Strange as it may sound, I just got around to installing a boost gauge on my car today.

Results are...interesting. Peak PSI at 7k in 2nd is between 9-10 on an Autometer Pro-Comp 0-35 PSI gauge, and right at 9 on a VDO Vision 0-15 PSI gauge. Went out and bought the cheesy looking Autometer gauge after getting what I thought were high readings on the VDO.

In 3rd gear, 6.8k redline, it peaks around 8 to 8.5 PSI on both gauges, with the VDO reading the low end.

Oddly enough, at 5k in any gear when I apply load, both gauges show 5 PSI.

Supercharger is also a confirmed V2.

In case anyone was wondering, the gauge vacuum line is tee'd off the bypass valve vacuum line (which is in turn tee'd off the brake booster line coming out of the side of the intake plenum). The tee off the BPV line is where Dinan recommends you install a boost gauge.

I'm wondering just how accurate these gauges are, because with a 5.14" (assumed) crank pulley and a (confirmed) 3.43" SC pulley, I should be about 2 pounds lower peak. Makes me hesitant to install the 3.33 pulley at this point.

Any thoughts?

are they both mechanical gauges?

Croak measure the diameter of that blower pulley, it sounds like boost numbers from a 6 inch crank pulley.
Also I know boost should be boost and vacuum is vacuum, but Dinan reccomends using the brake booster hose as the source and RMS recomends teeing off the FPR hose. MY prob is Paul's boost seems a tad high to me also and he has the same hose setup as you. I am just wondering if you moved the boost gauge hose to pick up your reading from the FPR line, if it might give a lower reading.This might be pure nonsense from me, but I am so curious because although Paul's HP numbers may support his boost contention, the pulley combo to me does not.
Either that or an RMS aftercooler wastes more boost than I think, and I really make 13 lb boost and are loosing 2.5 lb thru the aftercooler.
In that case maybe your boost is right with a smaller crank blower pulley.

BoostedhypeR: Both mechanical gauges

Marco: I think your theory is correct, I'm going to tee into the FPR line after the car cools down to see if that changes anything. I'm curious, where is your BPV tee'd off of on the RMS install. Or rather, where WAS it tee'd when you ran a BPV instead of a BOV? ;)

No offense to Paul, but I've always been a little, um, skeptical of his 8psi out of the box V1 readings. He's checked it with three seperate gauges, but as far as I know, he used the same tee location with all three gauges.

My other concern is my Aquamist pressure gauge, which is ALSO tee'd off that line. It's set to start spraying at 3.5 PSI, and while my butt dyno has been happy with the results, if my readings are a pound or two off across the board and the pressure switch sees the same thing the gauges do, that means I'm spraying pretty damned early.

The saving grace if that is true is that I'm using a 2d and mapping on injector duty cycle, which means early spray is fairly light.

Ok, quick hit of the crank pulley with a tape measure shows about 5.25", which makes me pretty sure I'm running the 5.14" if you figure in measuring error.

Croak I have always had some type of bypass valve returning to the the intake side of the blower. I have never has an actual blow off valve that blows off into the air.If I represented that here, that was my error.
But my source for all my vac / boost readings come from my FPR plastic line. I tee'd my plastic line and put the other side to my by-pass valve.I tee'd that line again right before the bypass valve to run my boost gauge and my Aquamist.
I set my Aquamist to spray @ 5 lb boost. I have an indicator light that lights my whole interior up @ night when it is spraying. It sprays exactly when my autometer phantom boost gauges reads exactly 5 psi boost.Therefore I feel it is very accurate.

Incidently I set the Aquamist switch with a MightyVAc with a Snapon gauge when I manually set it up to spray @ 5 lb boost.

Ok, just got done doing some testing.

Tee'd into the FPR line with the VDO 1-15 gauge, took it for a spin, boost dropped from 9.0 peak to 6.5-7.0 peak

Plugged the Autometer gauge into the same line, peak dropped to around 7-8 (hard to tell on the Autometer gauge, since it's 0-35, the resolution is a little coarse at lower boost levels).

Then, just to be really anal, and piss off all the dog-walkers in the neighborhood, I connected BOTH tees up. Ran the VDO off the FPR, the Autometer off the brake booster. VDO showed 2-3 PSI lower than the Autometer.

Reversed them, VDO off the booster, Autometer off the FPR, pretty much the same results.

Long story short, running off the brake booster line or anything downstream of that gives a higher pressure reading than running off the FPR line. In my case, it was on average about 2 PSI higher reading running off the brake booster line.

Things that make you go Hmmmmm.......

