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<Info>Combustion chamber design and 85 cams (c900)
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Posted by Ubipa (more from Ubipa) on Fri, 24 May 2002 13:23:39 Share Post by Email
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Finally I got a chance to look at the different c900 cylheads and study the flow designs. It's quite interesting how Saab progressed through the years w/ the different 16v cylheads for the c900.

The 85 being the first w/ a symmetrical four leaf clover design combustion chamber(picture to the left), equal ports, smaller lift on every other lobe(1st,3rd,5th,7th) of the intake camshaft, and external oilers.

The second was the 88 up w/ a similar symmetrical combustion chamber as before but more in a butterfly shape w/ two less unshrouding humps(no picture sorry, but 2 less humps explains the slightly larger combustion chamber volume), internal oilers, equal lift intake camshafts but w/ every other Intake port(I believe 1st,3rd,5th,7th) having a lower and steeper ramp in the port ceiling.

The third being the 90 and up 2.1L cylhead(or 90-92 9000 2.3L). This head had larger ports than the two previous and the ports were larger and symmetrical, also the cam lifts were symmetrical. The 2.1/2.3 was designed w/ internal oilers and a combustion chamber similar to the original pent roof(picture to the right w/ two combustion chambers). The 2.1/2.3L combustion chamber was designed as an asymmetrical chamber w/ only one recessed unshrouding hump in each chamber. The humps location alternate intake valves (2,3,6,7). This is compared to the humps at every valve on the 88+ w/ two humps and the 85 w/ 4 humps(also explains the 2.1/2.3L slightly larger combustion chamber from the previous cylheads).

So what does this all mean. I guess the best thing to do is try to understand what differences there are and why they changed them.
The one significant thing on all the cylheads above is the fact they were all designed to create swirl/turbulence in the combustion chamber to reduce knock but the method varied from each generation. On the 85 head it was the shorter lifts(1st,3rd,5th,7th), the 88+ it was the lower steeper ramp in the ceiling in every other port(1st,3rd,5th,7th), the 2.1/2.3L it was the asymmetrical combustion chambers w/ a single recessed unshrouding next to intake valves(2,3,6,7).
Combustion chamber design on all three show a definite evolution.
The 85 chamber is appropiately unshrouded for the valves but the squish is uncontrolled and would cause the combustion gases to center over each vavle instead of the sparkplug. In addition, the 85 symmetrical clover designed chamber would be more proned to predetonate due to the sharp angles of the arch intersections.
The 88+ symmetrical combustion chamber is a little better designed w/ removal of the 2 unshrouding bumps allowing more squish control (still slightly angling combustion gases over the the valve) and less chances of predetonation(but still greater chances due to the unrecessed unshrouding humps).
The 2.1/2.3L asymmetrical pent roof combustion chamber gets rid of one more unshrouding hump and then recesses the last one giving excellent squish control and an even less chance of predetonation to occur. Because of the asymmetrical design and the location of the recessed unshrouding hump a negative pressure zone can develop during overlap. This negative pressure zone will help w/ swirl effect/turbulence in the combustion chamber allowing more exhaust gases to evacuate and evenly mix w/ the new gases.

One question that arises is about the effectiveness of using 85 cams in the 2.1/2.3L head. Does the 2.1/2.3L have an nonturbulent combustion chamber? No. Does the 85 cams design synchronize w/ the design of the 2.1/2.3L comb chamber? No, because the 85 cam has smaller lobes at valves(1,3,5,7) where the 2.1/2.3 chamber is designed w/ humps at valves(2,3,6,7). So the questions that are then raised:
Is the effect of the 2.1/2.3 chamber design enough to create the proper turbulence?
If adding the 85 cams, will the effect to the 2.1/2.3 help every cylinder or just two?
Will the added turbulence improve hp over the deficient total lift of the valve?


posted by 4.40.3...

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