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MAG58's DOHC L28 Build Log


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I hope to make this my updated thread on my build log for my *choke* twin cam attempt(s). To Preface this, I fully understand the following before attacking this.


From 3 years of searching, aside from one machine shop, this seems like a hopeless, useless attempt.


This is not going to be quick, easy, cheap, or probably logical for that matter.


There is no reason what so ever to chose this over say, any choice of RB engines, or other 6/8 cylinder engines that actually make sense. However, I've got about a year and a half of solid research/drawings into this, so I'm in it for the long haul.


So here it goes.

As far as L series parts, it's going to be the block, crank, and rods, oil pan, oil pickup, and that's probably it. I've got an F54 block that saw about 10k miles before it sat in a field for 15 years, along with the crank and rods to go with it.


The Head: Currently the game plan is an RB head, as I really only need the bolt holes and cylinders to line up.


The front plate: This is the big killing point of the majority of these projects. So far I have drawn up a complete front plate, to block off all oil passages, re-route water lines, etc, and am mating the very bottom of the Stock L front cover so that the maximum number of stock parts from whatever engine can be used with a minimum of custom fabbed one-off parts. The plate I have drawn up will eventually be making it into CAD files fairly soon that I plan to make available to HybridZ members, for free, hopefully.


Block Modifications:

The big problem with this engine will be coolant, as external oil lines and oiling modifications have been beaten to death and tried and tried on hundreds of engine designs with success. That said, the problem with the RB and L series, is the direction of flow is essentially backwards, rotationally speaking. If this happened, there would be very little flow on the exhaust side of the engine, leading to a horrifically short engine life. The block will have to be filled with Devcon, or preferably welding on the intake side to rotate the direction of the water jacket, with new holes machined on the exhaust side of the block, along with being stabilized to allow correct flow throughout the cylinders.




There is a long list of modifications that I will be posting as I find time.


Moderators: I really wasn't sure an appropriate title for this, so change away.


I really don't expect this to be taken too seriously until I get some pics and dimensions up, but they should be coming soon.



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I have probably studdied L head combinations as much as anyone on here. I own 3 seperate RB heads. I have built my own twin cam head (only one in the world that is a KA head setup) and have looked using one of my RB heads. I can say this, it is easier to do the KA head setup than it is to use the RB head. The RB head is about 1" too short. None of the water passages line up. Nor the oil passages. Then the worst part is the timing system. RB is belt drive, and the snouts of the cams come way out in front of the block. The best way would be to build a jack-shaft that comes up and mounted in the block above the crank about 6". Then make a speacial timing cover that has support bearings and use an outboard shaft out of the cover. Then run a belt and tensioner system from that external jackshaft up to the cams. I will probably do it one day, just to say I did it, but I see it at about twice the work compared to my setup. Only benifit, is you can run manifolds that already bolt up.

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that sounds interesting. I've already got plans for completely removing the front cover and replacing it with a custom plate to remove water and oil pumps along with killing the dizzy. Oil passages i'm not worried about as i plan on running almost a wet/dry sump hybrid, maintaining all oil passages in the block but running an external oil pump with a line plumbing the head separately, similar to what some NASCAR teams did in the 90's. This also has the advantage of ensuring a constant, positive flow of oil to the mains and rods which would be compromised with what basically amounts to having to support an entire second head worth of cams and valves. Water passages, may be an issue, but i already plan on modifying the coolant flow in the block anyways, to provide better cylinder flow than the current design, so it will end up being modified anyways.


Currently I'm looking for a cracked 20/25 head and a pair of wiped cams/stock cam wheels to use for mock up, So i can start laying out the required machining steps required to get the kind of coolant flow i'm looking for.

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  • 2 weeks later...

Update: I went to Indy yesterday to see what's at the local Pull-a-Part, and found sitting there an M104 Merc Cylinder head. So for 40 bucks, I pulled it and nabbed all the goodies and brought them home to see if it fit. It did :eek:... The bore spacing is DEAD ON and the head bolts are very close, in the small thousandths , which is doable.


That said, I took some pics of the head vs a stripped P79 I had just sitting around and threw it on the block to see what it would look like. Right now it's just sitting there, because I couldn't find a stock set of Z head bolts. It would actually bolt up using them, but I'd like to bore for the bigger Merc head bolts since they're massive.


