Divided Intake and Swirl Flap Installation on a 1HD-FT

[VADUS]

A lot has been said and done regarding exhaust-side improvements for cylinder breathing in our remarkable four-valve engine. This naturally includes the twin-scroll manifold and turbocharger, which enhance wave dynamics in the divided manifold and protect the cylinders during the valve overlap phase, along with a separate exhaust path for the wastegate valves, significantly reducing backpressure behind the turbine wheel under high load. However, on the intake side, most research revolves around intercooler types and their characteristics, including the associated piping. Yet, a great deal also depends on the intake manifold, where similar wave dynamics occur, either hindering or helping to fill the cylinders. The intake "manifold" should more accurately be called a distributor or simply runners, as it doesn't "fold" (mani-) anything together but rather separates the flow. Nevertheless, I'll use the established term "manifold," which in our case is combined with a plenum chamber.

In a common distributor feeding all 6 cylinders, there is significant wave "interference," since all cylinders breathe from a single volume of the manifold-plenum. The basis of this "interference" lies in backpressure pulses from the end of the exhaust stroke during valve overlap, as well as reverse waves generated when the column of accelerated air reflects off the suddenly closed intake valves of neighboring cylinders. Furthermore, our engine operates on the Miller cycle with a shortened compression stroke, and it has a so-called "fifth stroke," where during the compression stroke, for the first 32 degrees after BDC (24 degrees for the FTE), the piston pushes the inducted air charge back into the intake manifold, causing yet another backpressure wave. This cycle is used to lower the effective compression ratio relative to the geometric one, and thereby increase the expansion ratio relative to the fuel's knock limit, as the piston's full geometric stroke is utilized 100% during the power and exhaust strokes. Hence the engine's modest specific power output and its legendary reliability. This entire wave "interference" in the intake tract significantly hinders cylinder filling, not only in naturally aspirated engines (as many mistakenly believe), but also in turbocharged ones. So, how can we reduce the influence of neighboring cylinders on each other to improve their air filling?

The answer suggests itself. And here, the firing order of our inline-six cylinder engine 1-4-2-6-3-5 plays just as positive a role as it does with the twin-scroll exhaust manifold. It's enough to simply divide the common manifold with a partition, and the mutual influence of the cylinders on each other drops sharply, since the consecutively firing cylinders will be located on opposite sides of this partition in their own individual manifolds. Many manufacturers use a setup with such a divided manifold, and the funny thing is that Toyota used the same divided configuration on the younger engines of the 1H series—the 1HZ and the 1HD-T: there, the manifold itself is completely separated, and the intake duct from the air filter is also partially divided. This is quite sufficient to dampen the backpressure pulses from the neighboring plenums to a negligible minimum.

 

[VADUS]

So, what prevented Toyota from applying a similar solution to the intake manifold of the 1HD-FT and 1HD-FTE? At this point, you could cry, laugh, or do both simultaneously, but it's clear that the obstacle was, once again, that infamous EGR system. The EGR introduction point into the manifold was designed in such a way that it would have been impossible to distribute the exhaust gases and soot evenly to both channels—the intake tract is simply too short, so the exhaust gases wouldn't have enough time to mix uniformly with the fresh air. Consequently, one channel would receive significantly more exhaust gas than the other, leading to uneven cylinder operation between the two groups, as well as increased soot buildup in the group with the higher exhaust content. The alternative would have been to create two independent and symmetrical EGR systems, each feeding its own cylinder group, but such a solution would have been overly complex and, from a commercial standpoint, an unacceptable luxury... Therefore, Toyota took the cheap and simple route—they simply abandoned the divided intake tract design.

This is partly why the torque curve of the 1HD-T has a slight advantage over the 1HD-FT at low rpm (up to 1500), a fact many of our forum members have noticed. Of course, this is far from the primary reason for this phenomenon, but it does contribute its part to the overall picture. In general, the influence of a divided intake is more pronounced at mid-range rpm and during transient conditions, although it is also noticeable at both low and high engine speeds. The main reasons for the 1HD-T's slight superiority at the very bottom end are, once again, that infamous EGR system (which most of us have long since eliminated), as well as the 1HD-T's two-valve valvetrain design, which improves cylinder filling at low rpm by creating swirl of the air charge within the cylinder. How does this work? During the intake stroke, the cylinder draws air from the manifold through only one port, which causes the air velocity in that port to become relatively high, forming an air swirl as it enters the cylinder. The narrower the port, the higher the air velocity, and the stronger the swirl in the cylinder. During the compression stroke, this swirl intensifies due to the sharp reduction in its radius (transitioning from the cylinder into the combustion bowl in the piston) and, upon fuel injection, ensures a more intensive supply of oxygen to the boundaries of the fuel spray. In the four-valve design of the 1HD-FT(E), two ports are used simultaneously for intake, which significantly reduces the gas velocity in these ports and hinders swirl formation at low rpm. This leads to the characteristic bluish smoke at idle on cold FT(E) engines, which many have long called the signature trait of four-valve 1HDs. It's simply that the fuel burns very slowly and incompletely, saturating the exhaust with unburnt fuel vapors. At high rpm, everything changes—the piston speed becomes so high that swirl forms even with two ports working, while the single-port design of the 1HD-T starts to choke due to the insufficient flow capacity of that single port.

This creates the impression that the four-valve design has a clear disadvantage at low rpm compared to the two-valve design. In reality, this isn't the case, and engineers learned back in the last century how to solve this problem for the four-valve configuration—and not just solve it, but achieve improved low-end characteristics compared to the two-valve design. The problem is solved quite simply: one of the intake ports (the wide one) is shut off at low rpm using a flap, leaving only one narrow working port for cylinder filling. This allows the air to be accelerated even better than through the single, thicker port of a two-valve design. When the capacity of the first port becomes insufficient (for a diesel, this typically occurs around 2000 rpm), the flap opens the second port. The system is not complicated at all, and in modern engines, it is controlled by the ECU. Every manufacturer has their own variations of this design under different patented names: at Honda, this function is integrated into the VTEC system, while at Toyota, it's called ACIS—Acoustic Control Induction System. The system has different variants, especially for naturally aspirated gasoline engines, where even the length of the intake runner can be altered. In our case, with a turbocharged diesel engine, simply blocking one intake port is sufficient. And Toyota finally came around to this solution in the very last facelift of the 1HD-FTE engine, which featured an EGR cooler and a variable geometry turbocharger.

 

[VADUS]

A common shaft with swirl flaps, installed in the thick, curved, and short intake port of each cylinder, is located between the cylinder head and the intake manifold. It is controlled by a common actuator with a vacuum solenoid managed by the ECU. However, nothing prevents creating a reliable mechanical control for it—more on that later. The only weak point of this shaft assembly is its susceptibility to coking from soot originating from the EGR system… Yes, yes… there it is again—that small but utterly vile eco-contraption, fouling with soot virtually every component downstream, slowly killing the engine.... The flaps become coked with soot, lose their mobility within the bushings (much like intake valves in their guides), while the actuator continues trying to rotate them. This places increased stress on the flap running bushings, leading to play. If the EGR has been deleted and the shaft assembly is in good working order, these problems can simply be forgotten. Incidentally, I suspect that the presence of the EGR system was the primary reason Toyota did not implement these flaps earlier, only introducing them on the version with the EGR cooler, naively believing that cooled EGR wouldn't foul engine components as badly, or that fuel quality would improve. Practice has shown that it does foul them, and fuel quality doesn't fundamentally change the picture…. In some cases, the shaft assembly requires replacement—when the combination of excessive soot (from poor combustion) and engine oil does its dirty work. Overall, our specific FTE flap shaft assembly is designed as simply and reliably as possible—it's all metal, except for the plastic bushings. If EGR soot doesn't settle on it, there's essentially nothing to break. In contrast to this assembly, the modern automotive industry is rife with plastic intake manifolds and flaps, packed with numerous sensors and servo-electric actuators, also made of plastic. The reliability of such flaps is rather mediocre, and they are often eventually deleted and removed. We, however, will do the opposite, because soot and other traces from that EGR-contraption have long been absent from our engines, and Toyota's all-metal execution of the flaps for the 1HD-FTE does not really invite criticism. In case of a perfectionist streak, one could machine more reliable bushings to replace the plastic ones.

 

[VADUS]

This is where we slowly transition from theory to practice. Given the interchangeability of the cylinder heads, Den and I got the idea to install the swirl flap shaft assembly on my 1HD-FT (even though I already have an FTE cylinder head fitted). However, it will have to be installed together with its original intake manifold, as the runner length on that manifold is reduced by exactly the thickness of the flap assembly and an additional gasket.

