No announcement yet.

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

  • Filter
  • Time
  • Show
Clear All
new posts

  • Well, mot passed with just 2 front tyres needed I was impressed with how clean this engine is running bear in mind there is no cat'. 0.53% CO and 58ppm HC. That's really good news!

    Went home with time to use up and dug out the TPS loom to check the wiring against the pic supplied. Yep, my wires on pins 1 and 3 were the wrong way round! quick bit of soldering sorted that out and all is good. I'm not going to make any bold claims of rediscovered horsepower but it does feel good. The proof in the pudding will be what the mapping software thinks is going on...

    Thanks again to SaBoy for the pic - spot on confirmation and just what was needed.


    • Good news! Put it on a dyno be interesting to know what its making?


      • So, revisit to check on what if any difference all of the above has made to the fuelling on my 7A.

        Previously a thorough check was made on the operation of electrical components, fuel pressure, flow,spark plugs etc etc. what didn't get done because it didn't seem to be an issue was an intake system leak test. I'd never seen one done before and was fascinated to see the kit and how it worked. Essentially a smoke machine and an air line just to push the smoke in a bit. Happily there was only the smallest of leaks from the breather pipe that pushes into the underside of the Michelin man hose, that was sorted just because it ought to be right and there were no other leaks. The small leak there was is not going to have any effect on fuelling once up and running but there was a slim chance it could affect idle.

        It was also verified that the ECU was responding to the ECU temp sender by installing a variable resistor test box to replicate change in coolant temp, wideband lambda confirmed the ECU did influence fuel correctly with a varying signal down that wire.

        So, out on the road what was going on with the fuel map by the end of the day?

        Consistently 95% fuel map used. Up by 5 % on the best reads last time but still not quite right.

        It was interesting to note that on planting the throttle, fuelling enriched to 12 - 12.5:1 AFR due to pressure regulator operating but then it leaned off until 5500rpm when it changed again to 12-12.5:1. This is what was happening last time apart from being that little bit further into the fuel map now.

        Sadly there are no obvious causes as to what is going on here but currently there are suspicions that as the only non standard parts in the engine; the 034motorsport injector conversion might not be 100% suitable for the ECU. Now, don't get me wrong, when I fitted the 034 kit - it DID improve the car and until i could see what was going on with the fuelling, I would have had no idea from how the car drives that it wasn't quite right. The issue might be the resistance values of the injectors.

        Standard 7A injectors are between 1 - 3 ohms running through a resistor pack of 5-8ohms. A Max resistance of 11 ohms within range. My old 7A injectors are 2.5ohms (not measured resistor pack as it is still on the car) Bosch red devil injectors 0280 155 759 are 14 ohm resistance but a resistance check when hot showed this had risen across all 5 to be 19.5 ohms and that is a significant increase over 11ohms.

        The working theory is that the ECU is not opening the new bosch injectors in the same way as the original items and this is why the fuelling doesn't seem to respond to inputs from the tuning software. Whether it is opening speed or opening amount is unknown but with the biggest fuel figure that can be put into the map, the car was not over rich despite the injectors claiming to be able to flow more than standard. You would expect a bigger injector open more would squirt too much fuel in wouldn't you?

        Some testing will be done on a flow rig with original set up, the bosch red injectors and some pink injectors which are 12 ohms. I also am looking out for some low impedance bosch injectors that might be suitable. The suggestion of Bosch Green giants '803s' is an option as they are 4.5ohms but they are 440cc/min flow and may be too much when standard figures are around 270cc/min. The greens could be turned down in the map but whether they can actually be turned down that much is unknown.

        Having said all that; once the tuning was finished I can confidently say the car is MUCH nicer to drive now and in headlines; the car has no sharp dip or 7A style peak in power. It starts to pull well from 2000rpm, is a lot more linear and interestingly at 5000rpm it sounds significantly sweeter and smoother. Having driven on my favorite roads now rather than the motorway home, it does feel 'stronger' and that is fine by me but the missing 5% leaves me wondering how much nicer it could be... Figures once it is all sorted and i have got some to share!

        Attached Files


        • Comment

          • Good stuff.Im glad you are enjoying the car after all the jobs done to it.Hope you will be 5% happier at some point


            • Cheers ​​​​​​
              Yes, liking the car at the moment and 95% happy is not a bad place to be fingers crossed there is some good news to report some time soon.


