Should I get a vac. modulator?
02-04-2007, 06:03 AM
#1
Should I get a vac. modulator? Having a 4L80E mildly built (red clutches & rossler T-brake, auto valve body). Car is currently setup for nitrous, but I plan on switch to FI later this year. I would like to go with a 2 bar speed density tune when I make the swap to FI.
2 main questions:
1. Is there any problem refrencing line pressures to vacuum in a boosted application?
2. Do I really need a vacuum modulator with no MAF, or is that a misconception?
I talked to a sponsor that told me the PCM just needs an airflow rate to adjust pressures, it doesn't matter if it comes from a MAF or from a speed density calculated flow rate. The car is an '01, so I think that is right, I think earlier years automatically switched to 100% line pressure w/ no MAF.
2 main questions:
1. Is there any problem refrencing line pressures to vacuum in a boosted application?
2. Do I really need a vacuum modulator with no MAF, or is that a misconception?
I talked to a sponsor that told me the PCM just needs an airflow rate to adjust pressures, it doesn't matter if it comes from a MAF or from a speed density calculated flow rate. The car is an '01, so I think that is right, I think earlier years automatically switched to 100% line pressure w/ no MAF.
02-04-2007, 10:22 AM
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I am going vac modulated in my 4l80 for a truck. Had one on the 4l60 for a while now and I love it. I cant accidently mess up any settings in my pressure tables and burn up the transmission and the computer cant get confused on which pressure it wants to send to the transmission. If the electronic pressure control solenoid goes out, you lose pressure to the transmission, if the hose comes off of the vacuum setup it sends full pressure to the trans. Sometimes the old way still works better.
02-04-2007, 02:05 PM
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Originally Posted by 02Reaper
I am going vac modulated in my 4l80 for a truck. Had one on the 4l60 for a while now and I love it. I cant accidently mess up any settings in my pressure tables and burn up the transmission and the computer cant get confused on which pressure it wants to send to the transmission. If the electronic pressure control solenoid goes out, you lose pressure to the transmission, if the hose comes off of the vacuum setup it sends full pressure to the trans. Sometimes the old way still works better.
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02-06-2007, 07:30 PM
#9
Well each person and builder has there own ideas about this ,I am the sponsor who said he didnt care for them and I am not going to get into the great modulator debate but I will say a few things and reasoning for why I dont care for them.
1-They are dependant on engine vacum to control line presure
This is not and issue if you are building a car that will see the strip WOT as its primary use since at WOT the vacum is low anyway and high max line is what you want.However for the street driven car you dont want the line high all the time like cruising.
The vacum modulator takes away the PCM control of everything except for shift points and lock up. But the other variables such as the minute ajustments it makes for each gears shift time(time between command an shift) are gone it does these by minute increases or decreases in line.
Presure ajustments can no longer be made for temperature by the PCM.
2-super charged and turbo applications.
At least from my experince which was mostly with buick grand nationals even under normal driving conditions boost can be acheived and it dosent only happen when you are at WOT therefore the vac signal may be low even just taking off normally causing higher than needed line and therefore more stress on the transmission than is nessesary in just normal around town driving conditions particulary coupled with high stalls which will raise the engine rpm and therefore boost,
The check valve or blow off valve does keep the presurized boost from reaching the modulator true. however it also leaves the modulator at high line causing again more presure than needed unless you are at WOT in which case the PCS would also be at 0 current and would be at max line also.
Yes I know there are several people who build using the vac mod and are having good results and thats fine its working for them as I said no debate just my reasoning explained.
3-Now another reason given for the vac over the PCS is if the PCS goes bad it goes to low line and burns the trans up actually that can go either way if the PCS opens up electrically it will go high line. It will go low line only if it has a mechanical failure such as the orifice become damaged or trash getting in it. But then either situation should be obvious to the driver in the sudden change in the feel of the shifts and should be addressed imeditaley as any shift problem should. Plus with the electrical failure you will get the check engine light.
On the other hand what about the vac mod. Well they can fail too the diaphram can go bad. This will cause max line in one case such as a complete failure or if it just leaks lower than expected line along with trans fluild taveling up the vacum line to the engine and with newer cars and the cats etc a small amount of fluild over a period of time may not cause the car to smoke out the tail and your first symptom of a problem that is noticable is when the pan is empty and the trans neutrals on a run.
