JacRyann

Yes, manual controllers are too simplistic. They only know about inlet-pressure vs. their own spring-rate. But nothing about spring-rate of wastegate or exhaust backpressure.

In my earlier testing with regulator-style manual controller, I had to send 4-5psi to wastegate in mid-range and reduce that to 0psi by redline in order to maintain 18psi flat boost-curve. Yes, I had one hand on the **** and adjusted it as the engine revved in order to get flat boost.

EBC was much better solution as it used feedback to adjust its solenoid's duty-cycle to maintained programmed boost.

thingo

With mine we considered 30 bp at 20psi at 6500 was to high, got it to about 25bp at 20psi ( (but twin scroll which might change things)though I've seen better. Back pressure went up when increased the size of the compressor wheel.

TurboTommy

I also had higher back pressure with a larger compressor. It's because it takes more exhaust gas energy to turn a larger fan, so to speak, even if it doesn't have to turn as fast.
25 psi back pressure for 20 psi boost at peak power is very good; the twin scroll makes more efficient use of the energy available from the exhaust.

Patrick;
I'm seeing a ratio of 20 psi bp to 20 psi boost at mid RPMs. As RPM rises, it reaches 25-26 psi bp to 18 psi boost at 6,500 RPM. I have a fairly large hot-side.
I was also told that for a good combination of spool and power a 2:1 ratio at peak RPM is very acceptable.

TurboTommy

Well, that's just it though, Alan.
What you were posting wasn't simple at all.

nick_968

What turbo are you running Tommy?

TurboTommy

61.4mm compressor with p-trim turbine wheel in a replica 10 housing

333pg333

Actually looks like we hit more like 50psi b/p @ 26psi boost. The 'Engine Coolant' pressure was actually the Xover sensor. TPS was playing up at the end too for some reason.

thingo

I guess those back pressure numbers might be typical, but they still seem far from ideal to me.

Geneqco

Here's a good article dealing with back pressure and, in particular, the interrelationship between back pressure and camshaft selection:

Turbo Camshaft Guide - The Truth About Camshafts And Turbochargers

http://www.hotrod.com/how-to/engine/...amshaft-guide/

Of particular relevance to this thread are the following excerpts:

Turbo Cam Basics
The one thing that all three of our noted sources emphasized is that the knowledge base established from turbochargers designed 10 or 15 years ago is antiquated when applied to the current crop of high-efficiency turbochargers—unless you’re trying to get by on old, cheap turbos. “In the old days it was typical to see 1.5 to 2:1 backpressure ratios,” Duttweiler says. “Today the backpressure is actually less than the boost pressure.”

All internal combustion engines perform best when tuned with a certain amount of camshaft overlap in which both the intake and exhaust valves are open at the same time. If the exhaust backpressure is greater than the inlet pressure, the exhaust will push back into the cylinder and (given enough time) up into the inlet manifold.


With newer turbos, the reduced backpressure also means the exhaust valve can be opened sooner and held open longer, which is generally accepted as beneficial to high-rpm power production, just like on a normally aspirated engine. According to Duttweiler, to make good power, turbo engine efficiency depends more on low exhaust backpressure than tricks with the cam.

333pg333

Yes, we do have too much backpressure and need to look at this. Don't want to spend too much more time and money on this motor though. Can only think by going to a larger .82 or 1.06 housing that we will lose a lot of response. Need a VGT type turbo as has been suggested.

gruhsy

20/18
20/17 ish

I posted on another post about this a while ago. Info is on here.

Jason

Dave W.

I may be pointing out the obvious, but the crossover tube adds backpressure due to it's length and diameter. It needs a small diameter to maintain heat and velocity to the turbo, but at the same time there's a large amount of backpressure simply from pushing 500HP worth of exhaust through a long 2" tube. Since the turbine also works on a pressure ratio anything you can do to improve exhaust efficiency will pay a large dividend in reducing backpressure. Have you thought about moving the turbo?

thingo

Lots of things you can do before going to Vgt, pipe sizes can be optimized and shortened twin scroll,but as you say it takes time to try things which has a cost,but I doubt Vgt would be as simple as it sounds anyway.

333pg333

Too hard to move the turbo on a rhd car without major changes. Have to stick with what we have for the time being.

Time and money spent on this motor which I don't want to do much of anymore Rod. I feel like we're pulling in the opposite direction to that of it's initial design.

Geneqco

+1 I think the biggest issue is finding a suitable vgt turbo without having to use a twin setup.

I think this is the thread Gruhsy referred to:

https://rennlist.com/forums/944-turb...-pressure.html

Lots of good info there including this from Raceboy:

My friend has an E34 M5 with fully built 3.6 engine and Borg Warner S480 turbo running at 1.6 bars of boost and having maximum of 1.3bars of TIP Full boost is at 4700 rpm all the way to 8400 rpm.
Standing mile speed is 331 kph and that is with full weight (1820 kg with driver).
I installed and tuned ECU on that car so I have 1st hand information.

1.3 bar of TIP was measured at 7000 rpm and it did not rise at all from then on.

Sid mentions his as being around 1:1