Building a Twin 4-BBL Throttle Body EFI Engine

Posted by Chris Myer 09/29/15 7 Comment(s) Custom EFI Configurations,

Building a Twin 4-BBL Throttle Body Injection (Dual Quad) EFI Engine

Building a Twin 4-BBL Throttle Body Injection (Dual Quad) EFI EngineUpdate, April 6th, 2018: Early last year, it looked like Holley was going to follow-through on plans to make building a dual-quad throttle body engine easily achieveable.  They released a new 2x4 Terminator injector harness that would allow a second Terminator throttle body to be connected via the main harness.  While there was no mating connector on the Terminator main harness into which to connnect this new harness, it seemed obvious that Holley would be redesigning the harness to add this connector.  One would only need the requisite components to put together a plug-and-play hardware dual-quad hardware setup.  And surely an update to the setup wizard would be immediately released so that the handheld could be used to set this up.

I was so confident of this that I re-wrote this article to imply this was reality.  Nearly a year later it turns out that was fake news (my apologies.)  The main harness is still connectorless and the setup wizard is still dual-quadless.  It turns out there is much work to be done before a dual-quad Terminator can be built.

However, this didn't stop several intrepid souls from asking if I'd provide the necessary hardware customization and custom configuration setup required for them to install twin Terminator throttle bodies on their twin-4150 manifolds.  And thsi has turned out so outrageouly well that you can now order a similarly-prepared twin Terminator setup as a kit, ready for installation on your hot rod.

How well has it turned out?  Well neither the customer or myself could be happier.  Here are some case studies:

Mike Leather's '32 Ford w/Twin TerminatorsMike Leather's '32 Ford w/547 CID Ford Engine: Bottom Line - Starts in all weather with a bump of the key.  Allows this 800 HP beast of an engine not only to be operated as a daily driver but it is even tame enough (until unleashed) to pull a trailer!  It has even vastly improved fuel economy over the original dual carburetor setup.  Read the full story here.

Ed's Hot Rod Shop Twin Terminator SetupEd's Hot Rod Shop Dual-Quad Rat Rod - Ed came to me early this year just after we'd had the success with Mike's '32 Ford so I was confident that we could do the same thing for him.  Ed produces some of the most amazing hot rods you've ever seen, incorporating body mods like radically chopped tops with crazy-powerful engines and a beautiful rust patina that evokes both nostalgia and awe.  Ed opted for the Stealth / double-pumper look that fit his application uniquely.  We'll do a full write-up on Ed when he gets this project finished but here is a video Ed posted on FB yesterday of the initial run-in.

The write-up below is the original instruction Holley provided me when I first approached them about doing a dual Terminator setup.  Because this involves buying the components ala-carte (versus kitted) it is far more expensive and doesn't include the 3.5-inch handheld LCD touchscreen display.  But it will allow you to accomplisth the same thing without having to build your own main harness.  But if you're seriously considering this, take a look at this page and contact me. I would be thrilled to get you set up!

Now here is the original write-up if you want to see how painful (not to mention expensive) this used to be...

Anyone who thinks that building a high powered EFI performance engine featuring two 4-barrel throttle bodies is going to instantly make them more popular and give them a lifetime of street credibility, well, they are probably right. Ok, I know that was wrong. I was supposed to imply that they are likely compensating for some other shortcoming but, come on, who isn't going to look at that and think it doesn't just exude cool factor?

There are plenty of oustanding bolt-on kits that allow you to install a single 4-BBL TBI Throttle Body in place of your single 4-BBL carburetor. Among the best of these are the Holley Terminator EFI line of EFI kits. Sadly, none of these gives you what you need to swap two carbs for two TBI throttle bodies. So I wanted to provide some insight on exactly what this upgrade would entail.

Intake Manifold Requirements

Building a Twin 4-BBL Throttle Body Injection (Dual Quad) EFI Engine You are going to need an intake manifold with twin 4-bbl flanges if you don't already have one. If you are tooling around in V8 American Iron this is probably not going to be too difficult. Amongst the major aftermarket manifold manufacturers a dual quad manifold has been produced for pretty much every US-made small- and big-block V8 ever produced. The image at right is the Weiand PN 1985 Dual Square Bore Intake Manifold for the Big Block Chevy. If you need an intake manifold, contact me and I can help solve that problem.

What You Need

Ok, this is what you were waiting for. While it is possible to start with one of the Holley Terminator kits and add the few more parts that you are going to need, it is actually going to be more cost effective to buy everything ala carte (individually.) The two biggest differences here (beyond the second throttle body) are the harnesses and the display/controller. The harness that comes with the Terminator kit has connectors for the four injectors that are on a single throttle body. The best way to resolve this is to buy a the universal ulti-point fuel injector main harness and an 8-cylinder injector sub-harness to connect to it. This uses all the same connectors as the Terminator harness but gives you the extra 4 injector connectors you will neeed.

