fussy IAC in LT1 stroker

More than 1 issue involved here, so please bear with me- I have seen several people mention drilling the IAC bleed hole oversize to allow greater flow for larger displacement LT1s. It seems to me like increasing the flow diameter would decrease the resolution of the idle servo, since there is more difference in flow between steps. I have seen in some other research, (and also seen used in carburetor applications) drilling small, fixed diameter bleed holes through the throttle blades to allow idle airflow with the throttle closed. It seems to me like this might be a good solution, allowing relatively fine adjustments with the IAC stepper without having it maxed out to accommodate the 383's airflow needs at idle. I bored the factory throttle body to 54mm (2.125") and used Holley throttle blades from a carb application. They came in a 4-pak, so if I drill too far I can swap them out for new blades. Is my thinking flawed on this plan?

Now for the other issue- In the course of installing a port-nitrous system under the manifold in the lifter valley, I was forced to machine away the underside of the manifold, thereby losing the small idle passages to the runners. I plugged the main passage that supplied airflow from the IAC to the small runner ports, and drilled a hole feeding the IAC air into the main plenum of the manifold below throttle body. I had noticed that one of the port feeds was a smaller diameter than the others, but I have no idea why. I realize that idle flow characteristics will be different, and that IAC stepper changes will have a slight delay vs feeding the idle air directly to the ports, but I am unsure if it will lead to other issues? Does anyone with more experience modifying the LT1 type manifold have any thoughts on this? I appreciate the input!

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Now for the other issue- In the course of installing a port-nitrous system under the manifold in the lifter valley, I was forced to machine away the underside of the manifold, thereby losing the small idle passages to the runners. I plugged the main passage that supplied airflow from the IAC to the small runner ports, and drilled a hole feeding the IAC air into the main plenum of the manifold below throttle body. I had noticed that one of the port feeds was a smaller diameter than the others, but I have no idea why. I realize that idle flow characteristics will be different, and that IAC stepper changes will have a slight delay vs feeding the idle air directly to the ports, but I am unsure if it will lead to other issues? Does anyone with more experience modifying the LT1 type manifold have any thoughts on this? I appreciate the input!

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yeah, i think that was a poor solution and it'll never idle properly again, i'd be buying a new manifold.....unless this is a track only car where idle isn't that important, in which case just jack the throttle plates open to attain good minimum air and stop worrying about it

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More than 1 issue involved here, so please bear with me- I have seen several people mention drilling the IAC bleed hole oversize to allow greater flow for larger displacement LT1s. It seems to me like increasing the flow diameter would decrease the resolution of the idle servo, since there is more difference in flow between steps. I have seen in some other research, (and also seen used in carburetor applications) drilling small, fixed diameter bleed holes through the throttle blades to allow idle airflow with the throttle closed. It seems to me like this might be a good solution, allowing relatively fine adjustments with the IAC stepper without having it maxed out to accommodate the 383's airflow needs at idle. I bored the factory throttle body to 54mm (2.125") and used Holley throttle blades from a carb application. They came in a 4-pak, so if I drill too far I can swap them out for new blades. Is my thinking flawed on this plan?

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what you're enlarging is the bypass hole on the throttle body that provides constant airflow to the idle passages despite the air going through the iac valve. all you want to do is allow the IAC to function properly by putting it 'in range' so it's never totally open or closed. with a non-butchered lt1 intake manifold you want nearly all of your idle air going through the idle passages rather than the throttle plates to allow even distribution.

I have several other LT1 manifolds laying around (I am fond of the system and my '55 chevy pickup has one in it as well), so I figured if it flat doesn't run I can just grab another manifold off the bench. But the whole point of the wagon is huge power with no visible signs (hence the name sleeperwagon) and there is no other place to hide 500hp worth of nitrous jets. I figured absolute worst case I can fab up more hardlines under the manifold and put the idle ports back in. I own a CNC machine shop, so there really isn't anything I can't make if I need to. For this project hiding everything is of the utmost importance. I have probably 150 hours invested in disguising or hiding every single component in the nitrous system. None of it is visible anywhere without disassembling the car.

