Carbs and an 2.7 RS spec engine
 

Anyone else running a 2.7 RS spec engine (2.7 RS P&C & S cam) ?

34mm VENTS were suggested and installed. I am now choking at 6000 RPM. I checked the TOMLINSON book and the chart shows that a 2.7 engine limited to 6000 RPM should use 34mm. My engine is specced to run to 7200 rpm. It looks like I need 40mm vents.

What are other's using for vents?

James

James,

I think giving some more info. will help get you an answer.

I can say from my expericence with a 2.7 and solex cams 34mm was fine.

I think you said you had very large intake ports on your set up and higher than stock 2.7 RS compression (8.5:1 is stock)

What c.r. do you have? and what size intake ports? and what is the extent of the head work done on the engine?

It also seems that you said you were having distributor / igntiion woes. I think you should get this sorted out before changing to much larger venturis. I ran a 2.7RS spec dizzy in my car. You should be able to find one of these w/o any problem.

Good Luck. ...enjoyed meeting you at the Houston pelican get together.

Jay

Are you running Weber 40's or 46's. There are no such things as 40 mm venturi for Weber 40's since by definition they wouldn't be venturi with the same ID as the carb body.

The Distributor is fine now! All fixed. Combination of the points installed wrong and the timing 180 out...

The 34mm vents are too small. I am choking at 6000 RPM. I am thinking of trying 38mm, but I haven't found any yet.

The intake manifold was opened up to 40 mm. The head was opened up to 40mm at the intake down to the the 2.7l intake size (I've forgotten the intake vale diameter, but I'll look it up...)

According to my Weber handbook, 2.7l running 7200 rpm should be using 40mm. By my calculations, if I'm choking with 34mm at 6000 RPM, then 38mm should allow me to go to 7200.

James

38's may be a big jump. If you want to make more power top end you may want to look into enlarging your Air Correctors. You did not say what size you were running. But a set of Jet drills to find what size you want then order the correct ones may be an alternitive to new venturis.

Using BA's venturi formula......using 6300 rpms for *max * power (2.7L RS MFI motor), I get a venturi dia of 33.67mm.

My 2.7L hot rod pulls strongly to 6500 (or so) and will rev to
7300 (MSD soft touch) but is not pulling hard there......it's an AX thing.... to avoid a shift.

My cams are Solex ( max power @ 6100) , but I'm using the 34s.

The max power RPM estimates are for the cams, irrespective of other mods, just to get one in the ball park.

Way too big. I have 36s on my 3.0 with Solex cams right now, and a number of people have told me that it's too much (I'm still playing with it). I would double-check your total advance at 6000 RPM...

-Wayne

If you are using Pat Bradens book the chart is calibrated for "race" motors, ie not much power below 5K but lots above it. Unless your ports on your heads are in the 38MM range and 10:1 comp and 906 cams I dont think you will get any bennifit from 38mm Venturis.

You may want to post your jetting set up, It may help.

34mm Vent, 145 Mains, 180 Airs, 60 Idles, F3 ET

The engine is 2.7l built with 2.2l heads modified to improve airflow, RS spec P&C, MFI S cams, Bosche "S" Mech adv distributor with 7200 rpm rotor.

You could always pull your "vents" out and see what that does to your problem.

BTW, with all of that work, you did match the ports up with the intake manifold didn't you?

David Duffield

34 mm venturis seems kind of small for me for a 2.7 that is using an S cam along with the expectation of 7200 RPM. For comparison...

1) BA's Book (2nd ed, page 141) has a neat chart comparing 32, 34 and 36 mm venturi. He doesn't give the engine size on the graph, but in the text on the next page he refers to a 2.7 with E cams, using 40 mm carbs, 34 mm venturi, F3 emul tubes, 135 main jets, 145 air correction. Going back to the graph, he showed the 32 mm venturi peaking at 6000 RPM and ~210 HP, the 34's peaking at the same engine speed and more like ~215 HP, and the 36's peaking at 6500 RPM and 225 HP.