The other difference is the display/controller. The Terminator kits come with this really nifty 3.5" touchscreen display that allows you to setup your EFI system without the use of a laptop. Unfortunately, since there is no Terminator dual-TBI kit available, there is no option to on the controller to automatically setup your dual-TBI kit so it will require you to do the setup with a laptop. Now, the 3.5" display would still work as a monitor and for much of the tuning, and you may wish to purchase one or even opt for the big 7.5" Digital Touchscreen Dash that Holley offers. But I only wanted to list those things you absolutely will need so I leave that off for now. Here is the list:

A few things from the list above will catch your attention. We have listed the HP ECU instead of the Terminator. In reality, the HP, Avenger, and the Terminator ECU (w/o transmission or drive-by-wire control) are absolutely identical. Because you will need the sealing USB cable that is not included with the Terminator ECU in order to attach your laptop as well as a copy of the Holley tuning software, we opt for the HP version which include both. We are suggesting the Holley-Bosch O2 sensor because it is significantly less expensive than the NTK version, and completely adequate unless you are going to be running racing fuel--if so, opt for the NTK.

Installation Overview

Installing a throttle body injection EFI system is not too much more difficult than installing a carburetor. The linkage on these Holley Terminator throttle bodies is just about the same as on any 4150 style carburetor, so getting one set up is a breeze. Beyond that, you are going to need a dual carburetor linkage kit that connects the linkages of the two carburetors. We can recommend a good yet inexpensive linkage kit.

Once the throttle body and linkage are installed the coolant temperature sensor is put in place. Reuse existing vacuum lines to provide manifold vacuum for the MAP sensor, mounted in the throttle body. As with the throttle position sensor, also installed on both throttle bodies, it is only necessary to connect to one or the other. The O2 sensor will require an appropriately-placed screw-in sensor mount. If one is not available this O2 sensor weld-in bung is available.

Your new Dual TBI Throttle Body EFI system will need a fuel system that is capable of providing adequate fuel for your projected HP rating at 45 PSI. If you are converting from carburetion, visit our Fuel System Kits page to get a better idea of what is required and see some systems that come kitted for your use.

At this point you can mount the ECU, install the power, main, and injector harnesses, and you're all ready for setup.


Holley Dual Quad TBI Setup Screenshot

As always, try to select a setup from the base and custom maps that are provided in your Holley laptop software's Global Folders that will get you in the ballpark. If you look under "Base Cals" you will see about 6 different TBI setups for various Small Block and Big Block engines, with cam durations ranging from 215 to 245 degrees. For the example above, I selected the small block 245 degree duration setup TBI base cal and worked from there.

Skipping down to the "Fuel System" section, I left induction type set to "TBI" but changed the "System Type" to "Terminator Custom." This sets all of the "Fuel Injector Information" (in the next section down) to the correct info for the Terminator injectors. I left the rest of the info in this section unchanged

Under "Fuel Injector Information, I checked "Dual TBI Setup".  If you establish a progressive linkage between the two throttle bodies then check "Progressive Throttle Linkage. Because the Terminator throttle body is a progressive linkage that brings in the secondary at about 36% throttle, you can leave the "TBI 2-4 Switch" at the default setting for now. You can confirm that later but it is close enough for initial start-up. There is a trick that will let these throttle bodies open synchronously but that is matter for another class!

Beyond this, normal tuning procedures apply. I wrote this fairly quickly, so if you see any errors or anything furhter you'd like me to cover, please leave a comment below. Thanks!


After I posted this, Holley gave me a call. While they didn't disagree with anything in the article, they wanted to make sure that I articulated that tuning a dual throttle-body setup is not trivial--as a matter of fact, it can be especially challenging! So, in my closing comment where I say, "normal tuning procedures apply", I should say that while they are "normal", creating a driveable twin throttle body setup can be a real bear. Like any project into which our customers launch, know that we are here to help make you successful.

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7 Comment(s)

08/28/16, 06:42:34 PM

I hope this is the right place to post this.I'm building a chevy 409. 0.30 4.ooin stroke.I would like to keep it looking stock.Dual quads make it right.However I'd like to kick it up a notch.dual EFI under a stock looking air cleaner.Would make my day.I'm thinking Holley HP for starters.I'm not sure about throttle body's.I have been told that it might be lazy off the line.I'm not going racing.But I would like lots of torque on the bottom end.My engine builder have not decided on the cam as of yet.It's going to be a hyd roller.Spec's to be determined. TomB

Chris Myer:
09/28/16, 06:59:08 PM

Thanks for your note, Tom--this is the right place for your question!