We should measure all of your pulleys against the diameters on mine. If you really are pushing 9+ psi, that wuold explain your pinging. Even if it was cured by replacing coil packs, the high boost could have been the thing that uncovered their weakness.

Or.... You are running more like 7 psi and all is well.

I'm running dead on 7 PSI at peak tapping off the FPR now, it NEVER goes higher than that, though it sometimes shows up to half a pound lower.

Pulleys are the correct size, SC is 3.43" for sure, and the crank diameter is LESS than the 5.25" I measured when you consider my "from the top and around shit, don't burn yourself on the hot block" measuring method, so I'm confident if it's not a 5.14", it's real freakin' close.

I'm now convinced that tee'ing off the BPV/Brake booster line gives false pressure readings on my car, on the high side.

I don't think the booster line itself is the problem (though it does pull vac from a different spot on the plenum), but rather the effect of the BPV valving is somehow causing the pressure to read higher.

Not too worried about the BPV alone being on that loop, since I measured idle vacuum on both the booster and the FPR with my Mityvac and both were identical, and the BPV is vacuum (not boost) actuated.

I am going to pull the Aquamist off that tee and run it and my boost gauge off the FPR line tomorrow.

Bring your stuff over tomorrow and lets baseline mine too. Plus I have to get my arse in gear and get serious about finding a boost gauge. You can surf the net and spend my money :)

Will do, when are you free?

Have to go in to work again tomorrow not much later than mid-afternoon, fookin' crunch time is teh ghey.

I've got another gauge coming, just because I couldn't decide which one I wanted. VDO Vision 0-15 PSI that reads "Boost" and a VDO Cockpit International vac/boost gauge that doesn't say "turbo". Along with two MORE VDO Vision Oil Pressure and Oil Temp gauges. And my Autometer Pro Comp silver/silver 0-35 PSI gauge that I'm now pretty much stuck with since I dropped it and bent the bezel ring. Heh, for what I've got tied up in gauges right now I could have bought most of the SPA gauges I really wanted. ;)

Frack frrrrrack FRACK! Forgot to go back to NAPA today to pick up my senders that I alread paid for. They open tomorrow?

NAPA on Mission by the cutoff toHome Depo? I think they are open.

PS: You left your phone on the other floor again. Call the house.

Hi Croak,

The real boost readings are at FPR ! RMS have right.
It's absolutly normal to have much higher readings at brake booster, because there the boost sender is to close to the supercharger outlet. You must measure the pressure much closer to the chamber, this is the only important thing.

Put a gauge in the sc charge tube, and you will see aprox. 20 psi :) is normal.
but after air goes in the intake manifold the pressure will drop, and close to your chamber you will have only 6-7 psi boost.

p.s. keep the VDO 15 psi boost, german look and quality is the best for your bimmer :)

Jim, Croak, and everybody. I can understand why boost pressure might read less... But how can two lines sourced from the same location read differently? I mean, both are manifold sourced. How could one line INCREASE the boost (and vacuum I suppose) over what is real?!

I need to understand this before I make the switch. MY initial reaction would have been that the fpr line is losing pressure somewhere, except Marco has found similar readings. Moreover, when dinan quotes his 6 to 7 psi spec for his kits, I assume hes using teh same line hes telling us to use. What does that mean? That his kits really only produce 4 to 6 psi?

This is too weird...

Dinan is weird... :)

vacuum readings can be different only if you have a hole in intake manifold. and also different engine size creat different vacuum readings

BTW, do you know that the new Vortech V2-Sq blower have 3,61 ratio, and an 2 years older vortech unit have only 3,45 ratio. maybe this is the reason of different boost readings. You guys have exactly the same vortech blowers ?

>>BTW, do you know that the new Vortech V2-Sq blower have 3,61 ratio, and an 2 years older vortech unit have only 3,45 ratio. maybe this is the reason of different boost readings. You guys have exactly the same vortech blowers <<

I dont think thats so much the question as is why would lines emanating from the manifold sourced brake booster line carry MORE pressure than the fpr line, also sourced from the manifold...

the brake booster line is prety much close to the throtle body. This is what I try to explain above. The readings from the throtle body are not usefull.

Another think, the power brake unit use pressure from the manifold, there can be higher readings generated by the power brake unit, works like an air pump... only a thaught :)

I've always tapped my boost gauges into the FPR line...

I tap into the FPR line as well, and have always recommended such.

I wonder if there could be a static pressure vs. dynamic pressure thing going on here? More "flow" in the brake booster line, as opposed to "pressure" only in the FPR line since it is "closed" and does not have the big reservoir?

