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.
(ô_!_/ô) (ô_!_/ô)
69' baja: kombi box, thing spindles, irs, disc front, type 3 rear drums, 2 inch lift kit, 31x10 rears. 👍
New engine in process: 94mm p&bs, 74mm C/w chomol Crank, 35.5x39 SP heads, turbo. Wierd combo, hopeful torque monsta!
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!
modnrod wrote:Measure the airspeed while your at it, and listen to the port, especially at low and medium lift off the seat.
😏
haha, I could guesstimate if i know what the air flow rate of the vacuum cleaner was :P The flow vs head graph of the vacuum pump would make it very possible.
What was very interesting is that SP head did have an audible woosh noise that i didn't notice with the DP head. Could only assume air velocity and thus turbulence was the reason.
(ô_!_/ô) (ô_!_/ô)
69' baja: kombi box, thing spindles, irs, disc front, type 3 rear drums, 2 inch lift kit, 31x10 rears. 👍
New engine in process: 94mm p&bs, 74mm C/w chomol Crank, 35.5x39 SP heads, turbo. Wierd combo, hopeful torque monsta!
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.
(ô_!_/ô) (ô_!_/ô)
69' baja: kombi box, thing spindles, irs, disc front, type 3 rear drums, 2 inch lift kit, 31x10 rears. 👍
New engine in process: 94mm p&bs, 74mm C/w chomol Crank, 35.5x39 SP heads, turbo. Wierd combo, hopeful torque monsta!
Good stuff!
westi wrote:That's mad Alan.
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.........
SuperOwen wrote: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.
hmm, thats a really good idea. Solving the mystery as to whether the SP intake manifold is where the restriction is at too. Will have to wait a little till i have some single port kadron manifolds, they are on the wish list but i got the DP kadron manifolds.
It would be ideal too if someone brought around a big valve DP to borrow for 10 minutes to complete the comparison
🙂(ô_!_/ô) (ô_!_/ô)
69' baja: kombi box, thing spindles, irs, disc front, type 3 rear drums, 2 inch lift kit, 31x10 rears. 👍
New engine in process: 94mm p&bs, 74mm C/w chomol Crank, 35.5x39 SP heads, turbo. Wierd combo, hopeful torque monsta!
modnrod wrote: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.........
haha, the whoosh was definitely coming from the SP intake port, I had OCD in checking that the blue tack was blocking all possible leaks and the vacuum wasn't significant enough to suck it in
🙂
Even in the big valve, ported/polished head it made that noise despite the less restriction compared to DP with the less vacuum pressure present.
There certainly is a HUGE space behind the intake valves in the SP. I'm not familiar with tumble ports, time to google!
(ô_!_/ô) (ô_!_/ô)
69' baja: kombi box, thing spindles, irs, disc front, type 3 rear drums, 2 inch lift kit, 31x10 rears. 👍
New engine in process: 94mm p&bs, 74mm C/w chomol Crank, 35.5x39 SP heads, turbo. Wierd combo, hopeful torque monsta!
bajachris88 wrote:There certainly is a HUGE space behind the intake valves in the SP. I'm not familiar with tumble ports, time to google!
Have a peek at 4V stuff for bikes.
Not that tumble is necessarily better really, just seems to be a popular way of doing things.
