Designing an engine built around turbo inlet restrictor

G'day all,

I'm new to engine building and have just started the fundamentals course. My question is based on the theoretical side of things and the maximum airflow through a 36mm inlet restrictor before sonic choke is reached. At this stage I don't have any ramp rates of the restrictor as the maximum flow will assist in determining turbo sizing. Once I have the flow rate, I can begin to calculate the engines requirements from there. From, what I've found, 194L/m seems to be the choke point, but I want to clarify this as these figures seem to be too small in the real world. Then there's the volumetric efficiency to take into account and an assumed value.

If anyone has any sources or formulas for calculating this, it would be greatly appreciated.

Wastegate control should probably be shaft speed and/or turbo inlet vacuum based in combination with target manifold pressure/charge density below choke to minimise unnecessary exhaust manifold pressure and pumping loss.

You probably want to minimise cam overlap too.

Depending on your rule set maximising pressure to, and minimising temperature of airflow to the restrictor and positioning/transitions in/out can make a significant difference to flow and pressure drop if there is freedom to do so. If that is the case looking at FSAE historical restriction designs may be worthwhile.

Can you link to your rules document?

Hi Michael,

Thanks for the info, I'm a bit unsure what you mean with your first point regarding wastegate control. Are you saying that I need to determine exhaust manifold pressure to come up with the inlet restricted flow rate? Please excuse my misunderstanding if wrong.

The category requires the restrictor to be mounted within 50mm of the compressor wheel, which I believe rules out FSAE for influence as they have a much more gradual ramp rate after the restrictor to minimise flow separation. The category in question is Improved Production. 2024 regulations (I'm unable to locate 2025, yet there are very minimal changes year to year) are as follows:

https://motorsport.org.au/wpblob0fe832abcb/wp-content/uploads/docs/default-source/manual/race/2024/ra26-group-3j-2024-1.pdf

The point on turbo control wasn't actually knowing EMP but not making it any higher than required to achieve peak flow or very close to it on the restrictor. Once the restrictor has choked there is no point increasing wastegate effort as the higher EMP doesn't result in increased mass flow, just more pumping loss on the exhaust stroke. Likewise as you hit choke IMP:EMP will degrade, so high overlap cams aren't desirable unless you think you can gain efficiency from additional pumping gas without unstable combustion or slowing flame propaganation too much.

If you do some searching you may find someone has datalogged or characterised a similar restrictor to work out how much vacuum is required to approach choke flow on the restrictor. Pulling any more vacuum at the compressor inlet is lost power and higher exhaust temperature for no gain.

Is the inlet side of the restrictor bellmouthed? It wasn't my area of work but from memory the flow standard nozzles (noting these were a straightpipe exit more like your restrictor) used to calibrate gas flow meters in a large switched array were close to choke with ~8X kPa absolute on the downstream side and absolutely stable with ~7X kPa absolute downstream. There are lots of papers on characterisation of these nozzles you can probably find or at least excerpts for free that should give you a ballpark target.

I might do a bit of searching on the laptop later and flick Khaled a message on LinkedIn to get a ballpark figure it's been quite sone time since I worked at Lindfield now. They used something like a 50kw roots blower for vacuum on the big rig IIRC.

Quick look at the rules shows an option with a decent bell so you should be at least in the ballpark with the ISO oe EN nozzles for performance vs vacuum. Doesn't appear to be anything stopping you logging compressor inlet vacuum either. Absolute worst case you could run wastegate duty vs rpm based on logged data while working out if you can come up with a better control strategy with available logic/tables in your ecu.

These kind of designs.