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Boring out a TBI TB

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Old 09-25-2000, 07:17 PM   #1
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Boring out a TBI TB

Has anyone tried to bore out a TBI to 2"? I did it on my TPI TB myself. I removed the shaft and had a machine shop bore it out to 52mm and make new butterflies for it. Why can't we do the same on a TBI and save the $300 Holley charges for a new one? Besides the injectors and the adjustable FP regulator, isn't the only difference between a stock and a Holley unit the bore sizes? Sounds like a good experiment.

Alex
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Old 09-25-2000, 07:36 PM   #2
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Yeah, but you cant bore it out to 2" you can bore it out to 48mm though which is the size of a stock TPI throttle body which you should have the throttle blades for. FYI, that should flow around 530 cfm give or take. Ive seen it done, it performs real well.
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Old 09-26-2000, 09:45 AM   #3
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Here's just some math I was messing around with. Should give you an idea of what your going to gain in flow.

inch mm info
1.6875 42.8625 stock
1.88976 48 tpi flaps

stock unit 400cfm

20% gain from ultimate bit mods on stock unit comes out to 480cfm.

12% gain from 48mm flaps size + 20% gain from ultimate tbi mods comes out to 528cfm.

Note: those are only rough estimates and not flow bench tests.
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Old 10-03-2000, 03:32 PM   #4
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This seems like it might be a little on the low side. Volumetric flow rate is equal to area x velocity, and assuming air velocity stays approx. constant, the flow rate advantage of the 48mm bores should be about 20% greater than stock. Then with injector spacing and inlet radiusing, I would guess closer to 575. If it's only 530, how could holley achieve 140 more cfm with only .11" more diameter? Also, are yall coming up with these 48mm plates from the 'yard. or what? I've been wanting to do this, and am having a hard time finding the tpi plates.
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Old 10-03-2000, 04:34 PM   #5
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i have a spare tpi setup so i have some plates but i havent actually had my TB bored cause I just ofund a 2" pretty cheap
What i was thinking for you guys who cant find the plates is taht you could contact one of the companies like SPD that bores stock throttle bodies and just ask them for the plates they take off the core units
If that doesnt work you can always get the plates from barry grant
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Old 10-03-2000, 04:37 PM   #6
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I also priced it at my local machine shop and they wanted 17 dollars per bore so you figure if you dissasemble it and get the plates for 10 dollars you could have a much bigger throttle body for under 50 bux
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Old 10-03-2000, 06:45 PM   #7
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This sounds like a better alternative than hunting down a 454 unit and the hassles of the electrical connections being different.

I'd be very interested in someones results.
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Old 10-03-2000, 10:59 PM   #8
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Glad this subject came up. I haven't been able to find a 454 TBI that I trust and I don't understand the wiring problems. Am also concerned with
problems with the Holley 670 injectors, wiring and throttle linkage hassels.
Would boring out the stock TBI to 48 mm and increasing the injector size to 65# be a good set up for a mild 350 (GM caprice crate engine), or do I have to bite the bullit
and go for the 454 or the holly to make the 350 worthwhile?
Dumb questions I know, but you guys have helped me out before......bob

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Old 10-04-2000, 04:08 PM   #9
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I think this is a valid option. Basically what you are doing is creating the turbocity.com tbi yourself. It just depends if an extra 11 thousandths of an inch is worth the extra $ you're going to spend on a 2". For a reasonable build up, say up to 300 hp, I think the difference shouldn't be that signifigant, and if noticable it would only be in the very top end. One question I have, for Pablo or whoever can answer it, is in my buildup I'm going with vortec heads and a fairly mild, hi torque cam (210/218 447/462), and assuming I bore the tb to 48mm, would it be best to go with 65 pph injectors at higher pres, or 75 pph at lower pres? I think I will be looking at about 300 hp. Thanks.
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Old 10-04-2000, 05:44 PM   #10
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454 tbi is overated, i have one. But. the wiring was not a hassle at all. AT ALL.
I was wondering, could you possibly bore it to an oval> and somehow re mount the injectors. hmmmmm.
But, 50 bucks is better than spending for holley. You are buying a name.
o yeah, and 454. wholesale cost is 460, and retail is 635. Thats just the tbi/wo injector pod. WOW.

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