Larger Venturi for Zenith TIN40 on a 2.4L? Options

[Tdskip]

The 74 914/6 build has a 2.4 L MFI engine in it that came to me running Zenith 40 TIN.

I spoke with a Zenith specialist and he said that a 34m Venturi would be a bette match for that engine displacement and it’s an easy change to make along with some jetting changes etc.

This seems a whole lot more cost effective than going to Weber 40ida that are $3,000 +/- just for cores.

Anyone here have experience with this they can share?

Thanks!

[930cabman]

I have twice overcarburatored Porsche engines. Once with a 2056, went with 36 vents, just wouldn't draw, ended up with 32's, runs great and gets great gas mileage. A fresh 2.7 I went with 36's and again, tooo large. Also ended up with 32's

My suggestion would be to try 28's and go from there

All the above is with Webers, 40 IDF and 40 IDT 3V

[Superhawk996]

I have twice overcarburatored Porsche engines. Once with a 2056, went with 36 vents, just wouldn't draw, ended up with 32's, runs great and gets great gas mileage. A fresh 2.7 I went with 36's and again, tooo large. Also ended up with 32's

My suggestion would be to try 28's and go from there

All the above is with Webers, 40 IDF and 40 IDT 3V

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America is the land of overcarburation.

The chart below is for Horsepower not drivability. Usually 4-6mm smaller than this chart recommends for good vacuum signal, air velocity, and driveability. You give up a tiny bit in the top end but unless you’re on track - you’re not running at redline for any significant part of a drive.

The TiN were for 2.2L and they come with 27.5mm Venturis. So are we to believe that Porsche didn’t know how to tune? By the time you get to 32mm you’ll start trading throttle response for top end gains.

For a 2.4L you’re going to be about 30mm which will probably work a little better with the MFI cams (assuming that’s what’s still in it).

[rgalla9146]

All three 2.4 engines came with MFI. Do you know if it's a T, E or S ?
The different states of tune will likely affect your choice.
Zeniths 40 TINs are not as well supported as Webers.

[Tdskip]

Thanks for all of the responses.

Not looking to over do it here, just get enough flow to better match the larger displacement. She is to lean know and I'm running out of mixture adjustment.

I do not know what cams the engine has.

Very helpful chart, appreciate you posting that.

[rgalla9146]

Quick edit here.
The 911 T in 1973 changed mid-year from MFI to CIS fuel injection.
I'll bet your engine has Zeniths on it because the original CIS was not operating well or parts were not available.
The engine type can be found on the flat surface to the right of the fan housing and down by the engine tin.
Should be 911/ 51 (T) 52(E) 53 (S) or 911/ 91 or 96 for CIS

[Ninja]

Thanks for all of the responses.

Not looking to over do it here, just get enough flow to better match the larger displacement. She is to lean know and I'm running out of mixture adjustment.

I do not know what cams the engine has.

Very helpful chart, appreciate you posting that.

New (old) member here. My last member name was technicalninja...
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The Hawk is right!

Street based cars require carbs set up for midrange power.

One thing that grabbed my attention is you're running out of "adjustment".

Are you trying to adjust mixture across the board with the mixture adjustment screws?

They only adjust in idle and when you're on the progression slots (little tiny bit of adjustment on progression). So, the mixture screws help during idle and low speed, low throttle opening operation.
This is 50% of a street car's performance envelope.

Past light cruise the jets themselves come into play.

I don't know anything about Zeniths but carbs are carbs.

If the optimal venturi is 30 (I agree with Hawk again here) I could make 27.5 work fine below WFO. The 27.5s might give me slightly better power below 4K and the proper venturi would have slightly better power above.

On yours I'd try bigger idle jets if the lean spot was below 3K, next step up on mains if it was lean between 2-6K and smaller air corrections if the lean spot was above 4k.

Are you using a wide band O2 sensor to verify mixture?
In my book it's the only thing that is really accurate.

You provision the mains and air corrections to the venturi size and changing venturis will often require main and air correction jetting changes. A venturi change from 27.5 to 30 would NOT be a big jump on either, maybe one step.

I'd jet to the venturis you have now before changing them.

You have the opposite problem that most members have. Almost all of the carb kits come with venturis for power/race operation, and they don't work well below 4K, REQUIRING a venturi change.

I had a customer with a built 460 ford (9.5comp, medium cam) who was using a 600cfm carb (baby small block carb). The computations showed a 1000 cfm carb for power and an 800 cfm carb for street use.
He never took it past 4K...
That tiny carb significantly improved low end torque and worked just fine for the customer. He WOULD not have liked a 1000 cfm carb at all.
This was in a Cobra replica (4 speed manual) and is the most violent car I've ever driven. At any speed below 100mph you could turn it sideways with throttle application alone!

Small carbs are NOT inherently bad!
Most of the time they are better than "oversized" carbs...

[Superhawk996]

New (old) member here. My last member name was technicalninja...
(IMG:style_emoticons/default/ninja.gif)


Small carbs are NOT inherently bad!
Most of the time they are better than "oversized" carbs...

Welcome back - what happened to old username?

Had a BMW 520 2.0L four in Europe - tiny carb. Great drivability and still would do 100+ on the Autostrada for hours on end.



Had a dual Stromberg setup when I first bought it. Never could get it sorted (I was only 19 & just learning) but switched to the single (Zenith?) setup, tuned it, and it was way more drivable. Wouldn’t mind having a go again at those Steomgergs (IMG:style_emoticons/default/lol-2.gif) that humbled me though!

This engine & carb (internet photo not from my car) but same setup.


Was a good intro to bigger carburetors not always better. Luckily I had some guidance on that from some of the diesel power generation mechanics and a guy with a 911 that took me under his wing.

[porschetub]

The 74 914/6 build has a 2.4 L MFI engine in it that came to me running Zenith 40 TIN.

I spoke with a Zenith specialist and he said that a 34m Venturi would be a bette match for that engine displacement and it’s an easy change to make along with some jetting changes etc.

This seems a whole lot more cost effective than going to Weber 40ida that are $3,000 +/- just for cores.

Anyone here have experience with this they can share?

Thanks!

Tom I have 2.2T motor , I went to 30mm vents ,125mains and 52 idles runs/starts great ,tryed 32mm vents but bottom end power was weak but top end strong ,too much lose of vacuum to work and fussy to tune.
In your case you may find the MFI cams could make a differance but I don't exactly know,cheers.

[Tdskip]

Thank you very much for the posts and additional considerations

[sixnotfour]

911/57 2.4 Zenith Carb 32,32 1972 2.4 911T, Euro version, Zenith 40 TIN carbs