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Can a cylinder head flow more than the exhaust can handle?

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  • Can a cylinder head flow more than the exhaust can handle?

    My car is a CQ20V and with around 1600 miles under its belt is running pretty well following an engine rebuild. Fitted to the engine are: brand new 034 injectors, Tommis adjustable cam pulley adjusted to compensate for maximum skim the head came with, cylinder head is nicely ported with different possibly supertech valves (i didn't get the head done it came on a spare engine), both manifolds matched to head, standard MAF, tubular exhaust, decat pipe and Tony Banks 2 box stainless exhaust.

    Yesterday i went for live remap and the following checks were made prior to getting the emulator plugged in and showing how the engine was using the standard map.

    Fuel pressure. 3.5 rising to 4 bar without vacuum.
    Fuel flow. 2.5 Litres returning to tank per minute
    Lambda control. value of 1 and reading 10 times every second, old one was only managing 5.
    Idle C.O - 0.5%
    HC - 95ppm
    VAG fault code reader plugged in. no faults.
    Coil voltage. 13.4 with engine running
    Alternator output. 14.2v
    Spark plug operation. consistent across all 5 plugs, coil building spark nicely - leads all good.
    TPS range. no problem. 3.3v at full scale deflection.
    MAF receiving 5V and responding but unsure of voltage output (beware not all operating MAFs are equal - some output more signal than others)
    Compression. 14.7 BAR across all 5 cylinders with only 5% max variation.
    Cam timing all spot on all the marks.
    Inlet air / vacuum leaks. None.
    Exhaust leaks. None.

    Results were interesting. Performance was pretty good mid range but on full load, the emulator was showing that the engine was only making use of 90% of the fuel map. It was interesting to see what was going live as it happened but no matter what was done with the throttle the car just wasn't using all the map. AFR was dropping to around 12 - 12.5:1 as it should when the throttle was pressed all the way down but then quickly returned to 14.7:1 or as near as damn it as revs built to red line. The emulator was able to make changes to the map in the first 90% so was talking and amending things properly but couldn't do the last bit.

    Much head scratching followed and the current working theory is that somewhere the car isn't breathing well enough for enough air to pass over the MAF to allow the last 10% load fuel to be generated. There are no obstructions in the inlet right from cold air hose behind headlight, through filter (which i will change as although it wasn't that dirty, for the price of a new one it is easily eliminated), through the michelin man hose and through the throttle body. both butterfly valves are opening fully to their stops on full throttle.

    I need to do some investigating before my return visit to finish off.

    The only thing which was harder to check was the internal condition of the exhaust. It is only 4 or 5 years old and the center section is straight through with inbound and outbound pipes in line with each other, the pipework over the rear sub frame mirrors the original and the back box has offset in and out pipes leading to a single pipe splitting to 2 tail pipes.

    Is it possible that the back box simply can't pass enough gas when on full throttle and high rpm? The offset nature of the pipes leads me to believe there is some kind of chamber in the silencer which the gas needs to navigate before exiting rather than rushing straight through. My idea is that now the head flows well, the exhaust which previously did alright with standard amount of air passing through is now struggling and becomes a limiting factor. There is no crush damage anywhere on the system but I've currently got no idea if wadding is escaping into the gas flow anywhere.

    Appreciate your thoughts!

  • #2
    What maf are you running?

    Are you still on a standard 7a airbox?

    The intake behind the headlamp is well known as flowing very poorly, and the maf is a limiting factor on even a standard engine.


    • #3
      Still on standard air box, paper filter and the 7A Maf. The hose from behind the headlight is indeed a bit 'rough' inside as it is made from foam of all things which is stopped from collapsing with a wire coiled up inside it. It is however quite a bit bigger than the hole through the maf housing. I did try the V6 maf mod a while ago and went back to standard as the only thing that happened was... Not a lot. It was also a bit tricky to connect up as the Michelin hose is way to small. A generic silicone hose with necessary take offs fitted for isv etc worked though. Going through a mapping process may be the time to try it again though as there would be no better time to make the adjustments!


