21.5 compression

[Marty Lau]

Okay so it seems as the ticket for performance is 18to 1 compression. These engines were designed with 21.5 to 1 compression there must have been some reason this was done. What are the advantage of 21.5 to 1 compression vs lower compression? I'm just currious, yeah and I know what happened to the cat! smile.gif

[slagona]

A big advantage of the high compression diesels is easy cold weather starting. I can start mine at 0 *F without plugging it in - no problem. Yes, it's not as easy on the engine as it would be if it were plugged in - but it does start quite reliably and "easily".

My 18:1 compression engine (when new) got finicky at temps under 40*F and almost HAD to be plugged in at anything under 30*F. This was also 7 years ago - our glow plug systems are a little more advanced now. Remember that these engines were designed in the early 90's - before the unlimited time glow plugs were readily available. The 9G's prohibited an extended glow time and make starting a little rough on those cold northern winter mornings. Even when plugged in it would take a few minutes to smooth out....

Short answer is that GM was going for easier starts and the general sense of a better running diesel as far as the majority of the public is concerned. I'm sure there were many other factors and compromising that led to the final decision for the compression ratio with the politics of warranty repairs and general public playing a major hand....

[eracers999]

What is the part no of the (unlimited time) glow plug ? Thanks
Kent

[cudaaa]

Kennedy sells a good set of plugs.

[tom.mcinerney]

Kent--the newer glows deliver more heat. actually i think slightly less heat for a much greater duration.

16Ga~-(Scott pretty well covered it) I think there's a very slight increase in efficiency with higher comp...marginally better mileage. But also increased wear/stress.

[G. Gearloose]

Originally posted by slagona:
Remember that these engines were designed in the early 90's I take exception to that

[slagona]

Well, since there are no smileys in that exception, I'll clarify my intended meaning......

The complete engine system (Turbo, glow plugs, larger displacement, .....) was introduced in the early 90's.

Point being with the glow cycle time available with the (at that time) stock glow plugs, the hard cold starts would not have been well received by the general public. I'm sure there are many other factors - such as no IC, overheating problems, ...... which played a role in that decision.

[markrinker]

Question: If the turbo fails on an 18:1 compression motor, will it even continue to run? One advantage to higher compression / lower boost is that they are not as dependant on turbo operation.

However, if engine wouldn't even run, more 6.5s wouldn't be on the road, operating with failed boost controls, belching black smoke (extra pollution) and running high EGTs that shorten engine life!

[CleviteKid]

Our engines will run just fine NA at 18:1, if you can get them started. And remember, when you ARE starting it, the turbo is doing NOTHING, so you are all starting de-facto NA engines.

The thermodynamically ideal compression ratio for a diesel is about 13:1, and that is close to what the large engines (think 17" bore, 21" stroke) run in 4-stroke cycle versions.

Dr. Lee :cool:

[Jim P]

My 18:1 engine has no problem starting even at -5 degrees F. as long as I use my manual glow plug switch to glow another 10 to 15 seconds after the light goes out. I don't think it turns over a whole revolution before it starts.

The one thing that I don't like is that it smokes real bad for 3 to 4 minutes after I start it when it is real cold. It also misses a little when you try to rev it up when cold but once it is warmed up, it runs like a raped ape. Actually I believe my engine is a little lower than 18:1.

[markrinker]

I should have dropped in 18:1 slugs when I did the motor swap.