Attachment 14905

As you can see, there isn't a lot of extra room underneath, but I think I can run 1/4" tubing between the nitrous system and the plenum floor if it becomes necessary.

Was it the #1 idle port that was smaller? I've heard that these ports were very inconsistently drilled by the original supplier. I opened up the #1 port on mine to the same as all the other ports.

Given that later design engines do not have a separate idle plenum, personally I don't think it's absolutely necessary. I suspect GM did it to minimize idle emissions.

On drilling the throttle plates, since you have replacements, that's a fine route to go in my opinion.

Is the engine already together and the idle is oscillating? Are you running a manual trans, or a smaller torque converter than stock? Have you adjusted the idle over/under correction values down to compensate for less rotating mass? Sure there's no vacuum leaks? Just covering all the bases here.

they did it because the lt1 manifold is a total shit show. try idling an lt1 on throttle plate air alone and you’ll see what i mean

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Originally Posted by sherlock9c1

Was it the #1 idle port that was smaller? I've heard that these ports were very inconsistently drilled by the original supplier. I opened up the #1 port on mine to the same as all the other ports.

Given that later design engines do not have a separate idle plenum, personally I don't think it's absolutely necessary. I suspect GM did it to minimize idle emissions.

On drilling the throttle plates, since you have replacements, that's a fine route to go in my opinion.

Is the engine already together and the idle is oscillating? Are you running a manual trans, or a smaller torque converter than stock? Have you adjusted the idle over/under correction values down to compensate for less rotating mass? Sure there's no vacuum leaks? Just covering all the bases here.

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It has been converted to a T56 manual, but the flywheel on the Centerforce dual-friction clutch is positively massive. I think it might weigh close to what the torque converter did. I modified the idle rpm targets to match an F-body PCM, but I haven't started screwing with the IAC values yet because I didn't know where to start.

Now that you mention it, it was the #1 that was smaller. Just to double-check I pulled the plugs under the spare manifold on the bench, and it also has a smaller #1 port so I'm guessing it was deliberate, though I have no clue what the reason would be.

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Originally Posted by steveo

they did it because the lt1 manifold is a total shit show. try idling an lt1 on throttle plate air alone and you’ll see what i mean

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I certainly won't argue that. But changing the manifold to a more modern design, (or adding a 24x ignition or anything like that) would pretty much let the sleeper cat outta the bag. I did port-match and blend the manifold to the heads and the throttle body, though I realize that doesn't fix the short-runner flow imbalance inherent to the design.

The later manifold designs without a separate idle passage are almost universally cross-ram setups. These have runner inlets positioned in a way that creates the longest possible airflow path between injector tips. I believe this is primarily for the purpose of preventing charge scavenging at idle and low loads.

The fatal flaw of the LT-1 mini-ram design is that it allows cylinders adjacent in firing order to rob fuel from their neighbors - with the open plenum design things are very "cozy" in there. Of all the adjacent firing cylinder groups, #1 is sufficiently far enough away not to rob (much) fuel from #8. Aside from this, there's a virtual orgy of fuel flying between ports at low flows. Take a look at your left and right O2 voltages while running a cylinder balance test - it should demonstrate this fairly clearly.

Any chance you took pictures of the idle passages as you were milling them away?

Neat idea with the covert spray setup. But I think you're going to see a very large BLM split below 20% TPS.

Edit: Another thing - if you've left the idle feed holes in the intake ports open (it seems like they are in the picture) you may want to plug them. Otherwise they're going to have free reign to ingest the fog of oil floating around the lifter gallery. To make your IAC motor work as "intended" you'll want to look at the TPI throttle body and duplicate the air passages it has to allow the metered air to get into the intake plenum, instead of feeding to the crankcase.