Here's the comparison of the Exhaust sides... The M104 has nice bigt exhaust ports.



On the intake, it looks like a big version of the L series intake port.



Here's what the CC's look like... The stock M104 has 88.5 Bores for the 2.8 and 89's for the 3.2 so the valves are good sized.



On the block it just looks even better, this is how the front cam drive lines up...




And the Exhaust comes out on the low side of the engine, so with the factory lean over on the L28, a turbo should tuck up nicely on the underside with all that room on the driver's side of the engine bay for intake.




That said, It's not going to be a cake walk by any means, I'm going to have to run an external oil feed and return and I'm going to have to close up the water jackets between the head and block to make them fit. For water return, it seems like pulling the 5 freeze plugs and tapping them for water will be the intelligent thing to do, coupled with the water exit that comes out between the two cam gears...

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Time for more pics i've taken after I finished for the day with a customer car...



The head is a little taller overall with the valve covers than the L series. The M104 engine actually lays at an angle as well and the side closer to the top of the engine bay (the intake side) is substantially taller. If anything the Hood Mount would have to be notched to fit the head.


That said, since there have been questions about bore space I dug up some old L28 Head bolts and bolted the head up. I had to remove the alignment studs on the block, but the head bolted right on up using the longer 5 head bolts. I flipped the motor over and took these pics...

Here's what number 3 cylinder looks like from the bottom :



And here's cylinder 4 as viewed from the same prospective...



Onto Oiling, The M104 uses HLA's which are very similar to the 1.8T 20V buckets if you've ever seen the Audi engine. They are fed by the bottom set of oil galleries (see below) and allows the valve to rotate. It feels as if the portion that engages the valve is actually on a thrust bearing, as it spins very freely on the 10 of the 24 I checked.



This is a shot of the back side of the cylinder head, The 4 small holes directly below the cam seals (two on each side) are the oil galleries for the cams and lifters. Since the entire head is fed off of two pressure holes that sit right next to the back two head bolts, I plan on plugging them and running hard lines to these to allow me to plumb them to the L's main oil gallery. Firewall clearance shouldn't be an issue, as the head is a little shorter than the L head.



Finally the head bolts, as I said earlier, using the longer head bolts, which this is a picture of, the head does bolt up to the L block without modifications to the bolt holes in either the head or the block. The threads on the the M bolt don't actually thread that far into the block, as the L series long head bolt is within .020" from the shoulder to the tip of the threads.


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HOW did we all miss this?


Bore that block a little to unshroud the valves, have a GOOD machine shop mod the head bolt holes in the head to center them up on the head bolt holes in the block, and then bush them back down to size, it would be the path I took to get the head bolted up, and check the water and oil passages.


Weld the water and oil jackets up completely in the head, and get an L-series headgasket lined up, mark and drill for L-series water passages, and then do the external oiling lines as you have already thought, BAM it's on there. Next, figure out the cam drive system...

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We all, defined as those of us who have looked for a DOHC solution for a while.


I mean, I figured that a mercedes would work, especially seeing as the L series is so similar to them (wasn't it based off of a mercedes M130?) but so many people claimed to have tried it and failed, that I stopped looking.


Anyway, looking closer it looks like a stock timing cover can be modified to fit the block, with a custom upper portion needed. Double row chain driven cams, I think? Count the cam teeth and compare to the L series, but I think you'll need some sort of countershaft arrangement to get it working correctly. If you can do it with just one chain, two upper drive sprockets, and one lower drive sprocket, look into the newer KA24 timing chain tensioner setup, it might be able to get it done.

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The timing Gears are incredibly close, though the L's crank gear is 20 teeth while the M104's is 36. I'm gonna go back soon an get the crankshaft sprocket and see how close that its, along with the tensioner since it is designed VERY similarly to the way the L series tensioner works. The front cover I think I can salvage, along with the Variable Valve electric over hydraulic system it uses for the intake cam :). The front cover doesn't bolt to the bottom cam drive cover like the L series does, so some simple machining, countersunk screws and an adapter plate should work. This will hopefully give me a standard timing chain setup without the bothersome counter shaft or jack shaft arrangement.


Thanks for all your comments and if anyone has any ideas on how to do anything differently than what I'm proposing, by all means speak up! I'm looking for all the input I can get.



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