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1988 Mr2 Supercharged
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Discussion Starter #1
Ladies/ Gents,

I have recently completed a 4AGZE build. The build includes a rebuild SC, new vacuum lines and a 176mm Fensport crank pulley. As part of the build , I've also installed a boost gauge (autometer) and have performed the "ABV mod". The schema below presents my existing setup.

ABV-MOD_SC4PLAY.jpg


Based on my current setup, I would expect to see 13psi of boost but it's limited to 10psi. In my perception, this means that there is something wrong with my "ABV mod".

I know that this topic has been thoroughly addressed in the past but browsing through the existing forum threads did not bring me the answer I'm looking for.

Just to clarify, my belt is not slipping and I don't have any boost leaks. The car pulls hard without hesitation but the boost is limited to 10psi. I'm looking for the lost boost.

Can anyone help?

Thanks
 

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There are a lot of variables that determine boost pressure of a fixed displacement supercharger. Atmospheric pressure, temperature, and intake plumbing can all lead to variable levels of boost. The supercharger is pumping the same volume per rev, no matter what the downstream pressure (within a reasonable limit). This means you get a volume of airflow (and power), not a preset boost pressure (like a turbo).

This is all to say, there is more likely nothing wrong and your engine can flow the amount of air your blower is putting out without requiring as much manifold pressure to do it.
 

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10psi is a very suspicious value. If the ABV mod is working properly, the boost should force the ABV valve closed and vacuum should open the valve. If the boost was attached on the wrong side of the diaphragm, vacuum would close the valve and boost would force the valve open at less than 10psi.

My best guess would be that the ABV valve diaphragm has failed so that vacuum does not open the valve and 10psi opens the valve. The SC will freewheel under vacuum and the engine will sort of run normally.

However, if you changed the cams during your rebuild, that might be just the way your engine is supposed to run now.
 

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1988 Mr2 Supercharged
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Discussion Starter #4
Thanks guys!!

I was looking for a logical explanation for the missing boost and I get what you are saying. The rebuild does indeed include stage II PDM Racing Cams (261 intake/ 268 exhaust) and 3mm bored throttle body.

Getting back to the point, does that mean that I have too much valve overlap? If the supercharger maxes out at 13psi, and I'm only seeing 10psi in my intake, than I guess that the rest of the boost just goes through my exhaust valves... Is that right.....? 🤔

Just to be clear, I'm not craving more power. The car is very fast already. I just want to understand what's going on so that I can learn for future projects.
 

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I am no cam expert but I think it's more complicated than just losing boost through cam overlap. A long intake period that starts early could also allow for more cylinder filling and thus allow more airflow volume. More volume admitted into the engine equals less boost. There is even a correlation between exhaust back pressure and positive displacement boost but that usually works out to more back pressure equals more boost (actually wasted boost).

The better your engine pumps air (volumetric efficiency) the lower the boost on a positive displacement blower even though HP will be higher with lower boost. However too much overlap will also lower boost.

The important thing is not boost. The important thing is mass air flow combusted into horsepower. That really can only be quantified on a dyno.
 
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