Oversized throttle body require a "re-jetting"?

Picked up my 81 GTSI a cople of days ago, fresly rebored to 69 mm by a small specialist here in Italy.

1) Seat of the pants feeling: very little difference low down, quicker progression from 5.5k to 8k BUT the car is not stock (cams, headwork, 10.4 comp pistons). I guess difference is noticeable because the car was actually restricted in flow after modding.

2) Kjet shoul be able to compensate automatically but since the car was modded the guy recommended increase in fuel pressure to avoid possible lean conditions at high rpms.

I was almost surprised car did not lose tractability at small throttle openings; the precise building of the butterfly valve seems to be critical.

 

Just thoretical here, but:
Agree that the K-jet will require no mods to keep up. It gives as much gas to match the air it sees going to the tb to give the appropriate air/fuel proportion. So, it shouldn't care what is happening downstream. Also theoretically, agree most of your change will come at higher open throttle settings where you are not already 'throttling' the flow with the butterfly valve.
Sounds pretty cool - good luck!

 

See http://my.ferrari328.com/manuals/boschtech-12d.pdf for an overview of the K-jet injection system (slow server, sorry -- try right clicking and save-as).

All the K-jet really knows is how much air is flowing, and it regulates fuel to match. As long as you're not outstripping the fuel capacity of the system, it should handle the change without problem. I don't think the minor bore change is going to affect it, as there were turbo installs and such back in the day that used the stock Ferrari K-jet implementation.

Can you do us a favor? Is there any chance you can do before-and-after dyno runs with the throttle body modification? Thus far there's been no hard data available on this modification.

Thanks

 

Merry Christmas all! I don't get over to this chat often enough, but I'll try and answer a few of the questions.
Tillman, I hope to have dyno figures on both 2 and 4 valve motors in the next 30 days or so. Durable 1 Inc. BTW is not a sponsor at this time. We have been doing the 70mm TB bore for some time now, and never seemed to be able to get car,65mm and 70mm together at the same time. So I picked up one of each model TB @ GT Car Parts, and I'll have them on hand. The 4 valve has been running very strong, and has a very noticeable top end . FYI: I did a 3 week extensive study on the flow bench while pursueing this . The physical size of the unit is the only reason I stopped @70mm. as the Bosch Distributor was not peaked at full out. I tested the entire air induction system from the rubber bellows on. It was quite en extensive tewst including the flow of the fuel distributor itself. I found a few suprises, and learned a tremendous amount in that time. I would note here that I ran the tests dry, as the flamibility ruled out running with a pump inline. Our web site:www.durable1.com has quite a few pics and text on the testing.
Reworking or "Tweaking" the Bosch to adapt the 70mm is not necessary.The Bosch unit delivers the right amount of fuel for the flow of air. Pretty simple. It would not know if the TB was the size of a Quarter, or 75mm. As the plate is drawn down by the force of the incoming air, then the distributor adds more fuel to match the air flow increase. So you have the basics right on the money there. Once you understand a bit on how the system works, one of the first things that are noticed is the fuel control pressures, which are the means of setting the fuel ratio. As I tested without the pressures against the plate (Sans fuel), and it still did not "bottom out" I plan on doing an adaptor for the 5 litre Mustang aftermarket TB's, so I can get more flow there yet, as it is capable of fueling it. I would certainly watch for fuel pump capability then, but it does not pose any problems in the 70mm's.
And yes, the throttle plate must be done right, starting with material selection, on thru to being lathe cut to diameter at a precise 8 degree angle. Anyway, you might be suprised to find out how many are out there now,but no one seems to be talking. I can't say I would either. Keep the intake tube on, and your trap shut, and no one knows that the last tuneup wasn't what woke up the ol' car! It is a great "sleeper" mod.
But before I get the TB's to the rollers, I have to finish a developement on a new design air horn for the carb cars that has shown a pretty substantial improvement on HP over stock. I'm goinmg to up one main this time, after trying to fatten it up with Air correction jet changes (Thanks for sending the jets Russ). While I was wating for the jets to arrive, I noticed the old screen left from holding sound deadening material. So out the screen went, which increased the air flow enough that the ACJ's didn't cover it. Another set of jets and a visit to the dyno! I think between Russ and I we do our fair share in supporting our local dynos,LOL. Come to think of it, the Bosch is a damn site easier to deal with!
Kermit

 

Hey Kermit,

Have you guys done anything with 348 throttle bodies?

 

I am in the process of taking my throttle bodies off the 348 to send to Kermit. I ended up being out of the country for an extended period of time so I couldnt get them to Kermit earlier but will do so here the first week of the new year.

Kermit - Count on me my man I will get them to you and call you next week with a measurement on the throttle body diameter. Hopefully I havent kept you waiting to long.

John

 

Ernie, John beat you o the opening this time. We had talked quite a bit, and came to the conclusion that the only reason I had not done them yet was simply access to them. AS I allready have the tooling for the others, it is going to be more or less adding bolt holes to existing proven jigs.
The throttle blades for example are turned on a lathe, but held the correct 8 degree angle in a jig, so adding a different pattern is a lot easier than starting from scratch. It was kinda a deal that fell together, he asked why I hadn't, I replied I needed a sample for tooling, and here we go. One thing I haven't crossed in my many hunts thru manuals, is the stock 348 TB blade diameter. If it happens to be 65mm, great. just happen to have a growing stack of them from the 70mm bores. IMO, based on what I have learned in the study of largeTB's, the 348 ought to really wake up. The stock Bosch units on these are a newer, and more refined metering system, and the added air ought to be easy for them to tune to. Now if I can squeeze in the time, I never have liked the 2 valve TB shaft being verticle. Seems too much like a rudder to me. Part throttle air distribution, and therefore fuel ratios between cylinders has got to be thrown off by this angling of the intake charge towards one end.....
Kermit

 

Matt - I will have the throttle bodies off to you first thing next week as I am in Germany at the moment and have them about half off the car before having to leave for work. I am trying to get a friend to drop in my garage for you and get a measurement on the diameter of the blades.

PS. Have you ever got your dyno numbers for the krank vents?

 

Not yet on the KV dyno numbers. I'll be running them at the same time as the TB's though, so it can get all wrapped up and done.

 

Well keep us posted on the out come. I am VERY interested in doing this. Also, on the inside top part of the intake plenum, there is valve that opens to even out the pressure between both of the intakes. If I remember correct it opens during full throttle between 3,200 and 5,500 rpm, to help with the torque. I'm wondering if it would be worth while to have that ported also? I know you have said that it the throttle body opening on the intake should also be opened up to help with the flow of the larger throttle body. So that is why I think that maybe the valve in the middle should be opened up too. What do you think?

 

Ernie, IMO I'd leave it be as is. Perhaps smoothe the transition, or rounded areas as they pass thru. As I understand, the valve is used to increase the effective plenum size for a side, Normally shut, one bank has one side of the plenum volume. When the valve opens, the plenum volume, and dampning efects a greater Perhaps there was a resonance that occured there, from the intake pulses.
HTH,Kermit