Restricting IAC passages

[Chewy1576]

I would be looking for a gasket mis-match or other vacuum leak. Air is getting in somewhere, I doubt you will need to restrict the passages. Any leak in the intake, brake booster, or other hose will give you the problem you are having.

The IAC is the restriction. It's a needle valve (more or less). Restricting the passage will do nothing.

You cannot oversize a throttle body on a fuel injected engine, be it port or throttle body injected. When a Carb is too big, engines will not idle since carb's rely on velocity and pressure differentials to draw fuel into the air stream. When fuel is injected under pressure by an electronic injector the old rules do not apply.

This might be my first post on this forum, but I have been working on cars for 40 years.

I did check for vacuum leaks, and I was fairly sure there were none. I had thought about air velocity through the throttle bores, and that could be a contributing factor to not getting fuel through the big block throttle body reliably at idle, but at off idle it ran great. Another factor could have been that I was running the big block throttle body over an unmodified Edelbrock 3704 intake with 1 11/16” bores. The step created between the bore diameters was probably causing all kinds of problems. Maybe just narrowing the passage wouldn’t be effective, I would need to make the port where the IAC needle seats against smaller.

I can see port injection being different as the injector is in the runner and the AF mixture only travels a few inches into the chamber. With TBI, the fuel has to mix with the air, travel down through the throttle body and runners to get to the chamber. I would think TBI still has a lot of the same rules as a carburetor.


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[skylark]

You cannot oversize a throttle body on a fuel injected engine, be it port or throttle body injected. When a Carb is too big, engines will not idle since carb's rely on velocity and pressure differentials to draw fuel into the air stream. When fuel is injected under pressure by an electronic injector the old rules do not apply.

I I have to disagree with you here. If you have to adjust the minimum idle screw down to close the throttle plates then there is too much air getting past throttle plates. When you close the throttle plates the fuel will pool on top of the plates and it will drip into the intake un-atomized.

 

[Spookers]

Well I ran my numbers with the CFM calculator. I haven’t had this engine above 4500, so I really only need 560ish CFM max. I’ll leave the small block one on for now.

Where should I start the bidding for all of these parts I’m not going to be needing now? [emoji38]


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wouldn't you know ! sounds like the 454 tbi is perfect!
the 305/350 TBIs flow 500cfm@3"/Hg compare that to carbs that are rated at 1.5"/Hg its roughly 350cfm (perfect for a stock 305 but under sized for any 350)
now look at the 454 TBI rated at 670cfm@3"/Hg or roughly [email protected]"/Hg

I would just fill in the passages with jb since it is easy to get back out with a little patience if you want to,
if your tuning it anyway definitely use the 454 tbi, id run the what ever injectors you want just tune for them, personally I think the 454 tbi is at the edge if you went for any more upgrades id be going bigger (750 q-jets are cheap and plentiful) especially since you only calculated to 4500rpm where its likely at some point youll want to go farther to like 5000rpm and have it fall on its face due to small tbi

Before the internet says crap like it wont work try it as its free or next to free.
Just so you know, I currently run a 454 tbi with 74lb injectors on a stock 305 minus the headers and intake which is a stock manifold off a 87 350 1 ton as it still ran the q-jet on the new intake manifold bolt pattern and the reason I used it was then I didn't need to bore the stock manifold (which I did to a couple and found that 1 7/8 is the max before hiting coolant) plus the 454 tbi came with the tbi to q-jet adapter (which came factory on 454 1 tons) which in a way acted like a 1" spacer, works awesome, since then ive ported the stock heads and got a huge gain from that for free and about 8-10hrs work (aprox 2hrs a night for the work week)
I watched a series on youtube by "Headbytes Porting" titled "350 TBI Stage III.5 portin

 

[Spookers]

To get the iac to flow less you will want to tune it to open less, play around with "initial IAC flow vs temp." under "IAC parameters" in "scalers" on Tuner pro, also "IAC counts vs desired airflow". I started by cutting back the stock numbers to 70% then fine tuned from there.

 

[Chewy1576]

Well plans are changing...

I bought a new truck this weekend and have to sell the truck with the 383. I’m not going to be doing any more work with it except for getting it tuned for the smaller injectors. I was planning on trying these suggestions, but came across a deal I couldn’t pass up.


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