Opening intake port timing earlier creates more torque?
Fabricator
Joined: Jan 2004
Posts: 1,322
From: Central Ohio (Hebron) Zephyrhills Fla.
Originally Posted by heretic' post='844476' date='Nov 7 2006, 07:41 PM
Where did your bridge ports close?
For what it is worth, a stock port with proper induction and exhaust will make peak power within a few hundred of 8k, depending on the engine. Most extend ports *will* make peak power above 8k. For an extreme example, I believe it was Mazdatrix that built an E/P engine that basically had NO power peak, at least not one that was useable. 10,000 rpm shift point and it could still make power past that if it could hold together. It's all in the closing line versus port area.
As regards your second question, perhaps it will better answer your first. Yes, at part throttle the power sucks due to charge dilution. But if you want more power the first thing you do is open the throttle more, so in comparisons about power (versus driveability) at a given engine speed, full throttle is assumed. Charge dilution is a non issue at full throttle. Intake manifold is at atmospheric and the exhaust is generally at or below atmospheric in the chamber thanks to the inertial "slug" of exhaust gases creating a vacuum behind them.
Look at the charts and do the math. You can't be getting high VE with enough exhaust dilution to kill power at the same time, and Mazda's research has shown bridge and peripheral port engines getting significantly higher VE than sideports as low as 2000rpm, which also pans out in the driving experience.
The SCCA E production engine is limited to a street port with a limited open line. In fact, Dave Lemon wrote
the rule for SCCA on street ports. The junction of the side seal and corner seal may not be exposed to the port opening. All are required to run either the stock Niki with all hioles the same as the secondaries, or the stock TB or a Weber with two 38mm chokes. This is probably what everyoe runs.
The 10,000 RPM shift point might be typical of large intake runners that allow high velocity flow very high revs. But anyway, its a street port, and it sounds real good at full song.
Lynn E. Hanover
Senior Member
Joined: Nov 2001
Posts: 2,284
What if your intake runners are on the small side, but your primary ports are tall, like the example on the left?
[attachment=40153:attachment]
Also check the differences between ports (Sorry about the camera angle throwing off the relative sizes).
[attachment=40154:attachment]
[attachment=40155:attachment]
The exhaust port timings I mentioned earlier. I'd like to use the J-spec timing because it's the same as GSL-SE and should provide great torque down low.
[attachment=40156:attachment]
Which manifold? The one with channels cut or the seperate runner?
[attachment=40157:attachment]
[attachment=40158:attachment]
[attachment=40159:attachment]
And there's a pic of the 4 bbl carb.
[attachment=40160:attachment]
[attachment=40153:attachment]
Also check the differences between ports (Sorry about the camera angle throwing off the relative sizes).
[attachment=40154:attachment]
[attachment=40155:attachment]
The exhaust port timings I mentioned earlier. I'd like to use the J-spec timing because it's the same as GSL-SE and should provide great torque down low.
[attachment=40156:attachment]
Which manifold? The one with channels cut or the seperate runner?
[attachment=40157:attachment]
[attachment=40158:attachment]
[attachment=40159:attachment]
And there's a pic of the 4 bbl carb.
[attachment=40160:attachment]
Senior Member
Joined: Oct 2003
Posts: 1,420
From: Boston Ma.
Originally Posted by Jeff20B' post='844878' date='Nov 11 2006, 12:26 PM
Which manifold? The one with channels cut or the seperate runner?
[attachment=40157:attachment]
[attachment=40158:attachment]
[attachment=40159:attachment]
And there's a pic of the 4 bbl carb.
[attachment=40160:attachment]
I can see cutting a channel between secondaries 1 and 2 on a really late closing port.
Senior Member
Joined: Nov 2001
Posts: 2,284
That's interesting that you guys would say that considering Mazda started adding channels in the late '70s and continued it through the 1st gen RX-7. What's more, of all the engines on which I've tested manifolds with channels, they've all had stronger than expected idles and good low RPM driveability. Seems a little counterintuitive, doesn't it?
Senior Member
Joined: Nov 2001
Posts: 2,284
No, I don't think that is the reason. The runners directly below the carb are the same diameter as the carb butterflies. No clearance issues.
The channels increase the vacuum signal to the carb. A better idle and low RPM operation result. At least that is the theory. Presumeably more low end torque is available. I think it has something to do with all four ports pulling a vacuum on the carb while actual flow stays mainly on the primaries. Perhaps this is the reason for the tall primary runners such as those pictured on the 12A intermediate plate above.
The channels increase the vacuum signal to the carb. A better idle and low RPM operation result. At least that is the theory. Presumeably more low end torque is available. I think it has something to do with all four ports pulling a vacuum on the carb while actual flow stays mainly on the primaries. Perhaps this is the reason for the tall primary runners such as those pictured on the 12A intermediate plate above.
