1997 Mazda MX-5 Miata Long Term Road Test

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1997 Mazda MX-5 Miata: Long Rod BP Conversion And Nerdery

February 23, 2012

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Hang around with engine geeks long enough and the conversation invariably turns to rod length. This usually happens not long before they start talking about poop, which is where every conversation in the known universe eventually ends up. 

When speaking of rod length, we're talking about something far less biological than it sounds. We're talking about the length of the engine's connecting rods. The influence of conrod length is real but often misunderstood, and by building a long-rod Mazda BP engine for Project Miata we have a platform to share some nerdy insights with you. 

First, an anecdote. Years ago in a previous motorsports life, I asked the race director of a high-dollar OEM effort about his preferences regarding conrod length, specifically rod ratio. He, a seasoned veteran of many top-level engine programs, replied, "Rod ratio? Hell, pick your stroke, pick your deck height, then put the wrist pin as high in the piston as you can. The rod ratio ends up being whatever it is!"

Not a bad approach, it turns out. 

blewed_up_miata_rod.JPG 

[Photo courtesy miataturbo.net]

Let’s back up and cover the very basics first. Until something very dire happens (see above; someone else's spectacular Miata assplosion), an engine’s pistons are connected to the crankshaft via connecting rods of a (hopefully) fixed length within a certain range of possibilities. The length of said rod divided by the stroke is the rod ratio.

In creating the Miata’s 1.8-liter BP engine, Mazda wanted to keep cost low, and rightly so. As such, the BP shared many components with the architecturally similar 1.6-liter B6 engine found in ’90-’93 Miatas, including the rods. The BP was bored and stroked (83 mm bore; 85 mm stroke) relative to the B6 (78 mm bore; 83.6 mm stroke), so this rod-sharing approach resulted in a lower rod ratio in the BP. No big deal as the BP was a fairly pedestrian engine, neither generating much power nor revving very high. 

Other qualities inherent to the BP – a closed-deck iron block, factory piston oil squirters, low compression ratio in early variants, a forged crank – have made it a magnet for boost. Get too chummy with the boost, though, and you’ll find that the BP’s weakest mechanical links are those rods. They'll take some abuse if you have modest power goals, but get the itch and the rods will express their displeasure by first folding like a cheap menu and then giving the block a Caesarian.  

Having the luxury of this foresight, we're replacing Project Miata’s stock rods and pistons with beefier bits as a preemptive strike, especially as our starting point is the later ('01-'05) VVT BP that came with 10:1 compression pistons. Actually, Keegan Engineering is doing all the work. While we’re in there, why not dial things in a little by lengthening the rods?

Lengthening the engine’s rods to the maximum possible minimizes the pistons’ peak acceleration. Side loading against the cylinder walls is reduced, too, due to reduced angularity of the longer connecting rod. Also, with longer rods the pistons tend to dwell near top and bottom dead center for a longer portion of the crankshaft’s rotation. You’ve heard of all this before.

However, the reality is that some of the above effects are relatively subtle. We’ll find that perhaps the most attractive benefit to a longer rod is something less arcane.

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After talking things over with the folks at JE Pistons, it was determined that the longest rod possible in a stock-stroke Mazda BP ends up being 7 mm longer than stock (140 mm vs 133 mm center-to-center length). Sure, you could cram a hair more rod length in there if you wanted to push the entire ring pack further up the piston by thinning out the top ring land, but this is ill-advised.

With 7 mm more rod, the piston still retains a healthy top ring land thickness of 6 mm. That said long-rod-specific pistons are based on JE Piston’s newest Forged Side Relief (FSR) forging is even better. More on the JE slugs to come. 

project_miata_long_rod_mil_spec_rods_all.jpg 

Now to find a 7 mm longer rod that has the same big-end width and diameter as a BP rod. The stock rod from the E30 BMW 3 Series' M40 engine is surprisingly close, though its big end is a hair narrow and it has a much larger wrist pin diameter. It wouldn’t be ideal but might be workable. Mil-Spec, however, had a smarter idea -- a billet chromoly rod just for the Miata that's 7 mm longer than stock with none of the compromises of the OE BMW rod. I'll expound on these Mil-Spec rods in a later post.

Plugging all the relevant data into an excel file I whipped up, the longer rod provides a 0.5% reduction in peak piston speed and 1.2% reduced acceleration. Directionally good, but a small effect. Piston side loading drops by 5.2%, which is nice. Lower side loading decreases losses due to friction between the piston skirt and cylinder wall, plus it slows the wear of the cylinder walls on the loaded side of the piston. 

project_miata_long_rod_je_pistons_fsr_stock_height.JPG  project_miata_long_rod_mil_spec_rods_stock_profile.jpg  

The aforementioned unspoken advantage to longer rods is the opportunity they provide to take mass out of the piston. In locating the wrist pin higher in the piston, thus shortening up the piston’s compression distance, the piston becomes lighter. The pin bosses and skirts can become shorter, and in our case the JE FSR pistons ended up about 10-15% less massive than they otherwise would have been. 

In addition, the Mil-Spec rods are, too, lighter than the stock bits despite their additional length. Reciprocating mass reduction through lighter and stronger pistons and rods lightens the forces in the BP's bottom end in direct proportion to the weight removed, makes for a less thrashy BP and safely allows more headroom for revs, if so desired. 

So, the best result comes from a combination of long connecting rods and mass reduction. And boost. And engine control. And revs. And volumetric efficiency. And friction reduction. And...

--Jason Kavanagh, Engineering Editor

Sources

JE Pistons - 714-898-9233  

Mil-Spec - erik@mil-spec.org

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