I thought i'd share my little investigation. I had to ensure my big valve SP head port and polish job was actually achieving anything.

So... i compared four types of heads at 85.5mm bore:

1: Stock DP
2: Stock SP (with mild polish by the looks)
3: Big valve 39x35.5 SP
4: Big valve 39x35.5 SP with amateur port and polish.



Experimental apparatus was:

1. Vacuum cleaner
2. Blue tack to block valve guides, spark plug bores.
3. Cardboard to block exhaust ports on SP heads (when valves not installed in either cylinder)
3. clear tube with black food colouring/water as pitot tube/U tube manometer.



This experiment was to compare the overall intake to exhaust port flow with valves not installed, so solely a comparison on port flow. The apparatus and manometer geometry/water volumes were constant across the four heads to be compared. The pitot tube / manometer tube was plugged in a valve guide to measure the vacuum pressure as a result of the vacuum cleaner. Basically, the theory is less vacuum pressure means more flow for this rudimentary case. (IE: less restriction to breathing and thus the less vacuum pressure means the vacuum’s suction is being satisfied by air flow).

RESULTS

1. DP stock head





2. SP stock head (with suspect mild polishing)





3. SP big valve 39x35.5 head (stock port finish and valve guide bosses)





4. SP big valve 39x35.5 head (amateur port and polish)



RESULTS SUMMARY



So, the results prove the consensus. DP flows better than SP, but SP can be worked to flow marginally better than stock DP with big valves and porting/polishing. If i had a big valve DP head then it would complete the whole comparison which beyond a doubt would decimate all, but my theory is I’d lose my air velocity and hence torque for my baja. This experiment also proves the flow benefits of port and polishing, which was mainly focused at the 'big loss' zones just behind the valves (seat blending and valve guide boss work). So now I can sleep at night knowing the many hours spent is paying off. Now to do the second head sigh!

So, SP vs DP in percentage vacuum head comparison (mm) compared to stock DP:
Stock SP: 35.3% WORSE OFF
Big Valve SP (39x35.5): 5.9% WORSE OFF
Big Valve P&P SP (39x35.5): 11.8% BETTER OFF

Worse off in these cases is in regards to air volume flow, there is no doubt the restricted flow of the stock SP will result in greater lower torque at bottom end due to air velocity.

These values mean nothing quantitatively, but qualitatively compare the head air flow between the four different variations/setups.

The big valve heads are a part of my 2054cc in the making. build to come.. (been saying that for a while now).

Thanks for reading.

Chris.

Measure the airspeed while your at it, and listen to the port, especially at low and medium lift off the seat.

Come on, you tease, we're waiting!

Quote:

Originally posted by modnrod Measure the airspeed while your at it, and listen to the port, especially at low and medium lift off the seat.

Results updated now, I definitely did forget to cover the spark plug holes the first time with heads 3 and 4. Feel free to comment, criticise or question

Interesting results though.

Good stuff!

Another worthwhile variation would be to run the same tests with manifolds, stock dual port and single port and maybe the same with kadron manifolds etc.

You are officially my newest-this-month hero!
What a great test!

The "whoosh" noise could be a restriction compared to the DP flow-through, but it could be an indication of a "tumble" port, something other siamesed port styles do very well (like Holden Red 6), which could explain the bottom and mid range expected of a SP due to the bloody big space behind BOTH intake valves.
Oh, or it could be the bluetack getting sucked through the guides too I guess.

More, more, more, more.........

Quote:

Originally posted by modnrod The "whoosh" noise could be a restriction compared to the DP flow-through, but it could be an indication of a "tumble" port, something other siamesed port styles do very well (like Holden Red 6), which could explain the bottom and mid range expected of a SP due to the bloody big space behind BOTH intake valves. Oh, or it could be the bluetack getting sucked through the guides too I guess. More, more, more, more.........