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TBI Porting

Discussion in 'The Garage' started by Cricket, Feb 19, 2006.

  1. Cricket

    Cricket 3/4 ton status

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    Anyone here cut the lip off and shape the top of their TBI bore inlet. If so were the gains worth the work?

    Seems to me you get as much change with an injector spacer, which doesn't seem to be that much.
     
  2. bigblzr

    bigblzr Registered Member

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    hell yeah, my buddy did one up for his 350 crate for an 88 4x4 truck and he also made one for my 87 blazer. dont be shy, grind the whole lip off and the buff it up nice with a flapper wheel and a die grinder. make sure you clean it very well after, the little pieces get everywhere
     
  3. Cricket

    Cricket 3/4 ton status

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    What kind of gains did you acheive with the porting? From what I've read the fuel pressure metering & balance is far more important.
     
  4. mstefak

    mstefak Registered Member

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    I am the freind that did it. It is not 'porting'. Porting would be boring out the throttle bores to a larger size thus increasing the intake charge amount. This process is simply called 'radiusing'.

    Radiusing is when you grind out the upper ridge of the TB and smooth it too a shine. Doing this makes LARGE increases in throttle response all through the RPM band as well as a better torque curve.

    This mod was definatly worth the 1 or so hours it took me and that is why I made BigBlzr one.

    You are correct fuel metering is important. That is why you should make or buy an adjustable fuel pressure regulator as well, to bump up the performance. I ran my 350 without the adjustable pressure regulator for 5 months then finally did the regulator swap and it made a MASSIVE difference mainly because it gave the engine the extra fuel it wanted.

    All in all I think the best idea is to do a few things at once.

    Radius the TB
    Make an adjustable Pressure Regulator
    Make an Injector pod spacer
    Install a TB Spacer

    Good luck, if you have any questions I'll be happy to help.

    If youd like I offer the 'service' of doing all this for you .. of course at a price. PM me or email me at Mike_Stefak@hotmail.com for more info and prices.
     
  5. BadDog

    BadDog SOL Staff Member Super Moderator Author

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    I've got the "radiused" TB that is also bored out with new plates and bushings, plus a pod spacer and adjustable pressure regulator run up to about 22 psi at the moment using a 93 454 TBI in-tank pump. I wasn't terribly impressed with the returns on any of it to be honest. And I actually lost some drivability since it seems the vacume pressure on the larger plates is too much for the stock return spring to close when running. Plates are centered and properly adjusted, and work fine when the engine is not running, but idle will not drop below about 1000 without an external carb style return spring. Now that I've gone to the trouble, I find that many people with bored TBs have the same problem. :rolleyes: Frankly, I've been thinking about putting the stock TBI back on.

    BTW, this engine has decked and mildly ported S/R Torquer heads and a crane roller (hot compu-cam) set 4* advanced on a ZZ3/4 short block.

    Anyway, by far and away the biggest return I got was doing a chip. I still haven't spent nearly the time needed to really wake this puppy up, but the chip is the end-all-be-all to modding a TBI. IMO not so knowledgeable (but having done a lot of research and some playing) opinion, anything more than minor tweaks and maybe bumping to 15-16 psi on an otherwise stock TBI is a waste of time...

    IMO, www.thirdgen.org is the ultimate place to learn about TBIs.
     
  6. mstefak

    mstefak Registered Member

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    Your problem seemsw to be strictly your plates ... you need a stronger spring. You have to remeber you has a TPS so the computer thinks you are moving becsaue your giving it gas it messes with the whole system. Yes theoretically the computer should correct it ... but the vaccum may be higher at times which can mess with teh plates some more. If you had it bored .. why not just use a 7.4L TBI ?

    PS .. is your idle set screw adjusted properly ? As well did you reset your base idle ?
     
  7. sled_dog

    sled_dog 1 ton status

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    have you ever played with the idle set screw? You can't unless you modify the throttle body. Also, the Computer will never "correct for TPS", TPS is an input it can't tell if the input is incorrect unless it is out of spec(say below .45Vs or above 1V). You have to adjust the TPS to make any change in it. As for the stronger spring, how does one do that? The TBI return springs are after all that wound coil on the end of the throttle shaft, never seen one for sale that you could place on there for a stronger spring. I believe what Russ is saying is true, strong vaccum could very well hold onto the blades and cause such a problem. Crappy but possible.
     
