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| Iron Duke Intake / Bore Size ... (Page 1/1) |
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82-T/A [At Work]
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MAR 10, 01:45 PM
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Would someone very much mind measuring the intake opening below the throttle body, of the 1984-1986 Iron Duke intake?
My daughter has an Iron Duke, so obviously I can get at one, but I'd rather not take the air cleaner and remove the throttle body unless I absolutely need to. Just hoping maybe someone has an 84-86 laying around, and they don't mind measuring with a caliper (in MM) the opening on the intake from where the throttle body connects.
Thank you!!!
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IMSA GT
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MAR 10, 04:21 PM
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Stingray92
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MAR 10, 05:27 PM
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82-T/A [At Work]
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MAR 10, 06:42 PM
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Thank you! That page is interesting... particularly because of what it says about the TBI units. I actually have a bunch of them that I've (for some reason) collected over the years, and when my daughter rebuilt hers, we did so with the best of the ones I had. I'm not really sure that page is entirely correct though. The Fiero TBI unit from 1984-1986 is identical... it's called the TBI 300 and is interchangeable between all three years. The one from 1987-1988 is the TBI 700 (both are from Rochester). But that brings me to the next point...
| quote | Originally posted by Stingray92:
Here ya go,
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... thank you! This really helps.
I wanted to know this because the opening on the TBI unit is exactly the same as the opening on the stock intake.
I recently bought a 1984 Corvette with Cross Fire Injection, and I also have some old parts left over from when I had a 1982 Pontiac TransAm with Cross Fire Injection. On a whim, when my daughter was rebuilding her throttle body, I discovered that the ones on the Cross Fire Injection engine are identical in every way to the ones on our Fiero. The one on the CFI are known as the TBI-400, while the one on the Fiero is known as the TBI-300. There's even a smaller version whith is called the 7-103... which is basically a shitty version of what we have (meant for the 1.8 and 2.0 liter).
But here's the cool thing. Everyone that has a Duke wants that Holley 3730 TBI unit... but they're basically impossible to find. The bore measures at 50mm, while the stock Fiero's bore measures at 43mm (just like the stock intake). Well, the factory TBI 400, which is every bit identical to the Fiero's TBI-300 has a bore of 46mm. All you really need to do is remove the dual-linkage bracket, and replace the throttle shaft with the one from the TBI-300, and everything else bolts up.
Of course, as I see now... the factory intake would need to be bored out 3mm for it to even make a difference... but there are options if someone wants a little bit more horsepower and they're feeling cheap.
Thank you!
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82-T/A [At Work]
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MAR 10, 06:47 PM
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She's going to post her next video (which is a "How To w/ Upgrades" for the Fiero throttle body) next Thursday, and I'm helping her do a little bit of follow-up on this.
Thanks again!
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82-T/A [At Work]
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MAY 15, 10:41 AM
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Just a quick update on this... there's a company that bores 400 TBIs for the 81-84 Corvette, and 82-83 F-bodies that have Cross-Fire Injection. For the most part, you can actually use a 400 TBI in a Fiero, but will have to re-route some of the vacuum lines for things like the ThermAC and such. But the guy offered to bore out my 300 TBI (the one the 84-86 Fieros use) to 45.5mm, which is the factory size of the 400 TBI. My goal is to increase the bore size from the stock ~43mm to ~45.5mm, while using a throttle plate from the 400 TBI, which is identical looking to what's in the 300 TBI. So I'll literally have a totally stock looking 300 TBI that's got a much larger bore, and reasonably provide more air to the engine.
He's charging me about $395, but it comes with a bunch of other things too... he bead-blasts the entire throttle body, cleans everything to like new, installs needle bearings for the throttle shaft, and restores everything on it to "like new." He'll also bore a set of factory gaskets for me to the same bore.
I'm also having a local machine shop bore the intake manifold to 46mm, and will re-use the little fuel foil, with the indent. I choose to have the intake bored to 46mm because unfortunately, the way it is now... (as you can see from the pictures above that Stingray92 posted) with the fuel foil in there, it actually blocks some of the intake... so you don't even get the full 43mm of the stock TBI bore.
In the end, I'll have a true 45.5mm throttle body, that looks 100% stock, and will most certainly provide a bit more horsepower. I realize it's not going to give me 100+ horsepower, probably not even 10hp, but if it gives the car a bit more power and drive ability, while looking totally stock, then it'll be a nice improvement.
He's concerned about 2 vacuum ports, the one that goes to the EGR, and the one that goes to the fuel evaporative canister purge valve. Both of these port into the bore (before the throttle plate... you may not even realize it), but he wants to make sure that boring the TBI out by 2.5mm won't destroy these ports. But I'm willing to take the risk. He says that if the porting goes well, he'll offer this service at a lower price to everyone else... which will support both Fiero owners, and everyone from like Citations to whatever.
I'll post when I get everything back and find out.
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82-T/A [At Work]
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JUL 20, 11:39 AM
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Ok... so I wasn't going to post this, and was going to let my daughter make a video of it... (which she will still do). But as you guys know... we suspect either a cracked head or a faulty head gasket. Carter Carbs sold me a basically perfect cyl head which I've sent off to the machine shop to clean up, deck, etc... and we'll install that new cyl head in place of this one. We keep getting compression into the coolant, which is causing seals and everything to blow out in the cooling system. Anyway... since we're going to have the head off... I wanted to see how we might be able to get a little bit more horsepower out of the engine, while at the same time keeping it looking stock and not disrupting how the car responds (it drives really, really nice with the 5-Speed gears we have, and the car is a *sh1t ton* of fun.
