Alright so I'm building a 7.3 turbo IDI and I have 3 different model cups I could use.

7.3 left, early 6.9 middle and late 6.9 right. The Late 6.9 cup seems to have the smallest throat and highest compression. The early 6.9 cup seems to have the largest throat and lowest compression. And the 7.3 cup seems to be in between the early and late 6.9 cup.



Which one will make my engine run the smoothest and quietest and give great power.

 

Generally speaking, the lower the compression the more boost you can run safely, but the harder it is to start.

 

If i get that far, i will most likely use early 6.9 cups. However the early 6.9 in our service truck, is the loudest IDI ive ever heard. Thing RUNS though

 

Noise is usually advanced timing. I want to use the cup that will make a smooth-quiet running motor. Not sure if I should use late, early or 7.3 cups.

the early 6.9 cups have the lowest compression and largest throat but late 6.9 cups have smallest throat but highest compression..

I know 6.2 guys run the 1990s Diamond precups in their 1986 6.2s and they claim a larger throat makes more noise. I want a smooth running engine, not a loud one. But also good power and drivability.

 

.. sounds like you need a 460


I know advanced timing causes an louder engine, i assumed that to be a given. Our 83 6.9 is timed correctly, also, if it were in a comparable pickup im not sure my ATS turbo 7.3 could beat it in a race. Moves that 17k service truck around pretty good and its geared pretty tall.

Edit: even your assertion about the turds with larger ports running louder backs up my point.

 

I could use a 6.7 Powerstroke LOL! tons of power and quiet.

The early 6.9 throat isn't much bigger than the 7.3 throat.. Don't see how it could make it louder..

How loud is your '83 6.9 compared to a 7.3?

I guess I'll use the small throat 6.9!

 

It is very loud, id say right up there with a cummins. Being familiar with IDIs i insisted something was wrong with it. We checked everything out and its A-O-Good, just loud. Really bizzare because i had assumed that if anything it would be quieter with the lower compression. My theory in this is that the larger throat gets the fuel charge into the cylinder faster, which means the fuel hits the piston faster and a more "violent" combustion. Whereas the other cups create a slow burn effect with a softer ignition and longer burn. I think a large cup AND large throat would be a win win for a turbod idi with more fuel. However, this is a topic that it very much up for debate, and given that i in fact do not have an engineering degree, my advice is little better than opinion, if at all.

 

The only problem I could see running the early 6.9 cup is the side walls are thin. And the notch that the cup sits on in the 7.3 heads are wider than the notch in the early 6.9 heads. Meaning some air would get caught under the notch during the compression stroke not swirl correctly. Not sure if it would hurt anything or not.

I was thinking the lower compression would mean lower noise..

If I did chose to run the early 6.9 cups would it hurt my engine at all? I mean it seats fine and is flush with the head surface.

Look at this pic I made in paint and tell me what you think.

 

Strange, my '84 was the quietest IDI I've owned.

 

I suspect that your muffler(yeah, you know, the one everyone takes off) has more to do with noise than any pre-cup.
Timing has a lot to do with it, too.

 

Mac, im pretty sure were all talking about engine noise, aka diesel knock... not the exhaust.

 

Lots of theory.. A bigger precup mouth would mean faster pre-chamber to main-chamber explosion.. While a smaller precup mouth would mean a slower precup to main chamber explosion..

Well I'm about to send my heads off to the shop.. Should I use the small throat or large throat 6.9 cups for smoothness and quietness?

 

Guardian, where did you find the 6.9 pre cups? If OEM can you post back some part numbers, or post a phone number of the dealer/vendor you bought them from?

 

Just disassembled some heads today and all pre-cups are cracked; some are beyond the fire ring, others are very close. Anyone have part numbers? These are from a '94 IDI-T, and have the "88" cast into them.

 

Hopefully someone has the part number for those pre-cups. Even if you can get the part number there might not be any available. Here's a good search engine to find IDI trucks in a salvage yard near you. Good luck in your search.

http://www.car-part.com/