ViribusUnits

I just saw a thing about the L88 corvettee.

According to the web site, the L88 had a 12.5:1 compression ratio. How did Chevy get away with that? I know that all things being equal, more compression = more power. (Assumeing the gasoline is up to the level) Of course increaseing compression requires a higher octane gasoline. That shouldn't be a problem for a 928 GTS.

The 83 to 84 Euro switched from a 10:1 ratio, to a 10.4:1 ratio, gained 10 hp, and 32 ft/lb tork.

The highest that the 928 GTS goes is 10.4:1. Would a higher ratio be possible? And if it is, would it help power? (me, with my 9.3:1 is kinda jealouse)

Incendier

The mechanic's rule - anything's possible with enough cash

Seriously, we're trying to squish the same amount of stuff into a smaller space. Domed, weged, etc. pistons, longer connecting rods and/or a longer throw crank, redesigned combustion chamber, etc., will all do it. Measure piston-to-valve clearance, block/skirt/web/pan interference from changes to the con rods and/or crank, make sure your head gasket won't grenade, work damn hard on your a/f ratios and...can you say "interference engine"?

Lotta work for 10+ hp, although the torque increase is always a joy. Would it make a difference? Sure.

Also, remember that turbo- or super-charging is really only compression increased from the other direction.

ViribusUnits

I know.

When I finaly get around to building my stroked and bored motor, I'm want to put a screw type blower on it, and run like a 8.0:1 or 7 something to 1 compression. That away I can run higher boost presures, and get more power out of it. Evencaly, but right now college has be broke.

What I'm asking is why didn't Porsche design the GTS with a higher compression ratio? For the 90s GTS, it's an interference motor anyways. They're regesigning it anyways. They're switching from a 10:1 compression ratio to a 10.4:1 compression ratio anyways. They're clearly doing the work anyways, why not take it to the fullest of it's potenital?

Was it a fuel problem? A emision's problem? Somthing else, or just because?

Incendier

Sorry if I went on about something you knew; your question wasn't clear.

My best guess that it comes back to the purpose of the 928 - GTing, not stoplight racing. With the last-generation GTS engine putting out well over 300 HP, why increase the operating stress, cost of parts, etc. for a nominal HP boost?

High compression would offer no significant advantage to luxury car whose job was to maintain moderately high speeds for extended periods.

My subjective opinion is that high compression engines can also have an "urgent" feeling to them, the direct opposite of what you want in a GT.

Quality gas/sufficient octane to control detonation is not available everywhere - there are many areas of my home state where 86 is all you can find for 150 miles - and evidence of Porsche's consideration of this is the inclusion of the "octane loop" to adapt early 928 management to handle lower quality gas. I'm sure this design consideration continued through later cars, and fuel/spark/knock management has improved considerably even since 1995.

Put it all together - engineering decision to cap the compression.

My guess.

ViribusUnits

OK.

I'm sorry about mot makeing my question clear. It wasn't exactly clear to me before I asked it.

OK. What defines a GTS?

To me Porsche is a sports car company. They build ONLY one type of car, fast ones. The 928 GTS should be Porsche's fastest car. To me, Porsche's best should be compareable to the best other fast cars out there. In other words, why not go all out and build your GTS to the limits of technology.

But guess they had their reasons.

Flormat

Yes all Porsches are fast. Speed was not the only consideration for the 928. It has already been said. Just because the 928 was the flagship Porsche does not mean it should automatically be the fastest. The 928 is a GT car. If you wanted ball busting performance there's the 911 turbo. Nimbe and forgiving- 944. The best grand tourer ever made- 928.

Incendier

I hope this is the question you're asking.

General useage: GT - Anglicized, Grand Touring. Originally from the Italian "Gran Turismo Omologato", or "Grand Touring Homologated" (racing approved), and later bastardized by Pontiac for the GTO, which was anything but.

Originally meant to define a car that could be toured to the track, raced, and driven back home.

Current usage is for any car that allows the driver to cruise long distances comfortably, quickly, and in sporting style. Liberally applied - witness the Toyota Celica GTS. Eech.

Tony

I dunno. Id be leary "Blow'N" a bored 6.5 motor. Wall thickness/integrety may become an issue as you add boost. Normally aspirated is fine..but some boost may change the longevity equation.
Id go 6.0 and a supercharger if you were going to do both. IMHO.

<img src="graemlins/beerchug.gif" border="0" alt="[cheers]" />

Rich9928p

Engines today can run with higher compression because the engine control has more power and can do a better job of preventing spark knock. At the time, the EZK ignition was pretty state-of-the-art, but compared to today's technology it isn't. Today, each cylinder has its own coil and spark timing is calculated individually. This is not a possibility for the 928 because it uses a rotating spark distributor. The 928 uses common gang fuel injection, where multiple injectors open at the same time. Today's fuel injection is sequential, thus each cylinder may get a specific measurement of fuel.

Today, high-end engines such as Porsche had variable valve timing and variable valve opening. The 928 never had this. The Porsche 968 did have variable valve timing and generated 230 HP from 3.0 liters displacement in a four cylinder configuration (1/2 the 928 motor). It is probable that "IF" the 928 would have lived longer it would have also had variable valve timing and thus HP would have grown, but we know that didn't happen.

Modern engines use new technology such as "EGAS" (fly-by-wire throttle connection), which means that the engine control can do what ever it desires as far as opening the throttle vs. direct control by the driver. Thus, even if a driver stamps the pedal to the floor, the control can retard spark and even only allow as much power to be made up to the spark knock level.

So, the 928 technology is frozen in time. As it neared the end of production it is likely that volumes were so low that it didn't make sense to make any changes to the engine ... even if that made it lower HP than the most modern configurations. We have what we have, the configuration is pretty competitive with most cars of today. But sorry, time and technology march on and we're left behind.

Look at Porsche's GT2, 456 hp. is made from only 3.6-liters displacement. This moves the 3175 lb. beast from 0-60 mph in 3.6 sec., 0-100 kph in 4.2 sec. Quarter mile times are 11.9 sec at 120.6 mph.

ViribusUnits

Thanks.

It seemed kinda crazy to me that a 60s Corvette would run such a high compression, while our cars would be reletivly low.

I guess I'm kinda stuck in the past of my understanding of engines. I'll have to work on it.

I feel better now though. I compared my car to the sports cars of the early 80s. Mine beats a 83 Mustang, or a Comaro, and is about equal, with maybe a slight advantage on a 82/84 Corvette. All with a smaller motor. I'm not realy sure what else existed at the time, but at least it makes me feel better that I don't have a 5.0l engine like the 85+

To be honest, I've yet to even seriously think about improveing the Lawn Ornamate's 4.7l engine. We're trying to keep her on the road right now.

Has anyone played around with bringing a 928 up to modern refinements? Say a 6.0l blowen engine, with all the refinements of a 968 engine? I've heard of a project to put a varo cam on a 928, and Devek's stoker and bored abilitys are kinda famous. F.A.S.T. seems to do good work with turbos, and there are pictures of even roots blowers attacked to the engine. Has anyone played with all at once?