Crossfire Engine Development

[Simon]

Hello Guy,

I am working on developing the engine in my Crossfire and would value some input. Firstly, the engine is a Mercedes Benz 112 V6 with the following factory spec.:

215 BHP (160 KW) @ 5700 rpm (67.2 bhp/litre)
Torque 230 lb. ft. (312 N·m) @ 3000 rpm

3.2 L (195.2 cu. in.) 90° V6
Bore 89.9 mm (3.54 in.)
Stroke 84 mm (3.31 in.)
Compression ratio 10.0:1
Cast aluminium alloy block with Siltec bore liners
Forged steel crankshaft
Forged steel connecting rods
Flat topped pistons
Dual tuned Intake Manifold
Twin ignition
SOHC per bank roller rocker arms
3 valves per cylinder; two 1.42-in. (36-mm) intake, single 1.61-in. (41-mm) exhaust

Development has been hampered by the car being my daily driver, but as of today I now have 3 cylinder heads; one bare, unused factory-fresh, and a 'reconditioned' pair including all the valve gear and cams. So I now have the opportunity to see what potential there is in the heads. Ultimately I want to run a set of 6 downdraught throttle bodies in place of the variable inlet system/single throttle body setup. I will contact you separately to discuss the possibility of some flowbench work (and the options on my 2L TC come to think of it!) I will try and take some measurements of port dimension and cam timing/lift etc. and post them up here at the weekend. In the meantime here are some pictures of the various features of the heads. Any comments welcomed.

Thanks,
Simon.

Factory-fresh head

head.jpg (131.21 KiB) Viewed 4429 times

Factory multi-angle valve seats, twin plugs

valve seats.jpg (210.84 KiB) Viewed 4429 times

Factory inlet port, note injector relief

inlet port.jpg (105.76 KiB) Viewed 4429 times

1:1 ratio roller rockers

roller rocker.jpg (91.22 KiB) Viewed 4429 times

Jig for mocking up throttle body manifolds

jig.jpg (97.22 KiB) Viewed 4429 times

Dual length inlet manifold, single throttle body at rear with 90 deg. MAF housing (restrictive?)

inlet manifold.jpg (431.12 KiB) Viewed 4429 times

[Guy Croft]

Interesting in that it has two inlet valves and one exhaust but I don't like the valve area ratio which is E/I only 65% (same as the Cosworth Sierra 16v Turbo head..) and that is way too low. Not good and I don't know why they did that. Maybe because they could just get away with it given that they have design envelope (whereas we want to exceed it!) but I cannot see that that engine would be developing anything like its real potential in terms of power and economy judging by that feature alone.

The flow ratio port-port would be interesting, ideally - in std form - it would be the same as the valve ratio - but it rarely is. Note that - if it is the same and you improve the inlet ports to give more you could end up with even less than that modest 64%. The flow ration (measured as BPF - bare port flow) needs to be much more, pref 72% or more. Certainly on turbocharged units more seems to be better. If the inlet port underflows its real potential the actual BPF E/I might come out OK.

The short splitter (now de rigeuron many multivalve heads) is to get rid of (or try to get rid of..) the horseshoe vortices in the barrels that long splitters generate and can be pretty well left alone. The valve angles (knowing Merc) are probably right, but there are plenty of areas where modest smoothing will yield gains, inlet and ex and in the chamber too.

FWIW Most heads look like this these days, barely touched by human hand, razor sharp and not a hint of deburring, all cnc, banged together at the factory by men with chain-mail gloves.

This is one that definitely needs some work but if you don't explore with a flowbench you could, oddly enough, make it worse, not better.

GC

[Simon]

Thanks for your insight Guy, and it was good to speak to you earlier. I think a session on the flowbench will indeed be time well spent rather than 'flying blind' as it were. I will aim to arrange something with you in the next few weeks. Very interested to see what the outcome is. I will strip down one of the heads tomorrow to get a look at the valves and post up some more pictures. In the interests of comfort I probably ought to spend a bit of time deburring too!

Regarding the valve ratios, MB's official justification for the triple valve design would appear to be increasing exhaust temperature for efficient catalyst operation.

From the MB press blurb c.2003:

Three-Valve Technology Reduces Emissions by 40 Percent

The new Mercedes-Benz V6 engine features innovative three-valve-per-cylinder technology which can reduce exhaust emissions dramatically over 40 percent particularly during the critical warmup stage, before the catalytic converter usually begins to operate. Catalysts must heat up to work effectively, and this takes nearly two minutes with most modern engines. Increasingly strict emissions limits mean this converter light-off time must be reduced to about one minute, and the new Mercedes-Benz V6 meets this tough criteria. There's about 30 percent less surface area at the exhaust port, relative to a comparable four-valve engine, which dramatically reduces exhaust heat losses between the engine and the catalytic converter. This translates to higher exhaust temperature (about 70 degrees Centigrade or 125 degrees Fahrenheit) and converter "light-off" about 12 seconds earlier.

In the new V6, combustion heat in each of the large, single exhaust valves is dissipated through a sodium-filled valve stem, while exhaust heat is retained and insulated by double-wall piping in the exhaust manifold. The seamless double-wall manifold is made by using high-pressure liquid-forming technology, for greater durability and lighter weight.

There is no tradeoff in horsepower and torque with the new three-valve technology compared to a four-valve design. In any efficient, well-designed engine, exhaust valve size needs to be somewhat smaller than the intake valve area, simply because the "leftovers" of combustion (especially in a modern, clean-burning engine) take up less space than the incoming fuel-air mixture.

[Guy Croft]

exhaust valve size needs to be somewhat smaller than the intake valve area, simply because the "leftovers" of combustion (especially in a modern, clean-burning engine) take up less space than the incoming fuel-air mixture.

haha - love it!!

GC

[Simon]

I thought you would have a chuckle at that last paragraph!

[Simon]

I have now stripped one of the heads down and removed a set of valves.

Crossfire-valve_640.jpg (78.17 KiB) Viewed 4263 times