Exhaust manifold (vs. headers)

[Warpspeed]

Rex, if economy is the absolute priority, headers still win out through lower total back pressure, although in practice the difference may be more theoretical than practical over what you are proposing.

Biggest gains will come from a much higher compression ratio and some really creative valve timing.

I suggest you research "Atkinson Cycle" and "Miller cycle" engines.
These usually always run a supercharger for best results, but you can do something very similar with a normally aspirated engine.

The trick is to close the inlet valve very late, when the piston is a long way up the bore.
This reduces the dynamic compression ratio because compression only begins after the inlet valve has completely closed.

This enables you to run a crazy high compression ratio with a very small combustion chamber without exceeding the temperatures and pressures that could reach the detonation threshold.

Once it fires, you can get a very high expansion ratio, especially if you keep the exhaust valve opening very late.
This won't get you any more power, but it can sure will help the BSFC and thermal efficiency if that is your main goal.

A much larger exhaust valve can also help reduce exhaust pumping losses which also helps.

[Boffin]

Rex

Be very careful restricting flow through your exhaust. I will guess that this will make you add fuel.

I say this from experience though and our exhaust efforts 09 to 10.
09 ran the 08 exhaust, which was a 1" primary (port is 1 3/8". It ran a tapered nozzle about 2" long to change size). It also coupled incorrect pairs (1&3, 2&4 i believe) in a 4-2-1 exhaust.

In 2010 we stepped up to a 1.5" primary (1.5" straight to the 1 3/8" port. There is a reason for doing this) and couple correct pairs together (1&4, 2&3), still keeping the 4-2-1. The intake and engine internals were unchanged from 09.

By doing this we reduced fuelling by an average of 27% (I have the data to prove this), while increasing torque and thus power everywhere.
Torque was 8Nm up to 75nm @ 7300rpm shifted 300rpm down (09 peak was 7600rpm).
Power increase 12-14kw, up to 68kw @ 11300rpm, shifted up 2900 rpm (09 peak was 8400rpm).

If you want to reduce fuel usage with a 4 cylinder car, choking flow to do it not a good way.
A very smart way to do it is to look towards Sophia and the way they use their engine and a lesser degree to Swinburne’s petrol car and look not at the engine but at the drivetrain.

[RenM]

If you are already running lean and still want to cut power reduce the rev range further. It will reduce mechanical and throttle losses.

If you have no problem with knocking increase your water temperature to reduce in cylinder heat transfer losses.

However at some point you will have to question your car concept...

[RenM]

Originally posted by Warpspeed:
This enables you to run a crazy high compression ratio with a very small combustion chamber without exceeding the temperatures and pressures that could reach the detonation threshold.

I reckon they are still running the 600 cbr, so extremly high compression ratios (>15) will geometrically not be possible due to the very small stroke. Everything beneath can possibly be run with e85 without knocking.

[Rex Chan]

RenM - I agree, a fast 4 cylinder is prob never going to do well at fuel economy (though UWA did it in 2011, by running in low RPM/low power range).

yes, we are running CBR600RR 03-06, with E85, so have had no knock issues (at stock 12:1 CR).

[Mbirt]

I'm with RenM. Instead of reducing volumetric efficiency, why not just shift it to lower revs to downspeed the engine? You could go beyond changing duct lengths and try concepts like closing the intake ports down with epoxy for increased charge inertia where you need it at low speed. I saw tiny diameter intake runners on at least Toledo and Stuttgart this year.

I have seen a healthy reduction in POT BSFC using atkinson cycle operation on another engine I'm working with, but the reduction in low-mid range torque will not be welcomed in an autocross car.

[Warpspeed]

Originally posted by Mbirt:

I have seen a healthy reduction in POT BSFC using atkinson cycle operation on another engine I'm working with, but the reduction in low-mid range torque will not be welcomed in an autocross car.

Atkinson cycle is a fairly radical approach to improve thermal efficiency at the expense of power output. No argument about that.

But as it involves valve timing and compression ratio changes, it does point the way towards a possible compromise situation.

As suggested earlier, a smaller bore longer stroke engine might be a better candidate for this type of approach.

[Mbirt]

Please, someone. Do it: http://www.ski-doo.com/technol...ogies/4-strokes.aspx

[Jon Burford]

Originally posted by Mbirt:
Please, someone. Do it: http://www.ski-doo.com/technol...ogies/4-strokes.aspx

Yes please.