I have a general question that I do not think I have seen discussed here. I am some months away from building my 1985 slant and my desire is to bring it as close to modern standards as possible with LS coil packs, a modern ECU, O2 sensor, knock sensor, MAP sensor, etc. I know that there has been a trend in automotive design toward thinner oils and tighter tolerances. Would the slant benefit from building on the tight edge of the tolerances and running a thinner oil? I realize it will still need zinc for the flat tappets. I am planning on "blueprinting" the engine.

It would seem that it might help with oil pressure and lubrication.

Would the hydraulic lifters need to be somehow modified?

Thanks.

Are you going to add 3 more main bearings and a crank with counterweights on either side of each rod pin so the crank doesn't flex and wipe out the bearings? Because that's where the whole tight bearing clearance thing falls apart. The crank flexes and edge loads the bearings.

Here's a proper I6 crankshaft. Both pictured are essentially the same except for stroke.

_________________
Joshua

See threads here and here and here and here and here and here and here since we're on the topic, read this, too.

_________________
一期一会
Too many people who were born on third base actually believe they've hit a triple.

A couple of ideas to build a slant using up to date methods would include:
* mpfi with an efficient ignition system as mentioned
* modern short skirt pistons with thin metric rings and floating pins like 2.2 L pistons
* using a torque plate when machining the cylinder bores and using a machine shop that can get the bores very straight and round. The cylinder bore final surface finish needs to be comparable with the piston ring coating; chrome or moly
* run the main and rod journals on the tight side of the tolerances, be sure the shop doing the work gets the journals roundness also on the near perfectly round side of the tolerance.
* clean up and chamfer all oil passages and exits
* start with a cast crank block with the narrow main and rod bearings
* use 5/16 or smaller diameter valve stems with positive valve seals, conical or beehive valve springs are lighter, there are custom length pushrods available that are stiffer
* be extremely particular on the weight balance of the pistons and rods
* install an electric radiator fan to limit parasitic losses. Consider an electric water pump and an electric power steering unit if PS is part of the plan.
* a well thought out cold air intake and a free flowing exhaust would also be important

You can also consider special coating on the plain bearings to reduce friction and in the combustion chamber to control heat losses.

Do all of that and you will have about as modern of a slant that is technically possible using items currently available.

_________________
Doo Ron Ron and the Duke of Earl are friends of mine.....
https://www.youtube.com/watch?v=uX8Nj8ABEI8

Please define "proper". The slant has been running and racing for something like 42 years. I'm no authority, actually quite a newbie with the slant. Before joining this forum I read at least one post on every one of the nearly 500 pages in the engine section. I don't recall seeing any discussion about "wiping out the bearings". My build will be practcally stock with the exception of modern engine controls. Maybe 230 HP tops. I saw the "Ruster" run a few weeks ago, and I suspect he has a bit more than stock horsepower, and yet did not "wipe out" his main bearings. Maybe he added those extra main bearings to make it "proper". Maybe he will pop in here and tell us. Maybe one of the Chrysler engineers will pop in here to explain how the slant was designed with improper main bearings, yet has run trillions of miles for 4 decades.

I was trying to ask what I thought was a legitimate question about a new build and with modern oils would it be a good idea to have the machine shop turn the bearings undersize with an eye toward running at the tight end of the specs. I am having a hard time understanding how my mild build would benefit from loose tolerances, to the contrary, it would seem puzzling to me how an engine "without proper main bearings" would benefit from a loose build.

I also asked about whether the hydraulic tappets, designed for 1970's oil, would function with maybe a 0-30 oil. I plan on running a hydraulic cam.

Thank you guys in advance for indulging me. I have appreciated the way this forum embraces people into "Slant World".

Typically, a racing or high RPM Slant will need more generous bearing clearances.

_________________
--> Check out my FI Turbo build <--
--> And the race truck build project <--
--> The Diesel Corvette <--

Actually, I thought that I had listed several modern style improvements that can easily be incorporated to a slant six engine build. At least two of those did address tighter crank main and pin tolerances.
Not speaking for Josh, but he typically makes good points using few words.
I will elaborate. Stock bottom end slants are plenty strong, that is shown by Turbo builds like Lou’s and others. But they can go south quickly when starved for oil, like Flying Brian’s did at an autocross event or Ryan C’s four wheel drive truck did at CC a few years ago. Slantzilla currently has an add here at .org for a rod and piston needed to fix a low mileage slant that spun a bearing when the previous owner let the oil level get low. The bottom ends of slant six motors are not bulletproof. Tighter crank pin and mains build oil pressure but reduce oil flow.
No you will not be able to add main bearing bridges, but modern in inline 6 cylinder engines will have seven mains. 4 will work, 7 are stronger and take more abuse.
Concerning your question about hydraulic lifters, thinner oil should have no effect on the hydraulic action. If you are cutting the head and/or block or using a reground or non stock cam be careful to get the pushrod length correct to keep the preload on the lifters around nominal. The lifters in my 83 D150 had around .080 of preload travel so there is some room, but keep that in mind. If things go wrong inside the hydraulic lifter it is more likely to be because the preload was way out than due to oil viscosity.
Also on lifters, I had not seen it mentioned much here but on other Mopar forums there are comments about the poor quality of some of today’s aftermarket hydraulic lifters. So if buying new buy from a known quality supplier or if your original lifters have life still in them, have them reground. Oregon does that.
And one more thing, perhaps you could just cut to the chase and list the tighter crank tolerances that you know of and are planning to copy in your build.
I believe that lighter weight oils are common today in modern engines to drive improvements in fuel economy. And the manufacturing changes that make the light weight oil use feasible are:
1) improved oil flow to bearings,
2) mating parts are matched better for size due to improvements in machine and gauge capability and repeatability,
3) more robust part designs,
4) improved assembly techniques concerning cleanliness and bolt torque.
If you want to run lighter oil apply the 4 points listed above. For item 3, using a high quality King bi metal bearing set and ARP bolts will cover that.

