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Boost pattern on my stock sequentials is 12-8.5-11.5 psi until 6000rpm then it drops to 8.5 psi and holds until redline. Boost transition is fine and the car pulls great untill 6000rpm where it starts losing torque.
The car is pretty stock aside from few bolt ons:
-oem decatted downpide
-midpipe in stock diameter 2.5"
-Powerflow catback in 2.25" (smaller an stock diameter..bought with it installed)
-cheapman airbox mod with kn drop in filter
-knightsports chipped ecu
-very little flow of water injection (0.4mm aquamist nozzle meaning 190cc/min) single stage post intercooler starting at 8psi to control knock at possible spikes. Remark that boost doesnt creep nor spikes at all.
-hks twin power (with all new 4xbur9eqp and new wires)
-new fuel filter and all maintenance done as usual to a recently bought car
-hks grounding kit
Knowing that stock boost pattern is 10-8-10 and 8psi again from 6000ish to redline and that in similar modded cars' logs that I was able to find they hold max boost until 7000rpm, what can be causing my boost drop? I thought about wg solenoid but it controls boost just fine until 6000rpm, also cheched the ypipe coupler for leaks..
Could the smaller catback be causing enoght backpressure to make the boost drop?
Regarding the wi, the ecu is not tuned for the water injection as it is injecting such a little amount of demineralized water to control possible knocking just in case of spikes happening (stock ic), not leaning out fuel maps or advanced ignition. I tested the car with a bigger nozzle (0.7mm aquamist jet flow checked near 350cc/min instead of the 190cc/min of the 0.4mm jet) and as spected boost was lower in every rpm when the wi was on, so was the power felt on the butt dyno. Could be the wi limiting the boost at high rpm with the stock wg duty cicles?
Or am I completely wrong and it is the usual boost pattern?
Thanks mate! That was one of the post that I mostly took into account to think everything is fine. Ecu prolly has stock boost control because I tested it before in a completely stock fd without any boost increase.
I floor it down in 4th gear from 3000rpm and got that 12 - 8.5 - 11.5 - 8.5 pattern. With a bigger jet of wi boost was lower so do power felt. It could be that the restriction the stock ic makes at high flows (top end + wot) supose a big difference in pressure between the source where the wastegate take the air from just instantly after the turbos and the pressure where I have placed the boost gauge at the uim with a difference in temperature also due to the ic and water injection doing its job..
I will try to install a mbc taking the boost signal from the uim directly to avoid pressure losses. Do an ebc working in closed loop fixes the matter or just the stock sequentials doesnt flow enought to mantain 12psi until redline?
Short answer (TL;DR): Try a less restrictive intake (mindful of spiking/restricter pills etc) before you try changing the wastegate position through boost control. Reducing intake restriction makes the turbo work less hard; boost control makes the turbo work harder.
Detailed answer: If you want to raise your boost, I propose that the first thing you try is to keep everything the same but remove your stock airbox and ducting and replace it with something that has less restriction. Follow me here for a bit because I'm going to go into some OEM level black arts of tuning and engine systems design, adapted from some work I did a couple years ago on an engine that recently went into production.
Here are compressor maps for the '92-'93 turbos (black) and the '99 turbos (pink). Look at the compressor speed isolines.
Find the 100,000rpm black line and follow it. Notice that as the pressure ratio goes up, the airflow goes down? The pressure ratio goes up as the wastegate position changes (compressor speed increases), but at a fixed compressor speed the ratio goes up due to the restriction across the compressor inlet. I'll skip the equations, but remember that the pressure ratio is the ratio of the compressor outlet pressure to the compressor inlet pressure. It is NOT the boost pressure after the intercooler or in the intake manifold; that is a misconception that results in people underestimating how hard their turbo is working. A given intake system (stock or aftermarket) has a mass flow vs pressure drop curve.
The image above are pressure drop curves for a series of OEM intake systems on modern factory turbo piston engines. The curves are measured in an engine dyno lab on a firing engine by plotting the engine mass flow and the turbo pressure ratio at discrete steady-state measurements on a WOT torque curve. Notice how the flow vs pressure drop curves are exponential rather than linear--they have that curve in them that makes the restriction shoot up. From the perspective of making power, you want to be more like the orange line, not the green line.
We can subdivide the stock intake systems into its components by putting each part of the system into a flow bench at a fixed airflow. Here's a fictional pie chart I made (inspired by real systems) that illustrates the principle:
On a stock intake system, when you change the air filter to a drop in K&N style, you change the green part of the chart.
