3.4l Modified Throttle Body...Problem with IM?

[60dgrzbelow0]

Gentlemen... Thanks to you all... I have some great news..

I followed your collective advice in the original post and took the UIM and New TB down to my machinist, who promptly looked at me something like this: and said, "Bobby... I'd have to charge you more than it is probably worth...Why not try your hand at this yourself...if you FUBAR this UIM...not to worry...you have ANOTHER one on the old engine you can use as a backup. Go home...get out your air tools and some grinding fittings and get to work...Just don't go crazy and mess up the face of the TB!"

And so... I did just that. First...I took a flat, opaque plastic lid and scribed out a 27.5 mm radius in the middle of it and then cut out the 55 mm circle. Then I laid that over the UIM TB opening and aligned it damned near perfect. Then I just dragged a permanent Sharpie pen around the outside of the circle and got a symmetrical guide this way to follow. Using all manner of grinding and polishing bits...I spent all morning into the afternoon working my way from the front to the back, gradually opening up the inside area of the hole to a depth of about three or four inches inward and refined this to the point of grinding down all the casting bumps and junk until it was very much wider, inside and out.

Then I took the edged circle out to 55.01 mm ...just to make sure that the inner 55 mm opening of the New TB would not butt up against anything when aligned over the gasket. I could NOT believe the pile of waste metal aluminum there was when this job was done! But having done this carefully and smoothing everything down on the interior, I think my first try at "hogging" out all the raw material and cleaning it up just might make for an easier "Breath of Fresh Air". Thanks one and all... and I think this thread has come to a very happy conclusion!

[SappySE107]

For future reference, it is not worthwhile to polish anything on the intake. A rough surface flows better than a polished surface on my flowbench.

[ForcedFirebird]

Simplest way to know if your TB is a restriction is to measure the vacuum in front and behind the the throttle plate. If there is ~2"hg or more of a difference then the TB is a restriction, somewhere around 1.5"hg is nearly perfect.

[60dgrzbelow0]

Ben...

Thanks... In future, I'll remember (turbulance is GOOD) and avoid all that unnecessary labor.

John...

(I thought you would be proud of me losing my 'virginity' in trying this out). I'll use the technique of measuring a differential vacuum to test and see if my handiwork...worked. Does the test have to be done at any particular RPM, or will idle suffice? Are the dual gauges expensive?

[ForcedFirebird]

Nothing wrong with opening up the TB, you know the iron heads can use all the help they can, but think about your question for a second. How could you test if the TB bore was too restrictive if you didn't have the TB in wide-open position? RPM doesn't matter, as long as it sees WOT.

No, you can buy cheap vacuum guages at a place like Harbor Freight.

I was just trying to say "bigger isn't always better". I doubt even a modified aluminum head engine would see a benefit from a TB let's say 75mm, but seems to really work well with a 65mm (~440cfm capability).

[60dgrzbelow0]

Understood. (Bangs hand into forehead for not seeing the obvious...) The only reason I attempted to work on the UIM at all was that the 3mm smaller opening in it seemed like it might negate any benefit of putting on the newly modified 55mm TB. I think it's a fairly modest Mod compared to the ones I've seen with the Lt-1 Engine TB boxes welded on, etc.

I'm not likely to do anything more dramatic, since the iron heads are "Stock as a Clock" as per the valve sizes (with the exception of adding in the stronger CompCam performance Valve Springs, Keepers and Valve Stem Locks) As you know from following the build all this time, the only other performance parts inside this DD will be the 0.75mm over bore and appropriate piston set and rings, the Cloyes True Double Roller Timing Set, a CompCam 260H Camshaft, the CompCam Roller-Tipped Rockers and a set of Pacesetter Ceramic Coated Headers encased in rolls of 2" fiberglass heat wrap with S/S ties. I don't expect to see anything more than 185HP even with all of this stuff... But as long as it runs and doesn't lope too much to a stall once it is broken in, this will more than suit our needs.

