simpson36

First, a bit of background on why in the heck GM made this crazy setup in the first place:

Crossfire TBs were not originally designed to be dual TBs. They are 4 banger TBs machined a little different with new linkage and balance ports.
GM needed fuel injection to meet mileage and smog standards and all they had at the time were 4 banger TBs. As soon as they finished developing the two barrel TBs, Crossfire disappeared and the more conventional arrangement of a dual plane manifold and a single TB went on for another decade.

The purpose of the IAC is to provide a method by which the car's computer can control the idle speed. The computer sends step commands to the IAC and it opens and closes an orifice that lets air into the TB below the throttle plate, thereby controlling the idle speed.

One of the causes of the Crossfire's infamous bad idle is the dual IACs. The reason is that the wiring for the two IACs is simply spliced together and both IACs get exactly the same signal from the ECM. If both IACs are in a good mood and do as they are told, they stay in sync and all is well. 20 year old wiring, connectors stepper motors (the IAC itself) can get an 'attitude' like a 2 year old human and just refuse to play nice . . . sometimes.

So let's start from a nice steady crossfire idle (OK pretend . . . ) . . the A/C compressor comes on and the idle speed drops. The ECM sends out step commands to the IAC to open a little and restore the idle speed. One IAC responds and the other is still having a tantrum and refuses. The ECM, having NO WAY to know that ony one IAC is being stubborn, just sees that the idle did not increase enough and sends out more step commands. As one IAC keeps opening more than the other, the all important TB balance is rapidly going south and the idle is getting rougher and rougher . . which effectivly slows the idle . . . resulting in yet more step commands from the ECM . . . which unbalances the TBS even more . . . makeing the idle worse (and slower), and on and on.

So there you have the Crossfire's little dance with the devil. Just for icing on the cake, the ECM frequently re-sets the IACs so the problem mysteriously 'heals' itself and you might get a decent idle again . . for a while. This has two effects, One: it makes people believe these things are posessed by demons (I'm in that group), and two: the problem is very hard to diagnose unless you 'catch it in the act' . . which I did, and that started my little engineer brain thinking of how to have only a single IAC.

In order to get two TBs to share an IAC, we have to somehow steal the air from one TBs IAC circuit and pipe it to the other TB.

That's doable, but it's going to cause some problems to fix. We need to make sure each TB gets the same amount of air from the single IAC or we defeat the whole purpose of the mod.

To accomplich this, both TBs should draw their IAC air thru the same sized hole, yes?

So we slap an orifice into the Momma TB that is the same size as the smallest passage feeding the Baby TB. Air comes into the IAC 'chamber' and then passes thru this hole to get to the throat of the Momma TB, so this is where we create the restriction:



Looks like this from the bottom:



Now we tap into the TB near the IAC chamber. Look closely to see the cross-drilled passage down to the IAC chamber.



I love the little fittings with the constrained O-ring gaskets. There is not nearly enough room for an NPT tap in this location, so these little guys make it doable:



We need to make a plug plug the huge hole where the IAC motor was screwed into Baby TB:



Put everything together:



And, of course, also close off the orifice where Baby USED to get fed from:



Enlarging Momma's IAC orifice and using a different IAC motor with a larger pintle finishes the mod.

Last item is to make up the transfer tube. So now you know what that oddball jumper is for . . no, it's not the carrying handle



This is obviously not a cross fire manifold, but any arrangement can be used by making an appropriate tube or just using barb fittings and rubber hose.

Dewey316

Fab work looks great.

I think the only thing I would do differently, is try to equilize the the length between the two of them, so that you maximize your chances of not having a strange surge happen do to the difference in IAC passage lenghts. That would be the only thing that I would worry about they way you do it. It may not be an issue though, only running it will answer that.

RFmaster

Very nice writeup - is it possible you can shed a light on throttle shaft, bushings modifications???

//RF

simpson36

Here is my tech series on Crossfire. There's a lot of throttle body stuff in there including installing shaft bushings.

http://www.thecubestudio.com/Crossfi...eriesIndex.htm


I prefer to add double sealed stainless steel ball bearings instead of bushings, but that's athread for another day:

chevymec

iTrader: (5)

Nice article !! I have only delt with crossfire once. It was on a vette that somebody had tried to "fix" the idle. Luckly we still had the Factory service manual and followed the directions to sync the TB's. Ran like a champ after that.

simpson36

Dewey;

Excellent observation. This is the second mod I've done like this. The first one has been on my wife's '82 Vette Collector Edition for many months. It works perfectly.

