FI Build-up - German Plenum/TB & MS-1

[Vee Dub Nut]

Ok.. been pondering on this for a few days, and talking shop with a coworker. Some comments to ponder on...

It's not really hard to figure out the why, you just have to think about it for a second. With the gate on a primary and the gate open to control boost pressure, you'll end up with exhuast moving quickly in 2 directions. One direction when that cylinder fires, then reversing as the gas from other cylinders make their way out the gate. In the collector is the best place for it, the gas does not have to reverse direction.

I have a difficult time seeing how the exhaust gasses could reverse flow in the header unless my turbo is HUGELY undersized. The momentum of the exhaust flow is going to be the dominate factor in the other three cylinders and is going to tend to push them through the collector to the turbo. Now, IF the turbo was grossly undersized and I had a high turbo back pressure, this could possibly cause some issues with flow. I don't see this as an issue with the current placement. The 38mm wastegate has plenty of capacity to remove pressure from the header, and if you remove the addition from one of the primaries, your removed 25% of the energy available for the turbo thus controlling the amount of boost it can create. I'd venture that if you completely remove the flow from one of your primaries, you would have trouble making boost at all unless again you have turbo that's really too small for your engine.

also, I believe Marty said that you can lean out that one cylinder under wastegate cycling...

I'm having trouble conceptualizing this one. Walking through a hypothetical scenario, say I'm under boost and I reach my prescribed boost level to begin opening the wastegate. The gate will open on the number two primary and then the exhaust flow will partly begin to be vented to the atmosphere. The actual VE of this cylinder should go down while the waste gate is open, cause part of the charge is going to be blown through the cylinder during overlap due to the lower back pressure. So with the drop in VE for that cylinder, I think I would be more likely overfueled.

if the exhaust gas from the other primaries doesn't reverse, it will at best be "dormant" at the collector... the pressure WILL go down, and thus slow down the turbo because of the loss of pressure on one primary. but it leaves the number 2 cyl with a rapidly fluctuating backpressure which can affect its volumetric efficiency.

Isn't that just the point of a wastegate? Its a pressure relief, its supposed to reduce the pressure in the exahust pre-turbo so that the turbo stops making more boost past the desired level.

think of the wastegate as a controlled exhaust leak, because that's what it is. if you have an exhaust port leak on a normal header, it hisses and lean-pops due to the irregularity in backpressure.

Yes I agree, a wastegate is a controlled leak. By the way, that's not necessarily a lean pop. The only way you can get a pop in the exhaust is if you have unused fuel to combust. Now this can happen when your lean because combustion isn't complete and pushing unspent fuel into the exhaust, or it can be when your over fueled and not burning all the fuel. When you have the unused fuel in the exhaust in an environment with plenty of heat, and you present the oxygen through a leak or similar, you get combustion and thus popping.

This all goes back to being properly tuned and having a good exhaust system.

while things will never be equal on an un-equal turbo header, you stand to gain a lot more efficiency and control by letting the wastegate bleed from all four cylinders. I think MinamiKotaro noticed that his boost was a lot more consistent too... less random spiking and creep.

This may be.. I think the biggest advantage would be more consistent VE's from cylinder to cylinder. How much more consistent and is it a big issue? Dunno..

For two reasons I'm going to leave it where it is right now. First, I'm not going to fix something that I'm not necessarily convinced is a problem. Call me hard headed. Second, I don't even have any of the materials to make the modifications on the header. I'm on a VERY tight budget to finish this out and am not going to be buying more stuff.

I'll be the first to post up if I decide to change it down the road cause I'm not happy with it for some reason, with plenty of info on the how's and why's.

[miniman82]

You just had a brain fart, didnt you?

[Vee Dub Nut]

You just had a brain fart, didnt you?

please explain?...

[Vee Dub Nut]

Got my stainless tubing in today. Got the charge pipes all cut and sorted out. The only thing I'm lacking is the 45 degree coupling between the turbo and charge pipe (ignore the straight coupling that is currently holding things together).





Getting closer!

[miniman82]

You just had a brain fart, didnt you?

please explain?...

Couple days ago you were cool with the explainations, now that you've had time to think about it you're second guessing? Just think about it a little longer...

