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Comparing OMP output: s4 FC vs s5 FC vs FD

Old 05-03-19, 11:45 PM
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Comparing OMP output: s4 FC vs s5 FC vs FD

Hi all

Before you think oh no not another OMP vs premix thread, this is not one of those discussions!

What I'm trying to figure is the best metering oil pump to run in my application.

I have a series 5 13B turbo block with EFR7670 IWG fueled by straight LPG (propane), which is delivered in vapour form via a gas mixer/throttle body.

Being straight LPG, there is no way to premix oil in the fuel tank so OMP is the only option. With a potential maximum power output of around 350-400rwhp, I'll want to make sure whatever OMP solution I go with can deliver sufficient output. Many assume that being a 'dry' fuel, LPG may require higher oil injection than a liquid fuel. I am inclined to think the fact LPG is delivered as a gas may lower lubrication requirements as it cannot wash the oil away from where it needs to go in the same way as rich fuel mixtures might (something which bumpstart can back up from pulling down engines run exclusively on LPG). Nevertheless, I want to get this part of the build right.

Being a series 5 engine, I already have a series 5 electronic metering oil pump but it has the same footprint/bolt pattern as the FD so I potentially have the choice of fitting a later model item. I will be running an aftermarket ECU with EOMP control, so whichever option I choose i will be able to tune oil delivery. For simplicitiy, if I switch to an early s4 FC front cover, I could also use a mechanical OMP from a series 4 turbo.

Now, the OMP Declassified treads on this forum posted by TTMott have been fantastic. I have read them thoroughly and spent a heap of time researching other old posts. The OMP Declassified threads conclude that the Mikuni OMPs on series 8 JDM FD engines has twin pumps and higher maximum output than the Denso OMP used on 93-99 engines. 2000 onwards engine also introduced changes to the oil injector nozzles and jets.

The higher output from the Mikuni makes sense as the higher spec s8 variants had the 280ps engine with approximately 20% power output than early FDs. On that basis, I may as well just upgrade to the series 8 item.

However, it's not that simple. Mazda were also trying to REDUCE oil delivery to improve emissions performance. This is shown by the new coatings used on the FD rotor housings and reducing the 4 nozzles in the FC to just 2 nozzles on the FD. With the Renesis, Mazda took this further, but there were obviously issues with the series 1 RX-8, and the series 2 Renesis features a much higher output OMP with pressurised output and 6 nozzles instead of 4.

What I haven't been find any information on is the maximum output of the electronic OMP used on the series 5 RX-7. Power output on the series 5 was considerably lower on the FC but the oiling requirements were likely higher due to the rotor housing surfaces and apex seals in use at that time. Likewise with the series 4 - I wonder whether max output is similar to the electronic items, after all the pump itself looks similar - what differs is the stepping motor that provides metering of the output.

I also haven't been able to figure out the effect of injecting via 4 nozzles instead of 2. I assume output to the rotor housing nozzles is effectively half that of the FD?

So I have a choice: keep the s5 OMP, convert to s8 Mikuni OMP (which I should be able to find, as s8 imports are plentiful in Australia) or convert to series 4 mechanical.

In making this choice, the great unknown is maximum oil output. Leaving output aside, here's my breakdown of the pros and cons:

Series 5 EOMP

Pros
  • Already have 1
  • 4 oil injector nozzles when oil more likely to be safely delivered even if 1 or more nozzles become blocked
Cons
  • Extra lines, means extra cost/time in replacing them (mine need to be replaced)
  • 2 manifold nozzles are not dripping oil where it is actually needed - at the apex seal
Series 8 Mikuni OMP

Pros

  • Simplicity - only 2 injector nozzles and lines
  • 2 new lines are pretty cheap to buy new from Mazda
Cons


  • No redundancy if oil injector nozzle becomes blocked, which is apparently quite common with the newer style nozzles Mazda sells now.
  • Extra cost
Series 4 mechanical OMP

Pros

  • Reliability - do not fail - they only leak and rarely if well serviced
  • Easy/cheap to replace
  • 4 injector nozzles provides redundancy in case 1 or more nozzles become blocked
  • Frees up 4 ECU aux outputs
Cons


  • Need to change front cover to s4 item to work
  • Need to rig up cable to operate metering lever
Anyone have any thoughts on the output question and recommendations on which way I should go?
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Old 05-04-19, 09:58 AM
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interesting, it seems like the S8 pump is the easy button, but it would be interesting to test the S4 turbo pump. the S4 turbo pump can flow a lot, but it would be nice to have a quantification instead of a story about how my old car used a lot of oil one night...
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