Cylinder head Questions

[anders]

And another picture..

[anders]

Ok, another question.. Tom Bryant has said if you want to build a "performance" D24T you should use a early 11mm D24 head because of the larger volume in the Pre-chamber. Does this mean the 11mm D24 heads have an even larger chamber then the 12mm D24 heads?

[745 TurboGreasel]

Is the chamber volume different, or the opening size?

[anders]

There's a noticeable volume difference between the D24T chamber and the 12mm D24 chamber. The outlet looks to be the same. I have not had the chance to compare a pre-chamber from a 11mm D24 head. When I do I will compare all three.

[v8volvo]

Interesting info. As far as I know the prechambers only come in those two sizes, 30mm and 32mm, but I didn't realize that all the NA heads had the larger chambers. I agree it is counter-intuitive that the NA head should have the larger chambers, since with forced induction the turbo motors should be able to more than compensate for the extra chamber volume. I would agree that having the larger chambers would theoretically be beneficial for getting max power out of the motor, especially for running increased boost.....

One other interesting point, I am in the process of sourcing a new head for a guy's D24T that I am working on now, and looks like we will be getting one through AMC Spain. All the heads they offer for this application list a 32mm prechamber size, including the D24 version and the D24T version. Rumor has it that in Europe both D24 and D24T engines always used the 32mm prechamber -- the 30mm chamber was only used in US export models, and apparently only US D24T engines! I believe the idea of the smaller volume prechamber was to sacrifice some HP in exchange for a slightly higher final compression ratio to improve extreme cold / high altitude starting, which they must have thought was especially important for the US market. Still doesn't answer why only the D24T got them though.

Maybe they thought the D24 motors couldn't afford to sacrifice any power so didn't want to put the smaller prechambers in there, or (more likely I think) they figured that the 240 series production would be ending soon and didn't want to invest in making changes to the D24 head for a model that they planned to phase out anyway. When the 740 series was introduced in '85 that was supposed to be the point when the 240 series would begin to wind down. I don't think they had anticipated it still being in such high demand that they would have to keep making it for another 8 years....!

[745 TurboGreasel]

If I had to guess, I'd say the smaller chamber helps complete burn on time in spite of US spec retarded injection timing.

[v8volvo]

Yeah, but the 10psi of boost would be more than sufficient to achieve that same effect, unless you are only talking about cold starting. In any case from an emissions standpoint higher compression and less timing advance should create similar exhaust gas composition to a larger chamber and more advance..... if anything I'd argue the opposite and say that the more retarded timing spec is a *result* of the smaller prechambers, not the *reason* for it..... but we're guessing here.

[745 TurboGreasel]

Definitely guessing.
It's also possible the concern was more about off boost emissions, after all, we got an EGR system that mostly works at idle.

[anders]

Okay, I checked a known 11mm head and it had the 32mm chambers. So I checked a known 12mm NA head and it has the 30mm chambers. So I measured the head bolt holes of the other three NA heads I have and there 11mm also! Funny how one those 11mm heads was bolted to a 12mm d24 that I just pulled apart.

So how are you going about getting one of those AMC heads? Did you find a importer or you buying from them direct? I might be interested in buying a head also, help save on shipping maybe?

[ngoma]

Current discussion at vwdiesel.net might be interesting to you:

Topic: More About Prechamber Inserts
http://www.vwdiesel.net/forum/index.php?topic=33184.0