Based quite a lot of testing on std, tubualar and kit car exhausts manifolds and dowmpipes systems, on a known engine, I would suggest leaving the optimised OEM casting with the custom down pipe. Based on what has been demonstrated on factory 8v engines on here and some modified ones, the cam in your engine would enable improved volumetric effciency after 4000rpm with a slower ramp off to the cut out. If the engine calibration has been optimised to suit this change in hardware , I would expect torque to increase over std to match the VE profile, thus leading to more power as the revs increase. Being limited to a STD rev limit makes switching to a different manifold, from "off the shelf" a risky business in terms of cost for benefit. What I mean is you might find the best exhaust frequency from your base engine, that a specific 4 branch system works best at scavenging the residulal gas in the cylinder, maybe mismatched based on the tube diameters, runner lengths or collector design . Directionally if the engine has a manfiold custom built for its spec, then you would tend to have a small benefit in midrange torque from a 4-2-1 system and if running a 4-1 again custom built, you can encourage improve filling at higher rpm, if most of your racing uses that range of the rpm range.
Wow! Loving that reply Here's my dyno plot from the current engine. Spec: 1mm overbore, Newman control cam (280 I think) with vernier pulley, exhaust as previously stated, modified std airbox with Pipercross filter. All optimised over a full morning on the rollers with an old school race mechanic who knows his way around a golf. How does it look to you & would a Piper 285 cam + custom race 4into1 manifold improve things as a package?
I thought the Newman control cam was a 288, but the memory is vague. As you're talking Piper 285 cam, does that mean the series is changing supplier next year, or offering a range of options? You might want to check the 'real' duration figures between your cam choices. A piper 285 Hydraulic cam is only 276 duration, I believe.
Not sure what the wording of the regs will be but the plan is to allow a better cam & 4 branch in order to decrease the ever growing gap between 8v & 16v classes. The idea being that at the right circuit in the right conditions (ie rain) there may be a possibilty of an 8v outright win which is currently all but impossible.
2.0 bottom end would help to even it up... or even allow ported heads on standard valve sizes - the cost jumps a lot when you go to bigger valves.
My thoughts summarised: Blue, this is the plot you put up of your current spec. Red your currect spec with an optimised or custom tuned exhaust manifold. Green your base engine with a piper cam and a custom tuned exhaust manifold. Bare in mind this is just a prediction based on your baseline, that assumes: The fuel mixture is around LBT between tests and dizzy is offset to give best ECU mapped timing where the engine's VE is at the highest. Dyno settings and conditions are consistant between tests. Exhaust manifold was designed for the rpm range (4-6Krpm) and cam choice. Cam choice is positively interacting with the rest of the hardware and ECU settings Hope this helps. Regards
Manifolds can be funny subject to the spec. Std engines in general can show an increase across the board with long 4-1's, stick a radical cam in and sometimes all the low and midrange can vanish. Fit a long primary 4-2-1 and all the low and mid can come back with the same top end as a 4-1. Forget all the old urban myths about 4-1 giving better top end and 4-2-1 better low and mid range...it can vary wildly from engine to engine...and capacity. .and that's without looking at collector style and both primary and secondary diameters.
Easy enough....there are a few good custom manifold makers out there, and you'll need to know the exact cam being used and the cam timing...I'll fire you over a message with the rest of the details later Alex.
You take the data that you posted and target the horsepower (torque) and rpm that must be the highest. You then determine: Cam timing. Post size and length. Valve sizes. You then use this information to input into a particualar equation or Matlab to determine the various lengths and diameters that would produce the highest depression at the next firing port based on if the manifold was a 4-1 and 4-2-1. For a given frequency you will get a few tube lengths that will achieve the same effects. You can repeat the calculation for different power peaks to get a range of lengths that would help tailour the best design for the specific application. Once this is completed you build the system and then fit to car then start to retest on the same rolls as in your dyno plot. You will need to know the fuel mixture at WOT as the vehicle is now and after when the "new exhaust" manifold is fitted. This is need tweaking if the manifold starts to scavenge i.e. the mixture will start to lean out. If you have the resources to weld and bend tubes then great. If not it could be expensive.
I wouldn't even attempt it myself, I think I'll be talking to BTB, they did a cracking job on my downpipe
This would be a better option vs buying a system from the auction. At least they know what you are trying to achieve.
According to this ad on here, the Newman cam is a 272. Appears that old age has set in on the memory http://www.clubgti.com/forum/showthread.php?t=244418
