Single vs Twin turbo setup

I have been thinking about the benefits of twin vs single for some time.

Because there was no hard proof, no dyno figures or any real evidence of one being better than the other I thought it might be worth looking at some compressor flow maps and plotting air flow requirements.

I used a spreadsheet to calculate the VR-4s air flow at pressure ratio. this allows me to plot the RPM line demands on the compressor flow maps.

According to a quick sanity check using this calculator, my projected air flow should be fairly accurate, if not a little pessimistic due to the calculated figures being 15PSI at 5500RPM.
As a third check, the power output figures approximately match my dyno figures (with exhaust + intercooler + MBC @ 15.2PSI) however my dyno figures were not at 15PSI as do to the turbos inability to hold 15PSI... more like 12PSI. (which is why the plot should be a touch pessimistic)

I have plotted on 2 compressor maps, both Garret BB turbos... The theory being that most of us want good response with good power potential. Personally I am willing to sacrifice top end performance for good response.

In this vein, I selected the GT25R for the twin turbos. Garret rate them at up to 270HP each, giving a total of 540HP total using 2.
This selection was based on the maximum response attainable from a good BB turbo.
For the mapping I based the calculations on 1250CC (half engine displacement)

For the single I used the GT35R. Garret rates this turbo at up to 600HP. Looking quickly at the compressor map it looked like it could be usable.

And here are the compressor maps with the RPM lines marked in. Obviously I cant say for certain that these are going to be accurate, but interesting nevertheless.

So, feel free to discuss :)

As you can see the result is interesting to say the least.

According to the plotted lines, the GT35R should give similar if not better response, not to mention better top end performance. (I assume due to its ability to generate more pressure)

What I am missing however is plots on the turbine flow and what real world response to expect from that side of things.

NOTE: This is not a 'I am right, you are wrong thread'. I did this for discussion purposes and the idea that it could generate some useful dialog. If people start getting worked up and getting too specific about who is right and wrong, ill start deleting posts. I don't claim this information is correct, only speculate that the air flow data looked to be fairly consistent with my dyno chart and other air flow calculators.

The GT35R can definitely go up to 2 bar without a problem, while it looks like the GT25R can only go up to about 1.5 bar, and only over a narrow range.

Those graphs don't really show how quick the turbos will spool up though, how exactly have you come to that conclusion? Those graphs just show what the turbo can do given enough time to spool up, rather than their actual spool characteristics. ie if you are going up a hill with the throttle open you might reach points lower down on the graph, but on a normal flat surface it would be more difficult

They seem to be very similar lower down, ie 1 bar at 3000rpm or 9cfm for the GT25R and 18cfm for the GT35R are in similar spots on the graph, if anything the GT25R has more room to the left of that point, so I would imagine that a pair of GT25R's would be capable of running at lower rpms and higher levels of boost than the GT35R without going into surge.

I've posted this page on the APS site before, I'll do it again now as well:

Obviously we have some packaging problems, a pair of larger turbos are probably best mounted either side of the transmission rather than straight off the side of the manifolds, while a big single is going to be best to go in above the transmission.



Single or Twin Turbochargers on a V Configuration Engine

[SPOILER]It is no secret that APS is developing a single turbocharger system for the Nissan 350Z, however this new offering may create confusion in the market place regarding the benefits of single versus twin turbochargers. The following discussion sets out to explain the strengths of each approach so that APS customers can make an informed decision as to which application is appropriate for their requirements.

Turbochargers demand a good deal of high energy exhaust gas to drive the turbocharger turbine. Exhaust gas energy is a function of the mass flow rate of exhaust gas, gas temperature and velocity. The higher each of these exhaust gas parameters, the greater the energy available to spin the turbines. This means that the exhaust manifold design and the proximity of the turbocharger to the exhaust ports is critical in the overall performance of the turbocharger.

A "V6" configuration engine such as that found on the 350Z has 3 cylinders on one bank and another 3 on the other bank. This means that for the optimum turbocharger operation in terms of turbocharger response and resultant engine power over the entire RPM range, a turbocharger must be located in close proximity to each bank of cylinders - ie twin turbochargers.



In a twin turbocharger configuration, each turbocharger is located close to the respective cylinder bank for the optimum exhaust gas energy transfer to each turbocharger

A single turbocharger configuration on the other hand necessitates the exhaust gasses from each bank travel a longer distance than that of twin turbochargers located at each bank. The total distance travelled is determined by the placement of the single turbocharger but in short, exhaust gasses from one bank must travel across the width of the engine bay and merge with the gasses from the other bank before finally entering the turbocharger. This has a negative impact on the total exhaust gas energy available to drive the single turbine.

