To TVIS or not to TVIS

TVIS - Toyota Variable Induction System

In this article I try to explain the how and why of Toyota's variable induction system as fitted to many 80's and early 90's Toyota engines. For example 3S-GE, 4A-GE, and 1G-GE and all their turbo- or supercharged variants. Some newer versions dropped TVIS. I will focus on the frequently asked question of whether or not to disable TVIS for best engine performance and back my opinion up with Streetdyno sheets.

Click here to skip the boring (tech explanation) stuff and go straight to the results.

About the 4A-GE

Now down to business. I'll be discussing TVIS on NA 4A-GE motors only. The general idea will be the same for other motors, but the numbers will differ. My test object is an '89 red top motor. If you are one of those people who think red tops don't have TVIS, you should read up on 4A-GE differences in any of the numerous websites on this topic. Just a short summary : somewhere in '87 Toyota built the 4A-GZE and at the same time, beefed up the 4A-GE's bottom end too, creating the red top motor. It wasn't until about 1990 that Toyota dropped TVIS and created the small port head which also has red cam cover letters.

A good article about the 4A-GE and its version history can be found on Steve Neese's AW11 resource or on Phil Bradshaw's site. There's also a similar page in the tech section of Club4AG but I believe this was incorrectly copied from Phil Bradshaw's site. I have verified and reported the differences, but the Club4AG site owner chose not to correct or even reply.

What's TVIS?

TVIS is a variable intake system designed by Toyota to allow an intake manifold using large intake ports in the head to perform well over a very wide RPM range. Any TVIS manifold has TWO runners per cylinder, one of which is blocked off at low RPM by throttleplate-like butterflies. These butterflies are mounted in a sandwich plate between the manifold and head which is commonly called "the TVIS plate". Blocking off one runner increases velocity in the other runner, making it operate much more efficiently thru scavenging effects. (until it reaches its flow limit of course)

Here's a picture of the TVIS plate. Click on the pic to get the fullsize version, with manifold. (378 KB!)

In case you're wondering : the two bolts on the upper left "ear" are the EGR valve mounting bolts. The black can in the bottom is the vacuum tank, the greenish thing (with plug) above that is the VSV. The actuator is on the top left of that. You can see the shaft run thru runners 2-7, with butterflies in runners 2,3,6 and 7 (open now).

In Toyota-Mods, Club 4AG and similar forums, the question often pops up how to disconnect TVIS, so the TVIS butterflies stay open, killing a presumed intake restriction. Some even go as far as emptying the TVIS plate of its contents. Usually, someone then posts an explanation that TVIS actually HELPS low end power by keeping velocity in the runners up, but without proof, not everyone will believe him.

I have not seen information on how much difference it really makes, and on whether or not TVIS switchover point is at optimal rpm from the factory. I was curious about all this though, so I decided to look into this. First about how the system itself works.

How TVIS really works

TVIS is held open by a spring, and shut by vacuum. Unplugging the TVIS actuator hose will therefore open up TVIS permanently. It has its own vacuum tank, enabling it to stay shut even at wide open throttle (where the engine has no vacuum of its own). An ECU controlled vacuum switch valve controls whether or not vacuum is applied to the TVIS actuator. Some have said that unplugging the VSV connector will cause TVIS to be shut all the time, but I have found the contrary : unplugging the VSV electrical connector leaves TVIS open all the time. You can find a vacuum diagram for a TVIS-equipped engine ('87 AE82 Corolla) here (52KB). TVIS is in the upper left corner, called "Intake Air Control Valve".

When wired as stock, the TVIS VSV gets power from an ignition feed. The other wire goes to the ECU, which grounds it when it wants to engage the VSV to shut the TVIS butterflies. So if you unplug it, TVIS will be open.

On my car (just like in the AE82 diagram) there's a T in the TVIS actuator vac line, leading to the EGR valve, thru a one way check valve. This T serves to disable EGR whenever TVIS is open. How? Well, with the TVIS VSV disengaged, the T is fed ambient air thru the VSV's little airfilter. That shuts the EGR valve (thru the check valve), whether the EGR vacuum modulator is trying to open it or not.

