legacycentral bbs

Although a majority of people choose to do it one way or another Id like to start a discussion as if none of us know how to do it and decide on the best place to measure the boost pressure.

From my point of view you have 4 spots that would be optimal for certain reasons.

Closer to the turbo
Just prior to the throttle plate
Just after the throttle plate
Close to a cylinder

I can think of good reasons for each location. Ill wait until others reply before posting my ideas so to not throw the discussion in the wrong direction.

Please post a location (or two) and why that location would be the best

I use the line off the intake manifold for cylinder #1. I think it's best because this is the line to the pressure exchange solenoid. So you see what the ECU sees. If I had to choose one other location I guess I'd say closer to the turbo. That way you had a true reading of what the turbo is putting out.

It's good to know what the turbo's doing, since that tells you where on the compressor map you are.

It's good to know what pressure's coming into the throttle body, since that tells you how much your intake tubing and intercooler are dropping it.

It's good to know what pressure is in the intake manifold (which ought to be more or less the same whether measured just after the throttle or measured near a cylinder) since that tells you what the ECU sees and indicates actual load on the engine.

I would ideally have three gauges... or one triple combo gauge. Right now I can just switch my one gauge between compressor outlet and manifold.

ciper wrote: Closer to the turbo
Just prior to the throttle plate
Just after the throttle plate
Close to a cylinder

When working with an intercooler, close to the turbo will show what output your turbo has. Anywhere after the intercooler would theoretically be a smaller PSI number due to the pressure drop of the IC.

Maybe i'm not understanding the actual question, but i think the most beneficial point to have your gauge would be in the intake manifold as this shows exactly what pressures your ENGINE is experiencing.

So you don't want any vacuum readings ?

If you want some, you dont have a lot of options : after the thottle body

Well, it's not about "getting vacuum readings;" it's about what you're reading. You'll get vacuum readings when you measure manifold pressure, since the manifold is sometimes under vacuum. That can be a useful indicator of engine health.

It's all about what you want to read... I guess one concise way of putting would be: If you want to know how hard the turbo is working, use the compressor outlet; if you want to know how hard the engine is working, use the manifold.

vrg3 wrote:It's all about what you want to read... I guess one concise way of putting would be: If you want to know how hard the turbo is working, use the compressor outlet; if you want to know how hard the engine is working, use the manifold.

Actually, last saturday a friend with a 1g DSM came over and we plumbed in an extra gauge into his car to measure his pressure drop. His current gauge is plumbed into the intake manifold so we spliced the 2nd gauge into the line running to his turbo (same line for his boost controller). Unfortunately, since the boost controller was manipulating the boost - bleeding some off, holding some in - we couldn't take acurate readings. We turned it up to 24psi in the manifold, but no matter what, the turbo always read ~.6 - ~.7 BAR. Then his clutch started slipping as all DSMs do, so we went him.

lesson learned: Always run a seperate line for your gauge for accurate readings.

There are places you should monitor for tuning, depending on what you are checking.
Lets seperate that from the best place to monitor for daily health.

I can see for tuning that you would want to set the boost by monitoring closest to one of the intake valves. Once that is set you would probably want to check the pressure directly out of the turbo. This would tell you the total drop across the system. You could then monitor just before the intercooler and so on to see where the drops are to find any potential weak points (a resonator that can be removed, a bend that could be shortened etc..)

The boost controller setup confuses me. I would think you'd want to measure boost closest to an intake valve. Problem is the boost controller would try to pressureize past a partly closed throttle plate! If you have the controller on the turbo outlet this isnt the boost the engine sees but just the pressure after it builds up in the piping. Im not sure what the lag time is but it seems to me that it would take a moment for pressure to equalize across the system which would cause the compressor outlet measurement to be partially invalid.

For the gauge and daily driving I think Id want to know what the engine sees so closest to the intake valve. Problem with this is that you can see how well the blow off valve is working (if at all). Pressure between the turbo outlet and throttle plate could be at some insane amount and start backing up into the turbo. It would be nice to know just after the throttle plate because this would be an average of what all four cylinders see. Although very fast the opening and closing of the intake valves on a cylinder cause pulsing. Most pressure meters interpolate the pressure. Depending on engine RPM the % of inaccuracy from the gauge changes. I dont know if its enough to matter though. Think of it this way, you can tune an engine for a specific rpm by using an exact length of exhaust piping, to optimize it for the pulse timing. Its similar for the intake.

