Hello all,

I've been doing a lot of research and reading, not to mention thinking about how to reach my engine goals. In that thinking, reading and researching, I've been operating on the assumption that the MAP sensor may not function properly should I use a cam with too short of a LSA and too much overlap. That brings me to my specific question:

At what manifold vaccuum will a OEM MAP become too unreliable to provide accurate information to the ECM?

If this is all misinformation, please do say so. In the interests of maintaining some manifold vaccuum, I've been figuring on using a LSA of no less than 110-112 degrees, with in the area of 30-40 degrees of overlap. One particular result I got on desktop dyno, which had nice power/torque curves had manifold vaccuum no lower thatn 14.29 in Hg running about a 112 LSA.

All input on this is greatly appreciated (and no, DD is not my end all be all of engine simulation, I'm just using it for ideas). :lala: Thanks!

 

The sensor is just fine.
It's what the engine is doing that's hard to manage.

The more radical the cam, the more each combustion event changes from cycle to cycle at idle, and low engine speeds.

In closed loop when the ecm is using feedback from the O2 the ecm is trying to target 14.7 and toggle the AFR across that, but with the big cam the motor is already doing that so the ecm is trying to over correct.

If you run Open Loop, the that removes the ecm from over correcting, and with some playing around, you can get a pretty good idle.

The other thing is that the engine has much less vacuum at idle, so the table space where you can tune with is smaller, so the tune so much more critical about being right.

On some really radical engines they just use the TPS, and RPM and ignore the MAP. But, driviblity can suffer.

 

Grumpy has told you what the problem is.

This has also been discussed on several threads before. Do a search on "reversion".

John

 

Ok, closed loop I get, but what do you mean by "with the big cam, the motor is already trying to do that" - it reads like 'the motor is trying to target stoich. ratio all by itself, so please clarify if you can.

Ok, here I take it your mean if I were to run open loop in all operating conditions and do a hella lot of messing around in the idle and part throttle tables. Right?


This statement probably is probably a better way of getting to what my original question's intent was. I guess I am looking to find out at what number, if there is one, where the manifold vacuum has limited the VE tables to where part thottle drivability is non-existent. IE, if the vacuum falls below 14 in Hg etc etc

While I'm looking for some serious power, it would be best to retain as much streetability as possible. I dont think an Alpha N system would be the most compatible with my goals, so I want to try to work as closely with the OEM ideas (closed loop, cruise mode, etc etc etc) as possible.

JohnL,

I did a search before and after this post, but nothing that hits what I'm trying to get at popped up. And part of the problem is that I havent done and hands on PROM tuning, so I gotta try to come at it from the engine goals, and then find out if what I'm thinking will get me there.

 

well the TPI intake design actually tends to thwart some of this by having a large compresiable volume of air to absorb oscilations in average manifold pressure. its not the absolute vacum thats the issue. its the osolations cuased by reversion and the large opening events that create the problem. the MAP sensor itself works quite fine.its the oscilations you have to contend with.

Some advice. Use a very large plenum. move the Map sample hook up line to a place where all the runners come to. try to use a soft hoses and make it a bit longer. you can try extremes like using a vacum resivor but the tunning for driveability can be a nightmare.

Stay away from single plane high rise intakes with short runner and small plenums. Stick with the TPI design or use a dual plane and get the REF line as close to the TB as possiable without having the vacum disrupted by incoming airlfow. the further it gets from the Intake valve the less pervasive the problem is.

This is one of the things Where MAF sensors shine. MAF dont worry about vacum or intake manifold reversion. They do have shortcommings but with the plethora of aftermarket components and other options comming to market id think twice about running S/D in all but the most radical street car.

If your gonna run boost of forced induction then S/D is your frined. But for a n/a application a MAF offers numerous driveability fixes that s/d would have a difficult time in remyding.

anyways those are my thoughts and opinions.


Oh and lets try a new phrase today VACUM OSCILLATION !!

 

The exhaust pulses are much more well defined with a big cammed motor, so the swings/ readings from rich to lean as the pulses go by the O2 sensor are more defined, and drastic. So the ecm is almost seeing too much feedback on what the AFR is.
If you want to keep closed loop then try moving the rich, lean O2 limits around some. With the increase in self EGR'ing you might want to raise the lower O2 limit, and slightly lower the high one, so the the ecm doesn't command so much of a swing across stoich..

There is no finite number for what will work or won't work.
There tends to always be an easy and close enough solution, and then a more time consuming better answer. It's a matter of what you want, and to what length you want to go to.

 

funstick,

thanks for the input. I'm leaning towards the HSR for this motor. It's got a nice plenum volume, and a runner length that looks like it will team up nice with the whole motor. Not really short, and definitely not the stock intake.

Grumpy. Believe it or not, I get what you're saying. Sounds like its really just going to take the invested time to try things out when it all comes together. Quick fixes are not exactly my style, so it looks like alot of burning and lot of learning is sure to come. Would I be correct in guessing that using a wide band O2 would be a better choice to try to work with on a more radical engine using SD? And many thanks for the input.

 

I'm running a cam that only gives me 65-70 kPa at a 900rpm idle. It runs fine and driveability is still good. Power Brakes were a problem though so I had to install an electric vacuum pump to power the brakes.

Tim

 

So, using the original location on the rear of the TB of a TB'ed engine for the MAP vac signal is not the best?

Would it be best to tap into the intake plenum? If that is better, how is possible for it not to be disrupted by incoming airflow?