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Originally Posted by spfautsch

The later manifold designs without a separate idle passage are almost universally cross-ram setups. These have runner inlets positioned in a way that creates the longest possible airflow path between injector tips. I believe this is primarily for the purpose of preventing charge scavenging at idle and low loads.

The fatal flaw of the LT-1 mini-ram design is that it allows cylinders adjacent in firing order to rob fuel from their neighbors - with the open plenum design things are very "cozy" in there. Of all the adjacent firing cylinder groups, #1 is sufficiently far enough away not to rob (much) fuel from #8. Aside from this, there's a virtual orgy of fuel flying between ports at low flows. Take a look at your left and right O2 voltages while running a cylinder balance test - it should demonstrate this fairly clearly.

Any chance you took pictures of the idle passages as you were milling them away?

Neat idea with the covert spray setup. But I think you're going to see a very large BLM split below 20% TPS.

Edit: Another thing - if you've left the idle feed holes in the intake ports open (it seems like they are in the picture) you may want to plug them. Otherwise they're going to have free reign to ingest the fog of oil floating around the lifter gallery. To make your IAC motor work as "intended" you'll want to look at the TPI throttle body and duplicate the air passages it has to allow the metered air to get into the intake plenum, instead of feeding to the crankcase.

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The original ports were drilled for #10-32 threads, tapped, and had set screws epoxied in place then ground flush with the insides of the runners. The idle passage in the throttle body is unchanged, but I plugged the main idle feed runner that used to feed the idle plenum under the manifold and re-routed the idle passage to feed the main plenum below the throttle body.

I understand what you are saying about fuel getting sucked around in the manifold, but I don't quite understand how the idle feeds affect that. If the adjacent cylinder is pulling fuel from the runner next to it during the intake stroke, wouldn't it also steal air from the idle hole in the same runner? Also, with a big (ZZ409) cam, the "backwash" (for want of a more accurate term) will likely not match what the original ports were designed for anyway.

I did take pictures of the idle and EGR passages under the plenum as I milled them away, and worst case scenario I just machine a couple aluminum bars to run down either side where the old ports were and essentially put them back in without all the odd casting shapes to get around the EGR passages that no longer exist either. The only reason for the weird angles on the idle ports was so they could drill 2 adjacent ports from the same access hole, and use half as many plugs to seal it up after machining. If I make new feed runners I can drill the new ports perpendicular to the wall on the centerline of the primary intake runners. I don't care if it takes an extra 5 minutes of machine work since I'm not trying to make 5,000 manifolds a day like the OEM

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Originally Posted by Sleeperwagon

I understand what you are saying about fuel getting sucked around in the manifold, but I don't quite understand how the idle feeds affect that. If the adjacent cylinder is pulling fuel from the runner next to it during the intake stroke, wouldn't it also steal air from the idle hole in the same runner?

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It absolutely will. Part of the fuel scavenging issue with the LT-1 manifold is created by the separate idle plenum. The purpose of the feed holes is to put high velocity air in the head ports to help get the evaporated fuel charge into the cylinder.

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Originally Posted by Sleeperwagon

Also, with a big (ZZ409) cam, the "backwash" (for want of a more accurate term) will likely not match what the original ports were designed for anyway.

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All valid points. It's just that you've completely altered the idle airflow characteristics of the manifold, so your individual cylinder trims are going to need lots of work. You might also benefit from experimenting with end of injection target.

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Originally Posted by Sleeperwagon

I did take pictures of the idle and EGR passages under the plenum as I milled them away

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I asked that because I'm interested to see how the idle plenum is routed. Mind sharing?

I will dig back through my phone and try to compile the pics. The quality isn't awesome because I was just documenting the flow paths for later, not planning on doing a writeup of the actual project. Although I bet it would be very interesting for others if I did create a thread about the mods. The EGR path was especially unusual, zigging and zagging all over the bottom of the manifold, then dumping the exhaust gases out 2 ports on the floor of the main plenum just behind the throttle body.