Autosport Labs

I finally got round to drawing up a chart from a log file, using Excel. After looking at it for some time, I realised that it seems to be working backwards when compared to the descriptions given previously!

If you look at the attached graph you will see that as the RPM rises (red), the load value drops (white). When I've taken my foot off the pedal and the engine's slowing, the load value rises again. Now, given that a low value of KPa means more vacuum, this means that I get more vacuum when the engine's accelerating (foot on gas) and the value returns to atmospheric when it's slowing (foot off gas). This goes against the advice posted previously!

If you haven't worked it out, RPM is on the left-hand Y-axis, Load and Advance on the right.

Time for some aspirin, I think... someone save me from a migraine!


David

So the MAP sensor is simply reflecting what is physically happening in the manifold runners - possibly affected by how you're balancing the vacuum signal across all runners. This is not unlike what dr.occa was observing during his experiments.

I agree that you should observe high vacuum at normal engine RPMs when the throttle bodies are closed, measured in the runner between the throttle body and the intake valve.

Have you tried temporarily attaching the map sensor tube right to the one of the runners ports, without using the accumulator? Might be a worthwhile test.

You're too far ahead of events Brent!

I've still got the single down-draught carb on (or at least I did when I made this log) so this is a reading taken at the base of the carb, just at the beginning of a shared 4-branch manifold.

I thought that I'd better get the current situation analysed before fitting the new carbs...

What - no response from anybody else?

Why do you think you're not reading high vacuum at that point of the manifold when you see high RPMs with the throttles closed?

Is the port you're using actually in the manifold itself, away from the carb, or is ported into the carb body? Perhaps your carbs are preventing it from seeing actual manifold vacuum under those conditions.

Fair question Brent.

The vacuum connection is at the very base of the carb, barely 1/4" from the mounting, so in effect it's a straight-through link to the top of the manifold. There's no reason to believe that it would go anywhere other than straight through to the inlet duct.

However, having said that, I will check that there's no silly nonsense going on there (but I don't think that there is).

cheers,
David

P.S. I'm seriously thinking of changing to TPS when I fit the bike carbs - there's a pot on the end of the carb spindle, but the only problem at the moment is that I don't have a connector.

Anyone got a TPS connector for a Honda CBR600F-X they don't want any more?

Brent,

Just a wild (and probably very silly) thought - does your log-writing code invert the MAP value so that it displays in the same way as the TPS reading? It's the use of the word 'load' that's triggered this thought.

If you look at my graph with the idea that the white 'load' line represents the amount of vacuum, rather than the absolute pressure, then it starts to make sense. You will see that as the rpm rises, the white line/vacuum goes down - as the rpm falls, the white line/vacuum goes up.

Or am I still clutching at straws!

cheers,
David

Nope, it's logging actual MAP value. Sorry.

If you'd like to satisfy your curiosity, attach a vacuum pump to the MAP inlet while logging, or, attach a vacuum gauge to the port on your manifold.

Unfortunately experiments will have to wait a while, as I'm currently in the midst of servicing and upgrading my car - it'll be a week or so before I can get the power on again

cheers anyway!

Rats!

Just had another look at the actual log file, rather than the Excel sheet. When I turned on the ignition, but before the engine started, I got a load reading of 101KPa (atmospheric pressure) which is absolutely correct - no vacuum expected at this time.

So my problem is with the engine, carb and/or the point where I'm taking off the vacuum.

Anyone got any bright ideas?

A fellow Locost builder has just installed MJ onto an engine setup very similar to mine - same engine and carb anyway. Knowing of my problems he took the vacuum off the point where the crankcase vent used to fit - and his load graph is the correct way up!

So the end of the story is that my vacuum is coming off the wrong point, which is down to me to fix.

Apologies for the long and misguided thread, and thanks anyway for the help and advice!

cheers,
David

That is interesting- to close out this thread it would be helpful if we could find out why the other port is different than the other takeoff point.

Is it easy for you to switch to the same take-off point as the other fellow? or, have you already committed to going TPS?

Thanks for the update.

Brent,

It looks like the distributor vacuum outlet is 'ported' - as far as I understand, it gives values almost the exact opposite of MAP (which is what I was seeing). I haven't managed to Google a good reference, but I have seen several articles that state "do not plug into the distributor vacuum supply as they are often ported. Use a direct connection into the manifold for MAP".

I've now decided to fit the new bike carbs, so I plan to use TPS as there's a pot already fitted to the throttle spindle. I've already modified the MJ board, so I'm committed...

David

hi david,
i have been gazing at your graph, and i dont see that it is so hard to explain (nothing like bravado is there?)

as brent touched on, and you have confirmed, you were using a ported vacuum takeoff,
which once was for the dist vacuum advance. for the benefit of other readers too,
ported vacuum simply means that the vaccum is taken from a hole which is covered
by the throttle plate, when it is fully shut. at that point, vacuum will be 1 bar, as the
hole is no longer on the engine side of the throttle plate. if the throttle is opened even
a small amount, the hole is then on the engine side of the plate, and seeing manifold vacuum.

implication: if you foot is off the throttle, measured vacuum will be 1 bar.

note: ported vac shouldnt give the opposite to manfold MAP. it is only different when the throttle is closed but the motor is revving. afaik, the main purpose, if indeed there is any other, of ported vacuum, is to remove all the vacuum advance at idle, as high advance at idle makes it difficult to set a steady idle speed. if you want to see how much of a difference that makes, just twist a distributor in the block at idle, and see how much the car revs up when you twist in 30deg advance. that is all assuming anyone on this forum actually remembers what a distributor is....

on your chart, you would expect to see measured MAP (from your ported outlet)
at a maximum, as soon as your foot is off the accelerator pedal. that is represented
by either idle rpm, or rapidly falling rpm. i think your chart shows high MAP in such circumstances.

next, you would expect to see low MAP ie high vacuum, when you have your foot lightly
on the pedal and the engine turning over at significant rpm. i think the chart shows that too.
every time you rev the engine up, MAP goes down. that would be counterintuitive if it were
on the road, because you would likely have the throttle wide(ly) open, which should lead
to high MAP, right? i am guessing however, that this was measured in the garage, in which
case the throttle is never really very wide open when reving the motor. so what you have is
part throttle and high engine speed, which should cause low MAP/high vacuum.

was this measured with the car stationary? i think that would explain it all. if not, let us know
so i can publicly disavow most of what i have said above .

regards
alexander.

alexander wrote: was this measured with the car stationary? i think that would explain it all. if not, let us know
so i can publicly disavow most of what i have said above .

regards
alexander.

Prepare to disavow!

It was taken on a short run up and down the road.

As for the 'ported' theory, someone else did a graph taken directly into the manifold (where the crankcase vent used to go) and had similar results to mine. So, no real conclusions I'm afraid.

In the end I wimped out - I'm fitting a set of 4 bike carbs to my engine, and they have a TPS on the end. Seems a shame not to use it!

cheers,
David