Just a thought on a foggy rainy sunday...

Steve

>>The real boost readings are at FPR ! RMS have right.
It's absolutly normal to have much higher readings at brake booster, because there the boost sender is to close to the supercharger outlet. You must measure the pressure much closer to the chamber, this is the only important thing.<<

Christian, how many cars would you say Dinan has supercharged. I dont know either. But Im sure its over a thousand. Even if it werent, though, wouldnt you think they have installed and measured the boost from enough of their applications to know if their recommended method of tapping the line feeding the bypass valve to pull boost measurement routinely produced reading 2 to 3 psi too high?

Sure, I measured 8 psi out of the box. But NOT ALL dinan sc cars, when measured this way, produces numbers like that. There are plenty of guys measuring 6 psi from this location... So, what, theyre really only producing 3 psi??

Just doesnt sound right. Im not disputing what Croak found. But its hard to imagine this is an endemic condition, true of all cars measuring the 'Dinan way'. They dont ALL measure high! HOw do you know the FPR line isnt producing LOW results?

I wish someone would contact Dinan with this. I would, cept IM personna non grata around there. Maybe croak, or somebody else NOT on bad terms with them can call them up this week, and tell them of the dilemma, and get a decent response from them, if thats possible.

Oh, one more comment. We know that most M3s using the Aquamist 1S based systems will start to sputter if triggered below around 3.5 psi. My hobbs switch triggers at 3.7 psi on the boost gauge. So, if this is 2 psi too high, then I d be really triggering at just about 1.5 psi, and I know for damn sure Id be sputtering and stuttering like a MF if I were really triggering that low.

This is driving me freaking crazy...So, maybe wed, wnen I get my wideband bung welded, the shop is the same one that installed my boost gauge 3 yrs ago; maybe Ill just try and get a freebee out of them to change the tap location :) If Im only running 9 psi when measured that way instead of 11, that would help explain alot.. :)

i need to install my boost gauge. hope you guys can tell me where the sender should go soon! :D

Who's got the boost bible? I have never unpacked mine since the move. Assuming a known and reasonable AFR, HP is pretty much a calculatable value based on boost. Since you know your last HP run and roughly know the drivetrain loss, you should be able to tell if you are dealing with a tap location problem.

At this point, Id go with the FPR line. I dont know if the difference you see will always be two psi, but if there is a difference, I would bank on the FPR, even though I dont know the reason. As steve suggested, flow could play a part in artificially jacking up the measurement from the brake booster system. But with a check valve, and under pressure, I dont see how flow would play a part. Just to play safe, tap the FPR line.

What Steve told is also my thaught.

And, the brake booster system is "under pressure" always...
No matter if is vacuum or boost, is "under pressure".

And the brake booster system have a valve that limit the vacuum (pressure), so is not afected by high pressure from our superchargers .

The brake booster system use the pressure from the intake manifold to have powerfull brake. So, vacuum is only a pressure from zero infinit (vid) to sea level pressure... 14.7 psi, and after that americans use to say have boost, but is only more pressure then sea level, in your case 11 psi boost => we europeans use to say that this meens 25.7 psi (1.78 bar) absolute pressure.

BTW, Paul what is the size of your supercharger pulley?

About Dinan, you have right they tuned well a lot of cars. And... I think they recomand to use the brake booster for the boost gauge only because is more simple to install, and they don't use to go with complicate things like a Stage 2 engine... just my thaught :)

Sure, you must use FPR line for boost gauge, please try.

If instead of 11 psi you will have 9 psi at red line, preaty sure at 3.7 psi where your WI will start, you will have something like 3 psi at FPR line. The wrong readings from brake booster are proportionals from zero to infinit, because don't exist -2 psi pressure...

>>
And the brake booster system have a valve that limit the vacuum (pressure), so is not afected by high pressure from our superchargers .
<<'

I dont know if this is what youre saying, but when I measured engine vacuum at the same place I tap for boost, it measured the proper amount of engine vacuum. I forget what it was, but 18 rings a bell.

>>If instead of 11 psi you will have 9 psi at red line, preaty sure at 3.7 psi where your WI will start, you will have something like 3 psi at FPR line. The wrong readings from brake booster are proportionals from zero to infinit, because don't exist -2 psi pressure...
<<

Yes, that is a good point. At that level I dont even think it will be that different; more like 3.5 instead of 3.8 or whatever it is. .MY theory on WI triggering anyway says to trigger as early as you can without creating a feelable stagger or stutter when it first triggers.