Initially, I planned to control the flaps using boost pressure and a standard wastegate actuator, which I intended to install in place of the original vacuum actuator. Such a system would operate smoothly, gradually opening the flaps as rpm and boost pressure increased, with full flap opening occurring during hard acceleration and pedal application around the 1800-2000 rpm range—just as we need. Fine-tuning the activation dynamics based on boost pressure could be achieved by selecting the appropriate actuator spring pressure and installing a return spring in conjunction with a standard bleed-type boost controller.

However, I then decided it would be more correct and simpler to keep the factory vacuum actuator and trigger it based on engine speed using a vacuum solenoid valve. This setup fully preserves the factory component design and, crucially, won't constantly flutter the flaps back and forth, which would increase wear. Switching based on rpm will also reduce compressor load and improve throttle response on the highway during sustained high-speed cruising.

An important detail: the factory flap assembly is designed to be in the open position when vacuum is absent. This means that in case of unexpected issues with the control system, simply disconnecting the vacuum hose will revert the engine to its original four-valve configuration.

For flap control, I found a ready-made solution—an on-board computer with a tachometer and a signal output that triggers above a set rpm. I'll just need to tap the W-terminal from the alternator for the tach signal. I plan to set the system to activate at 2000 rpm, although this might be a bit high. My friend Sergey's manual for his Mercedes OM642 diesel states that the flaps open at 1850 rpm. But that's a 3.0-liter engine, so perhaps in my case, with 4.2 liters, 2000 rpm might be the right transition point. I couldn't find any data on flap opening for the EGR-cooled 100-Series, and I'm leaning towards 2000 rpm because, in city driving, I usually shift before that point anyway, meaning the flaps would be almost constantly closed in urban conditions. But the best advisor will, of course, be practice, especially since adjusting the trigger rpm on the computer is straightforward.

Besides organizing the actuator control, I'll also need to weld aluminum mounting bosses with threads onto the manifold for the 1HD-FT's accelerator pedal bracket and throttle cable.

Well, since we're getting into welding, it's the perfect time to revisit the first part of this story about dividing the manifold. One could simply weld in a partition, but that wouldn't be enough—the separation would be too short. Even with the factory glow screen at the manifold inlet, the wave pulse would still enter the neighboring plenum, passing through the screen and reflecting off the intake neck. Therefore, a more radical and effective solution has been chosen: cut the manifold in half and create two independent manifold-plenums. The cut ends will be welded shut. The inlet to each plenum will be made from above using a separate 51mm pipe, which enters the plenum as a snorkel cut at a 45-degree angle, oriented towards the far end of the plenum with a 12-15 degree upward bend. From the plenums, two independent 51mm tracts will run parallel and merge near the intercooler via a custom Y-pipe (2x51mm to 65mm). This time, the "frying pan" (preheater) is being retired, especially since I haven't used it once in the last five winters. The engine starts without a Webasto at -20°C just as it does at +20°C. As a result of all these measures, I expect improved fuel combustion at rpm below 2000, with all the pleasant benefits that follow—reduced average fuel consumption, increased maximum torque at low rpm, and reduced vibration and engine noise. This mod is particularly relevant for manual transmission owners, whose operating range in the city is typically between 1400-2000 rpm. Automatic transmission owners are unlikely to fully appreciate the benefits of swirl flaps, as any noticeable throttle application in an automatic causes an immediate rpm jump past 2000. I also expect improvements at high rpm under maximum load due to the increased cross-section from the intercooler to the manifolds and the reduction of wave interference in the divided intake system.

We are planning to implement everything this spring or summer. The main parts have already been purchased. I'll be posting updates in the thread as the project progresses and results come in.

Another breath of air for our 1HD-FT )

With best regards,

Vadim Akopyan.

 

[Dougal]

About the same point in time (1998-2005) Nissan were playing with variable swirl valves on their 4 valve diesels and I happened to own a few. 1998 they had the variable swirl valves, a few years later they'd ditched them totally. I think it's one of those things they were able to tune around that ended up being marginal gain for extra complexity.
They all had EGR with and without the swirl valves.

BTW blue smoke isn't diesel, it's heavier oil.