Ok, so as explained above... the stock 300 TBI's bore size is 43mm. It's also correctly matched to the intake manifold... though with the fuel atomizer installed, you can see above that the actual measurement comes in at even lower (less than 42mm). Thanks Stingray again for taking those pictures!!!
So here's what I did. I know from my cross-fire injection days, that XPS performance does throttle body boring. If you saw the TBI 300 rebuild video my daughter made, the Corvette TBI units are nearly identical to the Fieros. You CAN easily adapt the Corvette TBI unit to the Fiero; however, she wanted to keep the stock 300 TBI because she wanted all the vacuum routing to stay in the correct place. So we shipped off her 300 TBI to XPS performance, and for $440... they cleaned and bored the throttle body. What's really cool too is that... because they professionally bore TBI 400s for Corvettes, they have a plethora of left-over OEM Rochester throttle plates from the Corvette... which are IDENTICAL to the factory Fiero TBI units, but just larger. So... without further delay... here's the picture of what I got back from XPS Performance:
Keep in mind, there is a detailed bulleted list of things that they do. In addition to merely boring it out, he also:
- cleaned the TBI housing (glass-bead blasted),
- refinished the aluminum,
- cleaned and tapped all the threads (mine had a broken TPS bolt in there),
- cleaned and re-installed the throttle linkage, and actually installed sealed / greased needle bearings (the throttle action is SO much more smooth now too),
- and completely cleaned all the throttle and vacuum passages of all the gunk and anything else that was there.
Here are some additional pictures:
A couple of things worth mentioning. You have to supply your own gaskets... and keep in mind, he also bored the gaskets to 46mm, which is now the larger and correct size of the throttle body.
In addition to this, I purchased a spare intake manifold off eBay and I sent that to my local machine shop to have it bored out as well. I don't want to run into the issue of also having that undersized because of the fuel atomizer... so I asked them to bore it to 47 mm, and then I'll be able to expand out the fuel atomizer, and still get the full 46mm bore size from the TBI unit through the intake. There's also a thread (somewhere), which I'll find that talks about how to modify the fuel atomizer to get a bit more flow through there and still have it function properly. Removing it all together usually gives about ~2hp (dynoed) on an otherwise totally stock Fiero. But the fuel ends up pooling and doesn't distribute properly to all four cyls like it should, so you really do want to keep it in there, you just don't want it to really restrict airflow (as it does now).
We also purchased an older ceramic-coated Fiero Store shorty header off eBay for the Iron Duke, which looks almost factory... so we'll install that too. I don't know what kind of power we'll get from the performance exhaust, but I'm estimating she'll probably see around 8-10 horsepower from the modified intake manifold and throttle body... which brings it a bit more in line with the newer Iron Dukes and gets it up over 100 horsepower. I'm being conservative, but it's probably a bit more. I'd love to do a before / after dyno... but probably not worth my time to do a before. I'll definitely look for a dyno after everything is done... since we already know what a stock Fiero's dyno chart looks like anyway.
Another thing to mention... because I'm upgrading to the larger throttle body that came directly from a Corvette... you can use one of the OEM injectors that's meant for a Corvette. It'll fit the 300 TBI neck, and will match the appropriate air flow increase for performance. Keep in mind there are two injectors on the Corvette... one for the front TBI (which produces a little more fuel since some of it trickles back) and the rear one which reinforces it. It's almost a negligible difference, but you'll want to get the front injector I'd suspect. My hope is that this will be a smooth and reasonable matched upgrade, that won't require any tuning to really be effective.
Obviously, there's better options, like the Holley 3739 that's 50mm, and lots of aftermarket intakes (if you can find them). But if you want to get back a bit more performance from the smog police, and have the car look totally stock... this is a fantastic upgrade in my opinion.[This message has been edited by 82-T/A [At Work] (edited 07-20-2025).]
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Dennis LaGrua
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JUL 29, 04:32 PM
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If you are looking for significantly more power from a Duke engine, a turbo might be the most cost effective way to do it. In 1984 there were two companies HItech turbo and Pfaff turbo, that made turbo kits that added 5 psi of boost and increased the Duke engine power to about 130 horsepower. When 1985 came around the V6 came out and offered 145 horsepower so interest in those two kids faded away. If you decide to add boost more fuel will also be required. I believe those Duke turbo kits came with larger throttle bodies. ------------------
" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, P-log Manifold, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, Champion Radiator, S10 Brake Booster, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "
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82-T/A [At Work]
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JUL 30, 08:04 AM
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| quote | Originally posted by Dennis LaGrua:
If you are looking for significantly more power from a Duke engine, a turbo might be the most cost effective way to do it. In 1984 there were two companies HItech turbo and Pfaff turbo, that made turbo kits that added 5 psi of boost and increased the Duke engine power to about 130 horsepower. When 1985 came around the V6 came out and offered 145 horsepower so interest in those two kids faded away. If you decide to add boost more fuel will also be required. I believe those Duke turbo kits came with larger throttle bodies. |
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Thanks Dennis... my daughter's goal is to improve the efficiency of the engine, while trying to keep it mostly stock. I reminded her that upgrading to a serpentine belt is not really stock... but that's kind of been her push. So, she's not really looking for real significant power adders, just things that will improve the performance of an already stock motor, while basically looking mostly stock.
We did think about a turbo also... but figured it would change too much in the engine bay.
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