2) Using that data and plugging it into my own calculations...
- 6000 RPM through a 32 mm venturi is a gas speed of 111 m/s
- 6000 RPM through a 34 mm venturi is a gas speed of 98.7 m/s
- 6500 RPM through a 36 mm venturi is a gas speed of 95 m/s
- 6500 RPM through a 34 mm venturi as in your engine results in a gas speed of 107 m/s

3) Looking at Porsche's other carb'd engines you'll see that they never even get close to that sort of gas speed through the carbs.
- '65 906 2.0 with 42 mm venturi: 64 m/s
- '70 2.2 T with 32 mm venturi: 87.9 m/s
- '68 2.0S Rally care with 32 mm venturi: 90.8 m/s

It just doesn't seem that common (possible) to be generating peak HP at RPM's where you are also drawing over 100 m/s in intake gas speed. Especially not with something like an S cam that has significant overlap. Basically as engine speeds increase, the venturi start to act like intake restricters which results in the a partial vacuum developing downstream of them. As the vacuum increases, it reduces the cylinder pressures (and thus torque) in proportion to the increase in rev's.

Yes, everything is matched up.

jluetjen:

This is what I'm feeling with the engine, once I get to about 6000 RPM, everything falls right off.

I'd be up to try just about anything at this point.

Does it fall off really sharply -- like something electrical?

Or more progessively, like the carb might cause -- "fall" off or "level" off?

The Power drops off. It is not a sharp cut.

David: Are you being serious? Should I consider pulling the Vents?

In a way it's a tongue in cheek statement. I said it in more of a trouble shooting vein to see if this gives you an indication of what you want the motor to do.

I believe that the "choke's" fit in from the bottom. If my math is correct the bore of the carb is 40mm. If you have "choke's" that have a bore through them of 38mm, the side wall of the choke at its thickest point would be approximately 1mm. or .040 thick. Not much usefulness in correcting the air flow.

I once put a set of these chokes, (actually they are venturi tubes) in my lathe and bored them as far out as we could. They were used on a set of side draft webbers for a mini-stock fiat running at the local fairgrounds. It was only ment to be run wide open but it did the trick. The owner/driver told me that, "It wouldn't be much different if we had just taken the chokes completely out of the carb."

I have come to regard all of the charts and graphs for air flows on motors as a starting point only. There are many other things that affect the settings of air/fuel metering devices other than what it says in some calculations. Right at the moment I'm consulting with a neighbor that has just installed an Electromotive fuel injection control unit on his LT-4 Trans-Am. BTW, it runs very well. We have tinkered with it for 2-3 weeks and now its just starting to be very smooth without hiccups. The graphs and charts were just a start.

My suggestion is just a cheap way to try out your theory to see if you need larger "choke's". I've looked at the specs for using 40IDA's on a 2.7L-3.2L motors and their similar sized motors. I seem to understand them. Then I go over and look at a 2L motor in a 914-GT using absolutely huge 46IDA's and all of my understanding goes completely out the window. I say to myself, "Is there any useable rpm range of this motor with these carbs?"

I do have a friend that is using the 46mm's on his 3.0L using the 20/21 cam grind. They were not an optimum choice.

Good luck, David Duffield

I am open to anything and I like the out of the box thinking. I'll pull them out and see how it runs.

Believe me, it's out of the box thinking here all of the time.

But its stll very hard to repeal the laws of physics.

Good luck,
David Duffield

Jetting looks like it is what is should be. I assume your fuel pressure is good, right? I know I am using a Holly Blue 110gph pump and at WOT the fuel pressure flutters so I run almost 4 PSI and it tapers back to 3.5 PSI at WOT and this made a big difference for me. As well as float height and fuel delivery are all ok right?

As far as 7200 as the RPM for venturi size. I thought it was supposed to be peak RPM not redline. I think peak power for a 2.7 with S cams is is about 6200 or 6300 RPM. But it should not fall off as you are expeirancing. If all of the above check out OK I would make sure you are getting full advance at 6000 RPM.

I'll pull them out and measure the OD with a caliper. Shouldn't take too long. And then I'll try to fire it up, and see what happends. This may be the least expensive carb part change I've ever done.

Dont be suprised if it runs like an a$$ biscut with the venuris out.