The real world effects of TVIS

So there are two ways of disabling TVIS. But is there a point? I think not, but wanted to know by how much. So I decided to test that using Streetdyno. Same car, same tank of gas, back-to-back runs on the same stretch of road, two for each situation ("perm open", "perm shut", "connected as stock") to find out what TVIS really does for engine power and torque. To shut TVIS permanently, I pried the pin out of the ECU connector (this is reversible if done right, and avoids cutting wires) and grounded it, so the VSV would be engaged all the time.

Testing was done using my laptop and Streetdyno 0.6.82. Test object is an 89 red top motor, with AFM (so any changes in airflow are compensated for in fueling). There are a few minor mods, but they shouldn't hurt the comparison. The mods : cone intake, full header-back exhaust, no cat. Not all that earth shaking, but it frees a few noticeable horses.

Results

Ok, I'm not gonna waste more time : here's the pic that tells the whole story. The power and torque curves in one graph:

As you may have guessed, green is with the butterflies shut, red with them open. Now where did the 4AG's nice and flat torque curve go? Exactly, it needs TVIS for that! Also note how badly it needs the second runner at high RPM... power maxes out over 25hp and 1000rpm lower with the butterflies shut! It makes your 4A-GE perform worse than a 4A-FE!

These are the power and torque curves, the blue one with TVIS connected, red open, green shut. The slight difference between blue and green below 4k rpm is probably a little glitch in precision. Drivetrain losses of ~15% were assumed, so these are estimated flywheel figures.

Conclusion

The difference TVIS makes is most evident in the torque curves. You do notice that the red graph never ever exceeds the blue one, so there is NO GAIN AT ALL in disabling TVIS on a (basically) stock motor. It's there for a reason, and does a fine job.

The torque curves intersect nicely at about 4400rpm, so the factory did a good job at setting TVIS switchover point to optimum. (1)

Here's the Photoshop PSD file (107KB) containing all graphs, each in a separate layer so you can compare them in any combination you want.

With big cams the story will change a bit, but I think runner velocity will continue to be a good thing for low end torque, although ideal changeover rpm will probably change. I don't own or have access to a cammed motor, so I cannot verify that.

You can also tell that top end power starts to drop off sharply at about 90-95hp with a single active runner. Before that, there's hardly any drop-off at all. So I wouldn't expect any gains out of emptying the TVIS plate of its contents (removing shaft and butterflies altogether) until about 170-180 NA HP with both runners active. But that is close to the limits of the stock intake manifold as a whole, so... (2)

Forced induction engines will probably behave like a stock engine, since only pressure and air mass change with boost, not port velocity and volume of intake air (post-compressor). So even those heavily modified motors will probably still benefit from TVIS in low end torque. I am currently in the process of turbocharging my car. I will verify the above theory as soon as that project is completed.

So, I don't think just disabling TVIS is going to gain power on ANY motor at all. Emptying the TVIS plate might gain a little bit though, but only at very high naturally aspirated power levels, where the whole manifold as a whole becomes a restriction and removing that shaft won't help it much.

Heck, these GIF's are so small (3K each) I might as well include a few more. Click here for more graphs: Power and torque curves for TVIS shut, open and working (stock), and all curves together.

Please e-mail me with any questions, comments or corrections.

Thanks to the people of Toyota-Mods who helped me understand TVIS. Comments, additions and corrections are welcomed... I'd love to get some feedback!

Footnote 1

One might expect a slight dip in the blue curve at 4400 rpm based on the red and green curves. But who knows, maybe the time it takes for TVIS to open closes that gap by allowing partial airflow thru the second set of runners? Anyway, that's a minor detail.

Footnote 2

The large port head is often said to be TOO large, in port size. Maybe there are gains to be had by extending the port divider in the head all the way up to the TVIS plate. This removes an area of expansion and turbulence, and extends the velocity idea of the single active runner all the way into the ports down to the valves. This could be the way to extract maximum power from the stock NA 4A-GE intake manifold, unless it has downsides I'm overlooking. Any feedback?
UPDATE : I now see this cannot work. Where would you put the fuel injector ? Exactly...