COMPACT THROTTLE BODIES

[PeterR]

A young friend wishes to convert a straight six carb engine to EFI while retaining an old style appearance.

My suggestion was to use two TBIs turned on their side and fitted with individual air cleaners so that the set up would look similar to twin side draft carbs.

I purchased a pair of single barrel throttle bodies from a cross fire Corvette so a common GM ECM, wiring, and sensors could be used.

However when the throttle bodies arrived I discovered they are much larger than I had expected from the photographs, and when mocked up beside the engine look quite out of place.

I have seen pictures of a later style single barrel TBI used on four cylinder GM engines.  This type appears to be much more compact but I believe it is produced by Delphi instead of Rochester, and I wonder if it is compatible with the older ECM units.

Any info welcomed.

[wayne petty]

yes... you can use smaller  throttle bodies off gm cars..

there are 1.8 OHC pontiacs... there are 2.0 OHC pontiac motors. both of those will have smaller throttle bodies..  and will plug and play using the corvette wiring harness..  the 2.5 four cylinders i think use a slightly larger bore..  but i could be wrong..

by the way.. the corvette injectors are fairly special.. they actually don't spray an even pattern.. they have an angled pattern..

do me a favor. before you do the install..  or perhaps even before purchasing them...  open the throttle part way and check for throttle shaft to bore play...  all GM throttle bodies experience this... i have a fairly easy fix..

it involves picking up from the local engine rebuilder.. if it uses a .372" shaft..   pick up a few 0.375 thin wall valve guide liners...  a LETTER Y drill which is 0.404...  don't give the rebuilder your shaft size at .372.. they will give you that size liner and it won't fit without going to ream it..

the throttle plate screws have DAMAGED THREADS that need to have the damaged area ground off..  or you will break them. .they are like M2.5 x .40 or .60.. i forgot..  with a giant pan head.  you can if you leave a tiny amount of thread exposed when you grind it off.. HINT loosen the screws... it may come back 1/2 a turn.. stop.. don't try to go farther.. just till  it tightens up coming loose..  grind the exposed threads off..   then tighten the screw into the hole and grind a tiny amount more.. this will leave you a small section to mash over when you are done..

drill the shaft bore to the letter Y drill.. stick the liners either on a 3/8 bolt  or 3/8 wooden dowel.. a worm drive hose clamp to stop it from spinning..  use a thin cut off blade to slice off the length you need.. you can trim the edges off on the cut off blade and drive it into the bore..

you may use grease to work the throttle shaft back into the bore.. i normally do both sides on a dual bore version. but not the middle..

reassemble.  you will want to back the throttle stop out.. and make sure the blade is centered.. since you will have the injector housing off the top.. you can use a steel bar or an extension in a vice with the throttle body upside down on it to back it up to mash the exposed threads..  

i can describe how to set base idle.. i always plug the idle passages with a shop towel to perform this..

why go thru all this... if the throttle shaft bore is worn and sloppy.. when you return to idle. it varies the closed angle of the throttle shaft.. and the TPS sensor.. GM TBI looks at the TPS minimum voltage to switch to idle fuel trim..  idle fuel trim is 1.2 to 1.4 MS pulse width.. off idle is 2.2 to 3.2 MS pulse width..


3 CYL GEO Metro  
Metro
1993-1989 1.0L (6) 61"
1999-1998 1.0L (6) 61"

Cimarron
1986-1982 2.0L (P) 121" TBI 500

Buick Skyhawk , Sunbird
1986-1982 1.8L (O) 112" TBI 300
1989-1987 2.0L (1) 121" TBI 700
1988-1987 2.0L (K) 121" TBI 700
1986-1983 2.0L (P) 121" TBI 500

this might help in your search...

http://www.championparts.com/tbi_applications.pdf



Cavalier
1989-1987 2.0L (1) 121" TBI 700
1986-1983 2.0L (P) 121" TBI 500
1991-1990 2.2L (G) 134" TBI 700

[PeterR]

Wayne, thanks for the quick response.  Unfortunately I did not express my question very well.  

The application requires a pair of single barrel bodies which together can support a 5.0l engine.  The major difficulty I face is that this style of hardware is not easy to find here so I have been relying on the text and images from internet searches.

The dual single barrel Cross fire bodies appeared to meet the requirements so I purchased a pair sight unseen. These share the same 400 series body as a heap of 4 cylinder vehicles but have the larger 45mm plates, and the rear injector is slightly larger lb/hr than the front.

After taking delivery of them I discovered that due to the offset injector pedestal these single barrel bodies are not much smaller in overall size than the common 2XX series 2 barrel and have an odd oval shaped aperture where the air cleaner mounts.    They really are too big and bulky for the intended application.

From further investigation, the later 700 bodies look much more compact and appear to have a circular air cleaner aperture which would make them more suitable.  

Even if they are more compact I still have a reservation. The intention is to use the older style ECM, and it is clear the injector, TPS and IAC are mechanically much different from the earlier series, and I do not know if they are electrically equivalent to their earlier counterparts.  

Also the throat diameter appears to be relatively small.  This engine originally had a pair of 52mm carbs, the cross fire is 2 x 46mm and I would not want to go any smaller.  I have read that the Fiero has 45mm and there is some mysterious 48mm lurking in the sidelines.