  8. BadDog

    BadDog SOL Staff Member Super Moderator Author

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    Yes, I've set the proper min-air and idle, it is strictly the larger surface area on the back side of the plates creating a strong vacume lever that holds the plates open. I know a stronger spring will work, that's what I'm effectively doing with the carb return spring. And there is nothing the computer can do to correct it since as far as the computer knows, I have my foot on the accelerator. The second you cut off the engine, the plates close normally. And yes, I've aligned the plates and there is no bore binding.

    I went with the bored TB because it provided all I needed without having to swap intakes (or bore) and find the somewhat rare 2" TB, deal with wiring changes, and so on.

    Frankly, I wish I had gone with a hotter cam with lower idle vacume. Doing my own chip I can easily correct for a hot cam, but it seems there is nothing I can do to stop this high-idle problem without having a 15 lb pedal pressure.
     
  9. BadDog

    BadDog SOL Staff Member Super Moderator Author

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    BTW, another "proper" fix would be to install offset bushings and custom plates to provide the right ratio of vacume induced leverage so that the pull-open pressure returns to factory specs and the return spring can again do it's job. Before I go to that much trouble, I'll install a hotter cam with more overlap... I always did like the rump-rump-rump of my old GTO 455 counting them off. :D
     
  10. Russ

    Russ 1/2 ton status

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    Did the extra carb spring fix your high idle problem? I have that exact same problem on an over-bored 454 throttle body on my 489. The plates close great with engine off, but hold open at about 1000-1200 rpms when I let off the gas with the engine running. I have been considering the extra spring, but did not know how much it would stiffen up the pedal. Thanks,

    Russ
     
  11. BadDog

    BadDog SOL Staff Member Super Moderator Author

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    I used the outer spring of a double spring q-jet return spring set. It makes the pedal quite noticeably more firm, but generally stoped the high idle most of the time. But, there were still times I had to "pop it" so it would snap shut to low idle...
     
  12. Cricket

    Cricket 3/4 ton status

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    I've spent several days over on 3rdgen, I understand those guys are looking for every last ounce of performance they can squeeze.

    My question was geared toward gains for a generally stock truck with wheeling and daily driving in mind. The radiusing obviously removes the stock obstructing lip but at what exact gain. From what I've read the most you'll see is crisper throttle response. I felt there wouldn't be any "significant" gains for my application but I wanted to double check with someone that had tried it on a 4x4 rig.

    I think I will just rebuild the throttle body I picked up for the swap, radius if I have time, and port match the stock TBI intake.
     
  13. BadDog

    BadDog SOL Staff Member Super Moderator Author

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    IMO, unshrouding the bores (that's what I've heard it called rather than radiusing) is mainly a mid-high rpm mod. From what I understand, in theory, it should actually hurt tip-in and low rpm just a tiny bit (air column inertia and all that). I did the work on mine because with my "built" 350, the engine fell flat on it's face at about 4k rpms.

    Actually, I should say I wouldn't have bought it for my engine. I got the "brand new" bored/modified TB for cheap enough it wasn't even worth bushing and refirbing my own. I did the rest of the work, but the bore/deshroud was not done by me.

    But I can't imagine it making any difference to a reasonably stock engine. In hind sight, I wouldn't even have done it for my engine. I should have just stuck with a stock TB using elevated injector pod and higher fuel pressure. I honestly think that would have been plenty to reach my goals, particularly with the new chip. And as I said before, because of the idle issue, I may well just rebush and clean up my old TB and sell this modified TB on down the line to someone who want top end. All I really want is for it to pull like a mule from 2000 through about 5000 (heck, even 4000 is fine as long as it doesn’t drop like a brick past 4k!), I couldn’t care less beyond that…
     
  14. Cricket

    Cricket 3/4 ton status

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    Hey Russ, I came across some interesting articles searching the Internet in regards to throttle body shape/ design. Here's a few portions from the Jenvey UK throttle body site, while not the same product it relates to my original question.

    "What is the best throttle body diameter?
    Factors influencing size are; Power output, RPM, cylinder head design, cylinder capacity, position of the throttle body in the inlet tract and position of the injector.

    Choice of bore size is a balanced compromise resulting from the following;
    1) A larger bore leads to lower flow resistance, but obeying the laws of diminishing returns.
    2) A smaller bore leads to better throttle control and response (never underestimate) and improved fuel mixing.
    3) The system should be considered in total - from (at least) trumpet flange to cylinder and proportioned accordingly.