[IsaacHayes]

I never measured the vacuum from my 52 to 65mm TB, and I think you'd need a pretty precise gauge to measure it.

But I did notice that the sound of the intake was no longer super loud when using an open cone filter at WOT. So there probably is less restriction there since it's not as loud.

[ForcedFirebird]

I never measured the vacuum from my 52 to 65mm TB, and I think you'd need a pretty precise gauge to measure it.

But I did notice that the sound of the intake was no longer super loud when using an open cone filter at WOT. So there probably is less restriction there since it's not as loud.

As long as you aren't getting a terrible stumble when you stab the throttle, you are within respectable limits And uhhhh, you don't exactly have a "stock" setup

[60dgrzbelow0]

IsaacHayes...

Now THAT is THE perfect mental image you just conjured up to mind. No more of those "sucking a Golf Ball through a Garden Hose" or labored breathing noises from the intake side of the engine. I have to laugh just a bit though ...because John (Forced_Firebird) must have been reading this thread from the jump and was probably scratchin' his noggin about the serious difference between this little problem of mine...and all that "Heavy Breathing" his Aluminum Solutions are able to pull off while making so much more serious Horse Power.

[ForcedFirebird]

I read almost every thread on this forum

There was no head scratching. Sometimes it's just better to sit back and observe

[IsaacHayes]

When I stab the throttle it moves nicely, no stumble.

[bszopi]

I read almost every thread on this forum

There was no head scratching. Sometimes it's just better to sit back and observe

Ditto. Except I read every thread. And I often sit back and observe more than I post...

[60dgrzbelow0]

My problem is ...I can't stand the suspense of a baking cake...let alone wonder if the information I've got that might help somebody will pop up from someone else. Still... In reflection... both of your comments seem to be very good subliminal advice.... I'll try to sit quietly and wait, guys...I promise!

[BillS]

Ben is of course correct on the finish question.

I used to see the old style old theory heads done for things like Cooper S engines and they were works of art with a mirror finish you could see yourself in. Then someone decided to actually experiment a bit on the flow bench and found that the very smooth walls promoted a thin boundary layer of static air, so you in effect had a slightly smaller diameter slug of actual flow than the size of the port would have you expect.

By leaving some roughness (how much and what kind can endlessly entertain geeks with flow benches), you break up that static boundary layer and get it flowing again.

On the intake side, my 62 mm TB required some fancy machining to get that throat on the plenum out far enough. See picture.

I have never bothered much on trying to fit larger bore TBs to the 2.8 engines as flow tests I have seen indicate that on the Fiero, at least, the TB isn't the bottleneck - the throat area just inside is (and it is hard to do much about it, hence the replacement Treuleo style manifolds). The 3.4 seems to benefit ast top end, however. Does anyone have any flow bench results on the 3.4 with stock and bored TBs?

[60dgrzbelow0]

Smokin' Joe Frazier! I just had a thought! I think I now understand why Ben and BillS are exactly right on the preference of having a bumpy surface over a smooth one...and the proof comes in a form we can all recognize:

A Golf Ball...

Because of “Bernoulli's Principle”...if a Golf Ball had a perfectly smooth surface, the trailing portion of the ball would invariably suffer a partial vacuum during its flight...and this back pressure tends to make the ball slow down while passing through the mass of air. So to combat this problem, the ball has all those disruptive hollow “bimps” that break up this venturi-like phenomena, allowing the ball to glide more smoothly through the air mass and thus, farther along. The inlet metal surface of a Throttle Body is like the surface of the Golf Ball...with the ingesting air acting like the air mass the ball must fly through...so the principle just HAS to apply, whether it involves a golf ball... or a Throttle Body...right?

By the way...if you'd like to see an easy demonstration to make of “Bernoulli's Principle”, just take a 1” by 11” piece of printer paper, hold one end of it draped over your index finger and thumb and while holding it close to your mouth, blow air over the upper surface of the thin paper strip...and watch what happens... (Works GREAT in standard carburetors!)