Since publishing this article, Holley has released their new "Stealth" Terminator throttle bodies.These things look exactly like a classic Holley Double-Pumper carburetor. (Check them out at the link below.)

As far as being lazy, that is really as much of an issue of the manifold as the fuel induction system. Tuning an engine with a huge plenum space and big air doors is always going to make crisp throttle response challenging. But as long as you come up with a combination that works I don't think you are going to find a huge difference in throttle response between a pair of dual quad carburetors and an EFI setup with a similar flow rating.

As you get closer to choosing stay in touch with us by phone or chat and we will be glad to answer all of your questions.. Thanks again for writing!

Terminator EFI, including the new Terminator Stealth Throttle Bodies

08/27/17, 05:39:09 PM

Hey, I saw this and had a question along these lines.

I have an all Original 1969 Z/28 Camaro DZ 302 car with a Dealer Service Installed date correct Winters Crossram and the dual Holley 600 DP's. Very cool setup, Looks awesome.....Runs great, Unless you take it to cruise ins and stuff like that.It is so rich at idle it fouls plugs right away.

I was wondering if a pair of these Stealth Terminators could be made to work on my Winters Manifold. The only thing I see physically is that i know the Throttle Arms are specific to the 4295 List Holleys for crossram, but I have a replacement set of brand new arms

. Can two of these be made to act as one?

Chris Myer:
08/31/17, 12:06:22 PM

I'm not able to tackle the throttle arm part of this question. But I am 100% confident that when you get to the point of connecting linkage that one of the technicians at Holley can tell you exactly what you need for that.

As far as making the Terminator's work, Holley has moved the ball considerably down the field since I wrote the original article above back in 2015. They added an additional connector to the Terminator harness that gives you access to four more injector outputs. Then, they released a harness that plugs into that connector and right onto the injectors of the second Terminator.

To summarize, you simply need to buy one Terminator EFI Throttle Body Injection System, a second (presumably matching) Terminator Throttle Body, and this Terminator Dual-Quad Injector Harness. Only the sensors in one of the Throtle bodies is used. Install the complete kit per the instructions, add the second throttle body with the additional injector harness, work out the linkage and you're all set.

Hey, don't forget to send some pictures of that sweet setup in your Camaro when you're all through!

03/6/18, 08:51:44 PM

I made an operational crossram on my 385. It was a pain to get any part to just fit. It is not for someone that cannot fab things to work together. I have searched all over and to my knowledge mine is the only functioning Crossram with EFI out there, unless someone hasn't posted.

Chris Myer:
03/9/18, 03:19:53 PM

Nice work! But this reminds me that I'm slacking since I've not yet posted pictures of any of the dual-quad setups that we've done for customers yet. That will change soon!

Harry Mummert Jr:
10/26/17, 11:26:21 AM

I came across this site and been looking in to putting on 2 Terminator Stealth set up on a 1968 Mustang coupe with a 347 ( its a street /strip car). So to do this and to make it work do i buy one master kit and add the 2x4 harness and the 2nd tbi or do I buy all of this:
554-113 hp-ecu
558-308 main power harness
558-104 universal mpfi with ls style map connector
558-200 inj harness 8 cyl
534-10 coolant remp
554-101 bosh wideband sensor
534-216 Terminator TBI Throttle Body (Polished) (2 ea) or
534-217 Terminator TBI Throttle Body (Hard Core Gray) (2 ea)

Chris Myer:
10/27/17, 12:17:59 PM

Well-timed question, Harry. We are just building our first 2x4 Terminator setup with the new 2x4 Terminator Injector Harness. This is far and away the better way to go. Instead of buying all of the components you list above, you just buy three items: a single Terminator EFI kit, a second Terminator throttle body, and the 2x4 injector harness, Because you're buying the components as a kit, you save right at $700 over buying everything individually.

We have only run into one challenge and it wasn't a big deal. It turns out that although Holley has started producing the 2x4 Injector Harness, they have not yet (as of this writing) produced any Terminator main harnesses that have a matching connector. So if you buy this anywhere else you will get a harness with no place to connect it. But if you buy your setup from us here at EFI System Pro, we will customize the harness, adding the necessary 6-pin GT150 connector and wiring it into the ECU J1B connector. This only costs $139.95, which leaves the majority of that $700 savings in your pocket.

After this, it is pretty much a slam dunk. The setup wiziard on the handheld display is not yet programmed to do the custom configuration required for the 2x4 setup, but we will configure the ECU here in our shop before we ship it to you, so you don't need the setup wizard. There is no cost for the ECU setup!

You probably already recognize that you have to mount the Terminator Stealth throttle bodies sideways to make them fit on the 2x4 manifold. With the standard Terminator throttle body it is sometimes possible to mount the throttle bodies so that the linkage is inline, but usually you either have to mount those side ways as well, or else use the offset plates that are available from Offenhauser.