_________________
Doo Ron Ron and the Duke of Earl are friends of mine.....
https://www.youtube.com/watch?v=uX8Nj8ABEI8

The slant 6 is an economy engine that happens to be quite durable. One thing it does not have is as especially rigid or fully counter-weighted crankshaft. Both the block and crank need to be highly rigid to use the small bearing clearances associated with 'modern' engines. Since we're dealing with an I6 I gave an example of a modern I6 crank to illustrate the path some Germans took, but the Japanese and Americans eventually made similar crankshafts. On the other end of the spectrum you can build an I6 crank with 3 main bearings and no counterweights yet still have perfect primary and secondary balance. The Chevrolet Stovebolt 6 was this way, but it was all of 50 hp so it mattered very little. A crank without counterweights flexes more and the bearings and/or the crank itself suffers (breaks). With 2.75" main bearings and 2.1875" rod pins the slant six crank is pretty good for what it is, but when you propose changing the design parameters that make it highly durable you can expect it to be less so.

Here are some words from Mahle on bearing clearances. https://www.mahle-aftermarket.com/media ... 1-205r.pdf

So that's the big problem generally speaking. More specifically, Where are you going to get the very precisely machined crankshaft and bearings? Modern tight-tolerance engines have a range of standard size bearings. Have a look at this Honda procedure: http://www.haccord.org/crankshaft_main_ ... t-798.html

_________________
Joshua

I would hazard a wild guess, that the light oils seen in modern engines are more important for the variable valve timing components and the benefits it brings to the driving experience, and to a lesser extent fuel economy. Tight bearing clearances in and of it self is not the goal, but an after-effect of needing thin oil to operate the timing components. Sure it helps economy a touch, but only on the scale of 100,000 units a year.

The same sort of thinking needs to go towards any 'modern' improvement. Thin Piston rings is another example. Yes you can gain a little bit more power, and some less bore wear... and a very slight improvement in fuel economy. OR you can spend the same amount of money on cylinder head work, and you might find 20 HP, which would be (in my wild guestimation) double what the pistons would give you. At the manufacturing level, do both, and win.. at the racing level, do both and keep up with the competition. On the street, do it because you want to.. because it would be fun, or challenging to try..

I would love to try and hot hone a slant six. It seems to me that the long stroke, and long engine block would benefit from it.. I don't have access to a torque plate, so it will never happen... but it would be fun to see if it would help, and it is one of those easy "things" that might make a difference on a 50 year old engine.

Thanks guys! You've given me a lot more to research. I have the money to spend. The 2004 RX8 (very nice) cost me $1500 and a very nice 1985 Dodge slant (light block and crank, narrow crank bearings, 150 psi cranking pressure, no smoke) cost me $600. I will have $2000 in a new ECU and other engine control items. I just bought a good Subaru throttle body for $35. A buddy is building me a custom intake. LS coil packs and wires. I am using the stock transmission and rear end on the Mazda. There will be some extra torque, over the rotary, but I don't plan on putting my foot down. I'll have $100 in my buddy building my motor mounts. About $1500 in a transmission adapter plate.

I plan on getting everything running then pulling the engine to have it rebuilt plus the head worked $$$$$. I hope to have less than $15,000 in a very nice daily driver that I can take to coffee and cars to be a real conversation starter.

I thought I might have the only slant powered RX8 in the world. Maybe someone else will beat me to it then give me a hand.

Thanks again!

The weight of the oil has little to do with cvvt requirements, the systems are designed with whatever the engine is designed to operate with. 10W-30 to 5W-40, the system has oil temp sensors and intake/exhaust cam/crank sync & responds accordingly & instantly. Whether using an OCV, or an electromagnetic actuator w/slip rings. Thin oil is for quick pumping & less friction, as well as cooling. Most have spray-bars cooling the piston crowns from below & more.

To back up the above, this is one "modern" feature that Your project can do without. Just get a quality, precision job done to standard specs and skip the 0W-20.