No doubt the twin turbocharger approach is the optimum configuration in terms of overall engine performance, however there are cost advantages to consider with the single turbocharger approach.

To offset the affect of lower exhaust gas energy available to drive the single turbine, the size of the single turbocharger must be reduced when compared to the total turbocharger capacity of twin turbochargers in order to achieve similar low to mid RPM engine performance to that of a twin turbocharger configuration.

Whilst the above discussion applies equally to any modern "V" configuration engine, when dealing specifically with the 350Z engine, we can now discuss turbocharger sizes that are meaningful and applicable to this engine.

The APS twin turbos have approximately 80 lbs per minute total air flow (resulting in 800 flywheel horsepower) where as a single turbo which would produce a reasonable low to mid range power curve would be limited to around 60 lb per minute mass air flow sized turbo (resulting in 600 flywheel horsepower).

There are 3 different scenarios to consider:

To achieve similar low to mid RPM power and turbocharger response as the APS Twin Turbochargers (80 lb per minute = 800 hp) one would need to specify a single turbo of around 60 lb per minute = 600 crankshaft maximum Horsepower.
To achieve the same outright horsepower as the APS Twin Turbochargers, one would need to specify a single 80 lb per minute turbocharger - which is a massive turbocharger for a 3.5 L engine. The down side is that the low to mid RPM response would be greatly compromised.
To achieve higher horsepower than the APS Twin Turbochargers, one would need to specify an even larger single turbocharger - say around 90 lb per minute mass air flow. This turbocharger would have an operational range starting at 5,000 RPM (no useable power to speak of below 5,000 RPM). In this case, the engine would need to turn out to over 9,000 RPM to have a worthwhile power band. This single turbocharger may be viable in a competition engine which spends little time below 5,000 RPM, but be unpleasant on the road in most driving conditions.
There will be a crossover point on the power curve if the single turbocharger is significantly larger than the twin turbochargers - and only in case 3 (where the single turbocharger is larger in total air mass flow rate than the twin turbochargers). This cross over point will be at some point around 6500 RPM (best estimate only).

But getting back to production specification - a single 60 lb per minute single ball bearing turbocharger (600 flywheel horsepower turbo) - its power curve will always be below that of the twin ball bearing turbochargers (800 flywheel horsepower total) at the same boost level. It's virtually impossible to achieve the same low to mid range power and turbocharger response from a large single turbocharger in a V configuration engine.

If the single turbocharger is matched to produce strong low to mid range performance (which would be the wise choice) then obviously the turbocharger specification will need to be precisely matched to the engine capacity. Bottom line, a large single turbocharger matched for strong low to mid range performance on the Z V6 engine will always have a power ceiling of around 600 flywheel horsepower (around 500 wheel horsepower).

Regardless of the single turbocharger size, the real issue pertaining to high horsepower on a single turbo conversion for the Z is the very limited space available to package an exhaust downpipe capable of producing over 500 wheel horsepower. This is where we see the real limitation of the single turbo design for the Z in comparison to the twin turbo approach, unless you're prepared to cut the body sheet metal and make some fairly radical mods.

It's all very well to have an large single turbo, but when it's limited to around 500 horsepower due to a restrictive exhaust down pipe, you'll never see the real potential of the large single turbocharger.

Hopefully this helps to put the single turbo in perspective - and to give one an idea of the challenges that are presented to APS as turbocharger system design specialist.

In our view, a well designed single intercooled turbo system with a well matched turbo would be a very streetable package on the V6 350Z engine up to around 600 flywheel horsepower. This is a great option for the 350Z enthusiast who desires engine performance that is superior to the lower cost supercharger options - but at a similar price point.

That said, the twin turbochargers utilized in the APS Intercooled Twin Turbo system deliver superior low to mid RPM engine power but with a higher power potential of up to 800 flywheel horsepower (rather than 600 flywheel horsepower of the single turbocharger configuration). This of course comes at a higher initial purchase cost.[/SPOILER]

Yep, have read that article before. (Some years ago actually)

I don't recall seeing any hard evidence of the performance differences on that engine when using 2 turbos vs a single... so I take it with a grain of salt that they may have been keen to make it sound as good as possible to sell the setup.

I do agree in principle with that article, however I am trying to be somewhat more practical. The VR-4 is a different beast, it has less displacement and the engine configuration is different (I think... isn't the 350z a forward facing engine?) which causes some packaging constraints.
As such I don't think that article should be held as a bible for the VR-4, though it is useful.

It seems the smaller turbos don't like to produce as much boost as the bigger ones, which is where I see a possible advantage to using a single in this case.