 

[VADUS]

Agreed, the gain is not drastic, but it is clearly noticeable in 1000-2000 rpm range. A lot of peoples, who deleted and removed flaps on their cars noticed, that they also lost a bit torque at low rpm range. And like I wrote above, for MT users - particularly these swirl flaps from Toyota would be a good upgrade. For AT users flaps do not make any sense at all. But divided manifold should help in both cases.

 

[chapel gate]

About the same point in time (1998-2005) Nissan were playing with variable swirl valves on their 4 valve diesels and I happened to own a few. 1998 they had the variable swirl valves, a few years later they'd ditched them totally. I think it's one of those things they were able to tune around that ended up being marginal gain for extra complexity.
They all had EGR with and without the swirl valves.

BTW blue smoke isn't diesel, it's heavier oil.

if im reading your post correctly i agree, any well tuned/fueled/boosted/intercooled 1HD FT would make these marginal gains irrelevant.

non the less, a very interesting read and thanks for taking the time to post Vadim.

 

[VADUS]

any well tuned/fueled/boosted/intercooled 1HD FT would make these marginal gains irrelevant.

It depends on the purposes and targets of your tuning. If the goal is to achieve the maximum power and torque at high rpm, then yes, these mods are slightly noticeable. But if you are building up the engine for a good balance of fuel efficiency and power, for real living and driving, including the cities, highways, traveling etc., then you will get a noticeable gain in MPG and low-end torque with manual transmission. For instance, I use my TLC80 on everyday bases, and 90% of the time drive it in city mode, where the rpm range is 1000-2000. The same thing is with off-road conditions - low range rpm mode is being used often, and even a small gain in torque will be helpfull. As a traveler, I am deeply concerned with MPG, because sometimes there are no gas station within a days.

 

[aztoyman]

I'm looking forward to seeing the results of your efforts. Thanks for posting.

 

[Dougal]

Agreed, the gain is not drastic, but it is clearly noticeable in 1000-2000 rpm range. A lot of peoples, who deleted and removed flaps on their cars noticed, that they also lost a bit torque at low rpm range. And like I wrote above, for MT users - particularly these swirl flaps from Toyota would be a good upgrade. For AT users flaps do not make any sense at all. But divided manifold should help in both cases.

I think also turbocharger design and efficiency improved a lot in that time which made them redundant. A more efficient turbine will bring boost on sooner with lower drive pressure across the whole range. No downside. Most engines went to VNT turbochargers through that time and while the first of those were quite inefficient they got better relatively quickly.

 

[VADUS]

I don't consider any VNT turbocharger to be a reliable unit. In our conditions, the blades quickly clog with soot and seize. That's why I chose a twin-scroll. It also has an off-boost mode, especially during calm city driving. So for me, this is definitely not a redundancy. But most importantly, a divided intake and swirl flaps improve cylinder filling and combustion, whether under boost or not.

 

[Dougal]

I don't consider any VNT turbocharger to be a reliable unit. In our conditions, the blades quickly clog with soot and seize. That's why I chose a twin-scroll. It also has an off-boost mode, especially during calm city driving. So for me, this is definitely not a redundancy. But most importantly, a divided intake and swirl flaps improve cylinder filling and combustion, whether under boost or not.

My two VNT diesels have over 300,000km combined with zero turbo issues. They are as reliable as any other turbo.

The clogging issues others have come from short and cold running. There were Garrett shaft breakages early on but those were sorted with stronger shafts.

 

[VADUS]

We live in different regions with different fuel quality. My friend's VNT just clogged on his LR Defender with 250 000 km on it. Thanks God it happened just beforehand our journey to Caucasian mountains.

 

[Dougal]

We live in different regions with different fuel quality. My friend's VNT just clogged on his LR Defender with 250 000 km on it. Thanks God it happened just beforehand our journey to Caucasian mountains.

Ford PUMA engine huh? Those seem to have issues in everything.

 

[bjowett]

Both of those systems are based on the technology from Toyota gasoline engine T-VIS and ACIS systems. The ACIS shows how the divided manifold length works and changes, or extends the useful power band and RPM range. I always thought it would be interesting to apply an ACIS manifold and nice dual exhaust to a 1HZ. Good reading on these systems T-VIS Tech Discussion - http://www.turbomr2.com/MR2/Reference/TVIS/TVIS.htm

 

[VADUS]

Thanks for this link. Another conformation of thread's actuality for turbo motors.