUTIFUL finish in those ports.

[strokher racing]

What are the low and mid lift numbers on the intake and exhaust? What are the ports opened up to? That number at 450 lift corresponds to about 270cfm at 28 inches which is right with the top head porters in the country with larger valves.

Eric Hood

[350HP930]

Its likely that my apparatus might be reading a few percentile points too high due to the boundry layer effects in the measurement tube. Even if that is the case the numbers are still very respectable.

I haven't measured the exhaust flows yet since I still need to build an adapter that will allow me to suck the flow out of the exhaust port but I should hopefully get around to that this week sometime.

I am planning on retesting my tweaked heads tonight so while I am at it I will get some numbers for 0.25" and 0.1" lift.

The intake ports have been opened up to 40 mm and the exhaust are still close to the stock 34 mm.

[snowman]

I suggest forgetting all the numbers at 0.4" and higher. THe really important numbers are at 0.1" , 0.2" and 0.3", thats where ALL the action is at, ie area under the curve. Getting 10% better at 0.1" lift is much better than any improvement at 0.45" lift. The reason is that the flow at 0.1" and higher is for much much much longer time than the flow at 0.45". And furthermore Porsche heads flow very well at low lifts, ie they are in need of duration, not more lift. All of the Porsche heads I have flowed indicated that the HP limitations of the engine are NOT related to head flow, but to rpm limitations.

[350HP930]

I am happy to report that after tweaking all my intake port flows are now within 1% of each other.

As long as I was testing I decided to take several readings at different valve lifts. I figured a graph would make it easy to see how the flow curve drops off as valve lift increases.

It looks like my new super cup cam intake valve lift of 0.49" is going to be well suited for the flow characteristics of my modified heads.

[snowman]

Since your the real science type of guy I would suggest that you do some calcs or use an engine sim program to do them for you and check out what the flow above 0.4" gives you. Basically NOTHING (execpt extreem valve train wear) unless your willing to rev the thing to 12000 rpm.

In terms of numbers its like 5% more flow, but for only 1% of the time, ie 0.01 times 0.05 equals 0.0005 which is pretty darn close to NOTHING. TO put this in HP numbers, say you end up with 400HP the extra lift will give you about 400.2HP, if you are lucky.

I am not a true expert on this subject but Dema Elgin of Elgin Cams is. I would also talk to him for real answers about your cam and heads. He is also a racer as well as a world renown mfg of camshafts. He is usually available and will actually talk to us peons if you give him a call. He is at http://www.elgincams.com/

[350HP930]

A higher lift cam gets to 0.4 inches faster and stays open longer than a lower lift cam will.

[snowman]

True, but check out the calcs. Marketing hype is still marketing hype. Only a world class racer pushing for everything at ANY cost might ask for the last 0.2HP, but some wiser ones opt for relialibity.

I also know of another tradeoff that adds even more hp, compression ratio. That extra valve lift will limit the compression ratio enough that you may even end up with a net loss of power. These ain't chevy heads. Chevy heads can flow at very high valve lifts because of their poor design.

If you haven't bought your cams yet, hear the Elgin guy out and then make a decision.

[strokher racing]

Are those numbers still at 25 inches?

[camgrinder]

The following two quotes are from Demas class notes,

• In a performance engine, the intake valve is lifted past its point of maximum port flow.
• This increases the amount of time that flow is maximized, limited by the port.

[350HP930]

Originally posted by strokher racing
Are those numbers still at 25 inches?

No, those numbers are for 28" and should be more accurate since I made some scientific calculations about how big the boundry layer is in my flow measurement tube. Since most of the other flow numbers I have been finding on line are for 28" I have been adjusting my flow numbers to that reference pressure for comparison purposes.

Since the boundry layer is about 1.25 mm thick where the pitot and static ports are located in my 50 mm measurement tube the actual flow area was only 90% of the total area I was initially using for my calculations.

The latest numbers should be very close to the actual CFM numbers my heads are moving.

[strokher racing]

Those numbers will support the hp you are looking for no problem.

Eric

[snowman]

The flow numbers given will support a fantastic ammount of HP, but at RPMs that the Porsche will not operate at. I would suggest picking your max RPM eg 8500 and work backwards. IE cut the lift back until you start to see a decrease in HP. You will be amazed at how low a lift you can live with and give up nothing. The only other way to actually utilize all that air is to increase displacement.

an analogy. If you have 3 horses that want to leave the barn at the same time, you give them 3 doors for mininum exit time. If you add 5 more doors, you still end up with 3 horses, no more. And consequently you wear out 8 doors instead of 3.

[350HP930]

This is forced induction Jack. I don't have to spin crazy revs to shove air past my closing intake valves.

[snowman]

Originally posted by camgrinder
The following two quotes are from Demas class notes,

� In a performance engine, the intake valve is lifted past its point of maximum port flow.
� This increases the amount of time that flow is maximized, limited by the port.

This is true only if the head is limiting the power, in this case the RPM and or displacement is limiting the power and the heads are a distant 3rd.

[snowman]

Originally posted by 350HP930
This is forced induction Jack. I don't have to spin crazy revs to shove air past my closing intake valves.

Another good reason not to wear out the valvetrain needlessly.

Put another way you already have 8 barn doors open, 3 horses normally aspirated and 5 horses forced induction, why the need for more doors? And furthermore why not add more force?

PS note, the engine breaks at 5 horses.