Under or Over: what is possible...

[Sykohtic]

I'm planning my NA M20 build and have stumbled onto a bout of curiosity...

I plan to do a 3.0L stroker. I plan to use a Dblias 296 cam with adjustable gear, wasted spark, itbs, Megasquirt, a ported head, 11:1 compression, and am considering Meth Injection.

So the question has arisen: how do I want to play the stroker internals?

I was planning on running a 89.6 mm M54 crank, M54 rods, and custom 84 mm bore pistons. This is a safe option that needs no boring and produces a displacement of 2979cc. This creates a noticeably undersquare motor. Benefits include the safety margin in the block as mentioned before, a motor that will tend to be fairly thermally efficient for what it intends to accomplish. This also means it will make better low end torque, which is really quite good since I'm running a hot cam and ITBs that will sap power from the low end in exchange for ultimate top end power. The cons: running that cam and itbs also inherently means I need to rev the motor higher. Because of the increased piston velocity, that's a considerable amount of wear and tear on the bottom end and piston rings, especially since the motor will be high compression. Also the undersquare does mean it won't be as rev happy, though I want to lighten the bottom end as much as possible and run a lightweight flywheel to make up for some of that. Another con is that I currently do not posses the M54B30 bottom end and sourcing one will cost extra money.

So then I started thinking. I already have a M52 84mm crank and M20B25 connecting rods (though I'd much rather switch to M/S5x rods for less weight and more strength if I were to go this route). How far can you push the displacement on the B28 crank? Well, after looking around for the maximum bore an M20 can tolerate, I found the 87mm number. So an 87mm bore on 84mm stroke comes out to 2996cc. So I get more a bit more displacement, and get an oversquare motor. It will like the revs more for the displacement it makes, and favor top end power where the cam and itbs will be catching their wind. I already have the bottom end in my possession so cost is gone there. However, the custom 87mm 11:1 pistons aren't gonna be cheap. I'm sure they fall into "special" in the Ireland Engineering handbook so I would expect to pay more than the listed 900-some dollars for a set. The M52 crank is also, I fathom, considerably heavier than an M54 crank, though I admit I haven't weighed either. Then I still have to source the better rods for the added security. The motor also will inherently be mess thermally efficient, which is dangerous as I read that an 87mm bore gets uncomfortably close to the oil gallerys, so it brings the question of block integrity into the equation.

So pulling that together, really either option has equal points in the pros and cons. I thought about doing an 86.5mm bore if that's even possible. That still a bit oversquare and gives a little more marginal security on the block, but it works out to the least displacement of the lot at 2961 or something close to that, and at that point I'm not making a significant difference over the usual 2.9L I don't imagine.

At the end of the day, the car has to be reliable enough to drive long distances. I know I'm asking a lot of a motor that borders on a race motor either way, but witch option would you guys go with?

[digger]

doubt youd see much from water/meth injection with those specs, since the CR is not that high and dynamic compression won't be particularly high with that much duration

what is wrong with long stroke and big bore e.g. 89.6 stroke and 86mm bore?

Also the assumption that the shorter stroke bigger bore will be more revy happy is only partly correct. You need to increase the inlet valve sizes for that to happen. If the displacement between the options is the same then you are only changing the rod ratio which actually has a very small influence on engine VE. The shorter stroke does offer less friction due to lower piston speed but it’s not really more rev happy just more efficient for the same amount of cubic inches.

[Sykohtic]

doubt youd see much from water/meth injection with those specs, since the CR is not that high and dynamic compression won't be particularly high with that much duration

what is wrong with long stroke and big bore e.g. 89.6 stroke and 86mm bore?

Also the assumption that the shorter stroke bigger bore will be more revy happy is only partly correct. You need to increase the inlet valve sizes for that to happen. If the displacement between the options is the same then you are only changing the rod ratio which actually has a very small influence on engine VE. The shorter stroke does offer less friction due to lower piston speed but it’s not really more rev happy just more efficient for the same amount of cubic inches.

The main reason I was avoiding the big bore, long stroke 3.1 was that the two options above present a similar cost. Using the 84mm crank means more machine work and piston cost, but to use the 89.6 mm crank I need to source the bottom end, which could cost up to an extra $500 anyhow.

