Chunkerz

This is a n00b question, but when people want to increase the boost in their 951, why do they need a special headgasket and an oringed head and ultra super low compression pistons and all that stuff? I understand the differences in the combustion process and the need for the special stand alone engine management and all that stuff to prevent knocking, but why do they need the other modifications? In my VW diesel I've been running ~24psi for 3 years with the stock headgasket, rods, and 19.5:1 compression pistons and I haven't blown the engine yet. I've seen pictures of the headgasket for my VW and it doesn't look like it's anything special.

bordin34

The basic principle behind how a gas and diesel engine fire are very different.
With gas you want to compress the gasses and ignite it via the spark plug before it has detonation or knocks. Knocking and detonation are bad, very bad. When you force more air and fuel into the engine it is more likely to knock, thus the lower compression pistons.
Diesel engines are designed to run off a detonation in simple terms. There is no spark plug or anything like that, discounting glow plugs when cold.
Normally a diesel engine will be made much stronger in every aspect than a gas engine because it has to control a lot more force.

86 951 Driver

If anyone could come up with a true answer this issue would be solved.

Allegedly the issue stems from cylinder movement in the 951. Since it is an open deck the cylinders move around under really high boost and power levels. People like Blown 944 and refresh951 have filled their blocks half way with cement. This helps to stabilize the cylinders. They are pushing some pretty power and have proven that this can fix one of the big downfalls of the 951. The other downfall is the #2 bearing.

doc_

Cement...wait, what?! Thread?!

Gawernator

Same reaction. Cement wtf?

Darwantae951

lol

pa944red

Do you ever get the feeling that there truly is very little new on this forum, just new people asking the same questions?

I guess that is what happens when you have been around since 1998.

Darwantae951

That's always been an issue, but I seriously think this is the first time I've seen someone bring up diesels in comparison to our cars.

Jamesr6967

An epoxy poured around the base of the cylinders to stabilize them. Look for the build thread that is labelled blown944 (iirc).

morghen

Who would say something like that? cement is brittle and weak...you cant have it handle non stop vibrations and/or heat exchanging from the engine cly as it will quickly develop wear areas and its effect of stabilizing will last say...5 minutes to 5 days ?
The answer for cyl movement is a metal fixture to fix the deck on top, near the head gasket.

The main difference between the diesel engines and the gasoline engines is the diesel compresses the mixture quite some more. Therefore alot more pressure and the need for more metal or stronger metal to hold it together.

Thats why the 2.0L 924 engine is much stronger than the 944 engine.
The 924 engine block was designed by Daimler to be a turbo diesel earth mover engine and the block is extremely strong.

Mark924S

Morghen is correct, Diesel fuel uses compression to combust, you only "need" glow plugs for start up after that it is all compression. Most diesels are all Iron to with stand the increased stress.However I see where Chunkerz is making a comparison, not just diesels but alot of the newer imports and turbo VWs when they dial up the boost run any where from 15-25 pounds depending on the application. We have to remember the newest 951 is over 20 years old, and they all have technology developed in the early 80s late 70s. My 951 doesnt produce nearly enough power to worry about the cylnder walls wiggling, but it is one of the many flaws you encounter on the quest for ultimate power, the heads dont flow very well, the stock ignition isnt all that grand. But that doesn't make these cars inferior, they just havent had the same R&D that other marques have enjoyed from the after market.
A 951 essentially needs to be modernized when it is brought to higher boost levels, through extensive engine work and computer tuning or standalone management a well sorted 951 feels as good if not much better than most modern cars. but thats my .02

User 52121

Combustion chamber design, new electronics, better materials, direct injection, better packaging... it all comes into play.

Diesels inject fuel directly into the cylinders, and the fuel injection is how they control combustion timing (since as mentioned, there are no spark plugs). The fuel is injected at MUCH higher pressures than gasoline (20,000+ psi) so that the combustion event is usually triggered right as it's injected. And if you look at a diesel piston, it actually has a funny looking bowl machined into it, to help disperse the fuel as it's added in.

A Ford Powerstroke piston:


And also as said, because of the much higher combustion pressures in diesels, everything is usually cast iron. Big beefy stuff.

16valver

Another thing to consider is just how different gasoline and diesel fuel are. I believe diesel is a much more refined product of petroleum. Its easier to ignite.

As stated above, diesel engines use compression to ignite the fuel. Glow plugs don't even ignite the fuel, they just warm everything up so compression will actually ignite the fuel.

Its funny, when you blow a turbo on a diesel engine, sometimes all the oil going in the engine from the blown turbo will cause the engine to keep running. Its rather funny. Lots of flames

NineFiveWon

[quote=OmniGLH;10425806]

Diesels inject fuel directly into the cylinders, and the fuel injection is how they control combustion timing (since as mentioned, there are no spark plugs). The fuel is injected at MUCH higher pressures than gasoline (20,000+ psi) so that the combustion event is usually triggered right as it's injected. And if you look at a diesel piston, it actually has a funny looking bowl machined into it, to help disperse the fuel as it's added in.

A Ford Powerstroke piston:

QUOTE]

You are referring to the later diesels with a revision of HEUI, or some form of "unit injection". Injection timing is important, diagnosing a diesel you will notice white/black smoke depending the on timing. (I.e. white smoke is too retarded and unburned fuel).
Those pistons aren't that wierd, you should see some of the new DI VW ones! Just to clarify, the "bowl" in the diesel pistons is actually the combustion chamber.

[QUOTE=16valver;10425866]Another thing to consider is just how different gasoline and diesel fuel are. I believe diesel is a much more refined product of petroleum. Its easier to ignite.
QUOTE]
Diesel is harder to ignite, but its' energy burns longer. Helping utilize the effiency of the longer stroke. Diesel's use manifold heating elements and glow plugs to help them start. And trust me, they're a bitch to start without em!

Didn't konw that, very very cool. Good info.

Anyways, why are we having a sophisticated diesel talk? Cement filled Porsches are much more entertaining!!

16valver

Whoops, yeah your right. Not as easily ignited, but burns longer. Didn't some of the early IDI ford motors not have glow plugs?

I agree, this talk about cement filled engine blocks sounds interesting.