Another somewhat related topic is the option of electronic senders. Running a long hose from the engine all the way to the gauge causes all sorts of problems. Why arnt electric senders more popular? You dont see people running oil lines inside the car any more for an oil pressure gauge!

Do you really think the pressure in different parts of the manifold is different? I would be surprised if that were true, given the volume and the amount of air that moves through there. I believe that you'd see pulses if you measured too close to an intake port, but that's about it...

If you read other posts I've made on the subject, you'll see I maintain that the correct placement (for almost all street applications) of a manual boost controller is between the compressor outlet and the wastegate actuator, for just the reason you give -- that referencing the manifold instead of the compressor outlet would make the compressor try to pressurize past the pressure drop of the throttle plate.

I don't understand what you mean by saying the compressor outlet measurement would be partially invalid... It tells you exactly what the pressure is coming of the compressor, doesn't it?

The problem with mechanical oil pressure gauges is that a leak results in scalding hot oil spraying about the passenger compartment. A leak in a boost gauge line results in nothing more than air spraying out. What sorts of problems do you see with running a vacuum line into the passenger compartment? If you're worried about response, it's not that big a deal if you use appropriate vacuum line. MegaSquirt (a semi-DIY EFI system) uses a MAP sensor soldered on the motherboard, and is meant to be mounted in the passenger compartment. The time offset between the TPS curve and the MAP sensor curve is measured in microseconds.

hmmm 2 boost gauges would be pointless but cool, I figure the more gauges the better. heh heh

ciper wrote:Another somewhat related topic is the option of electronic senders. Running a long hose from the engine all the way to the gauge causes all sorts of problems. Why arnt electric senders more popular? You dont see people running oil lines inside the car any more for an oil pressure gauge!

Good points all. This is a good thread.

As to your question ciper, electronic boost gauges tend to cost quite a bit more. The Greddy unit is usually between 160-180; more if you want warning and peak recalls and memory and all that jazz.. Compare that with an Autometer craptacular gauge that goes as low as $30.

A co-worker was reading over my shoulder and he pointed out that an electronic gauge is suseptible (sp?) to electronic interference. I understand his point, but i would hope that the higher quality gauges compensate for this somehow.

It's a pretty simple system and interference wouldn't really be a problem. It's not like it's a high-frequency signal or anything. The only concern about an sender-based gauge might be adequately grounding it, but I'd hope you'd run the ground wire to the gauge anyways, since you're running the signal and power supply lines too.

OK guys, let's get some visual aids in here. I installed my boost gauge yesterday, and am reading from what I thought was just behind the throttle plate:



But this must be just before the plate - because until I open the throttle plate the gauge reads 0. Boost is read fine, it's still useful - but I do have a vaccuum portion on this gauge and I'd like to see the relative health of my engine's internal seals and such. It's in inches of mercury - anyone have healthy vaccuum readings from an EJ22T?

Also - what fitting should I read from?

Yes, I do believe that nipple is before the throttle plate.

Most of us, I think, usually connect our boost gauges to the nipple on the front passenger side intake runner, shown here on Morgie's old car:

http://www.graphics.cornell.edu/~v/pics ... ge_tap.jpg

I get vacuum readings around 18 or 19 inHg at idle and around 20 or 21 inHg when coasting at high RPM, with my gauge tapped there.

Don't use that line... If it's not showing vacuum there's some sort of check valve on that line somewhere, use a vacuum tee and plumb your boost gauge into the line that goes to the MAP sensor, that way you see what the ECU sees.

vrg - I see, that'll give me the readings I want? Thanks... I will move it next chance I get and see what happens. I think my line will reach there.

white-t, is the link in vrg's post what you're talking about? Or are you suggesting a different location?

yes, do not use that line, it is actually right at the edge of the throttle plate. It's meant to suck the fumes from the CPV. You need to hook it up some place else.

So thats what the line is for. Thanks admin... I was wondering about it's purpose. Anyone have a diagram showing what all the fittings and smaller hoses' purposes are? Theres a rat's nest of rubber under my hood.

you can call me josh instead of admin if you want.....

What, you don't like Admin? I think it has an authoritative ring to it. OK, Josh it is.

Josh likes to feel like he's one of the common people. =)

free5ty1e - Yeah, I think you'll get the readings you're looking for tapped where I suggested.