Friday, December 13, 2013

What Have We Here? (Cont'd.)

One thing that's been confusing me lately is whether I have a Mk. IV or the next model, the "Spitfire 1500," which was introduced in 1975. One notable difference is the engine displacement: the Mk. IV continued the 1296cc engine from the Mk. III, while the 1500 had, you guessed it, a 1493cc engine. (Close enough.) My 1974 Spit says "Spitfire IV" on the badges, so it's the 1296cc mill, right?

Well, not so fast. A lot of very experienced Spitfire owners on the forums I now frequent talk of their "1973 Spitfire 1500," for example. How could they have a 1973 1500 when it wasn't introduced until 1975? Something didn't add up.

Tonight, I found some clues while looking for carburetor resources. Spitfire & GT6 Magazine has a page devoted to clearing up model year information, and right there in black and white, it says:

There is some confusion about what is a Mk4 and what is a 1500. All 1971 & 1972 cars were MkIV's.US 1973 & 74 cars (FM commission numbers) received the 1500cc engine (to try to counteract the strangulating emissions regs in the US) while the rest of the world stayed Mk4's with 1296cc engine. These cars were essentially MkIV's with 1500 badges and engine.

So what *do* I have?

My car carries commission number FM14102U*. The "FM" indicates it was a US model; the "14102" places its date of manufacture squarely in 1974, which jives with the March 1974 date on the VIN plate; and the "U" at the end indicates that it was equipped with "Federal" (that is, US government and not the stricter Californian) emissions equipment.

* "Commission Numbers" were the equivalent of VIN's (Vehicle Identification Numbers) used on pre-1979 cars. There are reasons that a commission number is not a VIN, but they are only interesting to insurance companies and government DMV offices. For our purposes, it's the same thing.
By the way, paint code #82 is "Carmine Red" - a deep red that looks exactly like what my car wears. This answers a couple questions, as well, because the bonnet shows signs of having been painted that color atop an earlier yellow coat. Was the car originally yellow? No, the paint code indicates the bonnet is off a different car, probably due to front-end accident damage. But the good news is that I can easily buy #82 paint in spray cans and touch up any areas I need to, like the bulkhead I recently primed in flat black.

Anyway.... FM14102U means I have the 1500 engine, right? Well, not so fast. According to the excerpt above, it should be badged as a 1500 if it has the 1500 engine. I've seen pictures of a 1973 Spitfire 1500's badges. Mine doesn't have them - it's badged as a Mk. IV. And besides, it's anyone's guess whether this is the original engine.

The only real way to tell what I have is to look at the engine number. I poked my head under the hood and found this:

Deciphering FM56740UE

"FM" means the 1493cc "1500" engine, and, again, the "UE" designates a US-market engine with the Federal emissions controls. So I have a 1.5L car. Bigger is better, right? Maybe.... The two engines are substantially the same - the difference is primarily in the crankshaft, which is modified to increase piston travel ("stroke"), and built more heavily in the 1500. This increased travel means that the piston must move farther, and is subjected to higher G-forces when it reverses direction, at the same RPM, than in the 1296cc. Coupled with the higher mass of the 1500 crankshaft, higher G's multiplied by more mass means that the 1500 is under significantly more stress, and thus more prone to high-RPM failures. Track racers strongly prefer the oversquare 1296cc engine for this reason, because high-performance track driving is much more often done in the vicinity of max RPM. You can read a detailed discussion of these issues here on the Triumph Experience Spitfire forum

But street driving is typically done between 2000 and 4000 RPM (2000-3000 for cars driven conservatively). And the longer stroke provides increased low-RPM torque, which translates to quicker acceleration from stoplights and such. The Spitfire 1500 was actually faster from 0-60 than the earlier cars, even though it was less successful (and less durable) on the track. The slightly overstroked 1500 engine should be fine for a lot of what I intend to do with the car.

It may not be fine for all of what I want to do with the car, though. The 1500 produced around 50 horsepower, compared to over 70 for the smaller Mk. III engine. That difference is likely to be apparent on long hills at highway speed, or, well, everything else done at highway speed. The Spitfire 1500's top speed is around 85mph, which is about the average pace on I-25 between Fort Collins and Denver most weekends. Highway driving will probably be a terrifying ordeal, with my poor little engine spinning desperately fast to stay out from under the hungry SUV's all around it. Maybe I'll take the back roads....

High Compression Model

But wait! The "56740" portion of the engine number implies that the engine is not original: this reference page puts the engine in the 1976 model year. The 1976 Spitfires were unique in the USA because they had a higher-compression head installed, running at 9:1 instead of the 7.5:1 compression of earlier and later US-model Spitfire 1500's.

I've always thought higher compression ratios were better, but I've never really understood why. So I looked it up on Wikipedia:

A high compression ratio is desirable because it allows an engine to extract more mechanical energy from a given mass of air-fuel mixture due to its higher thermal efficiency. This occurs because internal combustion engines are heat engines, and higher efficiency is created because higher compression ratios permit the same combustion temperature to be reached with less fuel, while giving a longer expansion cycle, creating more mechanical power output and lowering the exhaust temperature.

Non-US 1500 models with the 9:1 compression ratio and twin SU carburetors produced almost 20 horsepower more than their emission-control-choked and compression-tamed US brethren, and were capable of 100 mph top speeds. Now there's a thought....

Normally, an engine swap could mean a lot of things. It might simply mean that a previous owner blew an engine and pulled any old replacement out of a junkyard to get his car back on the road. But the 1976 engine number makes that unlikely. The 76's were rare and valuable enough that you probably didn't accidentally end up with one. This was probably an engine swap done as an expensive performance upgrade.

Whadya know? Apparently my car used to be somebody's baby.

Some Uncertainty Remains

Of course, it's possible that my fancy high-compression engine suffered a head failure at some point, and had a lower-compression non-1976 head installed. There's no sign that this has happened, but it can't be ruled out. I'd like to find out if there's a way to determine, empirically, whether the engine is indeed the high-compression model.

Anyway, that's what I have to work with. I'm inclined to minimize alterations to the car until after I've had the chance to drive it a bit. If it turns out to be sluggish and I plan to keep it for the long haul, I might look into a twin carburetor conversion, exhaust headers, electric cooling fan & fuel pump, electronic ignition.... Whoops, I just daydreamed $2000 of improvements for an $825 car. Easy, there, tiger.

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