These are the same type modulators used on the TH350 and 400 for years and this is not that uncommon a problem. I have seen many a th350 car come in smoking out the rear only to find the intake full of trans fluild.
Another possibility is the vac line coming loose in the trans which I know properly installed it shouldnt but if it does it could quickly empty the trans before you really even knew what happen.
I would say you can make a reasonable argument for either but at least with the PCS and electrical failure will turn on the ses light and let you know along with the change in shift quality that something is wrong , Where most failures you run into with the modulator you may not even notice till its to late to address them.
On the matter of speed of presure control well the PCS of course gets it signal at the speed of light and the VAC at the speed the engine raises or lowers vacum but real world other than the issues with the vac mode listed earlier they should be the same. As for which will make higher max line well in each case set up properly they both should max whatever max possible line presure is at wot Since WOT for the vac mod would be ZERO vacum and the WOT for the PCS would be 0 current.
Anyway personally I beleive that the PCS can give better overall control at all throttle postions and a more predictable transmission performance particulary in super charged and turbo applications .
Just my .02 cents well maybe a nickel
1-They are dependant on engine vacum to control line presure
This is not and issue if you are building a car that will see the strip WOT as its primary use since at WOT the vacum is low anyway and high max line is what you want.However for the street driven car you dont want the line high all the time like cruising.
The vacum modulator takes away the PCM control of everything except for shift points and lock up. But the other variables such as the minute ajustments it makes for each gears shift time(time between command an shift) are gone it does these by minute increases or decreases in line.
Presure ajustments can no longer be made for temperature by the PCM.
2-super charged and turbo applications.
At least from my experince which was mostly with buick grand nationals even under normal driving conditions boost can be acheived and it dosent only happen when you are at WOT therefore the vac signal may be low even just taking off normally causing higher than needed line and therefore more stress on the transmission than is nessesary in just normal around town driving conditions particulary coupled with high stalls which will raise the engine rpm and therefore boost,
The check valve or blow off valve does keep the presurized boost from reaching the modulator true. however it also leaves the modulator at high line causing again more presure than needed unless you are at WOT in which case the PCS would also be at 0 current and would be at max line also.
Yes I know there are several people who build using the vac mod and are having good results and thats fine its working for them as I said no debate just my reasoning explained.
3-Now another reason given for the vac over the PCS is if the PCS goes bad it goes to low line and burns the trans up actually that can go either way if the PCS opens up electrically it will go high line. It will go low line only if it has a mechanical failure such as the orifice become damaged or trash getting in it. But then either situation should be obvious to the driver in the sudden change in the feel of the shifts and should be addressed imeditaley as any shift problem should. Plus with the electrical failure you will get the check engine light.
On the other hand what about the vac mod. Well they can fail too the diaphram can go bad. This will cause max line in one case such as a complete failure or if it just leaks lower than expected line along with trans fluild taveling up the vacum line to the engine and with newer cars and the cats etc a small amount of fluild over a period of time may not cause the car to smoke out the tail and your first symptom of a problem that is noticable is when the pan is empty and the trans neutrals on a run.
These are the same type modulators used on the TH350 and 400 for years and this is not that uncommon a problem. I have seen many a th350 car come in smoking out the rear only to find the intake full of trans fluild.
Another possibility is the vac line coming loose in the trans which I know properly installed it shouldnt but if it does it could quickly empty the trans before you really even knew what happen.
I would say you can make a reasonable argument for either but at least with the PCS and electrical failure will turn on the ses light and let you know along with the change in shift quality that something is wrong , Where most failures you run into with the modulator you may not even notice till its to late to address them.
On the matter of speed of presure control well the PCS of course gets it signal at the speed of light and the VAC at the speed the engine raises or lowers vacum but real world other than the issues with the vac mode listed earlier they should be the same. As for which will make higher max line well in each case set up properly they both should max whatever max possible line presure is at wot Since WOT for the vac mod would be ZERO vacum and the WOT for the PCS would be 0 current.
Anyway personally I beleive that the PCS can give better overall control at all throttle postions and a more predictable transmission performance particulary in super charged and turbo applications .
Just my .02 cents well maybe a nickel
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Last edited by performabuilt; 02-06-2007 at 08:00 PM.
