roller lifters with 2.8

[Superdave]

Looks like Edelbrock agrees, the whole thing is a common plenum..

[ericjon262]

The problem is you are sharing 50mm between two cylinders that are firing consecutively, so you are actually sharing the carb as a small plenum. The reason the stock engine can use such a small throttle is because there is a common plenum behind the throttle to store air.

50mm throttle will flow 260.64cfm, but since the cylinders are firing on after the other there is no storage of air at the back of the throttle plate in your carb. This means each cylinder theoretically is only getting 130cfm.

Comparing to a 4 barrel carb is like comparing the 52mm throttle with a planum behind it because the 4 barrels are feeding more than one (or two)cylinders.

The optimum size for the 60 degree v6 for 6 throttles is somewhere in the 38mm range (six 38mm throttles).

I still feel feeding the pairs with 50mm with no plenum may be too small.

Curious to see what happens when the ignition is corrected.

keep in mind too, that we are talking about carbs, not TB's, a carb will flow way less than a TB the same size.

[veekuusi]

The problem is you are sharing 50mm between two cylinders that are firing consecutively, so you are actually sharing the carb as a small plenum. The reason the stock engine can use such a small throttle is because there is a common plenum behind the throttle to store air.

50mm throttle will flow 260.64cfm, but since the cylinders are firing on after the other there is no storage of air at the back of the throttle plate in your carb. This means each cylinder theoretically is only getting 130cfm.

Comparing to a 4 barrel carb is like comparing the 52mm throttle with a planum behind it because the 4 barrels are feeding more than one (or two)cylinders.

The optimum size for the 60 degree v6 for 6 throttles is somewhere in the 38mm range (six 38mm throttles).

I still feel feeding the pairs with 50mm with no plenum may be too small.

Curious to see what happens when the ignition is corrected.

This is by far and away the best and most reasonable declaration about this issue,
I have ever heard.
Thanks!

As I told you , I did make a plenum intake for the Ford V6 engine with this same carb.
And the max power was the same as is now with the GM engine.
And I bet you have never seen the German Ford V6 cast iron heads (design from the 60`s).
They are bad , and I mean really bad. They do not flow at all compared to the GM 60degree heads.
But the intake was much better than it is now.
That is why the max hp is the same with these 2 engines.

So , if we can not find problem elsewhere, the solution is to make a plenum intake to this engine.

[1988GTU]

So , if we can not find problem elsewhere, the solution is to make a plenum intake to this engine.

I'd start fabricating right now to save time.

[dskopek]

I ran a minisprint for awhile with a 850cc twin in it and ran 44mm flatslides on a standard motorcycle setup( 1 carb per cylinder) had a power band of about 6800-9500 rpm. We changed to a small tunnel ram style setup, looked like a "H" with the center crossover about the size of a can of beer. The end result was a much softer bottom end and a very strong top end that started about 7500 and pulled up to about 11,000--- No changes to cam or head and had to jet up on carbs a little. I don't know if the results would be anywhere close to the same in gains for Veekuusi (+ 1500 rpm on top) A 850 twin would equal a 2550 cc v6. With the 3 barrels on his carb spaced out across the top of a manifold I can see some troubles getting the fuel mixture right to all 6 cylinders if the manifold is short even with a plenum. Still a work in progress and with the work put in already it should only get better as you figure out what it likes! Thumbs up for the effort no matter the result.

[geoffinbc]

If you have to use those carbs then just make a common chamber for all cylinders. You may consider extending the runners across the valley and then into the common chamber to increase runner length. The problem as stated above is that the engine fires in number order and each carb supplies 2 firing events back to back and then waits for 4 other firing events before those two fire again back to back. In a common chamber this wont be an issue.

If you can change to 2 carbs but still be within the rules than you can try and separate the left and right side of the motor into 2 halves. This will give even firing events inside the intake halves and also give a nice strong signal to the carb at lower RPM.

I would suggest looking at some older Chevy 4.3L and 2.8L as well as some Buick 3.8L racing manifolds from back in the day because they were all designed for V6 Carb applications.