              • Glad to hear that sir,

                As this thread has certainly exposed some gaps in any scant knowledge I might have and is such a brain-teaser, It is indeed most interesting to hear of that improvement for your labours and attention to detail. I also believe you're correct that there's more to come, and perhaps from a couple of locations only slightly different from where you have mentioned already.

                - Did the smoke test include the oil fill cap and dip-stick? I mention them not because they could be vacuum leaks (they can't) but because they may be sources of un-metered air and your car appears to indicate symptoms that it may lack a healthy enough MAF signal at the ECM. As the cap seal ages it looses its' effective height/resilience when compressed. As I can fit 2 seals onto a fill cap (the original plus a new one), and still twist the cap back into place (although not recommended as it was slightly too stiff to close without bending the cap's metal tabs in my case), it illustrates that a new softer/taller seal may be required to keep this hole closed. If they've never been replaced, both the cap and stick will close with more authority with brand new seals.

                - Previously, you may have had some grey Bosch injectors? Where they those originally part of O34's early injector kit? If yes, why did you choose the red ones instead?

                My thoughts:
                - It may be that your car simply has to get a slightly increased MAF signal to the ECM at the very top end?

                - I don't think the WOT enrichment seen is due to FPR operation, for a couple of reasons. Is it not correct that:
                1.) Fuel is injected as a function of pressure wanting to equalize across the injector. The rate of that injection is strictly a function of the total pressure differential between both the fuel rail and the intake manifold; &

                2.) I believe our effective fuel pressure is near constant throughout the entire operating range as follows:
                a) As our fuel supply is constantly bleeding excess capacity by returning fuel to the tank, this marks it as the type of system which seeks to maintain constant effective fuel pressure across the injector at all times;
                b) At WOT when our Intake vacuum would be at zero, the rail pressure will be 4 bar, and so the effective pressure will be 4 bar; &
                c) As our FPR function is to provide reduced fuel pressure partcularly for idle and de-acceleration, and then simply vacuum referenced pressure thereafter, it therefore provides at idle with max. vacuum, the 3.5 bar plus engine vacuum (say 17 Hg) which equals approx. 4 bar effective fuel pressure across the injector also.

                As described above, AFAIK there is no ramping up of the effective pressure across the injector taking place which could account for the observed enrichment behaviour.

                - Is the (for acceleration) observed enrichment not simply due to the TPS which upon WOT, is temporarily given authority over the MAF, telling the ECM to go to open loop? However as the ECM may not have received a sufficient &/or sensible increased airflow signal from the MAF in the time allowed, it cannot yet command that last change in choice of map value (or table column?) which would drive the injectors towards the increased delivery for peak power at 12.x : 1 AFR. The ECM then simply falls back to the safe territory of closed-loop operation. This pattern is likely endlessly repeatable. Save for the lack of signal, I think it is working fine.

                - I don't think the new injectors are malfunctioning, either by being "under-driven" or causing any other issues such as your apparent one at the top of the range as:
                1.) Few have had difficulties with this arrangement and the mod has been widely praised for many years; but more importantly
                2.) It appears the current injectors still as previously, will deliver more fuel (if but briefly) when they are properly asked to; &
                3.) On my car (with the smaller grey injectors) uses markedly less fuel post mod, runs smoother from bottom to top end and at least subjectively, feels more powerful everywhere.

                - Similar reasoning might also suggest neither lack of fuel supply previously, nor the new fuel pump etc. now, is the reason for the small change in elevation of that same enrichment and then fade behaviour, as the supply capability increase would appear to easily exceed the necessary increased consumption taking place at the slightly changed point at which this same behaviour is now observed.

                - I do think the elevation of the enrichment & fade behaviour is due to the now reduced exhaust restriction, the resulting increased throughput being sensed by the MAF, and this increased signal allows the observed ECM map value or table column shift.

                - I do think that as the MAF has authority for open-loop operation and that it is exactly that which I don't think it can maintain at the very top end, this (even if you don't look at the actual MAF output), would point me towards revisiting its' operation as follows:

                1.) Is it possible that the MAF connector is somehow sub-par as per all aspects of Newsh's comprehensive thread here: &

                2.) Both the 7A and the 12v lack the air-filter box exit velocity stack that I believe the S2 and RS2 and others have. More recently even VWs have this component. Photo below (for the 12v) is similar to what i did for the 7A also. In my case I believe it is quite helpful.

                Fortunately these are all easy to check/do and not expensive to service. Don't laugh too hard, I used a paper coffee cup as my Mk1 velocity stack.