On a stock intake system, when you alter or delete the dirty air duct that goes from the fresh air source to the airbox, you change the blue part. So for example, a pod/cone style filter typically eliminates this duct altogether and that's one of their biggest benefits in terms of reducing restriction.
On a stock intake system, When you alter the duct that goes from the airbox to the compressor inlet, the clean side duct, you change the red part.
Any modification to a stock intake system that provides colder air is a good thing for performance overall, but if it doesn't change the restriction by altering the ducting, you still have mostly the same pressure ratios at a given compressor speed. Think of it this way (a little oversimplified): The restriction in the exhaust and the boost control tuning/wastegate position change the compressor speed (move you to a different speed isoline), but the position along that isoline depends a lot more on the restrictions in the overall cold side ducting (pressure drops before and after the turbo).
When you put a less restrictive intake on the system, you will flow more air at the same compressor speed. You're making the turbo work less hard at peak engine power. If you keep the compressor inlet side the same but move the wastegate position via boost control, you're increasing the compressor speed and increasing backpressure and energy wasted on pumping. You're just beating on the engine and turbo harder.
I am a big proponent of the stock boost control system, especially on a stock or chipped ECU, provided that you aren't catless + unported wastegate. Replacing a system with closed loop feedback, that is according to Mazda gear-based and synchronized with the turbo staging, with basically a hardware store contraption isn't ideal. The downside to the stock ECU/chipped ECU control is that nobody made a widely available solution for to tuning the stock boost control maps. People hate on stock boost solenoids + different restricter pill approach, but it's an intentional modification to keep the more sophisticated stock boost control logic in the system.
For more details on stock intake design principles, read my article on the design of stock intakes systems TWW - arghx's technical ramblings thread (scroll down a bit) for more details.
A-W-E-S-O-M-E thanks for taking your time to share your knowledge
I will try a mushroom type HKS intake I have here installed in another fd that is in dry storage, just worried about it sucking in hot air from the engine bay but I agree with less restriction it can flow more air at the same pressure ratio as you say, will also remove the secondary butterflies to enhance induction flow. If there are no spikes I will try to switch WI off for a small run in 3th gear after 5000rpm (I run a on demand type pump so had installed a switch just in case I run out of fluid in order not tu burn the pump trying to maintain pressure without fluid to feed the ssytem) to see if it has something to do with the pressure drop at the uim as it should be colder thus denser and at lower pressure for the same mass flow with the WI on.
Thanks a lot one more time I really love this kind of info and appreciate your kindness and selfless help
I think your catback is the problem. 2 1/4 inches is pretty restrictive. At 6k rpm, you have a lot of flow. Thats like trying to blow really hard thru a straw.
Do the math. Thats about a 20% reduction in flow capacity from a somewhat restrictive 2 1/2 inches (oem) to start.
If you can afford it, I would replace the catback before doing anything else.
The intake swap may very well solve your problem. I had a collapsing intake inlet (cheap bastard style, but not the real deal) that I had fabricated from too thin a material closing up with similar results. High rev boost fell off.
Before I figured that out I had tee'd into all my turbo control lines with a guage to verify they were getting vac/pressure signal, and tested the actuators. Turned out to be strictly a restricted flow problem.
The intake swap may very well solve your problem. I had a collapsing intake inlet (cheap bastard style, but not the real deal) that I had fabricated from too thin a material closing up with similar results. High rev boost fell off.
Before I figured that out I had tee'd into all my turbo control lines with a guage to verify they were getting vac/pressure signal, and tested the actuators. Turned out to be strictly a restricted flow problem.
Originally Posted by adam c
I think your catback is the problem. 2 1/4 inches is pretty restrictive. At 6k rpm, you have a lot of flow. Thats like trying to blow really hard thru a straw.
Do the math. Thats about a 20% reduction in flow capacity from a somewhat restrictive 2 1/2 inches (oem) to start.
If you can afford it, I would replace the catback before doing anything else.
Neither swapping out a catback nor an intake are especially time consuming. Give both a shot and report back.
