Intake Comparo question linkage type used

[FERoadster]

Jay: I realize that the tests on multiple carb intakes was done at higher flow rates but wonder how the linkage acted. Most of the RPM was probably above where the 2nd carb kicked in.
Did you use positive or progressive linkage? The book doesn't state. The cross rams almost have to be positive.
My question comes from the fact that I'm working on linkage for my 6X2 intake and would like to go with progressive. It would really be difficult to balance 6 carbs even though there are balance runners in the plenum.
The Edelbrock 3X2 I have uses 3 Stromberg WW's with positive linkage with ball bearing ends on the shafts. I'll probably never use it but it looks somewhat impressive on the shelf.

Richard

[cjshaker]

Not to speak for Jay, but all dyno tests are done at wide open throttle, so progressive or 1:1 linkage would have no difference as far as that goes. As for the 6x2 intake, couldn't 2 3x2 linkage setups be used? One on each bank? It would just require an actuator that ties them together.

[FERoadster]

Doug: yes I do have a couple of extra 3X2 linkages but just wanted to know if Jay thought some cylinders would be lean with just the middle two working until the progressive came onboard.
So on a dyno the change in RPM and output values are how fast the engine can come up to speed? I would have thought that it was a gradual increase in the throttle.
Novice here in this respect.
Richard

[jayb]

Doug is correct Richard, the throttle is opened all the way while the dyno holds the engine at the starting RPM.  For most of the engines in my book this was either 2500 or 3000 RPM.  With wide open throttle the dyno will hold the engine at the starting RPM indefinitely.  Then, to start the dyno pull you push a button on the console, and then the dyno slowly releases the brake, to allow the engine to accelerate at a predetermined RPM.  I used 300 RPM/second on all my tests.

[FERoadster]

If it is at WOT then aren't some of the lower RPM readings kind of off base? (I'd say bogus but maybe "less accurate"?)
Seems that at  maybe 3500RP , if carbs were 40% open they would have more velocity thru them and act differently.

Just my thoughts- not trying to re-invent dyno pulls. That's why I thought maybe different carbs may make a difference on some of the more marginal intakes.
I do realize there are too many different options to make that many tests.
From a prior post/question of mine it would be great to see what maybe 6 different carbs did on a RPM intake and something like a single plane Streetmaster or POS.

Just my thoughts.
Richard

[jayb]

Why would the lower RPM readings be less accurate?  The same torque measurement system is in place regardless of the throttle opening.

Are you suggesting that the engine would make more power with the throttles partially closed at the lower engine speeds?  That is certainly not the case...

[cjshaker]

The only time that may be the case is when the secondaries have to be tied open, which is often the case on dyno runs. I could see that having a negative impact on lower rpm numbers, but only until about 3000 RPM when secondaries typically start to kick in. Most dyno runs don't start much below that though.

[jayb]

Actually, in my dyno tests I've always picked up power by zip-tying the secondaries so that they open with the primaries on vacuum secondary carbs.  Sometimes picked up a BUNCH of power...

[cjshaker]

Right now a friend has borrowed your book, so I don't have it in front of me. What rpm did you start your dyno tests at? I was thinking along the lines of a multi-carb intake, so I would be surprised if you picked up power below 3000. I wouldn't be so surprised with a single carb because that would basically be no different than a single double pumper. Obviously there's a lot less venturi area with a single carb. Of course a lot of this would have to do with the CID of an engine. I could see a bigger stroker being able to make better use of the 2x4 down low, whereas a stock stroke 390 I'd think would struggle not to lose all port velocity with 8 barrels being open below 3000.

[FERoadster]

CJShaker: Thanks alot - that was my "kind of" inference the carb might get a different signal and open differently and be somewhat different in  availability in torque or HP available  at normal driving environments. From my book Jay used 2500 for some engines and  3000 RPM  for the modified stroke engines.

Not trying to extend the discussion but I'll most likely use progressive on the 6X2 intake and see how it responds in occasional street driving with a 4.57 gear and wide ratio trans.
Thanks all for the comments
More appreciated !

Richard

[jayb]

The amount of air/fuel mix that gets into the cylinders is going to determine the power production at any given engine speed.  This is going to be dictated by manifold vacuum.  If there is zero vacuum in the manifold then the full amount of atmospheric pressure is pushing the air/fuel mix into the cylinders.  On the other hand, if the throttles are partially closed and this results in some amount of vacuum in the manifold, then there will be less than the full amount of atmospheric pressure to push the air/fuel mix into the cylinders.  Less charge in the cylinders means less power.

I can see where at lower engine speeds, on a lower horsepower engine with a 2X4 system, if both carbs were open all the way on the primaries but the secondaries were still partially closed, there still may be zero vacuum in the manifold, so there would still be maximum cylinder filling.  But at higher engine speeds, on a stronger engine, less than full throttle opening is going to give some manifold vacuum, which will reduce power. 

Seems like velocity through the carbs will play an important role at low engine speeds also.  Let's take the case where the primaries are open but the secondaries are still not in a vacuum secondary carb, and there is still zero manifold vacuum at this point.  All the air will go through the primaries, causing a certain amount of fuel to be drawn from the venturis and into the engine.  Now, if we tie the secondaries mechanically so that they open as well, velocity through the venturis will be reduced, and we will see less fuel drawn into the intake, resulting in a lean condition.  So, if you take your vacuum secondary carb and try to tie the secondaries so that they open mechanically, the engine will be lean and the car will bog at low engine speeds.  To me this is a carb problem; mechanical secondaries need more accelerator pump shot to overcome the lean condition.  That's why there are double pumper carbs.

On the dyno, accelerator pump shot doesn't enter into the equation, because the whole run is done at wide open throttle.  Plus, if you have a lean condition at the starting engine speed, you can jet up to compensate. 

For the engines we work on, I don't think it is common to see a partial secondary throttle opening result in zero manifold vacuum at 2500 RPM or higher, even with a 2X4 intake.  I'm sure that's why I've seen power increases when tying the secondary throttles open on vacuum secondary carbs.