Sure, the single might spool up a bit slower, but does it matter? will that difference be great enough to be very noticeable, and will the top end kick of the single make up for any reduction in response?

As and addition, I am looking at the compressor map of the GT25R and it looks like they might surge on the VR-4 if they spool too quickly... You would want some significant increase in the air flow through the engine to avoid it.

As you say, the graphs don't show how quickly the turbo will respond, but it does show you what the turbo is capable of putting out flow wise. There is no point in it responding ninja fast if it just surges.

I would like to point out at this point that we aren't talking about huge HP figures, the gist of my thoughts were towards a fairly flexible setup with moderate power capability. (Economy I suppose... power vs cost vs practicality etc)
As such I am not really that concerned with the out and out performance bible as to how to get the best turbo performance possible with the biggest power output, but rather the capabilities of different setups and how the trade offs of each stack up.

the click to show thing isn't coming up, here is the link to it, scroll down a bit to find the twin vs single setup http://www.airpowersystems.com/350z/turbo/turbo.htm

The GT25R's are easily capable of 240hp on a 1.3l engine. While I'm slightly ashamed to admit it, I've been hanging out on starlet gt and suzuki swift forums a bit, and they find that gt25r's are great turbos for their engines with decent spool, normally around 3000rpm and are able to make 150kw atw without a problem. Typically they are running at about 1 bar, few people have gone above that because few of them have had forged internals.

My own personal view is that if you take a twin set up, you will be able to use small turbos, and 'generally' speaking, you will get a lower spool, and less lag. For an every day car, this is going to make it much more driveable.

If you take those twins and increase their size, then i think you are going to be closer to retaining that whilst also providing more top end power.

As for the single, whilst its total rotational mass must be less than two twins, i still think it is going to spool slower. My reasoning being that if you look at stock manifolds, the turbo has been mounted as close as it possibly could be to the ports. They have done this to reduce the energy losses from heat loss and friction (of the gas travelling through the pipework) to a minimum. They have been careful to keep gas speed up and not let it lose any energy. When you start putting on a single, your headers are going to be much longer, you are tempted to wider pipe which slows down the gas, and you will lose a lot of heat. Result, some of the energy you would have converted is lost.

What i am trying to get at is that if you have a 'right sized' single on a V engine, vs two right sized twins, then i bet the single spools later simply because of the additional loses involved on the hot side. As a result, you would probably need a slightly less than ideal size if you were going to run a single. I am talking purely about spool here- not out and out performance and hp numbers.

Just as an example, a customer of ours had a set of 15G's which he remote mounted up above the gearbox. This meant there were long runners leading up from the manifolds to the turbos. The set up could make 5psi max and no real boost before 4000rpm. The exact same turbos, not touched in anyway, then put back into the stock locationwill boost 20psi plus and start pulling just below 3k!

Hope thats useful.

Cheers,

Ben.

ok, so my idea of having two gt2554r's, one either side of the gearbox isn't a good idea then!

It lost 1 bar boost, just by moving them, that seems a lot.

It was!

Basically it couldnt breathe with the turbos so far from the ports, but back in the std location it performed as you would expect.

I would say for remote location the length of the primary and secondary runners, their ultimate and relative diameter and routing will all be critical.

I was always of the impression that the least loss before the turbo the better, so if you have to remote mount then you are getting into that right away.

Try driving something like an Evo, the turbo is right there as close to the ports as you could really get it. Now drive a scooby. There is a very significant difference, although remember the scooby exhaust route and diameter will have been 'tuned' by the designers. Come to think of it, lots of people put so called 'up pipes' onto scoobs expecting a power gain, but they often get a power loss.

Cheers,

Ben.

regarding the single vs twin debate by Ray Pampena (3si.org)

PS: ... I am from TwinTurbo camp ... :evilgrin:

thanks for the link valmes!

Useful link Valmes, thanks :)

Interesting that it brings up the point "apples with apples"

With the 2 compressor maps posted, I don't believe you are comparing apples with apples though. The GT35R looks to behave in a manner that says it is smaller than 2x GT25R but can handle higher pressure ratios better. If this is the case for small turbos, then in this case is it possible that we could get better performance out of the GT35R than 2x GT25R?

To be honest, I have always advocated going twins. Much for the reasons given in the links given. I did a lot of research a few years ago on the subject and it definitely seemed that going twins was the way to go.

In this case however, looking for only (compared to the likes of many, usually where this information comes from) moderate power figures delivered in a usable but not necessarily spectacular manner, who's to say that a single might not perform well?

Zentac, what turbo do you have? and what sort or RPM do you start getting usable boost?