another lot Zenith questions

[porschetub]

Will have my zeniths off my 2.2t soon to tidy up fan housing ,check altenator ,do the triangle,etc and will be giving the carbs a going over,they don't seem to bad but motor has been sitting for a while.
While I'am in there I would like to set up the jetting ,have trawled thru mods for using these on larger motors but need to know a proven method used to get better than stock running.
So far I understand a small increase in idle jet size will help (#50 or #55 ?),if someone has done this with good results I would be keen to know .
Are there any set rules by year/serial numbers for vent size as there are 2 apparently.
My engine is euro spec by the way so no enrichment systems.

[crummasel]

The carbs are set up as throttles by using 27mm venturies.
As long as you are not wanting to manufacture new parts find yourself a good machine shop (as far as you are not familiar with your own lathe in your barn) to open them up to 30..31mm which works well as long as you maintain the inner contour.
Further put in 125..130 mains and 55 idles to start with.
Thats still conservative but should give you a good step in front.
If you keep the 27mm venturies there is not much to do...

[porschetub]

Originally Posted by crummasel

The carbs are set up as throttles by using 27mm venturies.
As long as you are not wanting to manufacture new parts find yourself a good machine shop (as far as you are not familiar with your own lathe in your barn) to open them up to 30..31mm which works well as long as you maintain the inner contour.
Further put in 125..130 mains and 55 idles to start with.
Thats still conservative but should give you a good step in front.
If you keep the 27mm venturies there is not much to do...

Thanks for your reply,I didn't think that radical jetting changes would work without work on the venturies,I'am an engineer so I would look @ making a profile template of the vents and machining to the larger max internal size given the casting thickness,have use of several lathes to use so no prob.
Keen to squeeze a little power but don't want to stall the flow rate with big vents .

[VFR750]

Going to larger idle and mains do not require a Venturi change. All you are changing is the amount of fuel for the "fixed" amount of air controlled by the Venturi and air bleeds.

With today's E10 fuel, the original jets are too small. Experienced folks suggest bumping to 50 to 55 and 120-125-130 to get back to a rich condition. Which is safe.

When you start getting into serious tuning, you may find a better jet size, but it takes time, money, and several sets of jets (ask me how I know)

These recommendations will get you a good running engine, and err on the rich side.

[porschetub]

Originally Posted by VFR750

Going to larger idle and mains do not require a Venturi change. All you are changing is the amount of fuel for the "fixed" amount of air controlled by the Venturi and air bleeds.

With today's E10 fuel, the original jets are too small. Experienced folks suggest bumping to 50 to 55 and 120-125-130 to get back to a rich condition. Which is safe.

When you start getting into serious tuning, you may find a better jet size, but it takes time, money, and several sets of jets (ask me how I know)

These recommendations will get you a good running engine, and err on the rich side.

Sage advise from someone that has been there,thanks,was the venturie size year/engine number dependant? I understand there were 2 sizes fitted?
Maybe the euro spec motor had the bigger vents?,any idea's on that?

[VFR750]

Most were 27.5mm for the 911T. Don't know if they offered a 28.5 or not. Either size Venturi will benefit from the increase in jet size.

[crummasel]

28,5 were mounted in 2,4L euro - T

[VFR750]

Euro always got the good stuff!

[crummasel]

Given the casting of the small thing you can open 27s to 30 and 28,5s to 31. A quite special job on the lathe since the venturies absolutely do not run true and are tricky to chuck. But once you got the trick its a fairly easy job. Nothing to give to a shop (even if i said so above but its risky to crash them for unambituous shop people) but if you can do it by yourself its fun and useful to do.
A bit more than just blueprinting but by far not high end tuning or rocket science. Just worth doing and adds some fun to your car given a correct final jetting and fine adjustment job.

[DanPez]

Sorry to butt in again on this question of venturi resizing ....... widening the diameter of the venturies.
Where would this be an advantage?
The cylinder’s intake port diameter and camshaft lobe openings don’t change bore and stroke are designed so to fill the jug with certain volume of air.
Changing the venturi, wider, will only decrease the air speed going in the cylinder.

The only advantage I would see is air / fuel emulsion that will have an effect.

Could you or someone explain?

[VFR750]

The Venturi size will limit flow at the highest rpm. It works in conjunction with the main jets and emulsion tubes. Change one and adjustments need to be made

For the 911T the intake ports are only 32mm. So going really big is going to get you any added power, it is just going to kill drivability.

For the T, 30mm is likely a good balance. Where you sacrifice some low end torque for added top end.

[porschetub]

Originally Posted by VFR750

The Venturi size will limit flow at the highest rpm. It works in conjunction with the main jets and emulsion tubes. Change one and adjustments need to be made

For the 911T the intake ports are only 32mm. So going really big is going to get you any added power, it is just going to kill drivability.

For the T, 30mm is likely a good balance. Where you sacrifice some low end torque for added top end.

Thanks vfr750,this info backs up research and fills the gaps,the vent size is more important that most people realise,I see speedo fitted the larger vents to his Zeniths with good results but that was to suit his 2.4 build,makes sense.
The large aftermarket vents (32 and 34) must leave a big hole in the bottom end power?

[VFR750]

There are some good sources of info on carbs out there. Bruce Anderson performance handbook is very good for the Porsche enthusiast. He discusses venturis and engine size. It might be the best way to learn about how changes affect the balance.

[DanPez]

Originally Posted by VFR750

The Venturi size will limit flow at the highest rpm. It works in conjunction with the main jets and emulsion tubes. Change one and adjustments need to be made

For the 911T the intake ports are only 32mm. So going really big is going to get you any added power, it is just going to kill drivability.

For the T, 30mm is likely a good balance. Where you sacrifice some low end torque for added top end.

Thanks Mike,

[porschetub]

Originally Posted by VFR750

There are some good sources of info on carbs out there. Bruce Anderson performance handbook is very good for the Porsche enthusiast. He discusses venturis and engine size. It might be the best way to learn about how changes affect the balance.

Yea liked Bruces work on Excellence,sad loss that,people listened to what he said,remember in the early days he dissed the 2.7 as problematic and gracefully took the flaming he recieved.......he was right in what he said
Haven't read his book but will take your advise,carbs aren't new to me however as I'am in that age sector,thanks again.

[porschetub]

Are there any set sizings for the jets between euro and usa spec 2.2t carbs or are they the same across both types?
Thanks in advance.

[crummasel]

Basically they are the same with some minor changements (if i recall correctly) for California versions.
However, see above.
30 venturi will do no harm to torque and is a small but good step. Just do it right and it will work fine.

[porschetub]

Originally Posted by crummasel

Basically they are the same with some minor changements (if i recall correctly) for California versions.
However, see above.
30 venturi will do no harm to torque and is a small but good step. Just do it right and it will work fine.

Based on info from various sources on here (thanks) and other info I have ordered #55 idles and #130 mains and will bore to 30mm as to me it makes sense and is within the limits of the vent casting besides.
Have rebuild kits so its kind of a no brainer to not do it during a rebuild.