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Premium Member
718 Posts
Discussion Starter · #1 ·
As many of you know I had a failure of my Magnus SMIM, and put on a ported stock intake manifold as a replacement. I had to wait until now to do some real data logs because I wanted to put new injector isolators on befor going into heavy boost. I finished that up today, and the results are interesting!

Running a speed density system with the AEM EMS, so any significant changes in air flow will show changes in your A/F reading on a data log. Here is the jist of the testing.

Virtually identical spool up and A/F ratio until above 7000 RPM. At around 7300 RPM the ported stock intake manifold log shows a significantly richer A/F ratio all the way up to 8000 RPM where they showed an A/F ratio of 10.75:1 for the ported stock manifold vs the same tune with the Magnus SMIM where the A/F ratio was 11.3:1

My conclusion is that the Magnus has no downfall in the lower RPMs compared to a ported stock intake manifold, and shows significant gains in air flow above 7,000 RPM. Keep in mind this was one of the first SMIM's Magnus made for the EVO, and did NOT have the velocity stacks in it.

My impression from driving the car (seat of the pants "feel" of the car) is that the off boost throttle response of the Magnus SMIM was much better than the off boost throttle response with the ported stock intake manifold.



Premium Member
399 Posts
Interesting Keith - how much is Magnus paying you? :p Just kidding!!!!!!

The theory with aftermarket intake manifolds is that they usually always suffer on the low end. I know my Forrester manifold is not nearly as good with the off boost response as my stocker. This is good info.

70 Posts
Running a speed density system with the AEM EMS, so any significant changes in air flow will show changes in your A/F reading on a data log. Here is the jist of the testing.

If everything else is the same.
Air temp, baro reading, coolant temp, humidity.

But yes in all likelihood what you saw can be explained as you said.

It is interesting that the stock unit flows so poorly under boost.

here is something I picked up on the internet so take it for what you will. It is not really addressing Fourdoor's point, it was in regard to a question about runner length tuning.
Interesting on its own nevertheless ( for people like myself who need primers).


There are two basic problems here.

The first is to decide if you want to optimise your induction system for normally aspirated (off boost), or turbocharged operation.

The whole inertia and resonant tuning of an induction system is an attempt to stuff maximum charge density into the cylinder at the instant of inlet valve closing.

By far the most important parameters are induction path flow area and inlet valve closing point. The idea is that flow will continue into the cylinder long after BDC due to the built up inertia of the gas column in the induction runner.

If the runner is too small in flow area, pressure drop will be high and will restrict cylinder filling. Likewise too large an induction runner area will reduce gas velocity and inertia, and also reduce the ram effect. So there will be optimum runner velocity that will set an Rpm where Ve is highest. This is always the biggest single factor that sets where the torque peak will be for any normally aspirated engine.

If you don't believe this, try tuning an intake runner that was only half an inch in diameter for peak power at 9,000 Rpm. It simply will be too small to work, no matter what length you tune it to. An intake runner a foot in diameter would have such a low velocity there would be no gas inertia to tune.

So, there will be a definite relationship between cylinder capacity, induction runner flow area, and an Rpm where the torque and Ve will rise to a normally aspirated peak. You will see this when boring or stroking an engine. The torque peak always moves to a lower Rpm.

It is also possible to tune the runner length so that reflected pressure waves can add to the inertia tuning effect, but it is definitely a secondary effect.

The strongest tuning hump can be obtained by second pulse tuning. That is, the induction tract length is tuned to twice the induction pulsing frequency. With any four stroke engine, fundamental tuning is simply not possible. Second pulse tuning requires a total runner length (open end to intake valve) of 108,000 divided by Rpm.

This can rock the peak torque figure about it's natural centre, or spread the available torque over a wider range, but it cannot compensate for an induction system totally unsuitable in flow area for the application.

Third pulse tuning is also possible, and requires a much more practical induction length, which will be 97,000 divided by Rpm inches. but it will not be as strong a peak as second pulse tuning.

Fourth pulse tuning will be 74,000 divided by Rpm inches.

Fifth pulse tuning is 54,000 divided by Rpm.

The interesting thing is that although the higher order pulse tuning is weaker, the peaks appear closer together. For instance a ten inch runner length in a tunnel ram manifold will have peaks at 5,400 Rpm (5th pulse) and 7,400 Rpm (4th pulse) and might work rather well with a suitable camshaft. Ten inches is also a practical sort of length.

Or you might design a second pulse manifold to peak at 7,714 Rpm. It might be more powerful, but with less midrange torque, and will require a fourteen inch runner which may be difficult.

Anyhow you can tune your engine to have best Ve over any Rpm range you want.

A turbo is a whole lot different. It can produce massive peak induction pressures and flow at almost any Rpm, simply by sizing the turbo. But an induction system optimally tuned for lower Rpm may be very restrictive to the turbo at the top end.

You could decide to optimise the induction system for best torque to aid spoolup, but that will be restrictive at redline. A flat out balls to the wall turbo race engine designed only for top end power will have massive ports and short runners, and it will probably not gain very much from induction tuning. The turbo provides all the air, and the induction is designed for minimum flat out restriction.

So decide what you want. A good strong and responsive street engine with long runners and sensible cam that is "turbo assisted" at higher Rpm.

Or a genuine turbo race engine that relies entirely on boost pressure to make it go. It will be a slug off boost, but then it is a race engine, right ?
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