Mikkijayne said:
Ah so the IDIs can rev faster than the TDI? Why is that?
It is said that the IDIs (such as VW 1.9L TD or 1.6L TD) combust faster and more completely than TDIs at high RPM, even with less air consumed and more fuel consumed... at the same smoke level!
There's a bit more technical info
here.
The ability to burn fuel completely at high RPM is very important for efficient HP without needing transmission-shattering torque or useless FWD wheelspin (admittedly they look cool, but it doesn't make a car go faster). Ultimately consistent HP wins races - can't argue with the reality of 200ft-lb Hondas running 11's on the 1/4 mile track. However torque's more fun to drive, and is usually more practical in daily driving i.e. highway passes etc.
Here's a graph of my VW 1.6L IDI motor with a 1.9L head.. HP peaked at 5,250 RPM before dropping off due to what felt like a fuel cut-off.
It had a stock cam, stock fuel injectors with no smoke, Giles 9mm pump, a custom GT20 turbo (instant 15 PSI boost at idle - probably comparible to a VNT-17/22), and insufficient intercooling. Giles was going to sponsor a 11mm or 12mm 6,000 RPM pump for this project but the car went off the road.
The engine was in a 1997 Golf and it matched or exceeded the performance of a VR6 of the same vehicle weight, and it was only a 1.6L producing 0% WOT smoke.
There is another similar IDI that went on the road a few weeks ago with more headwork and a large turbo. It was still pulling at 6,000 RPM in 5th gear. 2nd gear was shattered on the 1st test drive. It's running my old 9mm pump although a 6,000 RPM 14mm pump is in the works. It will be very interesting...
FWIW I think my GT20 was one of my restrictions at 5250 RPM - like a VNT-17 or any VNT-17 hybrid it's far too small and EGTs were very high. For good HP a bigger turbo is necessary.
The 1.6L's short 86.5mm (?) stroke and lighter reciprocating parts may have contributed to better revs, along with IDI's theoretical better combustion at high RPM.
The video of a
mercedes peeling around, posted here a while back, is an IDI. IIRC it since got a supercharger (now twin-charged) and is smoke free at low RPM. The DIs however still produce impressive power. Is a fully worked DI ultimately better than a comparible fully worked IDI or vice versa? Who knows.. one thing though is that IDIs are already a dying breed (it's been 11 years since TDIs arrived in NA) and DIs are more readily available. The DI market has more $ for businesses and hobbyists to continue R&D in power enhancements. Both the DI and underrated IDI get the same level of support from me!