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

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  • thanks for the heads up!


    • As promised, some pics of the new back box. I should have taken them on the day it was fitted as now its got 700miles of winter roads having passed by, its a bit grubby. PD Gough do offer a polishing service and also a paint finish but this one is standard sheet finish.

      Tig welding pretty tidy, fit good, noise subtle and a lifetime warranty.
      Attached Files


      • I guessing that the one tailpipe being bigger than the other is an artefact of the phone camera


        • I had to go and check but, yes it is - in the real world they are bang on the same! (phew)


          • - Very interesting thread as it does challenge one to review how wide-spread the necessary conditions for proper power production are.

            - For example, the exhaust mods as an "input" towards the above , likely now less restrictive than both OEM or previous, so may provide a stronger return pulse back upstream, and may reduce pumping loss, so possibly a mileage improvement also? Workmanship looks 1st class, you have to like mods which over OEM (besides being near invisible), add reliability (via material & mounting here), likely remove weight and possibly boost performance without an associated (dare I say boost-like) penalty. Ought to compliment your slightly increased C/R and port-work nicely.

            - And are we correct to think you might not have ever looked at your exhaust had it not been for the suggestion of your tuner. To perhaps answer Error's question, was the exhaust re-visit suggestion due to the engine being somewhat reluctant to approach red-line?

            - I say all this as although these changes may hurry your ability to get up to speed, they will not add fuel (in relation to air); at least directly. So I'm looking forward to hearing of your results.

            -Does your exhaust also include a forward (a la OEM) resonator box?

            - My 7A is the early style, so no help there with wire colour. It also has the stock exhaust (very heavy rear section BTW), 034 injectors and I also had similar experience with MAF swapping.

            - Please explain how and exactly what the engine was doing when return fuel was measured. Can you possibly see / record fuel pressure &/or return flow while underway?

            - When I changed from stock to the 034 injectors (earlier grey versions of slightly less peak capacity than yours) I don't think that they gave me any enrichment whatsoever as my measured mileage improved thereafter, as did perceived response and power. Further, is it not correct that below peak power (i.e. in closed-loop) in a naturally aspirated system, that the O2 sensor has command authority and will drive the mix towards stoich.; despite -any- rating of the injectors' peak capacity?

            - Steve, as your WOT application at peak load during the re-map did cause enrichment if but briefly, did that not also prove your TPS is both operating and being read correctly, but that your MAF, which AFAIK should have picked up on that; may not be?

            - Further, (and anyone please correct me if I've got this even only partly right) WRT acceleration in open-loop, although it is a kind of chicken and egg situation is it not, in that:

            a) You need metered airflow to get more fuel; but you simultaneously need increased fuel to get more airflow; and

            b) From post #75 above:
            Originally posted by newsh View Post

            I have no idea if the 7A is similar to the S2 but I was told that in the S2 the MAF is not used at WOT. Closed loop only happens at part throttle.
            If your 7A is similar and if there was an issue with your TPS, then the ECU would keep trying to pull the mixture back to stoich even at full throttle.
            Just a thought.
            Newsh or anyone, as opposed to with super-charging where intake airflow might not always be a best indicator of load (and that perhaps intake vacuum or pressure might be in that case?), the MAF here must be given command authority in open-loop to sense load, after the initiation of acceleration by the TPS to break the chicken-egg cycle thing. Otherwise how else would one accelerate?
            Last edited by Lago Blue; 11th January 2019, 01:02.


            • My head hurts now


              • Yes, me too but it's an ice-cream brain-freeze kind of good hurt don't you find?


                • Yes


                  • Lago, I read all that several times and would prefer to have eaten ice cream, I'm a little lost! ​​​​​​ but you are right - it is good when something you bolt onto the car is a genuine improvement over factory. I remember taking the factory exhaust off my car. It had lasted 16 years and yes, it was REALLY heavy!

                    You are also right in saying I would never have revisited the exhaust unless prompted. I had no reason to as the car performed 'alright' with it on. The prompt got me reading about exhausts and there is so much information out there but reverse pulses and the tuned tubular exhaust manifold work hand in hand to draw spent combustion gas out of the head and now, I think they may actually be working together. My front flexi pipe bodge was possibly messing up the reverse pulse but now it is the same diameter all the way through there are no steps to bounce off incorrectly. The middle straight through box is still installed by the way.
                    Thanks for thinking about your tps wiring, it would have been great to verify that with an unaltered late 7a loom but alas not this time.

                    The easiest question to answer from those in your post above is about the fuel flow test. It was done with the engine off. My relay mod made it very easy to run the pump without engine. It would be interesting to see what was going on while driving but I don't really think I was on the edge of fuel delivery anyway, the swap was more just making sure. Those who do report improvements in performance with the relay mod are those running boost and big power outputs which do get towards what a pump can sustain. There was a thread on here not so long ago with a brilliant review of uprated fuel pumps. Well worth a read if you like brain freezing but it did show that even standard pumps are capable of way more than the 7a 170bhp.

                    3 weeks to go!


                    • Steve,

                      - Sorry, I was in a hurry to capture my thoughts yesterday, so I went back and re did them above to try to make my questions more clear.

                      - What I was trying to get to was that for Open-Loop, does one not have to give the ECM a plausible load signal so it can pick a load table column? Perhaps your MAF is doing that (supplying an acceptable load signal) up to a point, beyond which the signal sent fails muster at the ECM?

                      Then, lacking an appropriate load signal, wouldn't the ECM drop out of Open-Loop, back to Closed-Loop and you return to stoich. as Newsh explained in the last part of his quote in my post above?

                      I'm therefore thinking you may likely want to know how much signal your MAF is sending and when that occurs; before your next appointment. Could you use your spare harness to help wire in a DMM to then record voltage while underway? But I still suspect there may be a possible un-metered air leak somewhere which should be checked for 1st, it could be something like a too old, hard and ineffective oil fill cap gasket or similar.

                      - Speaking of appointments, to help me understand what goes on during these re-maps, WRT your part load table photo I see, RPM? on the y-axis and Load on the x, what are the Load units at the column heads? Are the figures in the table body itself perhaps injector timings in what units, percent? Roughly what percentage where the largest + & - re-map changes and where were they made?

                      - Now that you have some miles on the re-map and exhaust mods, what changes if any do you note, what are your impressions? Do you keep a fuel consumption log? Have you considered running the car up to that enrichment fade point before your next appointment sans instrumentation to see if it now approaches red-line with alacrity?

                      - I did see that fuel pump post and thought of likely more utility to me, would be to simply mount a 2nd stock pump (replicating the 20v sedan pump in its' position under the rear seat) plumbed in parallel as a select-able (via a dash-mounted switch) fully-functional alternate like any even basic powered aircraft would have.
                      Last edited by Lago Blue; 8th January 2019, 13:52.


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                          • Finally got a later 7a loom. Have connected tps so you can confirm which colour wires go where.


                            • Brilliant! Thank you very much for posting that up, it is very much appreciated it does get round the problem of the wiring diagram saying there are two black wires as clearly there aren't!

                              I shall dig into the loom at the weekend if I get the chance but it is mot day on Saturday and although I don't think there are any issues, you never know what this year's new rules mean...


                              • No problem. Hope all goes well with the mot.