Help with building your Speeduino, installing it, getting it to run etc.
By Tadream
Wondering something about anyone using TBI. I got my injector resistors today (7.5 Ohm) and wondering how hot they get when running. I'd like to mount them inside my case, but don't want a bunch of heat near the speedy. What is the experience with this?
By dazq
Tadream wrote: Thu May 25, 2023 5:07 am Wondering something about anyone using TBI. I got my injector resistors today (7.5 Ohm) and wondering how hot they get when running. I'd like to mount them inside my case, but don't want a bunch of heat near the speedy. What is the experience with this?
If you know the TBI injector resistance then you can calculate the heat dissipated . This will be a maximum as the injector(s) are not on constantly but it will be a good guide to the heat output.
User avatar
Use the Speeduino Injector Resistor Calculator to find the maximum watts of heat generated per-injector. It's a lot of heat. Inside the case is not suggested, and typically on a heat sink plate on the firewall or other suitable location is preferred. I have used them on the outside of the case, riveted over an insulator to limit heat into the case.

7.5 ohms is a lot of resistance for common TBI injector types. :? Most of them like at least 2 to 4A, and most TBI resistors are 2.5 to 5-ohm. What is your injector resistance and type?
By Tadream
I can't recall now, but before I ordered them I used the resistor calculator on the site and these are what it suggested. I do remember the current, though, and it was a 4 amp peak and then dropped to 1.5 amps on hold. I figured assuming a 50 % duty cycle the current would probably average somewhere in between. I wish I could recall the actual resistance now and/or where I saw those specs. But 4 amps would suggest about 3 ohms impedance opening and 1.5 would work out to 8 ohms once saturated on hold. If I assume an average current of 2.75 amps/ 4.6 ohms and these would drop that to just over 1 amp and dissipating around 5 watts if my math is right, so that wouldn't be that much heat, but yeah, it does feel like a bit too much resistance and maybe not enough current to operate them properly. Hmmm.... My plans were to change very soon to a port injection setup, but I don't have the pump, injectors, fuel rail or throttle body for that at the moment, but now I'm thinking of trying to scrape up the funds to go ahead and do that and forget the TBI entirely. It would also eliminate the need for the resistors. Oh, and I currently only have a Lambda sensor and really need the wideband. They're just awfully proud of all those parts, and this is intentionally going to be a REALLY one-off setup eventually. I was just hoping to get it running on the cheap for now. At least enough to move it around while doing body work. It hasn't been on a road in about 5 years and likely won't be for another 18 to 24 months, so there's that. I guess I'm gonna go back and try to find those specs again as I'm now in doubt about this. Thanks, everyone. Off to research some more.
User avatar
Don't get too bunched-up about it. The idea is to find a balance of enough amps to fire them at low cranking voltage, yet not burn them up at higher revs. I run the calculator for 14V at 85% DC as the most current the injectors will ever see, and all other operation will be less. Hopefully they get enough amps when cranking to open properly. I don't know your type, but I accidentally ran some GM TBI injectors while testing firmwares, for many miles with no resistors. No failures. :D Not saying you should, but I don't get stressed about it like I did. :lol:

I would try what you have, and do a quick bench-test for injector operation (clicking) in Hardware Test mode. Use a lower voltage if you can to duplicate cranking voltage; but in any case if you're cranking and not getting enough fuel you have a first point to look. Check that your injector latency (dead-time) is set well. Squirt some fuel in to get it running and raise voltage, and tune it from there for easier starts or until you change resistors.

TBI works far better than most assume, is easier (or more forgiving) for dialing-in, and is a very good first-step to get stuff sorted. I wouldn't throw it out yet. OT - I'm driving a couple hours away to pick-up a full V6/V8 TBI unit, fuel system and harness in a couple days for yet another TBI project. I like them as they are simpler and allow exploring tuning and the opportunity to tweak them to near MPFI levels with clearer cause/effect feedback. Anyway, choose your path and have fun! 8-)
By Tadream
Thanks. I got frustrated with my efforts to use the original 7747 case and just ordered the Hammond case that WTMTronics sells and ordered the lower value resistors at the same time. My work schedule is such that when I actually GET any time for projects I want to have the issues pre-sorted, so to speak. Waiting even 2 or 3 days to order something (Since there's no longer ANY SUCH THING as a real electronics store in my area) might mean waiting 2 weeks to get to try said parts, so... And yeah, I love the simplicity of TBI. Literally the only real issue I have is the lack of adequate airflow through the throttle body on GM's. It should have the good points of a carb, like intake charge cooling that makes carbs always make more power than injection, but I like the reliability, cold starting, etc of EFI. All the aftermarket TBI ones are too pricey and to spend that much, one might as well go ahead with a port system. Many times I've thought of getting a 2x4bbl intake and putting two of them on. If I could find one of the old Renegade intakes , or a heavily-ported factory intake I'd seriously consider doing a crossfire setup with the 2 hole bodies in place of the original singles, LOL. (I'm building a 3rd-gen F-body, so period-correct appearance is nice) Oddly, the site lists the resistors as 4.7 or 6.8 ohm,but what I got and what is pictured is a 7.5. I rather expect the 4.7 will likely be a 5, or 5.6 ohm.
By punisher454
So its not really that hard to build a custom TBI injector when you need one. The 4 injector carb spacer in the above link is a great example, with an inexpensive 4 barrel throttlebody on top. I like it. You can very easily run 8 skinny body injectors on a 4150 carb spacer, on a dominator you can squeeze in 12 with six on each side turning by rotating every other one 180 degrees so the connectors clear each other.
I did something a little unusual a few years ago on my brothers 67 Firebird with a blown 454. I used one Holley 670 TBI throttlebody from an old analog Pro-Jection and one from a TBI454 (both had 2" butterflies). Had a spacer/adapter underneath with 12 42lb injectors from some wrecking yard supercharged 3800's. The little Weiand 177 blower only has a 4150 carb inlet and we were just using what stuff We had laying around. The adapter plate was machined by me at work after hours. The ECUr was a GM '730 with an aftermarket addon control board (P4-EBL). A Speeduino would have been a lot better, but this was quite a while back.
Ran awesome though. Just dont be afraid to try something weird, it just might work out.
The pictures show some of the work in progress.
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By Tadream
Funny you should say "try something weird" since my plans for going to port injection soon involve a computer-controlled Quadrajet that I've gutted, milled away the boosters, etc. and plan to use as the throttle body. It, of course, has a TPS built in, so there's that. I've got a NOS Weiand high-rise single plane intake with the spread-bore Q-jet bolt pattern too, (rare) and that carb has two 57mm plates and two 34mm plates, so should flow plenty of air. So' other than the fuel rails that will be mostly hidden by heater and A/C hoses, etc, so it should look a lot like a factory carbed car. and work like a typical short-runner port system. Weird enough? LOL.
By punisher454
Yep, kinda weird, but really practical, I like it.
I like the idea of the Q-jet as a throttlebody. It will have a nice progressive throttle with good control of the off-idle transition with those small primaries.
It’s also real easy to fabricate a remote IAC using a 3/8 or 1/2” hose or hardline connecting it to the manifold.
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