I am hoping to completely re-write the above article soon with all of this info but here you have it in a nutshell. As always, call, email, or use the chat feature here on the website to catch me and go into details. I look forward to hearing from you!

John Barnes:
11/14/17, 11:03:55 PM

Good evening, I have been following this article since last spring. When I read about the harness for the factory dual Terminator...boom...I ordered everything up from my local EFI guy. So, I'm installing this set up the past weekend and did not see how to plug in 2nd throttle body. I just revisited your post and (as of 10/27) see Holley didn't fully follow through with the wiring. So... can I purchase what I need from you? Or send you my harness/ECU to upgrade for second TB? Let me know please and I'll get it sent with a CC#. Thanks!

Chris Myer:
11/15/17, 04:52:13 PM

Yes, in theory Holley is eventually going to offer a dual-quad kit that includes both throttle bodies, the Terminator main harness with the 2x4 injector harness connector, as well as the 2x4 harness itself. I am not sure this is the delay but they also have to add the functionality to the setup wizard to select a dual-quad configuration.

Being a little delinquent in my reply here, I've already responded to your direct question but I'll summarize here for those that might find it helpful. For those who want to do a dual quad setup we do offer to build out the existing Terminator main harness to include the connector for the 2x4 harness as well as per-configure the ECU so that the setup wizard is not necessary. While the wiring harness configuration is not necessary if you do the ala carte method and buy everything separately, that costs between $600 and $700 more.

I hope to get some pictures posted above of the ones we're doing now. :-)

04/4/18, 02:01:22 PM

Ive got a 1982 crossfire corvette 12k original mile survivor , I'm wanting to keep the original look of the car but would like to put some fun factor into the car.... with that being said I'm wanting to put in a big cube small block but cant seem to find the right fuel management system for my project, I'm thinking a twin tbi avenger set up would work and bolt to my renegade intake.

I know there are 2x4 setups out there but feel like that would be overkill for what I'm trying to do but cant seem to get anyone at Holley to talk to me about wiring harness. everyone here seems very knowledgeable any suggestions?

Chris Myer:
04/11/18, 01:57:47 PM

I can understand why you would be consider trying to do a dual TBI setup for your L83 Crossfire Corvette using the Avenger 2-BBL TBI system. That would probably be the closest starting point. But this is not going to be trivial--it is going to require a great deal of customization. That is really outside of Holley's business model.and is the reason they aren't able to support you. But I'm always up for such a challenge!

The standard single-throttle body 2BBL Avenger System is $2,129.95 as of this writing. I should add that exactly ONE is left in stock and Holley has discontinued this part number and says they will not produce any more when that one is sold. The second throttle body is just over $600. The custom modifications to the wiring harness and configurations changes to allow it to support a second throttle body are going to be in the $200 range. So you're over $3000 (and probably looking at some significant turning) to start.

Now I know you'd like to keep your 12K mile Corvette completely original, but I think if it were mine I'd just look at replacing the intake manifold with a square-bore carburetor manifold and installing the Holley Sniper EFI System. Even with the fuel pump upgrades you'd need you'll be complete for less than $1500 (not to mention the horsepower gain.) Installation would be a snap compared to what you'd be facing with a dual Avenger TBI. But if you're in a mind to do the dual Avenger 2-bbl, let me know soon. When it's gone that option is really off the table, practically speaking.

Daniel Harrison:
08/28/18, 01:00:26 PM

I have a '69 Camaro with a 427 sbc and run an Offenhauser Crossram with dual Terminators. I got it together in 2015 and have put about 6000 miles on it since. Tuning was a bit of a bear since I am running a large hyd. roller cam but it is way more streetable than it was with carburetors.

Chris Myer:
08/28/18, 06:31:36 PM

Great job--thanks for sharing!

04/12/21, 08:28:21 AM

With the Dual Terminator Throttle Body Injection System, is it possible to run: Dual sync distributor, with blanking cap, 8x holley coil near plug and 8 injectors? Does it leave room to control a trans later on?

Chris Myer:
04/16/21, 03:21:53 PM

Sure, with either the Terminator or Terminator X TBi systems you can use a Dual Sync Distributor and a blanking cap and run a coil-per-plug ignition setup. No second throttle body is required. Since the Terminator throttle bodies are only available with four injectors each the only way to run eight injectors would be to run a dual-quad setup, but of course the Terminator X Stealth throttle body is available with eight injectors. And while you could, in theory, set up 8 injector to fire sequentially if you had the Dual Sync Distributor it would not really provide any benefit on a shared-plenum manifold.

There is no practical way to add transmission control to one of our Dual Quad Terminator kits. It would be best to just start with that if you think you're going to use it. Otherwise you end up buying a new Dominator ECU and a transmission harness and that is going to be painful at that point.

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