[Vee Dub Nut]

Um.. ok

Never said I was cool with anything that I recall. You were the first person to actually give a technical reason for your argument and I appreciated the input. I did a bunch of searching on the forums about this topic and didn't find a shred of technical info about it. Maybe I missed some good threads on the subject, but my searching didn't turn up anything. I'm personally enjoying the technical discussions, that's why I do this kind of stuff.

For what its worth I have thought about, and likely will continue to do so. I shared my thoughts, could be wrong could be right. I don't profess to be an expert, but I've got a decent handle on engines and how they work. There very well could be some flaws in my reasoning (again, no expert), but it will take more than someone just telling me its wrong to convince me. No offense intended...

[miniman82]

None taken. I'm just saying; the collector is the best place for it, exhaust 'reversion' is the reason.

[Vee Dub Nut]

Remember that this is an unequal length tube turbo header, so reversion is theoretically already an issue. I may found out differently, but I don't think the wastegate location is going to make a big difference either way.

Since I have a fairly small mild cam with short overlap, this should help negate any reversion issues from the header design and wastegate placement.

[Beanbooger]

I say run it as is, but KNOW that it is sub-optimal. Whenever the wastegate is open, the VE for that cylinder will be different than the others, so its fueling WILL NOT be correct. And it may not be consistent across the entire rev/load range. VE may be up or down at different operating points and it is really hard to say without measuring and comparing (expensive), but I'd guess higher VE (and a resulting lean mixture) at high boost/high RPM due to the lower exhaust pressure leaving less residual charge in the cylinder. You may or may not even notice anything, but I'd keep a close eye on the spark plug for fouling or evidence of detonation, and watch for boost creep at higher RPM. Otherwise everything looks real nice and clean!! Nice Megasquirt kit Mario!!

[klatin]

...I have a difficult time seeing how the exhaust gasses could reverse flow in the header unless my turbo is HUGELY undersized. The momentum of the exhaust flow is going to be the dominate factor in the other three cylinders and is going to tend to push them through the collector to the turbo. Now, IF the turbo was grossly undersized and I had a high turbo back pressure, this could possibly cause some issues with flow. I don't see this as an issue with the current placement. The 38mm wastegate has plenty of capacity to remove pressure from the header, and if you remove the addition from one of the primaries, your removed 25% of the energy available for the turbo thus controlling the amount of boost it can create. I'd venture that if you completely remove the flow from one of your primaries, you would have trouble making boost at all unless again you have turbo that's really too small for your engine.

Reversion happens every time when the pressure in the cylinder is less than the pressure in the exhaust and the exhaust valve is open. Remember, a turbine is driven by mass flow and pressure. The mass flow of gasses through the turbine is about the same as the mass flow of air through the compressor (the little bit of increase by the fuel added does not count). Therefore the pressure ratio of the turbine must be higher than the pressure ratio of the compressor. And therefore the exhaust pressure pre-turbo is higher than the boost pressure. Otherwise you could connect the compressor outlet to the turbine inlet and the turbo would spool by itself, which is a perpetuum mobile.

[Beanbooger]

Therefore the pressure ratio of the turbine must be higher than the pressure ratio of the compressor. And therefore the exhaust pressure pre-turbo is higher than the boost pressure. Otherwise you could connect the compressor outlet to the turbine inlet and the turbo would spool by itself, which is a perpetuum mobile

I'll be brief here because it is off topic (we can start a new one if anyone is interested...), but the above statement is not true. There is an energy balance between the compressor and turbine and bearing mechanicals, not a pressure balance. It is very common on large diesels to have positive deltaP (compressor pressure higher than exhaust pressure).

[MarioVelotta]

Nice Megasquirt kit Mario!!

Thanks I say nice job Vee Dub Nut for doing a clean install!!

[Vee Dub Nut]

So close I can almost taste it...

Been busy the past few days. I dropped the engine out again and changed the engine mounts out. The ones I had in there were a little questionable, and the more I thought about it, even though that was the last thing that I wanted to do, I figured now was the best time to drop the engine and change the mounts.

Today I spend the day hooking everything back up, installing the header/turbo, dump pipe, charge pipes, diverter valve, etc...

Here are some pics:






Tomorrow I have to mount my full flow filter, and hook up my wideband. I'll also measure all the line lengths so that I can get my full flow lines, turbo feed and drain, and wastegate reference line made up next week.

I'm looking to start her up next weekend! Here's hoping I don't break anything!

[ToRy 70]

Lookin' good.

[MarioVelotta]

Lookin' good.

x2