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Renix MAP vac question

B

BlueCuda

NAXJA Forum User
Location
Tulsa OK
We just did a motor swap in a friend of mines 89. Prior to his old motor taking a dive it had a hesitation when you first hit the gas. We do the motor swap and of course its still there. The vac for his map sensor was hooked into the back(valve cover side) of the throttle body. So just messing around I moved it to straight manifold vaccuum with one of the ports on the intake. The hesitation is now gone. Just curious as if thats where its actually supposed to be. I have a Renix MJ for reference but it was a total 4.0 Aw4 4x4 conversions so its much cleaner under the hood than stock and its hooked to manifold vaccuum. Anybody got a stocker they can check for me?
 
The original Renix installation was to the throttle body. The throttle body has two holes there, but only one goes through - the other is blind. The original setup used a double rubber plug, with a solid plug in the blind hole. If the double plug was not present, is it possible that it was plugged into the wrong hole? Mopars from 91 to 95 use the same MAP sensor, and it is connected to the manifold, so maybe you should settle for the way it now is if it works correctly. Just make sure that you plug the hole in the throttle body, though, because it will leak vacuum.
 
I tend to agree with Matt, if it works keep it the way it is. I while back I remember reading a thread regarding porting throttle bodies and the MAP vacuum input. Someone mentioned that the throttle body is especially shaped to help with vacuum to the MAP, which in turn somehow affected the shifting of the AW4. I don't remember all the details now, but if your tranny starts to shift funny, it could be because the vacuum supply to the MAP got moved from the TB. I have no idea if these claims are true, just giving a heads up.


Best Regads,

SimpleXJ
 
The pic is from my '89. Is this what you are looking for?

688tb9d.jpg
 
Yeah, thats where it was. But it works much better now that its direct manifold vaccuum which is odd. Oh well, I think we will just leave it where its at lol.

However the MJ I mentioned above might have a vac leak at the TB as we didn't know this when we built it lol.

Thanks guys!
 
SimpleXJ said:
I tend to agree with Matt, if it works keep it the way it is. I while back I remember reading a thread regarding porting throttle bodies and the MAP vacuum input. Someone mentioned that the throttle body is especially shaped to help with vacuum to the MAP, which in turn somehow affected the shifting of the AW4. I don't remember all the details now, but if your tranny starts to shift funny, it could be because the vacuum supply to the MAP got moved from the TB. I have no idea if these claims are true, just giving a heads up.


Best Regads,

SimpleXJ
Click to expand...

If the MAP is reading manifold vacuum, it wouldn't have anything to do with the design of the throttle body. Manifold vacuum is present below the throttle plate and ported vacuum is above the throttle plate.

And, the AW4 is controlled electronically with no vacuum inputs. What they felt differently with the trans shifting has to be a perceived change.
It makes perfect sense to use a vacuum port on the manifold.

OP, how are you plugging that odd port on the valve cover side of the throttle body?
 
cruiser54 said:
If the MAP is reading manifold vacuum, it wouldn't have anything to do with the design of the throttle body. Manifold vacuum is present below the throttle plate and ported vacuum is above the throttle plate.

And, the AW4 is controlled electronically with no vacuum inputs. What they felt differently with the trans shifting has to be a perceived change.
It makes perfect sense to use a vacuum port on the manifold.

OP, how are you plugging that odd port on the valve cover side of the throttle body
Click to expand...


There is only one "true" port there, the other is just a keeper. The second plug in the TB is blank.

Spearfisher XJ said:
Some pics would help.
Click to expand...

Cruisers point about the difference in pressures, vacuums is ponderable, but there are a buttload of vac ports on the manifold, FPR, intake. Not necessarily attached to Manifold Absolute Pressure sensor. Could affect performance either way. :wierd:

100_0704.jpg


These are all I have of the area.



100_0705.jpg
 
The MAP wants to read manifold vacuum. That's how it compares atmospheric pressure compared to intake vacuum/load. You could feasably tap into any vac port on the intake manifold. I'm suspecting the OP had a vacuum leak issue at the throttle body or the vacuum hose was plugged into the wrong port initially. Therefore, when using the intake manifold port he noticed an improvement. The port in the throttle body goes right to the intake manifold anyway.
 
Actually, there is something about the vacuum being slightly different at the throttle body. It probably has to do with fluid dynamics and air flowing past the port in the throttle body. The 5.2/5.9L have done that for certain issued http://flatbroke69.tripod.com/tips.html#mapmove
 
Talyn said:
Actually, there is something about the vacuum being slightly different at the throttle body. It probably has to do with fluid dynamics and air flowing past the port in the throttle body. The 5.2/5.9L have done that for certain issued http://flatbroke69.tripod.com/tips.html#mapmove
Click to expand...

It would be interesting to do a side by side comparison with a vacuum gauge.
 
Alright. I just went out to the garage to look at a spare Renix TB. The lower hole is the actual port. The port is about 5/16" in diameter and makes a 90* turn to go straight down to the intake manifold area, an inch or more below the throttle butterfly. A 3/4" by 1/4" slot is machined at the bottom of the throttle body bore from the passage. Definitely manifold vacuum. BTW, poured myself a FireRock Pale Ale in a chilled glass on the trip.
 
cruiser54 said:
AA 3/4" by 1/4" slot is machined at the bottom of the throttle body bore from the passage. Definitely manifold vacuum.
Click to expand...

That doesn't mean it sees the exact same vacuum as the manifold especially when air speed picks up.
 
We need to do a comparison with a vac gauge.

The slot is so far below the throttle plate that I don't think it makes a difference as far as air velocity is concerned. It appears that they made the slot in the TB rather than modify the manifold. The slot is so big and so is the 5/16" hole from the MAP port to the bottom of the TB, I doubt it would generate any higher vacuum from the air flowing through the TB. I know what you're thinking as far as ported vs manifold vacuum when hooking up a dizzy vac advance line but I really don't think that's the case here.

I've been too busy on too many projects to test it myself.

Another thought: In later years, didn't they mount the MAP directly to the intake manifold?
 
definitely show the same MAP being attached to the T.B. in the Renix years and to the intake proper in the Chryco years, but there is still a chance that the ECU is expecting and interpreting the different vacuum signals based on original location/expectations.

I loaned out my vacuum gauge a couple weeks ago, of I'd run out and check each location-- thanx for the reminder to get the gauge back. If I get it before I see an answer, I'll check and post the results.
 
Well it seems to be working great plugged into the manifold and not the tb. Afterall, it's been four years since I made this post
 
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