[veekuusi]

Right now I am thinking of throwing that 3 x 50 mm carb away,
and buying a 4 barrel Holley 390 cfm double pump instead.
The price of that carb is no issue.

http://www.holley.com/0-80507-1.asp

And manufacturing a new intake manifold for that carb.
The calculatory hp for my engine is the same if the engine (6 cyl)
has 1-5 barrel carb. The weight of my car stays the same.
And the engine size stays the same.

And as you said, I can copy racing V6 and V8 manifolds from "the good old days".

[ForcedFirebird]

I would try to use the current carbs first since you already have them. They will flow enough with a plenum.

[dskopek]

I sat down with a cold one last night and looked thru this thread and found the bore/stroke and cr, cam spec's and installed cl. Head flow numbers, all that good stuff and punched it in to a dyno program just to see how it ends up. Real interesting on how it responds to intake style. Big change going from a IR setup to a single plane or tunnel ram set up. Makes you wonder just how good the dyno program is in this case! 330 HP 2.8 would be something to see.
This is with no changes to cam or carb!

[geoffinbc]

And as you said, I can copy racing V6 and V8 manifolds from "the good old days".

Don't copy any V8 manifolds because they do no fire bank to bank. Most people think a V8 is two I4 motors spread 90* apart when in fact they are two 90* V4 motors meshed together. What this means is that you have 2 cylinder on the same bank that fire back to back. For Example you have the typical small block which fires 18436572 this makes 8-4 and 5-7 fire back to back. You get 8-4 on one rotation and 5-7 on the next. So to compensate for this you end up with dual plane intakes with compromised runner design. On a V6 you have even bank to bank firing so you can design a high performance dual plane that does not compromise on runner design. Especially if hood clearance is not an issue. Now most would think that you should just jump right to single plane like you would see on a high rpm V8 but you can successfully make a dual plane V6 manifold with good equal runner length and you will have a great combination of high RPM power and good low speed signal to the carb because of the strong even intake pulses on each bank of the motor.

This one is a GM High Performance manifold for the 4.3L V6. It has a top that covers it and mounts a single carb in the middle. GM recommends welding in a divider to separate each bank for improved performance.

This is the Edelbrock 2.8L manifold. The carb mount is removable and underneath you will find the drivers side of the carb feeds the passenger side cylinder head of course the passenger side of the carb feeds the drivers side cylinder head. This is done to increase runner length.

[ericjon262]

I agree, don't copy anything V8, they just won't flow properly for what you want to do, think a 2 plenum setup, or a single plenum with a divider would be best, one for each bank, that will give you the better part throttle performance without sacrificing top end power.

[veekuusi]

I had a new dyno session today.
Better news than last time a month ago.
50 hp more power.
173.5 kW (7600 rpm) and 253.7 Nm (4860 rpm)
that is 232 hp from back wheels and some 250 hp engine power.

We have fixed the ignition problem, the advance was jumping all over the place in high rpm
last dyno session.
Wrong settings in the box (computer).
The first run was made to check if it has some effect to power.
But no, no gain to hp.

The second run was made with a extension to the intake runners.
The carb was some 4 inch higher.
And the result was no gain to hp.



The tubes between the runners was shut down during this session.

The third run was made with the tubes between the runners open,
the inner size of these tubes are some 1 inch.
And 50 hp was there.
So even tubes like so small gave pretty much more hp.



So the plenum tubes helped the engine to "breathe" better.
The problem definately is that the engine could not get enough air with
my first intake.
I am going to modify the intake now.
More hp are expected in the near future.
And I must give thanks to the experts of this very forum,
you were right.

http://www.youtube.com/watch?v=IVpICJFzKHQ&feature=youtu.be

The shift light starts to light 7000 rpm.

[ericjon262]

Now thats more like it!

[ForcedFirebird]

Glad we could help! Much better. Now time to make more adjustments to fine tune that monster

[3400-95-Modified]

wow 50hp from two 1" tubes... simply amazing isn't it...