      • #4

        What a marvelous thing that real time tuning you have access to! To your title's question I think yes certainly it can but didn't your countryman Mr. Ricardo say that the intake air flow was more critical as the air was thicker/heavier going in?

        - How much actual vacuum does the engine pull at full warm idle?

        - Does full warm vacuum get delivered to the FPR?

        - Does the engine get to full warm; and does full warm get correctly sensed?

        - Are there any untested opportunities for un-metered air to leak in (gaskets, vent pipes, small hose connections, O-rings, etc.), in between the MAF body and the throttle valves?

        - Do 7As' over there have that flexi warm air duct between the alloy header shroud and the air-box?

        - Perhaps the air-box been invaded by nest seeking squirrels?


        • #5
          Ha haaaa. Squirrels have been eliminated from being the problem ​​​​​​

          Actual vacuum figures weren't actually tested but the vacuum being applied to the FPR was good. Using the garages very good quality pressure gauge,(not my DIY gauge with erratic undamped needle) I could see a steady reading and it was 3.5bar idling and straight up to 4bar with vacuum pipe disconnected. Both readings rock steady and switch off holding pressure also good.

          The ecu coolant temperature sender is new and working fine. They are only £9 for bosch item so it had one as a matter of course during the rebuild. I forget the figures but when I tested it, the resistance started off where it should and ended up where it should. A short test lead with compatible plug on end makes this very easy, holding multimeter leads on the sensor in situ is not. Engine temps for coolant and oil both seem spot on as well. First line on oil temp while out testing and coolant just past the first thick line on coolant.

          Flexi hose from manifold to airbox hot/cold flap is there but to be honest, I've fixed that so it only ever gets 'cold' air. It never gets that cold here so it's never been a problem.

          With regards to air leaks and un metered air, I don't think so. I have been very careful to check every single pipe and replace where even surface cracking appears. I have a rare unsplit Michelin man hose doing the business at the moment. I did have a small leak around the idle speed adjuster screw but new o ring there has fixed it.

          The only thing not connected is the tank vent line from the blue solenoid. It isn't causing a leak because where it is connected is blocked off. I am reinstating it as part of investigating but I get no vapour smell through the open solenoid at the moment so I don't think it will contribute any extra 'fuel' ?

          I've been looking at the v6 maf conversion again and it is a considerable size bigger than the 7a. It just seems incredible that it came out of the factory with such a restricting route in for the air. Maybe now is the time to get a set ready to swap over on my revisit to the tuner and see what happens. It's not a big job to switch


          • #6
            The figures mean very little to me but they represent the map and when running, the blocks of map being used turn red and move according to load etc. Getting On and off the throttle is how we could tell only 90% load was being achieved - the red blocks just didn't get all the way to the right
            Attached Files


            • #7
              Your post give me some kind of cancer but after 5 minutes I understood your issue :-D

              Lean out mixture at high revs? - your fuel pump is dying. Possible the fuel filter is only "full of dirt" but I believe more in the fuel pump.

              You can't change the last row in your injector table? Sell the scrap ECM at ebay or somewhere far away.
              What a ****** software :-D

              At my V8 the maf load raised after updating the cats a little bit but not soo much.
              Air cleaner could make some issues or IAT sensor - don't know what you are using and what is really the issue to be honest... brain cancer... :-D


              • #8
                Now that is interesting. The fuel pump theory does sound possible (filter is brand new. The 034 injectors were making the car rich everywhere apart from high load and rpm. That could suggest that they weren't getting enough fuel to satisfy them. They weren't massive spec as there is no point in my car but it is strange how they made it rich everywhere apart from when you could expect them to need the most feeding...

                Sorry for what my post did to your head, get well soon!


                • #9
                  Measure the fuel pressure during driving and you will see what happens

                  Melted cat could cause some issues but I'm sure you would notice glowing parts of your exhaust.