You're right about the length of the runs, but I made the orifice in the 'Mommy' TB just a tiny bit smaller to compensate. I'm about as **** as they come, but taking the setup from 'very close' to 'perfect' in this case would require a ton of flow work and sophisiticated equipment and would not net any noticeable result.

simpson36

Chevymec,

You are right on the money! The Crossfire setup is extremely sensitive to balance. So much so that the factory WELDED the adjustment screw to prevent people from 'tampering' with it.

Unfortunately, that did not stop some backwater mechanics from just bending the arms to 'adjust' the balance . . I kid you not.




I ran into that so may times, I had to add an article just for that!

http://www.thecubestudio.com/Crossfi...eShaftArms.htm

89_RS

I know this thread is old, but last the last I heard of this project, it hadn't got past the getting the ECM to run both TBI stage.

I just saw an episode of Wrecks to Riches and they had a 426 HEMI with dual TBI's on it. Maybe we need to look at what the Mopar guys are up to.

simpson36

I'm still out here and still doing Crossfire work. I just sold my last ported Crossfire intake on ebay and I have my last set of TBs on there now also. I am just too busy these days to participate in forums.



I don't understand what you mean by the ECM not running the TB's.

You can run two injectors off any TBI ECM that runs two injectors, which includes any original crossfire ecm or any two barrel TBI ECM, which covers just about all of them.

You can put a pair of Crossfire TBs anywhere a 4bbl carb would go and I can get 1,100 CFM from a pair of croddfire TBsdual carbs would go and still have a good idle and good off-idle and tip-in, . . which is just about impossoble with than much carb.

The reason is that TBs don't rely on air flow to pull fuel so they idle fine with huge throttle plates and the resulting slow airflow at idle.

My crossfire tech stuff is a sticky on one of the big Vette forums, but I don't know if it is OK to post links here to other forums.

chazman

iTrader: (4)

I know everyone says that doing stuff like this is "easy", but I wish there was a shop here in the Chicago area which had the expertise to go through my TB's, rebush and rebalance them.

RFmaster

Contact TGO member dctrumpet - he does TB rebuilding, including re-bushing, etc. I do not know if he does - crossfire re-balancing - just PM him.

//RF

simpson36

It doesn't matter where you live. I ship worldwide. I have enough TBs that usually I can ship the new TBs before I get the old ones back. I often torque the new TBs with new gaslets to a cleaned Crossfire lid and pre-balance them on a balancing rig and ship the whole assemly to a customer. The customer swaps the whole assembly ans sends back his TBs and lid. No down time waiting on TB work and no balancing for the customer.
IO

simpson36

I don't do re-bushing jobs because I think it is a wastse of money. The performance killer on Crossfire it the tiny 4 banger TBs. If you are going to go thru the trouble of taking them all apart, it just doesn't make sense to put them back together with just some carb bushings (which don't fit a TB shaft anyway) and have the same size TB that just works a little better.

The 'vacuum leak' story that I see circulating is nonsense. There is no such thing as a shaft leak problem on a throttle body. The IAC dumps raw air into the TB at exactly the same spot so quite obviously it is not causing a problem. The problem is the shaft getting sloppy in the TB and preventing the TB from maintaining balance when the throttle is moved. The cure for that is not bushings, but double sealed stainless steel ball bearings.

The other thing is the TB size. Once the TB is apart, it may as well be bored out. so far as I know I am the only one who bores them without destroying the balance port. Without the port, you can't balance them. There is another story that you can balance bored TBs with a slip of paper, but that is just more nonsense. The balance is critical on multi carb (or TB) setups and feeling the drag on a slip of paper is nowhere near accurate enough.

Boring without destroying the balance port requires making new custom throttle plates. Even my big sleeved TBs have a new balance port micro machined into the sleeve.

Here is a pic of a fun project I did for a Forth Gen guy. I call it 'Mini MOATB'. The ultimate expression of the single IAC mod and proof you don't need to spend big bucks to get a trick looking setup that performs well.

simpson36

Here are some links for you Third Gen guys. At the top of the first doc is a lingk to the whole Crossfire Tech series. There are more links in the other papers to more related papers in the series.

