Thread: Dual Quad Sequential EFI

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Gotta lot going on right now, but I had to post this.

I've never really liked the look of the LS engine that I'm using in my build (or LS engines in general for that matter). I appreciate the technical stuff inside and they make lots of easy horsepower, but they are pretty ugly on the outside which is why even the factory usually puts covers on them. I've been pondering for some time how to make mine look more appealing to the old school traditionalist in me.

When this setup came up for sale on another website, I just couldn't resist. Now I'm normally "Mr. Practical" when it comes to street engines, but this thing just looks cool(to me at least).



It's a dual plane intake so low end torque should still be good. It has port fuel injectors so I also get to keep the sequential injection.

The throttle bodies are billet with progressive secondary linkage like a traditional 4 barrel.

The engine looked a ton better the instant I dropped it on. Looks like something that should be on a 60's era Cobraesque sports car like the Healey I'm building.

The only downside is that the throttle bodies flow an insane amount of air for a 350 CI engine. 1000 cfm each! I'll most likely have to limit airflow to make it practical for everyday driving and I'll most likely go with a progressive linkage between the Tb's like a traditional 2x4 carb setup, too. It's set up for simultaneous opening now and that will most likely make the throttle response a little touchy.

I'm thinking about converting a set of finned big block Chevy valve covers into coil covers to hide the ugly exposed coils. Add a big oval dual quad air cleaner and it should look reasonably traditional, while still keeping some of the high tech guts.

I'm really looking forward to see how this turn out.

Just thinking out loud, but I wonder if something like the NASCAR restrictor plates could be used to cut the air flow down to a more acceptable level.


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Mike, that's a good idea. I could probably fabricate something along those lines.

My idea was to fab some aluminum restrictor "plugs" that fit in the TB bores. The bores appear to be just straight holes with no venturi shape at all, which stands to reason. There's no need for a venturi if you're not trying to draw fuel. The plugs could be accessed easy to work out the restriction size.

I have only found one reference on the web to a similar setup. That was an article in Hot Rod (I think) about an LS powered Nova. The owner said he wished the power delivery was more progressive, because the throttle was kind of like an "on/off" switch, which made the car difficult to autoX. I've also heard the same thing said about regular LS intakes when very large TB's are used. The throttle response gets touchy. Apparently, port fuel injection with a dry intake isn't doesn't suffer from the throttle response problems that can sometimes happen with very large carbs.

I'm pretty excited about making this work (can you tell). These kinds of things are what makes hot rodding fun for me. Too bad I've still got a lot of work on the car before I get to really work on this.

That looks like an awesome set up! I'm right there with you on not liking the looks of the LS engines. I have been looking for BBC finned valve covers too. I have some SBC ones but they just aren't big enough to cover the areas that need it IMO.

Glad you folks find this interesting. Been doing a little seat of the pants thinking about what bore size will be required to restrict this setup to a usable flow. Feel free to comment on my theories.

I measured the throttle body bores and they all came up within a few thousandths of 1.75 + or - .003. Now is where I have to make some assumptions and yeah I know what happens when you assume.

Holley rates these TB's at 1000 cfm and all the bores are the same, so that must mean that each hole flows 250 cfm.

If I was building this with carbs, I would either use 390 cfm Holleys or 500 cfm AVS Edelbrocks. The Edelbrocks are rated with dry flow and the Holleys are rated with wet flow, so they are most likely fairly close in practical air flow. I'm going to go with 800 total cfm to start with, which I would think is plenty for any 350 that won't see the north side of 6500 rpm. Calculated required air flow for this engine is about 650 cfm. This means I need 100 cfm per bore to get 800 cfm total.

1.75 (existing bore size) works out to 2.4 square inches and 100 cfm is 40% of the existing 250 cfm rating. 40% of 2.4 is .962 square inches. If I round that up to 1 square inch it works out to a bore size of approximately 1.125. This converts to roughly 103 cfm per bore and 824 total.

I'm not sure if the cfm flow scales up or down like I have figured.

Anyone have any thinking on this?

Mike I’ve been kind of thinking about this since I read your post this morning. It seems to me that as the throttle bodies are only passing air that a simple option might be to simply disconnect the secondary throttle plate linkage and build progressive linkage for the TBs.

Basically you would now have 2 500 CFM TBs (1000 CFM total). Using progressive linkage to the secondary throttle body would determine when and how much additional air is allowed into the engine. How you design your linkage will determine the kick in point and total amount of air the secondary TB will provide. In theory the secondary TB should be almost infinitely adjustable to provide up to an additional 500 CFM.



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Mike, that's probably how I'll start out and see how it does. The secondary linkage is pretty easy to remove.

I definitely feel that it needs progressive linkage between the TB's. One 500 cfm TB should be plenty for normal driving unless it causes some kind of strange distribution problem and I wouldn't think that would be an issue since all it's passing is air. I have to say though, that I'm still intrigued by the prospect of getting this thing working with all 8 barrels, but it's something that I can experiment with any time once I get the car on the road.

I've been giving the total flow some more thought, too. These TB's are similar to the one's that came on some SBC multi port injections. Unless Holley engineering is being driven solely by bigger is always better marketing, then there has to be a reason that they felt 1000 cfm was a good flow rate for a wide range of engine sizes. I don't think they even offer a 4 barrel TB in anything smaller than 1000 cfm and with the CNC construction they could have built them any size they wanted with just a few simple tweaks to the basic program.