    What is the best Air horn ( / Trumpet / Stack / Bellmouth )?
    The air horn serves three main purposes; 1) To convert the pressure difference between bore and entrance into air velocity with the minimum of energy loss. 2) To act as the interface between the induction system and the atmosphere, i.e. the point at which pressure waves change sign and direction. 3) To complete the system to the required overall length.
    For ease of description the air horn may be considered in two parts; the 'flare' and the 'tube';
    The main job of the flare is to spread the low pressure zone over the largest possible area - to reduce local pressure reduction - whilst guiding incoming air into the tube with minimum disruption or induced vortices. The flare should be shaped to encourage air to enter from the sides, but not from the rear, of the mouth. This is achieved by either finishing the mouth with a sharp edge when the arc is a little beyond 90 degrees from the air horn axis or by folding material back, parallel to the axis, when the arc is at, or just below, 90 degrees to the axis.
    The main job of the tube is to accelerate the airflow smoothly and progressively. This is best achieved by an exponential shape - i.e. one where the radius of curvature is increasing constantly until the angle of the sides matches the next part of the system, usually the throttle body. At the intake end this should blend smoothly with the flare."

    This second part is referencing an induction horn that would sit on top of the TBI but it seems to me by radiusing the TBI lip you are eliminating the so called "flare" effect and changing the low pressure area. So unless you added an aftermarket flare or extension over the radiused inlet you'd lose some response. Does that make sense?
     
  15. BadDog

    BadDog SOL Staff Member Super Moderator Author

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    That's what I was getting at by saying:
    Perhaps it's more than "a tiny bit"? That's getting beyond what I can guess at...
     
  16. Cricket

    Cricket 3/4 ton status

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    "That's what I was getting at by saying:
    Quote:
    From what I understand, in theory, it should actually hurt tip-in and low rpm just a tiny bit (air column inertia and all that).
    Perhaps it's more than "a tiny bit"? That's getting beyond what I can guess at..."

    I understand, that's what made me curious enough to search even further. Makes me wonder if adding material between the bore and raised offset lip then shaping to create a better funnel instead of a step would improve the flow characteristics more than a radius. By adding you'd retain the flare and make it better, versus subtracting it completely.

    Addy: after staring at this thing it would be far more trouble than it's worth to attempt that change with who-knows-what result. The injector pods would have to come way up and the whole design would change. Ah well, never hurts to contemplate.
     
  17. BadDog

    BadDog SOL Staff Member Super Moderator Author

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    Frankly, unless your going for increased max flow for high rpm WOT results, I would suggest leaving it alone. These things were originally designed with focus on low-mid rpm performance and general driveability, so I don't think your going to get much for low-mid out of removing the wall. Just my opinion, mabye an engineering type can offer more...
     
  18. Cricket

    Cricket 3/4 ton status

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    Here's a picture of that procedure. Left side stock - Right side radiused.

    I settled for polishing the stock inlet, and injector pod housing.
     
    Last edited: Apr 15, 2006
  19. jekquistk5

    jekquistk5 Weld nekid Premium Member GMOTM Winner

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    I'm going to have to do this.
     
  20. BadDog

    BadDog SOL Staff Member Super Moderator Author

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    Are you in a position to test this back to back with no other changes, not even fuel pressure? I would be interested to know if it makes a difference. Especially if you have access to a dyno (I wish I did) so that we can get real numbers. It's so very easy to "feel" a difference when there is none, I think it's wired into our brains to justify work and/or money invested. :D

    From my reading and modest understanding of physics, I'm betting it makes little or no difference to low rpm and tip-in performance. It might even hurt due to air column dynamics. I have to believe they put that there for a reason, and my best guess is that is improves low rpm drivability at the expense of high rpm (which is typical of many of the related systems) and why the 3rd gen guys do it to help their quarter mile times. I’m only mentioning this again since that is counter productive to what most of us want in a 4x4, while being bang on for the “hot rod” types, so their reports of improvement should be taken with a grain of salt…

    Oh well, I've got nothing to offer but opinions, in many cases based on reading opinions of others, so I'll move along now... :D Good luck and I hope it works well for you, but as I mentioned earlier, I'm getting ever closer to making the call to reinstall my stock TB. Hmm, one good thing about that, I'll at least have the chance to test it back to back while changing only one (well, roughly one) variable...
     

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