Booost is a funny thing...Not funny Ha Ha, but funny as in strange, in that you can get many different boost readings from a similar setup. For instance, I can be driving at 2000 rpms, with no boost, and then step hard on it for a second, and watch the needle spike up for a second in resposne to the sudden throttle. Lets say it spikes to 3 psi. Thats a far different form of 3 psi than what you get if youre doing a normal WOT pull to redline, and it occurs at a very different rpm point. Thats what I mean about measuring boost. I do think its IS possible to compare so long as youre talking about a floored, WOT, run to redline. So long as youre floored, results can be comapred. Under these conditions, Im very surprised the results are as different as 2 psi. Perhaps one I could understand, but two suprises me.

I also think you might have a point about the dinan recommended tap site... But it still surprises me that they recommend this in all their instruction packets, so its being heeded by a lot of people. If it were only the simplicity though, which prompts them to instruct this way, there are a lot of other procedures in the installation packet which could be well simplified also, which they do NOT recommend.

Hell, we can analyze this til the cows come home. When push comes ot shove Im not sure how mcuh difference it makes. Your dyno result and tuning process isnt going to change much regardless of which psi your producing on the gauge.

I think the one thing this demonstrates, though, is that we need to be Very Careful about asking and recording peoples boost pressures.... we need to be sure and verify Which Kind of boost is being measured....If its the Dinan method of brake booster tapping, we will know to make the mental adjustment of 1 or 2 psi when comparing results. Given that, IM not sure how crucial it is that I even change my tap location. I may do it anyway, just to see.

If I do it on my own, do you need to jack the car or can you access it under the driver seat with it on the ground. You know where IM now tapped. I dont want to have to re thread the skinny boost line which runs into the cabin up to the gauge; Id rather just remove the line from where it feeds the boost guage near the bypass valve, and then attach that to a line T'd into the FPR line . Whats the best route to follow to run the line from the FPR line into the engine bay... where would you tie wrap it so it doesnt drag or interfere? Then, when its up in the engine bay, Ill just cut and tap again for the Hobbs switch. Guess Ill have to get a T plug to terminate the current boost connections....

>BTW, Paul what is the size of your supercharger pulley<<

Crank pulley is 6" and SC pulley is 3.33"

Funny, the same setup like mine, 6"x3.33".
My boost gauge is instaled in the FPR line, and also I have the BPV there, and at 6500 rpm I have 10.5 psi and at 7000 rpm I have 11 psi.

I have the OBD 1 intake manifold and also I have low compresion pistons 9.7, and high flow headers and 3201 cmc, all this must reduce the boost, but the boost is still at 10.5 psi at 6500 rpm.
But I don't have yet the water injection installed, maybe the air is prety expanded.

You can swith also the BPV and boost gauge from brake booster to FPR line. I'll like to see your results.

>>Funny, the same setup like mine, 6"x3.33".
My boost gauge is instaled in the FPR line, and also I have the BPV there, and at 6500 rpm I have 10.5 psi and at 7000 rpm I have 11 psi.<<

WHAT?????!!!!!!!!!!!

So, then, since that mirrors exactly what I have in the Brake Boost sytem tap, whas the dif???!!!

Hope all this wasnt for naught!!! I have the OBDII mani and stock internals, but regardless, the fact that were making very similar values with the different tap locations puts a dent in the theory that there really is a substantial difference!!

YES :)

But, my boost results was at 10 degrees Celsius (very cold, in Winter) and sea level.
Yours?

10 degrees C is 'very cold'.. Hell, thats only 50 degrees F! Its been near 0 degrees F on several occasions in the last 2 wks.. I seem to see a peak of 11 psi all year round. the fact that we have similar boost readings with the same pulley size, taken from the two different sites proves once again that you simply CANNOT make any valid assumptions with just a couple of data points.

I think was very cold for street racing. 1 mounth a go here was also 5 F (-15 C), but I don't use my bmw on ice...

Anyway, we will not know exactly until you will try FPR line.

My engine si slighty different then yours, and can have different results in boost readings. Also my sport cams can rise the boost with 0,5 psi, headers will drop the egt and this can also rise the boost, etc, etc.

Anyway, your setup is perfect with 3.33 inch (45.000 rpm) for Vortech efficency, and normal fuel. So, don't worry more than that, if you have 10 or 11 psi.

Next step is with the final 3.12 inch vortech pulley (48.000 rpm), but here the heat will go up exponential with the rpm, and you will need aftercooler and low compression pistons for better results.

Just sidelining back to gauges for a second.