I've come to the conclusion that TB size just doesn't matter as much with multi port injection as carb cfm does since there is no need to maintain good velocity through a carb venturi. The 1000 cfm throttle body has 9.6 square inches of throttle bore. A stock 78 MM LS1 has 7.4 and some stock parts are almost 90mm. A 102 mm TB is a common oversize aftermarket part and has 12.6. That works out to over 1300 cfm. A lot of those 102's are going on 5.3 (323 CI) LS engines and even some 4.8's (292 CI)! Based on that, I'm not going to worry too much about flow at this point.

The air cleaner will be in this week. The first big issue is getting it all to fit under the hood. Gonna be close.

Mike like I said I'm really looking forward to see how this progresses. Pretty much like you said in your first post, you can't argue with performance (and reliability) of the new stuff......but like you, to my eye most of it is at best pretty blaaaa, and at worst damn ugly when you open the hood. I miss the days when opening the hood of a car was as much of a selling point as what the exterior and interior was....so I really like the idea of what the whole project is about. I also fully understand about making all 8 barrels work.


".....The air cleaner will be in this week. The first big issue is getting it all to fit under the hood. Gonna be close......"

Yeah, I've got a few of those ...... It's how the Plymouth ended up with a scoop. It definitely wasn't my first choice but I was bound and determined I was going to run the Tri=Power.

I know I've mentioned it before, but what turns out to be really handy for me in those situations is setting my phone up in the engine compartment and using the video function to see just how much room I actually have when the hood is closing.



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Luckily, I have the inner fenders removed so I can just stick my head in under the fender and see the clearance or lack of. I'm pretty sure that the air cleaner will need to be modified.

It is set up to clear the choke housing on a Holley carb, so I think I can trim the base down some based on the pictures I've looked at. That might gain 1/2" and just might be enough.

Worst case, I'll have to drop the engine down a little. The motor mounts are still just tacked to the chassis and I have about an inch that I could use before the accessory drive hits the steering shaft. I would rather not do that for a lot of reasons, though.

If the air velocity thru the throttle bodies really does not matter, maybe you could just build a throttle linkage with a very “slow” ratio? You could make it so that when the gas pedal is on the floor, the butterflies are only half way open, or somewhere in that range.

Disclaimer- I’ve never done this, I’m just thinking out loud.

Well, the air cleaner came in yesterday, but like many things in hot rodding, it needed a little "rework". In other words, it didn't just bolt on.. This part was much worse than most because it didn't just not fit my application, it wouldn't even work as it was intended on ANY application. The dang filter wouldn't fit in the housing and it was off by a bunch!

I wanted a large oval finned aluminum dual quad filter similar to the old Cobra filters, except I wanted fins all the way across the top. The only one I could find made like that was sold by Speedway Motors under their house brand. The quality was, uh let's just say, less than great.

I knew that I was going to have to make a few mods to get it to fit under the hood, so I bought a "scratch and dent" item off of their Garage Sale page to save some money (cause it's ridiculously expensive) . I even called to check the condition before I ordered and was assured that it only had a couple of minor scratches, but that the filter element was fine. Well, it came in and had a partially crushed filter and I know why. The filter is too long and wide to fit in the recess that was cast for it. Someone attempted to force it into the housing and crushed it. So, before I could even attempt to get the thing fit to my car, I had to make the filter fit the housing.

Thank goodness I have a 2-axis CNC mill. Doing this job on a conventional mill with a rotary table attachment would have taken at least twice as long. Getting the pieces tied down to the mill table without breaking the thin castings was tricky and cutting it with a less than sturdy setup was a nail biter. It may look tied down well, but I didn't want to cinch the clamps down too tight and break the casting.

I even attached a dial indicator to the table so that I could monitor for the setup slipping. As it was, it took a long time to cut because I didn't want to get too ambitious with my cuts. Where most parts could be done in 1 cut, I took several lighter cuts just to be on the safe side.

Here is what I started with. You can see that with the filter element stuck in one end, that it hung over on the other end.

Here is the base section being machined. I had to skim cut the bottom to get it to lay reasonably flat on the table.



Then I flipped it over and machined the filter part. I wound up taking it out about 1/8" on each side and taking out the taper that was left by the mold plug. This is called draft and is on nearly all mold plugs to allow the plug to be removed from the molding sand.



The filter fits in the base, now on to the top.

It's always something! Nice work! I would have had to use carbide bits and finished it with small flap wheels. That 2-axis CNC mill would be so awesome to have! Those oval filters never ship well either. They never put anything in the box with them and they get slammed around and smashed! Drives me nuts. I finally got an ok one on the 3rd trip for my 40.

The top section was more difficult to tie down since there were no openings to use. I had to clamp it on about 1/8" of the outer lip. That's all I had to work with, since that lip also had to be cut. The cutter barely cleared the clamp! I was worried that the lip might fail during cutting. I've seen castings let go during cutting if too much material is removed, but this time everything went well.



I also had to shim the casting up with cardboard because it was warped and wouldn't lay flat.

40, I would have hated to remove that much material with a die grinder! It was a bunch of material. You're right, it's always something.