I was having a discussion with a local garagista who on seeing my TB before the S/charger said it could be a problem if one of the transfer pipes or intercooler punctured as the engine would immediately go full throttle. Can anyone explain the pros and cons of placing the TB before or after the s/charger? I note Moss on their kits place it before - there must be a reason for that? TIA. Cheers, Pewe
if one of the transfer pipes or intercooler punctured as the engine would immediately go full throttle
Theoretically yes, depending on how you measure the engine airflow. The danger would be if using speed density based engine control and a pressure referenced at the manifold. A large enough air leak post throttle would in effect cause a condition like running the engine un-throttled as the manifold pressure would be equal to atmospheric pressure. The ECU would continue to adjust the fueling according to the readings from its sensor inputs. with a MAF sensor setup, as the airleak in this cause would be post MAF sensor (assuming the MAF is positioned pre S/C), the ECU would not account for the increase in air flow caused by the air leak and the fueling would not be adjusted.
Can anyone explain the pros and cons of placing the TB before or after the s/charger?
With a throttle post S/C there can be an issue if not running a bypass valve. Because the S/C is not decoupled from the crank it will always boost the pressure in the inlet air path when the engine is running. When closing the throttle at high engine speed (i.e. full throttle gear shift or throttle lift at high speed) the manifold pressure will drop to below atmospheric while the pressure in the air inlet path continues to increase. This can cause a large pressure ratio to form across the throttle which will have some effect on tip in performance and general drivability. There is also the potential to cause mechanical stress to the throttle (although I would assume that possibility is very limited).
Moving the stock throttle to the blower inlet was how the Jackson Racing supercharger kit did it, and that was pretty much the only game in town for a decade.
I never heard of a supercharged '5 with anything other than a single pre-blower throttle until Performance5 developed the twin throttle idea, and never heard of one with just a post-blower throttle until Salon developed a kit like that.
I've still never heard of a runaway engine because a boost pipe broke. I have heard of a few cases where the throttle itself broke, though usually after it had been modified in some way and not necessarily connected with supercharged cars.
Actually, on reflection, I do remember someone who had no boost because a pipe clamp came loose and it just leaked air, but the off-boost effect was that the suction pulled the pipe and hose back together so they didn't even notice it.
The development of a post-blower throttle setup was a reaction to Track Dog Racing's highly effective over-the-radiator intercooler kit. Its longer pipework (than non-intercooled or chargecooled setups) increased the throttled volume and made the lag in throttle response more apparent, especially with the smaller M45 blower, less so with the MP62. Having two throttles, balanced so the pressure between them was partway between atmospheric and manifold pressure, mitigated that.
I always liked the simplicity, low noise and cool running of a single pre-blower throttle.
Last Edit: May 19, 2019 20:13:35 GMT by martiny: speling
Thanks for the replies even if some are outside my limited understanding of supercharging. Given tales of problems with tickover, driveability and s/chargers cooking themselves due to trying to pump dead air held back by two closed throttle bodies I'm sticking with a single one upstream of the s/chargger. We'll see if the bigger injectors and i/cooler live up to expectations when it's dynoed...
Sensible chap, what ECU will you use? after 18 months of fettling I have finally got my idle as I want it!
That's the problem (or one of them) its a Link G4. Few tuners around but fortunately one not far from me in RG1. So far they're doing a good job though we're now looking at moving the TB back to the original position i.e end of the plenium as it won't run smoothly on the dyno (probably too much volume to shift through the inter cooler). My gut feel is that it will end up having to run twin throttle bodies. In the meantime the meter's running.. Failing that I'll cut my losses and go turbo!
Finally have it running. Have to say Horsham Developments have done a bang-up job both in the fitting and tuning departments. TB now back in it's original position on the plenium. Runs like an absolute train as soon as the throttle's cracked open and the only problem is you can really hear the s/charger whining and the by-pass valve is mechanically noisy when it shuts as you come off the throttle. I might see if a piece of rubber tubing on the by-pass stop will cushion it. Can't wait to try it out at Combe later this month - providing it passes the noise test... Better watch out Horney Charles will be hunting you down
Ah... the noise, you don’t get that with the TB pre charger.
Correct. It was quiet to the extent you could hardly hear the s/charger whining. Now sounds a bit like Kraigy's so I'll be checking his out next time our paths cross... Need to put the noise metre on it before trackday at Combe later this month
Not my decision but the guys who were setting it up. Apparently it refused to run properly once the intercooler and pipework was connected - possibly not enough pressure into the plenium as I'd gone with a rel. large i/cooler (Audi 80 from memory). Also they expressed concern that if the pipework or intercooler was punctured, split or pipework slipped off the engine could go rogue with a free air supply. Found that one quite difficult to comprehend but took their advice. Seems to run like a train now and will be stretching its legs at Mission Motorsprt's Launch Control event at Abingdon this Saturday morning. Cheers, Pewe