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| 89Vette | What are the holes in plenum floor (behind TB opening) for? | ||
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Senior Guru
 
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I had thought they were intake ports for IAC-fed air. And, that the air made it's way down to the small holes in the floor of the base (small holes located in each runner).
In a recent CF thread, it was stated that EGR gases are forced up/out thru those holes. And, that the "blocks" commonly removed for porting were put there to aid in mixing. I had gotten the impression the blocks were there to slow -- and help divert -- air into (down thru) those tubes for IAC idle air. What's the scoop on EGR, IAC, and CSI passage-ways in a TPI base? IOW, what's their routing? (I already sold my oem base, so I can't go recheck myself. And, FWIW, the question goes to heating/cooling the intake.... Does EGR air raise intake temps at all? Does the coolant [under the TB] do anything more than prevent TB-blade icing in severe cold?) |
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Posted on: 2011/2/5 4:56
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383 stroker -- Semi-Siamesed SLP long-runner intake, AFR 195's, 4-2-1 headers, and a SR cam. ZF6 short-throw shifter. Custom body/interior. |
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| bogus | Re: What are the holes in plenum floor (behind TB opening) for? | ||
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Grand Imperial Pooh-Bah
 
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there is coolant in the base.
the tb coolant is theoretically designed to prevent blade icing, but gee, do you know how cold that would have to be? And after 10-15 min run time, it would have migrated enough heat to melt anything, anyway. EGR heat? I am sure there is some, but I doubt it's that much by the time the gas gets to the intake. I have never put a temp probe on it. Also, the egr inlet has a heat proof gasket that I suspect stops some heat, too. You can figure the intake manifold is going to be about 200F at full op temp. As for the rest of the ports, I would have to take a look. |
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Posted on: 2011/2/5 5:37
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| vetteoz | Re: What are the holes in plenum floor (behind TB opening) for? | ||
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Senior Guru
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Quote:
If that was the case why do early model TPI's have the IAC passage from TB directly to a passage under plenum that takes IAC air down the tube on drv side runner to the CSI location. Little opportunity for idle air to follow path you suggest [IMG]http://i171.photobucket.com/albums/u309/vetteoz/Engine/85-88plenum.jpg[/IMG] Quote:
Only have to look at intake to see the obvious. Holes in floor behind the TB connect to a passage cast on pass side under plenum , connects to the tube down pass side / connects to EGR valve fed from EGR connection at rear from riser on exh manifold All TPI have this EGR setup.( F-Bodies source EGR gas from crossover in head ) By your analysis where do the EGR gases enter the intake? [IMG]http://i171.photobucket.com/albums/u309/vetteoz/Engine/Plenumunder.jpg[/IMG] [IMG]http://i171.photobucket.com/albums/u309/vetteoz/Engine/intakeon3.jpg[/IMG] The late model intakes without CSI don't have the 2nd pathway from the TB to drv side .Compare the TB mounting face of the plenums, early vs late ; you will see there is no hole in plenum behind TB on late style [IMG]http://i171.photobucket.com/albums/u309/vetteoz/Engine/plenum.jpg[/IMG] |
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Posted on: 2011/2/5 6:45
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| 89Vette | Re: What are the holes in plenum floor (behind TB opening) for? | ||
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Senior Guru
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I see your point about the IAC passage for the CSI plenums. I wonder why they dumped that passage though? IAC didn't go away.
O.K., So what comes out of the small holes in the base runner tubes? (Qualify if different on CSI vs non-CSI units). Blow into the EGR inlet tube hole (not valve). Where do you feel the air come out? Doesn't it seem like those holes would be perfect for idle air and/or EGR air? |
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Posted on: 2011/2/5 16:09
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383 stroker -- Semi-Siamesed SLP long-runner intake, AFR 195's, 4-2-1 headers, and a SR cam. ZF6 short-throw shifter. Custom body/interior. |
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| flyboy | Re: What are the holes in plenum floor (behind TB opening) for? | ||
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Elite Guru
 
