3.5 conversion to 4.2 possible? - Mercedes-Benz Forum

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post #1 of 6 (permalink) Old 01-23-2011, 05:18 AM Thread Starter
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3.5 conversion to 4.2 possible?

As I want to stay with my original (cast-iron) engine and his 5-speed box in my engine bay i am looking for some alternatives. As i saw the long stroke 380 versione of the m116 has a different crank and shorter connecting rods. The pisiton fixing point has 26 mm as the 3.5 (420 has 23mm). So I thought to use this with the original pisions of the 3.5 and this heads (high compression) but the hight of the pisions from fixing point center to top of the pision differs about 2mm in the euro version.
They where different pisitons for the USA version (low compression) that coupd be used.
I don't think that I coupd use the 420 parts complete becouas the pisions are designed to be used in the aloy block and not cast iron block as the 350 is.

Any idea?

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post #2 of 6 (permalink) Old 01-25-2011, 02:38 AM
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The 3.5 is a high reving performance engine. Swap out the fuel injection and use a more modern one. There are many different ones available and they are all customizable. You will need an O2 sensor for feedback to the ECU. You'll get much better mileage and performance.

You could even make your own, Megasquirt is one such.

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Not part of the in-crowd since 1952. It's BRAKES not breaks. You break a bone, use brakes to stop your car. /rant

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post #3 of 6 (permalink) Old 01-25-2011, 08:44 AM Thread Starter
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I use MSIII with M119 injectors and open aspiration manifold and heads. So I will have a gain of 20 hp. But with a 420 conversion it would be more torque and hp too. But the proble is that the pistions are 2 mm to high with the connecting rods of the 380 (88mm bore) and the pistions of the 420 won't function in an iron block. So perhaps there are low compression pistions in USA for the 3.5 which could clear the 2 mm?

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post #4 of 6 (permalink) Old 01-25-2011, 09:31 AM
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If you have a 5 speed mated to a 3.5, it won't mate to a 4.2 block as the the bolts in the bell housing line up. 5 speed manuals were never offered with the v8's so you may have something different there already such as an adapter plate. If you can find one, the 5.0 liter engine from a 450SLC 5.0 is all alloy and the block design is the same as the 3.5/4.5 engines so it will mate up. This will give you another 40hp over the stock 3.5 engine.

I believe that GGR is doing this conversion for his W111 coupe.

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Check out the W114, W115 /8 enthusiast forum http://www.stroke8.org

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post #5 of 6 (permalink) Old 01-25-2011, 11:24 PM Thread Starter
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Non alloy block

This is why I am looking for to change the internals of my iron-block with the 380 long cors crank and rods. Staying with the original M116980 pattern. The only problem are the pistions which are a little to high and you can't use the early 380 ore 420 because of the different material used.

5.0 blocks from 450 SLC aren't easy to find and you see the difference immediately.

But if you use the M1168980 block with 380 internals (not 420 because of the different pistion fixing 23 mm instead of 26 mm) and 3.5 pistions you end up with 420o cc and 9.6 compression. Perhabps I found the solution, the 4.5 low pression pistions.

M116980 was fitted from Mercedes with 76/27 A5 but on request and only for VIP ore government cars. Ther are several cars arround (mainly 300 SEL 3.5 and one 350 SL)

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post #6 of 6 (permalink) Old 01-28-2011, 03:10 PM
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Pistons shape the cylinders in an ovalized pattern. This is part of the break in process and increases with mileage. This is more true for iron lining than Alusil. So if you change the stroke you need to rebore the cylinders and use oversize pistons so that pistons break in a new pattern corresponding to the new stroke.

I know Alusil requires specific rings, but I'm not sure the pistons per se are different. May be you could use 420 pistons and conrods, and 3.5 rings, or whatever rings for iron lining that would fit the 420 pistons. Rings are available in all kind of shape, diameter and thickness.

MB modifies the compression ratio by making the cup in the center of the piston more or less shallow. But the clearance for the squish area (crown of the piston) remains the same whatever the compression ratio. May be you could use some US 8.8:1 CR 4.5 pistons, shave the crown to get the correct clearance and increase the CR a bit at the same time as it will make the cup in the center a bit shallower. You may then have to make some cuts on top of the piston for valve clearance, especially if you use hotter camshafts.

Honestly, after sourcing another crankshaft, conrods, oversized pistons, rings, rebored the block plus all the work involved you will have paid for a 450 5.0L engine which has more displacement and is lighter. If like me you fit a 4.5 Djet intake manifold and keep the 3.5 cam covers and air filter housing and paint the block in black, only specialits will be able to tell it's the wrong engine, as it will be 1cm wider on each side. Other than that, it will look just like your 3.5 (the block is basically a 4.5 alloy block rebored to 96.5mm).

As you don't seem too concerned by weight an easier option is to source a good euro 4.5 engine. Your trans would bolt straight.

Last edited by GGR; 01-28-2011 at 03:22 PM.
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