Current condition
change airbox only
change catback only
change airbox and catback
I will. By the moment I have squeezed some minutes today to remove the secondary batterflies. As soon as the two little beasts I have at home allow me to spend some spare time out I will put some washers on the catback bolts to void that restriction, then bolt it back and test the hks mushrooms and both of them done
Will let you know, I cannot promise when it will be possible
The ducts are paper based with spiral wire reinforcements, they dont collapse under load at all. I took the idea from here http://www.rx7.org/Robinette/coldair.htm
But kept the original duct opened to warranty enought flow. As the surfaces on both sides of the airbox are similar it should be enough to not catch fresh aire feom the Intercooler. Although I will try the hks filters and if the airbox is the problem I will make sth to isolate the hks filters from heat, but I would prefer to keep the stock airbox if the others doesnt fix the problem
This is the intake I have handy. After those aluminium pipes they are attached to the same rough stock plastic hoses. I did felt a considerable difference in a 2gen turbo when swapped the original TID for a more suitable hose
I have to say that when I did the airbox mod there was a slight improvement on lag and butt dyno
If your cardboard is rigid, your airbox will flow plenty of air. The airbox is not causing your boost drop. You dont need to replace it. I still think the cardboard is a bad idea.......
Most likely the problem is your restrictive catback. I suggest looking in the classified ad section for a good used Racing Beat catback.
You are right it is not the best material but was the most suitable I had handy in the house when doing it. I will take this magnaflow 2.5" catback from the donor of the hks filters
Today tested with the magnaflow 3" catback (not 2.5" as I thought) and the secondary butterflies removed. It pulls harder and the drop has been retarded to 6200rpm or so and but it is still there until redline. It sticks on 9psi from 6200rpm to redline so it is better than the previous 8.5psi from 6000rpm on. I switched the wi off from 5500rpm to redline but it has marginally effects on it, very little boost gain if any so wi is not guilty either (as it is a single stage wi the amount of water is constant so at higher rpm wot the percentage of water is substantially lower than at mid range wot due to the increased airflow).
Here is a picture of the two catbacks side by side, weight reduction is noticeable too
Hks filters installed with new aftermarket bov. Yesterday while removing the secondary butterfies I put another ground to the coils (I had ignition break up at high rpm before changing wires and plugs and installing hks twin power, also replaced the leading coil by that time with a newer looking one).
Boost drop remains there. It maybe stretches 100rpm later or so until it comes in. This time I couldnt record it. I taped the hole from the intake to the stock airbox to send all that fresh airfow to the intercooler. I noticed some spikes to 13psi or even a lottle bit further so I changed the wi jet for a bigger one (0.4mm to 0.5mm) just in case...
Boost bounces in some pulls between 10psi and near 12.5 psi, not jumping but wiggling smoothly in 3secs intervals or so. That makes me think something is wrong with the boost control system, maybe the wg solenoid can be causing this kind of weird things?
After some tests, boost didnt go where it should (9-8-9 pattern) and then after leaving it idling for two minutes everything was back to normal. Something to do with the hot air being sucked or a soaked intercooler?
I also found an intake leak, some watter spitted over the throttle bodies leads to find a leak under boost, specially after lifting the throttle when the pressure between the throttle body and the turbocharger arises during a fraction of time until the bov is fully opened, leaving some water drops there. I have used joint paste for it, at the junction of the throttle bodies and the plastic elbow coming from the intercooler.
Have checked the dtcs and only "speedo sensor" fault was found. That should be the reason why cruuse control is not working neither the odometer.
Tomorrow when the joint paste is dry I will test the car again
I will test from the y-pipe to the throttle body, the cur runs smooth so there should not be any leak after throttle bodies.
It is a second hand car so cannot warrantee the mileage on the turbos, ot is possible they are old but they dont do any siren noise nor burn oil. I will have a look to the shaftplay of the primary turbocharger
Will let you know
There is an overhaul improvement over the situation at the beginning. Today check it in warm weather with the bigger wi jet and it doesnt spike at all. Boost now doesnt bounce between 13psi and 11psi like yestarday did... It holds 11.5 psi until 6300rpm or so then tapers down to 8.5 at 7000rpm
the other thing is, since we can't datalog a stock ECU/chipped ECU, we don't know what the boost control feedback is going. It could be reducing boost duty cycle in order to achieve its targets.
Do you see a difference in say 2nd gear vs 4th gear? Lug the engine at about 1500rpm and go all the way up to redline for 2nd, 3rd, and 4th.
I did the runs in 4th gear to take out of the equation the effects of the high rate of changes (fast acceleration) but will check it in other gears.
Does someone know where the boost maps are in the stock ecu? I mean and xdf file for tunerpro or the address and offset to change it in winols. Could try just increasing cycles of the wg at the rpm where the drop is found. I think this car boost control system works in open loop mode so the higher than stock readings should not affect the wg solenoid duty cycles. If increasing it doesnt work maybe the turbos are not in their best moment or there is a leak somewhere as you both suggest.
I will try to check the intake from ypipe to throttle body today and check the primary turbo
04-30-17, 02:44 AM
I really love this kind of info and appreciate your kindness and selfless help