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[quote]
89Vette wrote: O.K., So what comes out of the small holes in the base runner tubes? (Qualify if different on CSI vs non-CSI units). Part of the idle air circuit. I'm glad I didn't plug them when I was porting mine. I didn't know what they were. Small hole in lower rt. of pic. Here's a link to an interesting discussion. http://www.ls1tech.com/forums/lt1-lt4 ... ifferent-than-oem-tb.html I did plug the egr holes in the plenum and ground that dam away. I don't have egr. Attach file:  P1010009.JPG (144.29 KB) |
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Posted on: 2011/2/5 16:52
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| 89Vette | Re: What are the holes in plenum floor (behind TB opening) for? | ||
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Senior Guru
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Quote:
That's what I'm talking about. If the IAC air goes thru the plenum and out those small runner holes, how does it get into/thru the TB on 89-91 plenums -- where that center triangle hole was deleted? [IMG]http://i171.photobucket.com/albums/u309/vetteoz/Engine/plenum.jpg[/IMG] |
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Posted on: 2011/2/5 22:43
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383 stroker -- Semi-Siamesed SLP long-runner intake, AFR 195's, 4-2-1 headers, and a SR cam. ZF6 short-throw shifter. Custom body/interior. |
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| vetteoz | Re: What are the holes in plenum floor (behind TB opening) for? | ||
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Senior Guru
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Quote:
If the IAC air goes thru the plenum and out those small runner holes, how does it get into/thru the TB on 89-91 plenums There are openings either side of the bores on the back of the TB that put the IAC air into the plenum Quote:
As I have argued for many years; on the early setups IAC air is directed to the CSI then to the holes in base to distribute CSI fuel. IIRC the later bases are identical still have the holes drilled and passage but the supply is blocked by the plate on the drv side runner that replaces the CSI |
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Posted on: 2011/2/5 23:28
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| flyboy | Re: What are the holes in plenum floor (behind TB opening) for? | ||
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Elite Guru
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So on the later cars that don't have the CSI the IAC air follows the same path as all the other air? The little holes in the intake base runners are not used.
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Posted on: 2011/2/6 2:50
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| 89Vette | Re: What are the holes in plenum floor (behind TB opening) for? | ||
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Senior Guru
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Quote:
Why do you think GM decided to stop using those holes? Without the CSI, did they discover a better path thru the more open plenum-to-runner-inlet? And, why change the plenum casting w/o changing the base casting? Seems like there's more to the story. |
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Posted on: 2011/2/6 3:32
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383 stroker -- Semi-Siamesed SLP long-runner intake, AFR 195's, 4-2-1 headers, and a SR cam. ZF6 short-throw shifter. Custom body/interior. |
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| vetteoz | Re: What are the holes in plenum floor (behind TB opening) for? | ||
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Senior Guru
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Quote:
89Vette wrote: As I said ; I believe the IAC air to base holes was for CSI distribution to the individual ports, the early cars just used that for IAC air when the CSI was not operating because it was there Quote: 89Vette wrote:And, why change the plenum casting w/o changing the base casting? Seems like there's more to the story. $$$ GM already had a working TPI base so why spend money to change it, the redundant holes were easily sealed by the later style drv side runner |
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Posted on: 2011/2/6 4:42
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| flyboy | Re: What are the holes in plenum floor (behind TB opening) for? | ||
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Elite Guru
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Those redundant holes had me baffled. I thought they were maybe part of the casting process.
I wonder if one IAC system was better than the other? Mine makes a whistling noise?, which I think is normal. Did the earlier ones do that? |
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Posted on: 2011/2/6 5:07
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'91coupe, LT4Hotcam, some other stuff. If it's too loud, you're too old. "He works on old cars, then junks 'em" |
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| CentralCoaster | Re: What are the holes in plenum floor (behind TB opening) for? | ||
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Senior Guru
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What about PCV? Where does that enter the vacuum side of the intake?
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Posted on: 2011/2/6 5:42
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| 89Vette | Re: What are the holes in plenum floor (behind TB opening) for? | ||
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Senior Guru
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Be nice if someone with their base off (or a spare one), would shoot/blow some air thru the various holes to "map" out the flow.
I'm, of course, very curious. |
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Posted on: 2011/2/10 5:17
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383 stroker -- Semi-Siamesed SLP long-runner intake, AFR 195's, 4-2-1 headers, and a SR cam. ZF6 short-throw shifter. Custom body/interior. |
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| vetteoz | Re: What are the holes in plenum floor (behind TB opening) for? | ||
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Senior Guru
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Quote:
Ask and ye shall receive http://forums.corvetteforum.com/c4-te ... cooling-egr-port-too.html Make your own mind up on the OP's conclusion |
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Posted on: 2011/2/10 10:39
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| flyboy | Re: What are the holes in plenum floor (behind TB opening) for? | ||
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Elite Guru
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Quote:
Here is a link to a discussion on TGO. Be sure to click on the link "here" in post #1 to an earlier thread, and there check posts #13 and #14 especially. http://www.thirdgen.org/techboard/tpi ... lease-explain-tpi-tb.html |
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Posted on: 2011/2/10 14:18
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| 89Vette | Re: What are the holes in plenum floor (behind TB opening) for? | ||
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Senior Guru
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Well, I don't agree with C4Betts. I don't believe the TB needs cooling because air would accomplish that. I also (now) believe the EGR routes air to the ports in the plenum floor (behind the TB openings). (FWIW, Last year, I asked about IAC operation and didn't get any knowledgable replies.
The TGO posts I found (last year) were obviously wrong too. One "proved" you needed air routed all the way to each runner to get a smooth idle. IIRC, This person built a copper-tubing maze thru the plenum to get it!) My problem is I remember finding the tiny runner holes by blowing into the base somewhere. I can't remember how and this thread (or others) don't solve my personal mystery -- especially for an '89 setup. |
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Posted on: 2011/2/10 18:58
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383 stroker -- Semi-Siamesed SLP long-runner intake, AFR 195's, 4-2-1 headers, and a SR cam. ZF6 short-throw shifter. Custom body/interior. |
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| flyboy | Re: What are the holes in plenum floor (behind TB opening) for? | ||
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Elite Guru
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Quote:
The passage to the right of the gusset in photo P1010015 connects to the horizontal passage down the center of the base and the small holes in the runners (I should have highlighted it with a marker). When I blew through the tubing in picture 22, air from the small hole in the runner moved the paper. This is a stock '91 setup. Attach file: P1010013.JPG (136.02 KB) P1010014.JPG (141.32 KB) P1010015.JPG (142.37 KB) P1010018.JPG (117.61 KB) P1010022.JPG (124.32 KB) P1010023.JPG (116.35 KB) |
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Posted on: 2011/2/17 0:33
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| vetteoz | Re: What are the holes in plenum floor (behind TB opening) for? | ||
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Senior Guru
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Quote:
Exactly as I said above BUT only is functional on CSI equipped cars because as your pic # 13 shows , the passage is blocked by the plate on the runner flange where the CSI would be on a early car It is obvious GM only made/machined one Vette TPI base ,85 - 91 and when the CSI was deleted they did not bother to make changes to that system in the base ; instead just blanked the supply port off |
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Posted on: 2011/2/17 2:10
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