L Jet to B35 Conversion

[decoupe]

I've started the conversion to a e32 735i B35 intake manifold/fuel rail from an early 80's L jet - everything run by an after market engine management system. Having pulled the L Jet set up last night I have some observations, comments and inevitably questions.

First off, the cylinder head is an AMC (were these ever used as OEM on any 3.5 litre BMW's?) with intake ports considerably larger than the L Jet or the B35 ports. The PO had a very nice twin Weber 38/38 downdraft carb set up on it when I bought it and possibly they have been ported but there is no machining grinding marks so I don't think so.

Comparing the ports of the L Jet and the B35 intake shows about a 15 - 20% larger cross section on the B35 but the AMC intake port is probably another 10-15% larger still. The second picture shows the relative size using the gaskets as templates next to the AMC port.

When I cleaned up the B35 manifold I scribed the outline of the gasket on the flange and ground the flange to match for a smooth air path. I put the same gasket on the AMC head and marked the sides where the inlet is inside the gasket and will grind them as well. The picture below shows this area.

All of this is interesting and pretty straight forward, but because the AMC port is significantly taller, the lip of the injector port and the gasket of the B35 port relative to the AMC inlet could be relieved to bring the air/fuel mix closer to the roof of the intake (which has a ridge that runs to the valve stem). Is this a good thing to do? As it is the stream of fuel will be closer to the centre of the AMC port. Trimming the B35 outlet will tend to raise it. The pictures below show the extent of the manifold that would get ground back effectively raising the airstream in the AMC port (I think).

So, I'm new at this porting business and I'm assuming that improving the flow of air is desirable. I'm not trying to increase the volume, just smooth out the path the air follows. Should I grind back the upper lip? Is it better to keep the fuel stream closer to the centre?

One thing I will be able to contribute is the relative out put of the weber downdrafts to L Jet to L Jet with e21 runners to B35 manifold using the same dyno machine - all else being equal with the engine. In case that is of interest to anyone, that is.

Thanks,

Doug

 

[Sven]

Doug,

My engine rebuilder did a partial port to the L-jet manifold. He filled in the small void between the opening and the top bolt hole in the manifold and then ground off some of the portion that protrudes into the opening, leaving about 1/8" of metal between the opening and epoxy fill. This section is also tight to the injector port on the inside.

A custom redesigned manifold to transition between the "C" runners and the larger B35 ports in the head would be nice. Then one could also eliminate the 10° angle of the injector ports (so it would be easy to use a fuel rail instead of short hoses).

 

[decoupe]

Thanks Sven, I'll continue the research and ask elsewhere. Port matching in the search function provided this from X_atlasO on a similar question:

“If the ports on the head are wider than the ports on the manifold, then the gains from porting the manifold will be minimal, unless you increase the width all the way through the runner. Flaring it at the end of the runner won't help much, especially since the head is the wider point. The only loss this will induce is to create a small stagnant volume directly in front of the intake flange where the head is open. While it is certainly not optimal, I doubt it would have enough of an impact on performance to be a problem. Now, if the sizes were reversed and the head was the smaller port, you'd have a much bigger problem, as the intake boundary flow would run smack-dab into the head surface, significantly disrupting the intake flow.”

Doug

 

[decoupe]

Update on Conversion

I had a message to update the L Jet to b35 conversion - here goes:


Fast forward to October and the conversion is long since done and all the little issues (largely of my own creation) sorted out.

In the end, I decided not to do any DIY porting mentioned above as most of the matching between the head and the b35 intake would involve pulling the head and disassembling in order to keep any of the metal from getting into the valves/pistons etc. May do it this winter as part of a performance upgrade with a fully ported head and possibly a custom cam from Miller Performance.

Most of the remaining work to convert to the b35 is to make room for the position of the intake manifold relative to the engine. Unlike the L Jet/D Jet intake log and runners which sort of wrap up and over the head - the b35 intake sits completely on the brake booster side of the engine. The advantage to the b35 configuration, you can remove/ service the wiring harness, fuel rail or remove the injectors without removing the throttle body or intake manifold.

It is simplest to remove just about everything on the left side of the engine to provide room for what follows – power steering pump, reservoir and hoses, overflow tank, Master Brake Cylinder, brake booster and booster mount/pedal box.

First step is to find a good Tii Booster and shorten the neck of the brake booster mount/pedal box assembly. After removing the MC mounting plate, I believe I ended up cutting 3 1/2” out of the neck in total. Weld the MC plate back on complete cleaning, service and repaint as well. This should provide about ½” clearance between the intake and the booster. The brake lines were hand bent in place to line up with the MC ports.

Next item was to fabricate a rotating plate to accept the linkage bar from the accelerator and the Bowden cable to the throttle body. Most of what I used was from the L Jet version (which originally ran the 38/38 weber carbs) but wouldn’t work here as the throttle bodies are reversed in terms of which side the hardware operates. The photo below is self-explanatory in what was my solution to this problem but almost every other installation I’ve seen has used their own unique solution (check out Sven’s). One change I will make is to move the pivot connection to the Bowden cable closer to the main pivot point (with the spring clip) in order to bring a little more pedal travel to get to full open throttle. It’s a little sensitive.

Along the way, I found my driver side motor mount was cracked
across half it’s width, which I welded and reinforced.

Also, modified the L Jet water return (overflow and heater hoses back to water neck - bottom of the picture) adding about 3” in length.

Left to do is a cold air box to shorten up the aluminum intake pipe and keep the combustion air cooler in stop/go traffic. Still hoping to dyno this thing to see if there is any increase but the weather is getting wintry here and it may have to wait till spring.

 

[echappe]

I noticed the same issue when attaching my carburetted 3.0 intake manifold to the B35 head, openings in manifold much smaller than head. In fact if the manifold was ported to match the head the wall thickness would be down to almost nothing. None the less with the weber 38's the car really scoots. Has anyone ever heard of a custom manifold being fabricated to fit correctly?

 

[Denis McGrady]

b35 conversion

thanks Doug(again) now i see if you want to efi your 02 or coupe, you have to dig for a tii booster. its known as an ate T51 and apparently was used on other cars(volvo & merc). what are the specs on the air intake pipe? i want one. denis