The engine gets intake porting either way. I was planning to do +1mm oversized. I don't know the math for how then extra like 400mm^2 of overall inlet port area effects the VE and if that's significant enough to make the difference you're talking about.

If I had money to just throw on a fire it'd be cool to do an 89.6mm stroke and an 87mm bore to get a true 3.2L, but that's entirely a race motor at that level, and would require serious porting I would imagine to tap the full power it could make.

[zaq123]

Using the 84mm crank means more machine work and piston cost, but to use the 89.6 mm crank I need to source the bottom end, which could cost up to an extra $500 anyhow.

Aren't you going with custom pistons anyway? Or you are planning to use OEM pistons? I'm not sure I got your first post right....

Anyway, you said that you have 84mm crank and 135mm rods already.. Since your goal to save $$, use them. As far as machining work goes: doing anything more than 0.03" (less than 1mm) in the bore, will typically cost you about the same, 85mm, 86mm etc.
IMHO, If you want to save money....use what you got (if it's in acceptable shape) and go stock pistons. Block needs some machine work anyway to make it fresh, spend $200-250 more and take it to the bore size for the pistons of your choosing.
By the time you add the head porting on a flow bench (the only way to do it), ITB etc etc on your list, $300 for the bore job will be like a bucket of sand on the beach.... Don't forget to recondition your used rods (will cost you close to buying new ones if you don't have the equip. and have someone else doing this work for you)


I'm doing the "budget" strocker myself right now and this is what I'm doing:
M52 crank, new rods, custom 85mm pistons, stock and completely rebuilt 885 head (supertech valves,new oem springs, HD rockers, Dbilas 276 cam, Nuke adj.gear), OEM intake/TB (for now), MS2, headers. Wish I could do ITB right now but other little things (clutch, gaskets, seals, bolts, bearings etc etc etc etc etc add up quick).

[digger]

Yeah from a cost point of view custom 84mm vs 85mm vs 86mm is small difference maybe 100 if that, depending who makes them etc.

Same with stroke 84mm vs 89.6mm I doubt cust difference between them is much, if you have one crank you can always sell it and by another.

If you want a "more revy" motor then choose the smallest cubic inch it will make maximum hp at higher rpm if everything is is the same and you won't really give up any hp at all. The only thing you lose is torque.

[Sykohtic]

I think going with the 87mm bore idea is my best option then. Doesn't MM say 87mm still leaves safety margin as there's enough material to almost bore to 88? I haven't seen it myself, just something I read on an old forum.

It seems like that's the most cost effective and seems as if it will yield the best overall motor as long as its reliable. Which bring is the question of estimated reliability/lifespan? With occasional track time, maybe a long road trip, and some spirited driving, with proper maintenance and such, I'd like the car to make it 3 years without needing a rebuild, at which time I could pull it apart, update the bearings, do a light head refresh, routine maintenance and put it back together.

To review specs: 3.0L, 87mm bore, custom 11:1 pistons, M52B28 crank, lightened and balanced internals, 296 Cam, pump gas. Ported intake, upgraded valve springs, itbs

[digger]

i'm all for pushing the envelope but this sounds like unnecessary risk, the difference between 86 and 87 is not much like 2% and what is to say you dont lose that 2% due to increased bore flex reducing ring seal etc its a slippery slope when you start going down that path as wall thickness is a non linear relationship to strength and stiffness and then you add the fact that things are going to run hotter locally and this affects mechanical properties even more. 86mm is proven for NA if you dont do anything stupid with it as the walls have plenty of meat. just way up RISK vs REWARD

you only end up with 0.9mm from edge of bore to the oil gallery at 87mm and we are talking cast iron here not a nice drawn tube, its not that strong or stiff, more succeptible to flaws and what is to say that there isnt 0.1 or 0.2 mm of deviation in the gallery on some blocks.

MM said they dont use 87mm because then you cant overbore it if something goes wrong. they probably sonic checked the blocks they experimented on and may have offset bored it IDK.

why not a square engine 86 bore 85.8 stroke ? probably much same capacity as 87 bore and 84 stroke

[mr2peak]

What's wrong with doing a more standard 3.1L S52 crank motor? Not sure it's worth reinventing the wheel here.