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Hi Guys, How would you expect to see a map sensor respond on a wot ramp run. Attached is a graph of my map sensor - it looks like to me it starts to create vacuum as the rpm increases - Is this a sign of the throttle body being to small?- the engine Is a 450cc mx bike modified for motard racing. I was hoping to see a larger power increase with the mods I have done over stock. Ive got an extra 5hp over stock. I even tried VP ms103 fuel and got nothing at all - tried timing from -10 to +15 and lambda from .750 - .900 and saw no increase at all over the pump fuel E10 im using.
should the map sensor just stay at atmospheric pressure once the throttle is crack wide open?
Yes that would be a strong indication there is a restriction in the intake. And man, that is a significant one, you are down nearly 20Kpa, so that is potentially 20% power you are missing out on.
It may not necessarily be the throttle body - it could be airbox/filter/pipework etc - so basically you need to scrutinize anything before the intake valve.
I agree, that's a pretty significant restriction. I've seen similar drops in manifold pressure in logs from student Formula SAE cars which are required to have a 18mm-20mm restrictor plate in the intake. One log from a 600cc 4-cylinder engine shows MAP=970 mBar at 5000 RPM and MAP=820mBar at 13000 RPM.
On a naturally aspirated engine without a rules-mandated restriction in the intake, I would be concerned about a 50 mBar decrease from low RPM to high RPM at full throttle.
Yeah I have removed the filters and emptied the airbox from all the restrictions i can (but this particular model of bike is known for its small airbox volume) I have also fitted a scoop to the air box lid with the cooling fans directed to it. I have done runs with the air box lid off with no change in power but at that stage I didnt have the map sensor data logged.
If the sensor is in an intake runner as suggested, is this not simple Bernouli? as the velocity of the air increases, the dynamic pressure reduces. It's not uncommon to have port velocities approaching sonic.
Done some more testing tonight. ran the engine with the airbox open so only restriction can be the throttle - same results with the map sensor. I think ill get the TB bored
Good point about the Bernoulli effect. Some OEMs intentionally use a MAP sensor near the intake port (in the intake runners) for detecting TDC intake, this allows the ECU to find which stroke the engine is on without needing a separate camshaft sensor. A MAP sensor mounted in the intake plenum near the throttle should see less influence from Bernoulli effect.
I dont what you mean by "bernoulli effect" does it mean the map sensor reading is normal - no restriction? the dirtbikes only have a crank sensor. map sensor is located just in front of the butterfly, fuel and timing is done off the tps. Alpha N system I Believe
Husky Mike, I've tuned a few 450 and 250cc mx bikes. They're usually tps based every time. Quick revving. If you gained 5whp. Thats pretty good. On a 450cc bike using the Athena Get ecu, Ive gotten similar results. They are race designed engines, and finding 5whp is great. They don't leave much on the table from the factory.
Mike, Bernoulli is a physics principle which states that where any fluid (air is a fluid) travels through a venturi, as the velocity of flow increases, the dynamic pressure will decrease. As such, if you're placing a feed to a MAP sensor in a part of the inlet tract with sees high velocity (relative to the pressure in the plenum) it may be possible that you are seeing this reduction in pressure as a consequence of the increased velocity, ergo it may not be a bad thing or a restriction per-se.