                  • #10
                    Everything I have ever seen or heard of with 034 has been awful, just not quite right. Exhaust manifolds that have not been finished/cleaned up inside, injectors that don't work correctly. Just not finished off.

                    Have you tried running it without the exhaust system on just to rule that out?
                    If the fuel pump is suspect or the original then yes I'd look towards that as a possible cause.


                    • #11

                      I thought the opposite (incorrect air signal vs lack of fuel) but I admit my understanding is really very limited, please help me understand your current issue and say if I'm way off base here as follows:
                      - the map you refer to doesn't / can't know you've fitted higher than stock capacity injectors, correct?;
                      - your healthy but not excessive (over stock) fuel delivery capacity is perhaps evidenced by the negative trimming you have had done at the low end (and mid-range too, correct?);
                      - is not the current issue that it is lean just at the top end, correct?
                      - is it possible that the engine may be fully capable of breathing at the top end, but that it is acting as if the air flow is not being measured correctly (i.e. it is being under-reported) up there?
                      - is the above perhaps because the MAF's hot wire may be just insulated enough with baked-on crud to under-report?
                      - might the sensor (and the above symptoms) respond somewhat to a cleaning to at least indicate this as a possible causal issue?
                      - what are the current MAF outputs at idle; and at WOT?
                      Last edited by Lago Blue; 11th November 2018, 20:31.


                      • #12
                        I don't run a cat' so can rule that out.

                        My flexi joint in the pipe where the cat used to be is crusty on the outside, i haven't had the time to take parts of the exhaust off to look inside yet but one of the ideas suggested by tuner was to try removing rear box and see how it is. (other than NOISY ) I would need to rig up a way to support the back end of the middle section while doing that but shouldn't be too hard.

                        Dave, I hear things about 034 as well but the injectors are Bosch items from a different supplier now so the only part they are responsible for are the aluminium adapters. Simple things turned on a lathe, it seems they have been able to get those right at least.

                        I'm probably going to get a new fuel pump to try first, i can't remember how long ago this one went in and although it does seem to pump out enough, it can be noisy sometimes, usually when there is not much in the tank on a hot day. Currently it is quiet but pumps are items that wear so I'm happy to gain a new one in the course of trying to get to the bottom of things.

                        cheers for the ideas so far!


                        • #13
                          Lago, I'm not really anywhere near being an expert on the matter but i can appreciate the science and enjoy discussing it.

                          You are right; the map that comes from the factory wouldn't know the injectors have been changed but that is the good thing about getting a new map written - it can read what effect the injectors are having via the signal from the wide band lambda and adjustments can be made.

                          Top end is where the issue is, the problem is the software doesn't seem to be able to adjust that. It is communicating properly as adjustments can be made elsewhere but, no matter what figure is typed into the block on the screen, it has no effect on the reading from the wideband lambda.

                          I don't believe it is the software, it is something else interfering with the injectors delivering enough fuel. Fuel pump is a possible, MAF signal not indicating enough air is passing over it to allow the injectors to trigger enough is another possible but that is where my understanding gets sketchy. If it is a MAF thing, is it faulty or is there simply not enough air flowing? (It has been cleaned with carb cleaner as part of the fault finding on the day) I couldn't find any test figures anywhere for MAF outputs - it is just voltages at certain pins rather than what can be expected in service.

                          PLAN: I'm going to look inside the exhaust / run without it, fit a new fuel pump (found receipt and it is 9 years / 55k miles old), fit new paper air filter (no oil to dirty MAF wire) and reinstate the Evap circuit and see how things are with that.
                          Last edited by steve briance; 11th November 2018, 21:01.


                          • #14
                            Have you checked the wiring to the fuel pump? If it's stock make a New wiring with separate relay and thick feed and ground to it.


                            • #15
                              Originally posted by Airwolf View Post
                              Have you checked the wiring to the fuel pump? If it's stock make a New wiring with separate relay and thick feed and ground to it.
                              Good thinking !