I would be grateful if anyone can provide further guidance.

Regarding worn throttle shaft bores, the pair of Cross fire bodies were fully reconditioned including rebushing before I purchased them.  If I end up with bodies that have worn shaft bores I will machine out a pocket for a sealed ball bearing so they should last for years.

[wayne petty]

the injectors for GM TBIs as far as i know.. are all the same resistance...

the TPS sensors all use a 5.0 volt signal, a ground and the third wire to return to the computer...

the idle air control motors are 4 wire stepper motors...  as long as you are using the EFI computers that commanded them.. like the 7747 computer.. i think that is the number. you should be OK..

the corvette crossfire version used a second idle air control valve somehow..

you must want something like this..

http://www.ebay.com/itm/Rochester-700-single-barrel-tbi-fuel-injection-Take-offs-For-cars-and-trucks-GM-/171541554276

if you are just adjusting air flow.. you could in fact knock out some on a mill.. and use carb throttle blades.. but mount the GM TPS sensors. on the linkage.. you only need one sensor for both throttle bodies..

that is only a few holes to mill.. if you are not going to use fuel thru them.. just air.  perhaps i am wrong in what you were planning..

one could even go to a crazy thing and wedge in a tapered plug in one side of a twin when the throttle shaft is out .  

i always wanted to snag a crossfire unit to install on my corvair.. but i probably would have also used the 700 single barrel throttle bodies instead of the cross fire..

just thoughts..


page 6 has a bunch of different throttle plate sizes available.. for carbs.. but that won't stop you from using it on any other device

http://www.walkerproducts.com/wp-content/catalogs/oxygen-sensors/high-performance/full.pdf

[GPster]

Maybe you could look in an another direction. Do the newer Harley Davidsons use side draft throttle bodies? Something from a source other than strictly automotive might open up new horizons. GPster

[PeterR]

Maybe you could look in an another direction. Do the newer Harley Davidsons use side draft throttle bodies? Something from a source other than strictly automotive might open up new horizons. GPster

Just to step back a little.  

The original intent was to use hardware compatible with the old TBI ECM because these are a well proven swap to EFI, onboard data logging with self learning is a readily available mod, and wiring looms are an off the shelf item.

When fitted to GM V6 and V8 engines these normally run a two barrel throttle body with the two injectors.  The plan was to use two single-barrel bodies from a 2.5L four banger set up in such a way that the ECM would see them as a single two-barrel body      

A little later I stumbled on the Corvette/Camaro Cross fire which uses two single-barrel bodies in an arrangement similar to that I was planning.  They have a shared fuel pressure regulator, a single TPS and a vernier adjustment for throttle blade setting, so instead of messing around with modifying 4 cylinder bodies I purchased a set of reco Cross fires.

All this was done without ever having clapped eyes on the hardware and when they arrived I discovered they are too big and clumsy for the intended application.

Further snooping on the internet revealed the later 700 series single barrels which appear much smaller. I had already seen the eBay listing Wayne spotted, and was quite interested because these are new and unmolested

However there is still the issue that they have different injectors, TPS and IAC, which may not be compatible with the earlier ECM which I am committed to using.

If any one can offer guidance I would be grateful.

[GPster]

I don't keep up very well on any computer activated equipment so I don't usually come up with good suggestions. That is why I visit here (and here only) 5 or 6 times a day but don't open my mouth much. On an fundamental question, I thought you guys ran mostly on propane. Do these different fuels run on the came TPIs ? My main understanding of TPI is Tramatic Brain Injury. GPster

[wayne petty]

peter.. do you have any suzuki sidekicks down there.. those were also available with the 1.6 motor with TBI.. using what looks like the smaller 700 series ..

[PeterR]

I don't keep up very well on any computer activated equipment so I don't usually come up with good suggestions. That is why I visit here (and here only) 5 or 6 times a day but don't open my mouth much. On an fundamental question, I thought you guys ran mostly on propane. Do these different fuels run on the came TPIs ? My main understanding of TPI is Tramatic Brain Injury. GPster

First to eliminate some potential confusion.

Gasolene    
USA known as gas.............     Aus known as petrol

Liquified Petroleum Gas  (propane/butane mix)
USA known as propane............   Aus known as LPG or gas

Conversion to LPG was encouraged at the time of the fuel hungry carburettor equipped  smog engines because it has lower emmisions and Australia has enormous reserves of petroleum gas (enough for for another 50-60 years) while almost all petrol is imported.  

The road tax levied on petrol was not applied to LPG making the price at the pump about half that of petrol.  Even though the LPG returns much lower miles per gallon it was still more economic.

Over recent years road tax has been incementally applied to LPG nudging up the pump price. meanwhile EFI equiped petrol cars have become more fuel efficient making the true operating savings marginal.

Presently about 3% of passenger cars and light commercials are on LPG.    The vast majority of these are high milage vehicles such as taxis or courier vans running up around 100,000 miles per year.

If an old car has the the original engine replaced with a more modern engine, it is easier to meet the emmisions rules with LPG than petrol so some rodders have  taken advantage of this.