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Air-restricted turbocharged single-cylinder...

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New to this forum, reading all I can on tuning, but haven't had time to gather a ton of practical experience with tuning yet.

I'm a Norwegian mechanic, now turned engineering student, competing in Formula Student. The cars are small, single-seater open-wheel cars. Tracks are very narrow with short straights, so driveability and a wide powerband is important.

The competition rules limit us to 710cc, and a mandatory 20mm air restrictor in the intake. However, turbocharging is allowed (same size restrictor as NA engines!). The restrictor is to be placed before the turbo.

The engine we've chosen is a KTM 690cc single cylinder. (Bore x stroke: 102x84.5mm, CR: 12.6:1)

2020 will be the second year we're using a turbo. The setup we competed with this summer consisted of a completely stock engine, a Garrett GT1241z turbo (internal WG), BOV and an intercooler. This setup was kinda built by the "just slap a turbo on it"-approach. When looking at the compressor map, this turbo seemed like a good fit for the engine, assuming we have a max air flow of approximately 7-8 lb/min and a PR in the area of 1.75, based on a maximum boost level of ~0.5 bar (~7-8psi). See attached picture of compressor map.

This is a very time limited project (design and build a car during one year of school), so we only had time for one half-proper dyno session before driving. We hit boost almost straight away and got quite good torque figures early on. However, the torque (and power) drop off quite early, and exhaust pressure shoots upwards. See the attached graphs.

The internal wastegate has no chance of venting enough air to cope with the pressure, it opens fully straight away. (5psi/0.35bar spring).

The dyno results I posted doesn't show it, but we tried a run up to 7500rpm, only outcome was a decrease in power and rapid increase of the exhaust pressure. The engine revs to ~9000rpm in its standard application, so we were initially hoping for something close to this.

Any thoughts on these results? I suspect the turbine housing is too small/restrictive, but really not sure this is the only problem...

Should we see an early decrease in power like this only from rising backpressure? Or would you suspect that there is a relating issue with something like cam overlap, undersized (wrongly calculated) compressor side, or something else?

If anyone has some general input, thoughts or ideas regarding both turbocharging with an air restrictor, and turbocharging single cylinder engines, please speak up! :)


I realise we still have a lot of unknown variables in this system, trying to lock a few of them down:

- Our engine sponsor has a Superflow engine dyno, we are 95% finished with making adapters to hook up the motorcycle engine to the flywheel connection of the dyno, so hoping to be able to do some trial and error ehrm... R&D there throughout the winter.

- We are going to have a pressure sensors between the restrictor and turbo inlet, and an extra pressure sensor right after the turbo, to get true pressure ratio from the turbo. I'm not sure if we'll be able to implement a turbo speed sensor, but will look into it. If I recall correctly, the dyno has air flow meters to use during dyno sessions, so that might be all we need.

- We/the engine sponsor are also making a measurement jig for the head to get some realistic measurements of cam durations and lift.

Attached Files

Dyno chart wasn't included in first post - here it is

Attached Files

I believe there is a decent FSAE forum these days if you haven't looked there yet. Any decent engine, even NA will flatline or fall of power with the restrictor. The challenge is to select a turbine and control the wastegate to minimise losses after reaching choke flow. Probably worth looking at the strategies employed on restricted rally cars too.

Not much knowledge to gather there I'm afraid.

The restrictor is not close to choke flow for this kind of boost/power/rpm, so I'm fairly certain this is not the limiting factor. 20mm should theoretically flow enough air for 100+ hp, there are teams making ~90hp with turbo setups. (Usually not single-cylinder engines though)

I'm finding quite little specific info on tuning restricted turbo engines, there's a few threads on here and other forums. I also remember reading a good, detailed interview with a Prodrive engineer in the Race Engine Technology magazine (highly recommended btw).

The essence of this information is mostly keeping the restrictor choked while monitoring/managing "vacuum" between the restrictor and turbo, as well as using anti-lag.

We will look into using anti-lag, but before being able to exploit the full advantage of that, I think we need to look more into the dynamic compression, and deciding what boost level we want to run at. We were quite knock limited on this year's engine, as you'd expect..