Previous results: The next few posts will feature dyno data I have from previous testing. All dyno runs were done on the DynoDynamics brand of rolling roads. From when the test vehicle was fitted with a Mk2 8V GTI exhaust system, the shortened runner manifold was tested back to back in the same evening: Both of these were mapped on the road and then had development time on the dyno, so can be considered fully optimised. With the shortened manifold, you can see an improvement in low to mid rpm torque, which backed up how it felt to drive. The standard manifold with less torque, has a more linear feel to drive as a result. While the manifold was being switched between tests, rubjonny's Mk2 ABF digifant w/ Jetex was tested. Although this vehicle did not achieve the same torque output, it produced higher peak bhp. It was the belief of all present that the larger bore aftermarket exhaust allowed the engine to breathe better at high rpm (6000+). A 2.5" Jetex exhaust was purchased, and all tests after this feature system. New baseline Due to the hardware change the vehicle required retesting. This was run on the CGTI 8v vs. 16V dyno day. We can now compare the best result from before (shortened runner intake w/ Mk2 8V GTI exhaust) against the same intake with the upgraded exhaust system. The effect of the exhaust upgrade can be seen below: It was expected that the 2.5" exhaust would aid the higher rpm outputs, but an improvement was seen throught the entire rev range. This provides the results for the Shortened system as per Post1. The results were from the same dyno as in the previous tests.
Good results. So how much of a %age improvement in torque over the rev range at each stage? Seems to good to be true though looking at the graphs.
Larger Plenum Increased volume plenum was fitted (exact cc increase to be measured later). Initial road mapping suggested that it breathes better at the top end, which was indicated by leaner AFRs seen. Once optimised, the drive feel throught the rev range at wide-open-throttle had changed. It felt like reduced torque at low rpm but stronger at 6000rpm to 7000rpm. Due to timing, this setup had to be tested on another DynoDynamics machine. While comparing this result with previous may not be as accurate as using the exact same machine as before, the brand is the same. Assuming results are comparable, the change in drive feel matches up correctly. Reduced low rpm torque, increased high rpm torque, resulting in higher bhp. This trade-off is acceptable for track use as the increase occurs where it is needed & mid-range has not dropped significantly. G-meter testing will confirm if this is actually how the larger plenum setup performs. Standard ABF Intake Not forgetting the original item, which has been tested on the G-meter already this week. Without a dyno test we can only predict how this might perform. Taking the % increase for each measurement point from the exhaust upgrade and adding to the original manifold results from earlier we get: A good match to how it feels in car. Flatter, with less peak torque, it still pulls well as is an ABF in a Mk2. Interestingly the predicted BHP was 166, which was what rubjonnys car achieved (ABF, Digifant, chip, Jetex) Next steps are to complete validation by G-meter
That could be the graph scaling, which is like that in order to clearly show that there is a difference. % increase vs. RPM (first test 8V exh: OEM ABF Intake vs. Shortened ABF Intake) 1.065 5.1 4.8 5.1 4.4 5.5 6.1 4.4 4.4 3.4 1.0 0.7 2600 2900 3100 3300 3600 4100 4450 4900 5200 5400 5750 6100
Just worked out that on the increases higher than 1% (2900- 5400) Average increase is 4.8%. Using all figures is 3.8% (whole range ) Just for cutting out some metal.
Every little helps with a NA engine! lol +1% here, +1% there Does require tweaking, so standalone is required & maybe even K-Jet cars could benefit. Will test on a Digifant Mk3 16V, as although it's a struggle to alter anything ECU wise, it does heavily overfuel @ WOT post 4500rpm for component protection. Could add breathing to areas that already have more fuel, or it could run like a bag of spanners. Before anyone gets the hacksaw out: Disclaimer: This thread is for the interest of enthusiasts & should be taken simply as experiences shared in a report format. I am not to blame if you go chopping up your intake & do not get the results you desire. Examples from myself, Smudge, & Steve R serve to highlight the effects intake manifold geometry can have on engine output.
Standard ABF plenum volume measured to be approx 2 Litres Need to fit standard plenum back on the engine and measure the larger plenum volume
Results are in. For all 3 tests the following conditions were met: Same driver & passenger Same fuel level in fuel tank Same stretch of road used ECU optimised to hardware being tested Tested in 2nd gear of 02A CDA gearbox 195 45 R15 tyres fitted G-tech Pro used for G measurements (borrowed from Toyotec) It is clear to see that the most modified intake has the highest output, and the OEM manifold has the lowest output. The short runner, big plenum setup has an offset of nearly 0.02Gs, this can be felt as the OEM manifold drive feel is flat in comparison. The intake manifold that is simply shortened slots in between the other results, strong in the low rpm, higher peak than OEM, but the mid range doe not match the punch of the larger plenum setup. All 3 results converge as they near 7300 rpm due to the soft-cut rev limiter.
Encouraging result & commendably explicit denoeument! Thanks to Ben and associates for publishing these results - - development work of this type demands loads of personal time, engineering resources, and no little financial contribution - - you appear to have created a viable performance oriented single plenum alternative
Thanks Tigger, comments like that confirm that this was worth sharing to fellow enthusiasts. It would be good to see others put the information here to good use. An average improvement of 5.6% over the standard part can be achieved for the cost of some fabrication, with the packaging & testing already completed. I believe this would work well on non-standard engines also with increased compression, head work, performance camshafts etc. Perhaps a racecar that retains a single throttle body setup. For completeness I have to measure the volume (increase) of the larger plenum
Appreciate your efforts Ben, it's curious to note the shape of the curves haven't changed substantially, just moved up the range. I guessed it would move the peak higher or lower.
Excellent work and I'm sure I speak for everyone - thanks for sharing Time to look out those two spare ABF manifolds I have. Damn wish I hadn't sold those last two 051 heads as now I have no jig
When starting this a while back, I too expected the torque curve to move more in terms of rpm where it rises, falls, & peaks. We know that with very short manifolds e.g. the 1.8T item onto a 16V the torque curve changes significantly. However, changes here in terms of runner length & plenum voulme are relatively minor. From past experience I have found that shortening the runners may move the torque curve towards higher RPM, reducing peak torque, but widening the effective curve. Increasing the plenum volume has been observed to amplify the torque curve, but compress the curve width in terms of rpm, while slightly shifting the peak to a lower rpm. In this thread, both shortening and volume increase effects have been combined, so perhaps this is why the result appears as an overall increase / positive offset with similar rpm points.
Can I ask a question? Was the extended plenum material taken from a spare ABF inlet as I have a couple here that I could have cut up and modifed at a local eng place?
The manifold was taken to a professional welding/engineering shop and modified to Ben's requirements. Material was added at the shop.
Yes I used a small, highly skilled welding company who I have used before. I simply marked out where to cut, which was along the edge of the raised section where the VW DOHC writing is. Here is a better view of the plenum:
