Benchracing a theoretical engine build.

So, I've been playing with numbers in my down time at work and I think I have an idea for a theoretical Miata engine build. Nutty, but it's just far enough off the reservation that it might work. So far the math says yes, (capability for overbore, rod length, compression height) now I just need to see if it's practical.

The issue I'm dealing with is the rods. Off the shelf rods that I've found are 172-.256 thicker than the stock application. I'm thinking that can be skimmed of easy enough, and use rod bearings that are called for on the original stock rods. The small end of these rods, have a 22mm bore, and the pistons I've found have a 20mm bore for the wrist pin. Stock piston the rods have a press in wristpin, and the new pistons use press in wrist pins. So would bushing the small end boring it for a 20mm pin be the answer?

If this works, I would theoretically have a 1.9L long rod, (~1.76 rod/stroke) high compression motor, for $100 in hardware outlay. $60 pistons, and $40 rods. Rods in stock applications support 450 whp, and 8000rpm with a much heavier piston, longer stroke engine. I think they would be understressed in a 7000rpm lighter piston, shorter stroke motor. What am I missing?

Yes, I'm playing this one kind of close to the vest, as it may end up in a challenge car.

No one huh?

Stroke is controlled by the crankshaft not the rods. So in order to maintain or increase compression ratio, the pistons will have to make up for the change in rod length.

Mixing unrelated parts does not always work out well.

I'm not sure what you're trying to accomplish here. You've found a rod and piston combination that you think will give you a better rod length to stroke ratio if you can narrow the big end 0.256" and reduce the small end diameter by 0.0787"?

It might do better to talk with your machine shop. That way they can check the small end and see if they have a bushing to work there*. As to having the rods machined so their girth is less at the bearing, that should not be an issue.

• • since you are installing a bushing in the small end, I'm pretty sure it would need to be floating - I don't think (not sure) you can bush the rod AND make it a fixed pin

Oh... don't forget to include these costs in your total. The last set of rods I had worked done on was a set of TRD rods opened up to take a 20mm floating pin.

In reply to APEowner:

Yeah, that's pretty much the gist of it. I'm not that worried about narrowing the big end. It's the small end that has me concerned. Different years of this engine used different big end thickness rods.

In reply to oldeskewltoy:

I'm planning on having that talk soon. I'm just trying to do as much homework as possible before buying parts, and approaching a shop with a crazy idea. I may have to look into converting a press pin piston to full floating.

And yes, I'm going to document it, and the costs. I've been down the boost rabbit hole a few times now, and while it works. And works very well. It's just been done. An all 4cyl motor build hasn't been done very well at the challenge. So it will be documented.

Spinout007 wrote: In reply to APEowner: Yeah, that's pretty much the gist of it. I'm not that worried about narrowing the big end. It's the small end that has me concerned. Different years of this engine used different big end thickness rods. In reply to oldeskewltoy: I'm planning on having that talk soon. I'm just trying to do as much homework as possible before buying parts, and approaching a shop with a crazy idea. I may have to look into converting a press pin piston to full floating. And yes, I'm going to document it, and the costs. I've been down the boost rabbit hole a few times now, and while it works. And works very well. It's just been done. An all 4cyl motor build hasn't been done very well at the challenge. So it will be documented.

OK. That's a lot to take off the width of the big end but if the meat is there then it certainly can be done. You could theoretically bush to a press fit and it should work but then you're looking at a press fit into a press fit (actually a shrink fit to a shrink fit) and that just feels wrong to me. I'd rather put a conventional bushing in there and machine the pistons for retainers. That assumes that there's material in the pistons for the grooves.

I suspect that the machining costs are going to make this seem like less of a bargain.

Also, while rod ratio can have an effect on engine performance (or longevity) it's really a small part of the overall package and I wouldn't sacrifice much (time, budget or other parameters) to optimize it. Particularly since the definition of optimal rod ratio is fairly nebulous.

The big question for the big end is whether there will be enough meat left around the rod bolts to retain them.

EDIT: Also, will the nuts be too big for the resulting end cap?

So far the budget stands at 300 for the car. Purchased with a no start condition, that turned out to be a busted timing belt. I've got a line on an 02 engine and trans for 600. Don't need the trans, flywheel, etc I'm hoping to minimize the hit to the budget for it by parting it out for the pieces not needed. MS2/3 and VVT controller is going to run me ~3-400 for a kit unless I can find a cheap used box. I can do the suspension for ~$110 with more spring rate and shocks to handle those rates than I can likely use for this build. ~$100 in pistons and rods, pistons come with new rings. Then it's bearings, gaskets, machine work. I can get away with doing A LOT of it myself. The overbore, and bushing the rods/machining for floating pins is what's going to push the budget. With some diy head porting, and a little luck I'm hoping for one heck of a fun little car.

In reply to Jerry From LA:

Good point. I'll have to look into that. I'm sure it will be ok.

You're also going to need to get that balanced so don't forget to add that to your cost benefit analysis.

Planning on balancing the rods/pistons myself, then letting them balance the assembly before punching the block just in case this doesn't work out.

The small end of these rods, have a 22mm bore, and the pistons I've found have a 20mm bore for the wrist pin. Stock piston the rods have a press in wristpin, and the new pistons use press in wrist pins. So would bushing the small end boring it for a 20mm pin be the answer?

1 mm wall thickness isn't enough, it won't last if you can even get it to work

I'm curious how this will turn out. I've been thinking of doing a long rod build for the Protege, using custom rods with an FP crank and FS pistons in an FS block, eventually adding a turbo. Or two.

In reply to Zomby Woof:

So the answer is to open the piston up to the 22mm pin size then?

I don't think so. Maybe I need to read through the thread again, but I don't really understand why you are doing what you are doing. Sounds like a very difficult way of getting the job done.

Zomby Woof wrote:

The small end of these rods, have a 22mm bore, and the pistons I've found have a 20mm bore for the wrist pin. Stock piston the rods have a press in wristpin, and the new pistons use press in wrist pins. So would bushing the small end boring it for a 20mm pin be the answer?

1 mm wall thickness isn't enough, it won't last if you can even get it to work

When you bush rods for floating pins you bore them oversize and then install the bushings. I didn't think of it earlier but if there's enough meat on the piston pin bosses then boring them out to the larger pin size is and keeping the press in the rod certainly a viable option. That'll probably be cheaper than modifying the rod end and machining retainer groves in the pistons.

Zomby Woof wrote: I don't think so. Maybe I need to read through the thread again, but I don't really understand why you are doing what you are doing. Sounds like a very difficult way of getting the job done.

Better rod ratio for a theoretically better engine. 1.54 stock vs 1.74 hopefully. Longer rod allows for more dwell time, at the top and bottom. Which in theory should allow me to advance the exhaust cam timing a bit, and hopefully if I can get to the 02 motor with VVT before he sells it. (he's been trying for a few months, but not very well advertised) along with a VVT module for Megasquirt I'm hoping I can advance/retard the intake cam enough to create a WIDE torque curve that still makes decent power up top.

EDIT: I had a long post explaining this, but the power went out and killed it. The above is the BERK it post. Basically, BP engine is kind of an orphan, we all love to boost it, and therefor it's unloved by the aftermarket unless you want pistons and rods for boost. And they're PRICEY! The above is my back of the napkin engineering attempt at "off the shelf, parts bin displacement increase". I used the 85mm bore as the go to size as per FlyinMiata's page on the stroker kit, that's "safe". If I get this to work, I MAY try to boost it after the challenge. I don't have a lot of tuning experience with high compression and boost. But that's far in the future.

APEowner wrote:

Zomby Woof wrote:

The small end of these rods, have a 22mm bore, and the pistons I've found have a 20mm bore for the wrist pin. Stock piston the rods have a press in wristpin, and the new pistons use press in wrist pins. So would bushing the small end boring it for a 20mm pin be the answer?

1 mm wall thickness isn't enough, it won't last if you can even get it to work

When you bush rods for floating pins you bore them oversize and then install the bushings. I didn't think of it earlier but if there's enough meat on the piston pin bosses then boring them out to the larger pin size is and keeping the press in the rod certainly a viable option. That'll probably be cheaper than modifying the rod end and machining retainer groves in the pistons.

That's what I'm hoping for. I wanted to go to full floating just because it's "better" but OEM's have used press fit pins forever. I'll survive.

That's an awful lot of effort and expense for gains that aren't likely going to net you much in the way of performance improvements.

Zomby Woof wrote: That's an awful lot of effort and expense for gains that aren't likely going to net you much in the way of performance improvements.

That's pretty much the opinion of the internet when you do anything with a miata that doesn't involve boost.

It's also fact.

Those are the kinds of things you look for when you've reached the point where the easy stuff is all done and you're looking for an edge, the last 1 or 2%.

In reply to Spinout007:

That wasn't the opinion of "the internet". That was the opinion of a machinist.

It's a pretty well known fact that the cork in the BP power scheme is the head. A stock 1.6 head doesn't hit 180cfm on the intake side till .500 lift which is more than the stock cam will provide. A 1.8 will BARELY get to 180cfm with a stock cam. The head is the key to making more power. Whether stock, boosted, NA etc. You've got to open it up somehow. Heck most of the "fully ported" heads reported on MT.net barely get over 180cfm. You're probably wondering what my obscession is with 180. It's an "attainable" goal for a ported 1.8 head. Theoretically, if you can get 180 cfm out of a bp head, with the rod ratio, etc that I'm aiming for it should put down 180hp at the crank. At least according to the engine calculators I've played with so far.

I'm reading, listening, learning A LOT about port work, flow etc on small displacement engines. The one guy that's been the most enlightening so far made his name racing mini's.

The rod work, is being done to aid in cam tuning. In theory, you don't need a huge exhaust port with higher compression, and the sooner you can open the exhaust valve, the better. The same with the intake valve. With more "dwell" at TDC and BDC of the stroke, the sooner the ports can open to do their job. So even if you don't have the greatest flowing ports, you can still get air in and out of the engine.

So in essence, yes. I'm looking for that last 1 or 2%. The BP is pretty much maxed out in NA format in stock trim. The spec miata guys have engines putting down 160ish crank IIRC. I'm looking for that 160 without the rule book, teardowns, and 5-30k price tag a full tilt spec miata engine will run you.

As far as the stuff available to purchase for the miata. The cheapest forged rods won't hold up under 450hp and 8.5k rpm if the various forums are to be believed. The modified off the shelf rods hold up in a bigger displacement engine with a much longer stroke @ 8500 rpm and "stay under 500whp and stock rods are fine". Hypereutectic pistons and those rods should hold up fine for my uses tuned conservatively and keeping detonation at bay. Not to mention that if I ever decide to hang a snail off the side of the motor, a 5-6 psi setup should be almost instantly spooled with the higher compression, and slight bump in displacement. Not getting into the effects of forcing air into the cylinders and the ability to open valves sooner.

Even if this motor makes it to the challenge, it's destined for a street car that see's occasional autoX duty. So a wide streetable power band is what I'm shooting for. If it makes it run like I'm hoping it does. So much the better. My wife and I both miss having a Miata. The one I drug home from the challenge last year is sitting out back waiting for it's next lease on life.

Just curious, what's your plan for cams, lift and duration?