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porting precups

Matt Bachand said:
How much do you think is too much? How much more meat was in there for boring out?
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Matt, wouldnt know, there is still more material to be removed. I would like to do another set but bigger, to see if bigger is better. The best way would be to have a dyno set up and have a bunch of different variations
 
At what point do you think hogging out the cups will damage something? Is this an experimental engine? :eek:

I am familiar with terms like 'flame front' and fuel atomization, is this what you are effectively changing?

Interesting stuff.
 
THIS POST FROM MGW is really good about precups so thought I'd add a link here.

Because its more fitting than in the cam thread.

I am very interested in this, because I have an "S" engine, and they are supposed to have smaller precup openings and this next summer I was maybe going to be playing in there and change them.
 
Lesser diesel/air charge interaction. More delayed onset of combustion knock.

The downside to IDI cups is that the design has more surface area/overall cylinder volume. So proportionally more heat energy is lost out of the chamber via the greater surface area - also why IDI cooling systems have more heat to move/shed than DI engines.

The upside is that chamber promotes a lot of turbulence to better mix the diesel/air. So better combustion efficiency at the relatively low (by modern stds) injection pressures the cost-effective Stanadyne IP's of the time could reliably provide.
 
3500GMC said:
At what point do you think hogging out the cups will damage something? Is this an experimental engine? :eek:

I am familiar with terms like 'flame front' and fuel atomization, is this what you are effectively changing?

Interesting stuff.
Click to expand...
You would'nt damage anything, you would just get to a point were size of the port would lose its efficiency there fore lose power.

And no this is not an experimental engine, just an everyday truck
 
Definitally more info would be great, anything in the quest for more power! I wonder if running bigger precups on a high reving engine (like heaths 4500rpm racer) would make up for the loss in turbulance...

Justin
 
I think without "pre-cups" you would get more smoke and ineffecient combustion due to lack of "swirl/turbulence". Or poor emissions NOx 's and stuff that might not be visible expecially at light to moderate fuelrates.

The smaller the "pre-cup" hole the more mixing but higher heat lost to the heads through the "pre-cup's" material. "Pre-cup" port size should be somewhat inversely proportional to the amount of boost and "extra" power you want to build. Leave them smaller for better effeciency(mpg light load)/emissions but with less "overall/extra" power.

Might blow head gasket don't know but that would be for other reasons I would think. Direction of gas expansion at firedeck area????
 
I believe the reason Bison said it would blow the head gaskets is because about 1/2 of the precup overlaps the head gasket, which if not there would surely burn the gasket out.

One thing that I find interesting is that a smaller port would create a higher velocity of burning gases exiting the precup and forcing the piston down harder and faster. However, there is a point at which this would be bad, for example if you had a precup port the size of a hair it's going to throw out a hell of a lot of pressure out of it, but at a much lesser volume. I have been playing around with the shapes of precups a bit (more to come).
 
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If it were my truck I wouldn't go much bigger than the diamond precups. Those things were designed to send a specific patterned flame at the pistons swirl pockets. I know of one instance where a few pistons were put in the wrong way and burned up in no time flat. By making the precup to big it would surely effect how the combustion process works with the piston releafs and I am not so sure it would be a good thing. Plus if you remove to much material you are looking at having cracked precups and 1/4" crack is the max GM says is ok to have on a precup. Good luck with the project and I am looking forward to seeing the results.
 
The same pistons have 3 or 4 different precup opening sizes, and you have to believe GM was conservative with just about everything.

Are those swirl pockets or have something to do with valve position? If they have to do with valve position could destroy the piston real quick.

I guess Hank can get a good look at the pistons after trying different sizes.
 
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