More stupid questions

[Guest Anonymous]

Boy, it's intimidating reading some of the posts you folks put up re: manometer readings, fluid and static pressures, etc. I hope you don't mind the less informed coming with newbie stuff. I just keep telling myself "there;s no such thing as a stupid question...". Excellent site, fellows; keep it coming.

Anyway, is there anything inherently inferior to using carbs with a turbocharger vs. programmable efi? Obviously the efi can measure and automatically adjust for variables such as air/fuel ratio (o2 sensor), knock sensing, airflow/speed density, manifold pressure, water temperature...but shouldn't a well set up carbed system be able to be tuned for these factors? Does anyone have any experience setting up carbed turbo systems? Carbs are just so much cheaper...I'll cut the question off there before I say something stupid :} Thanks for the input.

[Drax240z]

First, there are no stupid questions.

 

Here is a bit of info I had posted on zcar.com about a week ago. Going to cut and paste it.

 

---Start quote---

 

There are two basic layouts of a turbo system using carbs. One is a "blow through" system, where the compressed air is blown through the carbs and then into the engine. This system allows intercooling, and is all around a better setup than the other carb'd system.

 

The other system, and the one to which I think you are referring is a "draw through" system. The carb is mounted BEFORE the turbo charger and the turbo sucks air and fuel through it. This is really a poor design. 1st, your 1 carb will ice up at any ambiant temperatures below 60F. 2nd, you run the risk of the fuel puddling out of the air/fuel mixture as it travels. Any low point in the system will end up with puddles. One way around this is to heat up all associated piping after the carb to keep the fuel vaporized. Umm, adding heat to a turbo system is NOT a good idea. 3rd you cannot intercool. Period. 4th, the way it works out is that each cylinder is fed by the 1 carb, 1 at a time. Over carboration to the extreme.

 

You'll find that carbs are difficult at best to setup for either type of system. The key is to maintain the proper air/fuel ratio throughout the rpm range, which can be tricky. Often carbs are setup for the correct a/f ratio at idle and part throttle only, and what happens at full throttle is a mystery to most people! Also when using a blow through system you need to be sure the carb is prepped to handle higher than atmospheric pressures. If you were to use SU's for instance, you may notice air and fuel leaking out around the throttle shafts when the carbs are under pressure.

 

Trust me on this, an EFI turbo system is a much better design. Fuel is not carried throughout the system, instead injected at the point of combustion. Intercooling is easy. You can more easily adjust air/fuel ratios at any rpm. No special carb prep.

 

If you want to learn more, try reading Maximum Boost by Corky Bell.

 

---End quote---

 

I hope this helps, and I really very highly recommend Maximum Boost! Yes indeed, one excellent book.

 

Feel free to ask away if you need more info.

 

------------------

"Gimme Fuel,

Gimme Fire,

Gimme that which I desire"

-Metallica

 

Drax240z

1972 240z - L28TURBO transplant on the way!

http://members.xoom.com/r_lewis/datsun.html

http://members.home.net/drax77/newpage.html

 

[This message has been edited by Drax240z (edited December 09, 2000).]

[John Scott]

My .02 is carbs are second rate to a good FI. Don't get me wrong a carbureted system can support plenty of hp, but not as precisely as FI thoroughout the rpm and boost range. I've been running a blow through (centrifugal) system for 5 years. My conclusion is even though it works very well, it only really addresses two ends of the fuel/air mixture spectrum. First you jet the primary side for cruising around with no or low boost. Since as air density increases, so must your fuel, the secondary side must be jetted for maximum boost, which occurs of course for mine at max rpm. Detonation at high boost due to lean mixtures tends to hurt your expensive reciprocating pieces. Everything in between is a compromise. The system works pretty well, but a FI that adjusts throughout the boost curve would be superior. Now if I only had lots of $$ laying around I could advance into the computer age. In the meantime I'll live with a carburetor's shortcomings.

JS

[Andrew Bayley]

I will totally agree with John, the carbs are a distant second next to EFI. However, carbs are a heck of a whole lot cheaper. Back in the days, I ran a draw-through L28 turbo system. I'll agree that it wasn't the best way to do it, but it did work. I never had any problems with icing, even at temperatures below 20F. I used to drive this setup 500 miles once every month driving to/from school in the dead of Michigan winters (three feet of snow, uphill, both ways, yadda yadda yadda).

 

The reason I decided to go with draw-through, is for the ease of air / fuel mixture control. Basically, the turbo would pull as much air as it needed from the carb. Therefore, I didn't need to do anything too fancy to the carb. I did richen up things a bit, but I later found out that was just fouling out plugs and causing me to run too rich.

 

It was a good system. Not great, just good. Even though it never iced, there would be a VERY long period of warmup time before I could drive it without stalling. Also, throttle response was totally lacking. In the end, I debated about switching over to a blow - through (I even bought the 4 barrel manifold for the L28) but I eventually "saw the light" and put in a V8

 

In my "retirement" garage though, I see at least one turbo L28 Z.

 

-Andy

 

[This message has been edited by Andrew Bayley (edited December 11, 2000).]

[Guest Anonymous]

Thanks for the valuable input, guys. I've decided to sell my P90 head, F54 block, T3/4 turbo and blowthrough carb setup and go with a Ford 5.0 efi 1991. Anyone interested in buying my turbo stuff? I'll post it in buy/sell once I get a chance to sort out exactly what I have (likely in January the way things are going...:])