AFM- major restriction? - Page 2 - Miata Turbo Forum - Boost cars, acquire cats.

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DIY Turbo Discussion greddy on a 1.8? homebrew kit?

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Old 05-23-2007, 12:26 AM   #21
Ben
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the supercharger has a throttle body pre blower. the tb before the intake mani is replaced with a dummy tb

that's why you always hear about the "throttled volume" issues with superchargers and not turbos

Quote:
Originally Posted by samnavy View Post
I don't get it. I understand the theory in the second sentence, but it says in his sig he runs a JSRC. They're still a hot-side install in this order, right:
Filter-MAF-intake-compressor-charge pipe-TB-intake manifold-runners-engine.

That would be sucking through the MAF and blowing through the TB. What am I missing?
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Old 05-23-2007, 12:39 AM   #22
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The fixed displacement pump is the biggest difference with a super. If flows a fixed CFM relative to engine RPM at whatever the inlet pressure is. If you lower the pressure at the super's inlet, you flow less air mass, and get less boost and less power. With the turbo, less inlet pressure will just cause less drag on the compressor wheel, causing it so spin faster, and still suck in nearly the same amount (mass) of air. The boost on a turbo being controled by a wastegate also ensures you still get the same boost pressure even with more intake restrictions. You will need more pressure in your exhaust manifold to generate that boost and you will loose some power, but not nearly as much as a super charger would.

Super chargers are much more sensative to air restrictions before the inlet of the pump, which is why a more free flowing AFM has more effect on them.
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Old 05-23-2007, 12:45 AM   #23
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I never knew that about the JR setup. It makes some sense now.


Last edited by samnavy; 08-01-2007 at 10:53 PM.
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Old 05-23-2007, 12:48 AM   #24
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that's superchargers in general, not just the jrsc.
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Old 05-23-2007, 02:23 AM   #25
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From reading this forum it's my understanding there are a lot of dummies with superchargers. Now we know that sc's have dummies x 2.
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Old 05-23-2007, 09:53 AM   #26
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There are only two ways I know of to really tune an AFM. One involves a dyno and a spec Miata shop and they make 'adjustments'.

The other way is described here:
http://forum.miata.net/vb/showthread.php?t=144196

The 'frankenmeter' is sitting on my shelf and is still available. (I don't have enough posts yet to use the classifieds, sorry!)
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Old 05-23-2007, 10:19 AM   #27
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Quote:
Originally Posted by Braineack View Post
those runs are at 12psi, ive taken multiple logs, with 203cc and 265cc injectors and couldn't get any good midrange AFR with an acceptable high-end AFR, so the compormise was 12.5:1 from 3-5K and 10.8:1 from 5.5-7K
No at 8 psi not with MS though with the stock ecu and RRfpr. I essentialy just maxed what the stock injectors where good for. This was with the stock AFM BTW.
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Old 05-23-2007, 10:30 AM   #28
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Quote:
Originally Posted by Chris Swearingen View Post
Let me start by saying my information is based off a Supercharged application which means that I am pulling air through the AFM not pushing it, but ...

I found the AFM to be a restriction. Swapping to an RX7 AFM made about half a pound more boost with no other changes. My before and after dyno runs show about 10 HP difference, but I had also added an EMU so my A/F ratio was much better.
snip
Quote:
Originally Posted by Braineack View Post
snip
I don't doubt it your results; however, it also looks like (12) runs were done in between the comparison with the addition of the EMU, and there is a significant change in the AFR.
You are correct. It was not back to back runs showing horsepower changes. We were talking about restriction. The increased boost especially on a supercharged application is evidence of removing the restriction. You can see by the air fuel lines that considerable tuning changes were made between the two runs. Turning up the boost in supercharges only happens with changing how fast you spin the blower OR how well the blower can get the air. The comparison of boost values is a direct comparison of the restriction of the air flow meters and air filters. The dyno runs were not done for the sole purpose of measuring the efficiency of the AFM but do show some empirical data. The link shows further restriction evidence from removing the AFM completely although that data is based on EMU logs not dyno runs.

To summarize what I was trying to represent, the supercharger saw less restriction with the RX-7 AFM as evidenced by the increased boost pressure.
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Old 05-23-2007, 10:34 AM   #29
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I believe it form the blow-through perspective, and even on turbo setups because talk of increased boost without and AFM or a larger one (i never really paid attention).

but I think on a turbo setup, since only a bit of vacuum even exisits behind the AFM at something like 12psi, it's not a big deal.
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Old 05-23-2007, 10:35 AM   #30
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I believe the results of DIY auto tune and link showed that afm removal made about a 10% difference in stock trim with out major timming changes, and just some more sorted fueling. So in short the only reason for rx7 afm is to modulate fueling better. I love to hear the noise of my intake sucking in the air from my filter. even with the AFM installed
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