Billy W

Jake951 if the shop put the sniffer after the cat would the A/F ratios be correct? I have been told that after the exhaust passes through the cat you will not get an accurate reading and that you must use the test port before the cat.

toddk911

"Otherwise, you can get an electronic boost-solenoid from BoostSciences for about $30."

WOW, so their electronic controllers are the same price as the MBC?? I have the Reliaboost 1 for $30 and I have had absolutely no problems making steady boost and holding steady boost to redline.

So, both individuals that are loosing boost up top with GURU wastegates also have stock cats still in??? Interesting

And, I don't see how anyone with their stock 15+ year old cats do not have some level of clogging. Especially when you have Danno and Travis both saying "get the cat out", that has got to be a contributing factor.

Any forced induction motor is greatly inhibited by heavy backpressure in the 3 big areas: Spool up time, total boost achieved, and increased heat. With a more free flowing exhaust, you get better spool up, usually 1-3 more psi back that were being robbed from a restrictive exhaust, and therefore, much less heat build up (due to the turbo not having to work as hard to achieve the same level of boost.)

Jake951

I think I have a clue now about that weird spike up in boost pressure near redline. I looked again at the datalogging file and I see from the TPS data that at redline I let up on the throttle. The last data point in that boost plot was taken just as I was closing the throttle, which must have produced some transient condition that momentarily spiked up the boost and was caught by the datalogger.

I looked through my other dyno runs and I don't see that spike, and those other runs all had consistent TPS data showing WOT throughout.

Jake951

I had not heard that before, so I'm not sure if it's true or not. However, it's easy to check by doing the measurements both at the test port and at the tail pipe and comparing. I'll ask the dyno shop next time I'm there.

Jake951

I uncovered this thread from a few months back and it may explain at least part of the boost drop observed at high rpm. Basically it says that the MBC senses the pressure before the intercooler (through the banjo bolt) but actual manifold pressure is measured after the intercooler. There is a pressure differential between the two because of the pressure drop through the intercooler. Furthermore the pressure differential is not constant with rpm because as air flow increases at higher rpm's, there is more pressure loss through the intercooler. Since the MBC controls the WG opening based on boost pressure before the intercooler, at high rpm it sends too much pressure feedback to the WG based on the reduced manifold pressure downstream. It seems the correct thing to do is to tap the MBC into the pipe after the intercooler and not before.

Does this make sense?

JustinL

Jake, I had given that some thought as well. In fact I had asked about running it right off the manifold after the throttle body, so the wastegate would only get the signal to open after MAP was at a maximum. The reason that someone gave me is that you want to keep the feedback loop as short as possible to avoid boost spikes.

Justin

Edit: HA I misquoted myself! here is the thread I was talking about:Click me!

toddk911

I might have been the one that started that post or I know I mentioned it here before.

Yes, there is anywhere from a 1-3 psi difference in pressure drops across Intercoolers. Even brand new, high end IC's have a 1-2 psi drop, as quoted by Corkey Bell.

I told people in those posts to mount the wastegate line AFTER the IC. I did, and it is pretty easy. This means now that when your boost guage reads 15psi, 15psi is actually what is going into the manifold, not 14,13,12 etc.

toddk911

yep, here is one of my psots from that thread:

"Nope, your not missing anyting. I have posted on this before but it did not get much attention due to the fact that I first read it at a ricer site, but then found the same info in Maximum Boost.

I have my banjo bolt set in the IC pipe AFTER the IC due to the 1-2 psi standard pressure drop. So, as you mention, if you WG sees 15 psi before the IC and opens as planned, only 14 or maybe 13 psi will actually get into the intake. Corkey Bells says that even brand new modern IC will still get about 1 psi drop, so I would assume our IC's see a good 2 psi drop, if not more."

NZ951

I just tapped the port on the manifold with a banjo and two ports for the boost, figure you can get more accurate than that. It was the port that went to the ISV.

Duke

It actually doesn't matter since the boost gauge doesn't take it's reading from there, but from the KLR line that is mounted directly on the intake manifold. The only scenario it would make a difference if you would have psi/bar-settings on the boost controller. But with a mechanical boost controller you set it by "turns" and how many turns that is necessary for a certain psi-value differs from car to car anyway.

The reason why it's placed there is because it's closer to the turbo and that makes the reaction of the wastegate faster.
On many turbos the pressure line to the wastegate is actually placed directly on the compressor housing.