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Pre-cup Questions

6.5L

6.5L

Old Iron Runner
Messages
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Location
Northwest Wyoming
Hey. So I have a 6.5L with square stamp pre-cups in them. I may have a chance in the future to get a set of Diamonds that have been ported out a little bit. What are the advantages of having larger pre-cups. I am looking for some pro's vs cons here. I have searched around and got a lot of different information. I am looking for just a simple straight forward answer. Any advice is welcome. I just want to know what I may get from swapping pre-cups. Has anybody done a square to diamond change? Is there a noticable difference?
 
Ok, so I read through there, and a lot of great info, but just need some clarification of things. Would the diamond pre-cups run cooler than my t-stamped ones? I am going for power here, and I do have very easy access to a dyno, so I plan on doing a before and after dyno result. Well I got my before results I guess. I plan on adding slightly more fuel in the future. Maybe another 1/8th turn to maybe try to squeeze some more power from the air from the ATT when I install it. Would the larger pre-cups help burn some more of that fuel and in turn increase any spooling?
 
I just answered this question. You hit a wall where fuel and timing no longer matter as you have run out of precup flow. Bigger can hurt economy, but, smaller and you hit a limit where you 'run out of air.' Simply can't move enough air in the time allowed after you hit the limit of a smaller cup. Huge custom precups over what is made in a production run have other interesting side effects.

Precup engines are for economy and low pressure injection and the starting problems that come with low pressure injection. The small precups mix the air better and get great MPG. Going for power and RPM - air will only go through a specific size hole so fast.

http://www.thetruckstop.us/forum/showthread.php?32683-Payson-AZ-Turbo-pull-off-*Round-1*
 
How much would just regular diamond pre-cups help as far as power goes? How much would diamond or even ported diamond hurt mileage? Maybe a stupid question but wouldn't diamonds help mileage a little since you are making a little more power off of the same fuel input? Sorry, I am not smart about this kind of stuff. Aren't the Square stamped pre-cups meant for NA motors? If I replace them with diamond pre-cups, which were meant for turbocharged engines I believe, help power and mileage since they were designed for use with a turbocharger?
 
In very general terms:

The "smaller" precombustion chambers (square) impart greater velocities/turbulence in the chamber which enhances mixing. This leads to a better burn and good economy. It's about exposing as much of the fuel to O2 as possible. This is accomplished by the high velocities breaking the fuel into smaller droplets which increases the surface area of the fuel exposed to the O2. One would logically think that this would lead to more "power" but the down side is the smaller mouth limits exit "pressures" to act on the piston. You can only cram so much mass/energy through a soda straw sort of idea. So the small mouth becomes a restriction on the power stroke.

The large chambers (diamond) allow more exit at the cost of mixing. Result is often more "power", less economy due to less efficient mixing..

The chambers are also, as WW mentions, necessary due to the injection scheme (low pressure, coarse spray pattern).


How much difference between them?

Too many dependent factors to give you an honest answer there....a guess would be a couple MPG either way.
 
I know exactly about the 6.2 wall,but somehow I got past it with extreme fuel and boost. It probably had enough fuel to cause a good sized explosion in the cyl. I also wonder if the compression is low enough,precup size would be less important ? I also thought that with very high compression,and high boost,there could be a pretty serious pressure spike,when all that air tries to get in the prechamber hole.
 
Well since I am going to be running an ATT, which in theory should increase mileage, then replacing my t-stamps with diamonds pre-cups, which should lower mileage but increase power, would I really be sacraficing mileage for more power? Just a question
 
What is the goal of the engine, is it power and then MPG or MPG then power?
You already have a big battle finished with the bigger turbo for MPG. GM was aiming for power with larger precups.
I am running 1/2 a turn on the 6.5 NA precups and it can use more fuel in the top end. You would also need to play with timing and add some radical advance to the DB2.