The last one I am going to do is on porting the crossfire manifold, but I just have not had time..

This stuff is a sticky on a big Vette forum, but I don't want to ruffle feathers by linking to another forum here in case it's a no-no, so here is a few that will get you into the others.

http://www.thecubestudio.com/Collect...structions.htm

http://www.thecubestudio.com/Crossfi...nstallTips.htm

http://www.thecubestudio.com/Crossfi...tialTiming.htm

89_RS

I meant a single ECM running two 2bbl TBI units. So one ECM controlling 4 injectors.

And I wasn't halucinating either. I also see that you have taken a different approach to this as well.

Ronny

iTrader: (1)

I believe the EBL-ECU may be able to handle that. But port fueling may be a better thought.

Nice to hear from Simson again !

chazman

iTrader: (4)

Where are you located?

simpson36

Yes, it was my understanding that the EBL had an 'option' that you could get that would run 4 injectors. My recollection was that it was a question of avaialble amps to power 4 solenoids.

That said, I see no practical purpose to doing so since a pair of the 4 banger Crossfire Tbs can be made to flow way more than a pair of small block 2bbl TBs (which cannot be bored out much at all). A pair of my smallest TBs 'SuperMOD' are equivalent to one big block 2bbl TB, which people (including me) pay stupid amounts of money for. A pair of SuperMOD TBs on a stock Crossfire setup is equivanet to, but much cheaper than this setup that I built with an NOS big block TB . . . sweet! :



My big UltraMODS go beyond anything that can be done with small block two hole TBs. If you go with a pair of big block TBs and (4) 90lb injectors, then yep, you pass me on CFM, but only by a little and you better have a big wallet, that's all I can say . .

Incidentally, you ThirdGen guys would be amused to know that my Wife's Crossfire Vette cracked a cylinder so I just slapped a store-bought rebuilt 305 short in it. (original block is now repaired sitting on an engine stand). The 305 loves those SuperMODs. The crossfire manifold gets a lot of undeserved bad press. It is actually a decent street manifold that becomes a great street manifold with an aggressive port job, BUT . . TBs are THE restriction, so don't waste money on manifold or exhaust mods until you get the TBs bigger.

The 4 banger TBs are pretty simple gadgets with no 'vital arteries' that get in the way of boring the crap out of them.

The larger problem with using two-hole TBs is that they have no provision for balancing and these setups are stupidly sensitive to balance. The farther apart the TBs are, the more ciritcal it becomes. With crossfire, you can't get them any further apart unless you mounted the things on the fenders and ducted them over

I am located in North Texas.

Think that answers the Qs. Thanks for the compliments.

RFmaster

Thanks for all updates (and very good photos documenting your work). As a side note, in the past four month I have seen several crossfire vets in local graigslist listings with 'dead' (or poorly functioning) motors. I saw one couple weeks ago - poor kid (20 year old college kid with no money) had to park it because he could not pass bi-annual smog that we have here (SoCal). I was tempted, but with all other projects and family responsibilities I had to pass (for now)!

BTW, I do not have cross fire equipped car (or truck), but I love what you have done with 28+ year old hardware!!

Mary XMAS and happy new year.

//RF

KITT1983

iTrader: (3)

Can you bore out my tbis?

ploegi

Don't hold your breath. He hasn't been here in almost four years.

gheatly

iTrader: (2)

there is a place in Houston, TX that still does this.


http://www.musclecarsoftexas.com/cro...ction-1982-84/


I have not used them before, so I can't comment on their work. My tbs were done by DCS before they went under and were bored to 2.0 inches.

KITT1983

iTrader: (3)

thanks

OUANANICHE 1982

Can you tell me please # item number of direct reading pressure gauge ( and tuning ) from DCS ( or other trade ) that connect in the front between the two TBI units of a 1982 CFI ?
Have you photos exactly where and how I can install that gauge on a 1982 CFI like mine in France ?
Would it be avantageous for me to replace the EP 386 original fuel pump with a DELPHI FE0110-11B1 rather than an EP 241 ?
I thank you in advance for your technical advice.

Ronny

iTrader: (1)

I replaced my original CF fuel pump with a TPI pump.

I now have a Accel high pressure pump since now is a 396 SB port fuel with single plane. I have the TPI pump that I could sell to you. It is 5 years old when removed. Maybe 20-30 hours on it. ronfehn@gmail.com