                I don't think you need different injectors (either larger, different impedance or alternate type), more fuel or much else. I do think you need to ensure you can best measure the air the car has shown it will already flow and that that signal is simply properly delivered.


                Test fit 2019-02-04 at 16.16.36.png


                • Time to revive this thread.

                  Finally got back to the tuner with a selection of different parts to bolt on and try, primarily centred around a different set of injectors. Bosch 0280150803. These were picked for two reasons - low impedance and more fuel. Depending on where you look they are either 380 or 440cc a minute compared to the 315cc /min curently fitted.

                  Now I fully accept that I took a punt on these not fully knowing what would happen and whether it would make any positive difference but if you don't try, nothing will change so I gave it a go.

                  The factory 7a injectors have a 2.5 ohm impedance but running through a resistor pack of 6.5ohms.

                  The 034 motorsports bosch 'reds' were 16ohms.

                  The 803 bosch green injectors are 4.7ohm but running through a new resistor pack made up at 4.7ohm. I.e virtually same total resistance as original set up....

                  With standard fuel pressure regulator and the new injectors; air fuel ratio was around 9 - 10. It was obviously very very rich and lumpy.

                  I fitted an adjustable fuel pressure regulator and turned the pressure down to 3 bar from the 7a factory 4 bar. Much improvement and now making 14.5 afr in closed loop lambda control. The fuel map was then charged with taking care of the fine detail out on the road.

                  First full throttle run and the afr was 11.8-12.0 and remained there for the duration of full throttle. This is a very obvious improvement as
                  ​Previously it was making the same figure but rapidly weakening off as it just couldn't maintain it.

                  Further runs to tweak the map and it is improved in all areas. Partial throttle, on off light loads and cruising is smooth and progressive, afr and therefore economy on a steady run should be OK.

                  There needed to be a tweak to the MAF and Idle screw though as returning to Idle was slow with it remaining at 1100 for a few seconds then slowly settling to proper Idle speed of 850rpm. Going through the 7a 'base line setting' procedures showed that without lambda and ISV control the setting was 4.5% Co and a fairly reasonable explanation as to why it was slow to return to Idle. Adjustments made and back out in the road - return to Idle momentarily catches it at 1050rpm then straight away controls down with ISV to 850rpm. Very happy with that, no droopy 7a Idle here now!

                  One day I will visit a rolling road to get a graph and some figures but overall I am very happy indeed.

                  I can't really give a definitive answer as to why it worked this time, the suspicion is that the ecu was unable to open and hold open the previous injectors when needed for prolonged periods of full throttle. The tuner said he adjusted the injector open times right down but by virtue of their higher flow was able to achieve decent results. Nothing else in the measuring of airflow was changed.

                  Stand by for figures!


                  • Wire wound resistors 50w rating. Same location as outgoing resistor pack.


                    • Well done sir! Congratulations for assembling and demonstrating a third injector option for 7A owners. Most interesting and somewhat confounding too! Are you saying that perhaps the increased resistance of the reds is linked to their lack of response beyond 95% map use and inability to continue delivering 12.X AFR @ WOT? One might therefore suspect the greens are not heating up and becoming even more resistant like the reds, or do you yet know that?

                      It's hard to argue with your success but IIRC, aren't the stock 2.8 12v (same displacement per cylinder) injectors only spec'd for 170 - 220 and actually only good for about 155 - 170cc/min?

                      With the above in mind, could the red's vs green's behaviour be explained by as follows:

                      - As the original factory kit is low impedance injectors plus resistor pack because the Hitachi ECM (licensed from Bosch, likely a Motronic clone) was actually built to drive more typical period correct high impedance injectors with a saturated signal and no resistors; and

                      - That the above allows the 034 reds, which are a high impedance injector to operate w/o resistors here, but with a higher than OEM peak flow rating (the reds are the Mk II injector for this mod, even larger capacity than the larger than stock Mk I grey's) and at our low delivery rates (relative to their capacity) and at the high frequency of peak revs, (could it be there is not enough fuel cooling of the injector due to short-cycling?), they overheat somewhat, become more resistant and they (and / or the ECM's drivers?) are hot enough to become temporarily unresponsive, and as you have said only noticeable with instrumentation, and at the very top end;

                      - Whereas the greens, being low impedance and consume less current with the inline resistors, don't overheat (and / or bother the ECM less) when asked to also under-deliver (again relative to their peak rating) similarly, at the high duty-cycle of max revs plus WOT?

                      Very interesting what you are doing here, looking forward to hearing what you learn next.