Intake manifold pressure query

[ChrisR]

What would it mean if my manifold pressure dropped as the revs increased?

Readings are approx.

906 mBar @ 4500 rpm
877 mBar @ 7400 rpm

Less pressure less air getting into the engine?

I've no idea how this kind of thing effects things

 

[drhowser]

surely as the "suck" from the engine increased the preassure would drop. isnt this is how the intake restricts the engine?

 

[ChrisR]

I've no idea lol, surely more pressure means faster moving air, thus more into the engine, isn't that how a turbo/charger works? I feel like such an idiot

Although I see what you mean, the engine is sucking more air in that the manifold can't replace quick enough, hence a dip?

 

[Pep]

i would think lower pressure means it is sucking more air through so the air pressure cannot keep up with the sucking rate?

seems the logical explanation but not sure if it is right.

 

[LegoHead]

The air is sucked into the engine, so there must be a slight vacuum in the plenum.

Is this absolute or gauge pressure?

 

[ditz]

its negative pressure ie vacum - and if you think about it more pressure (vacum) means there is a resistance causing said vacum to build in the plenum as opposed to flowing freely in to the engine.

 

[edde]

The engine is always sucking so as rpm built the restriction means it cannot fill the manifold as well so pressure drops.

 

[ChrisR]

I see, not good then

 

[Dan@SJM]

engines dont suck as such, they just get filled with air/fuel at atmospheric pressure.

 

[edde]

engines dont suck as such, they just get filled with air/fuel at atmospheric pressure.

Well call it a vacume then you give engines a manifold and they aren't able to breath as efficient though Im sure we both agree hecne the reason intake mnaifol work and bodies reduce restriction and increase power as such.

 

[Dan@SJM]

aye, a vacuum is formed when there is a restriction and the engine is actually sucking rather than being filled at atmospheric pressure.

you want to find where the restriction is, you need a glass tube about a meter in length bent in a u shape around a 25mm pipe, with indecators at 10mm apart, and fill it half way with water. The have a rubber pipe going to several different points:
1-Air box
2- air intake pipe
3-throttle body
4-intake runner
5-by the inlet port.
Take the different readings, this will tell you where the restriction is.

 

[ChrisR]

aye, a vacuum is formed when there is a restriction and the engine is actually sucking rather than being filled at atmospheric pressure.

you want to find where the restriction is, you need a glass tube about a meter in length bent in a u shape around a 25mm pipe, with indecators at 10mm apart, and fill it half way with water. The have a rubber pipe going to several different points:
1-Air box
2- air intake pipe
3-throttle body
4-intake runner
5-by the inlet port.
Take the different readings, this will tell you where the restriction is.

Cheers, the original figures came from log files from an RSTuner box so will do a run without the standard air feeds first to see if they are the problem.

 

[Dan@SJM]

thats usually where the restriction lies,

i noticed a big difference on mine when i took them off...

What part of your inlet has been ported? has the plenham chamber been done too? i did both of mine(not by a little either, matched it to the gasket all the way through the inlet runners.) if not i suggest getting that done...

 

[ChrisR]

Alex did the top and bottom bits of the intake for me. He said he'd never do another afterwards lol, took him a while so guessing it's had a lot done to it.

 

[Dan@SJM]

Aye they arent too nice to do lol, i showed Ads_172 the difference between mine and his when i fitted the inlet to his. the casting on the ph1's is shocking...

ive got another inlet which im cutting open to port ALL of the inlet runners on the plenham. Im not sure where the restriction is on mine yet, i'll be conducting a few tests to find out.