You will not be running in boost all the time so smaller precups have the MPG advantage. Unless you are heavily loaded or in WYO on the hills there all the time then in boost the larger does you better. So it can be specific to what you are doing. This is why GM had so many of them including really small econo cups for high MPG engines. A dyno is not going to tell you what driving will be like with both. Peak power is interesting but MPG and intended use are a better measure for you. By all means post the results though.

Cold fender air vs. the open filter in your pic can affect MPG. Colder air is better for both power and MPG.
 
This is my daily driver. I am going more for power though. I was thinking since I got the larger turbo which will help my mileage definately over that GM-3, that even if the diamond pre-cups decrease mileage, as long as I drive her nice and easy, and stay out of the throttle, I shouldn't really decrease mileage more would I? Even in town I shift low, and hardly go beyond an 1/8th into the pedal. just enough so she is slightly above idling around. and I maintain about 14-15mpg in town doing so. Highway I just set the cruise at 65mph as is, and it will get anywhere from 19-22mpg highway, depending on the wind and hills and such. And that is with a GM-3. I think it would be better with the ATT. Also WarWagon, that is a bit of fuel to be running with the 6.5NA cups, which is what I believe I have right now, are you saying that it is still managing to get all that air in from the ATT? How much boost are you running?
 
I don't have the time to answer in depth but. Here is the long and short. Another thread will explain what I have learned. Larger pre cups sacrifice efficiency at lower rpm but give you a better burn at higher rpm. SO that being said larger pre cups also give you more low end grunt depending on how large you go. But and here is the big but. BIG gives you great idle great off idle but dies for pulling at 1200 to 1700 with our cam shaft. Fuel economy should not suffer much with out going way to big.

A custom set is nice and 70 thousands larger than a diamond may not make to big a difference .

Smaller pre cups with lots of fuel make high egt after a certain point War Wagon confirmed this. LARGE pre cups make higher egt all the time but more under load, but have inherently better combustion times at higher rpm.

Almost a year and a half running a REAL large set of pre cups. All the data to know why and what might work. There is more but time is not on my side this morning.
 
I tried porting one precup, but ruined several grinding stones. A carbide die grinder bit worked,but barely. I believe the best way would be edm. The cups I have are o stamped,and fairly small.
 
Will get something to look at later, You HAVE to make sure they are equal, just porting with a hand grinder is asking for problems, power pulses can be greatly different if the openings are not real real close. Ask me how I know. :)
 
So with what Slim Shady said, I might almost be better off if stay with my square stamps. WarWagon said he is able to run about 15-17psi through an ATT with the NA 6.5L cups with 1/2 turn on the pump and is still a bit undefueled. So what you guys are saying is that I shouldn't need to upgrade to larger cups unless I am doing some serious performance work. I wouldn't mind a set but I do not want to sacrafice power up high with ported cups. If I am going to redline, I want there to be power from idle to redline. I really am not to thrilled about the idea of it dying off after 2000rpm haha
 
You would benefit from the larger pre cups with the setup you have especially in the higher rpm range 2000 to above 3500 rpm.

You will not lose anything that you will even notice at lower end with a precup slightly larger than a diamond. You will love how smooth a 6.5 can be at 2000 to 4000 rpm. The 6.5 will run as smooth as a Cadillac at 2800 to 3000 rpm on the highway and do it all day long. You do get some small increased fuel mileage at higher rpm by having a larger pre cup. At least that has been my experience.
 
I thought it was kind of clear I meant under fueled haha and I never usualy have my engine above 2400rpm(65mph) unless I am trying to "play" with other vehicles on the road and I don't usualy tow super heavy. The one thing that worries me is I kind of hear that EGT's will rise with the larger pre-cups. With my current setup I have never exceeded 1100* even with very hard throttle uphills. I know this is till very safe which allows me to kind of keep my eyes off my gauges and drive the truck however I want. I really don't want to have to drive the truck by EGT's when I do tow or want to have fun just for the sake of more power. Any thoughts? I know the later model 6.5L made a bit more horsepower and slightly less compression. Was this acheived through going to the larger diamond precup?
 
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