I had good results with the Greddy PH/W gauges, and others swear by the Defi units. Either way, get yourself a gauge with peak hold so you can focus on the *road* while you're driving. 8)

Gruppe-S (www.gruppe-s.com) have good pricing on both.

I do not think so Paul,His modified engine may process boost different than your car. He has cams and headers and it is stroked to boot. Cristian said himself he measured 20 lb boost at his blower, maybe he loses less boost, because his engine can process more. or maybe it is possible his gauge is not correct.

There are far too many people running more consistent boost numbers with pullies that give the expected boost range, than this one isolated car of Cristian's who has alot of mods we simply do not have.
Do not hang your hat on one similar boost claim, when there really are no similarities between his car and yours.

Croaks car is more of the correct example. Afterall what have you got to lose the change the vac/boost source and see what it says?I hope you are not afraid your boost gauge says 9, because that would say you make excellent power for 9 lb boost!

Also a reminder Paul, I made 9.8 lb boost on the Dynapack and then put headers and front pipes to the cats ONLY and 4 days later my boost was 10.3. That was with 3.25 pulley, aftercooler loss and 6 inch crank pulley.

I personally think you make 9 lb boost.

exactly, my Hiop High Flow Headers also rise my boost with 0,5 psi :)

just playing devils advecote marc, but dont forget, there are loads, most infact, of dinan cars measuring between 6 and 7 psi, using the 'dinan ' measurement method. that would mean theyre really making 4 to 5 psi, if all this is 100% true. Not Likely, with the stock pully combo, I think you will agree. Only way to know for sure is to try it one of these days.

just playing devils advecote marc, but dont forget, there are loads, most infact, of dinan cars measuring between 6 and 7 psi, using the 'dinan ' measurement method. that would mean theyre really making 4 to 5 psi, if all this is 100% true. Not Likely, with the stock pully combo, I think you will agree. Only way to know for sure is to try it one of these days. I dont think one car by itself necessarily proves anything, even combined with circumstantial evidence. Need more examples and results. :) Although, I agree, I think there very well may be something to this story.

I tee off the bypass valve and see 6.5 PSI..
I dyno 307 rwhp sae / 258 rwtq on 6.5 psi,,,if its high...Im looking at that power on 4.5 psi of boost.

If anything the fpr line does not have flow and should show the highest numbers than the brake boost/ bpv, which has flow and larger volume to fill and has a chance of a leak.

Could it be that Croak's FPR line has a leak?

It's not quite as linear as that Paul.

Just to clarify this again, the big margin of error seems to be past the 5PSI point..up till then, the readings seem fairly consistant between the two locations, as far as I can determine using analog gauges.

It costs less than $5.00 (tees and couplers) to test it if you already have gauges.

Me, I'd LIKE to see some more results, because I'll be the first to admit one instance doesn't make it a truth. But I'm positive of the results on my car.

WOW, right now I discovered that Paul's vortech blower is not V2, exactly what I sayed above. So, I have right, we don't even have the same type of blowers.

Exactly like Croack say, the V1 has only 3,45 gear ratio, and this my friend Paul will give you only 43500 rpm at blower at 7000 rpm instead of mine 45500 rpm. Well, is clear now that you don't have 11 psi boost in engine chamber.

Now, I'll modify my declaration, your car has more room for improvement until 45000 rpm at vortech blower. You can use 3.25 pulley for 44500 rpm. The smaller 3.15 pulley will go to high at 46k rpm.

Or, right now I find on Vortech website, they offer an Upgrade kit to change your gear ratio from V1 3,45 to V2 3,61, and not need another pulley. I don't now exactly how much Vortech will charge you if you have the supercharger in perfect condition. Ask them how much will cost you only for a change in gear ratio.

>>WOW, right now I discovered that Paul's vortech blower is not V2, exactly what I sayed above. So, I have right, we don't even have the same type of blowers.<<

Christian, how do you know I have a V1.. V1s are NOISY, at least the ones Ive heard... Mine is silent. I cant tell by looking since the shield has been rubbed clean by the hfm over the years. But, it was purchased in 11/99, which is right near the cusp of the cutover.. Judging by sound, relative to V1s Ive heard, this has to be a v2.

Croak, if i already asked you this, forgive me.. But, where do you tap your fpr line? I think under the hood you should be able to reach in and grab it, right next to the brake boost line feeding the bypass valve....That would make it alot easier than having to grab it down by the fpr under the car.

Paul the FPR line is under the intake. It is easiest to get from under the car, Trust me.

OK, Paul, I don't know if is V1 or V2. I just read what Croak said.
You can verify, rotate the sc pulley for 10 times, and see if impeller will rotate for 34 or 36 times.