Its all about fuel... Peak cylinder pressure is determined by the amount of fuel and air in the cylinder, respective to timing...

As of right now, I see no reason to drop below 20:1 on an IDI... But well see how the HG's like 400whp on stock compression here in a couple weeks, I think the key is keeping timing reasonable, which is key if you want any sort of drivability/torque anyway....

 

the compression issue was pretty well covered over on PSN on NMB2 build. get a cam that reduce compression for boost only

 

I just shaved the pistions in mine and if it is below 30f out side it needs to be pluged in to start it with the glow plugs working.
You can't compare an idi to a psd. The psd combution camber is way more efficent than the idi.
I have not had an idi yet that would run on ether alone. A psd will run on ether alone
A psd without glow plugs with over 100,000 mile are tuff to start when it is under 40.

 

I might be wrong, but I don't think you can "reduce compression ratio" using a cam change, that would only limit the amount of air going into the cylinder, not limit the ratio.

 

You reduce effective compression ratio, the static CR stays the same. But in reality all the engine sees is the less compression, aka, lower compression.

As far as cold starts, derricks 7.3 had 80thou milled and after an 8 second glow fires right up down to freezing. Below freezing its takes a little cranking 3-5 seconds. The 97 ranch truck -psd- takes about 10 seconds of cranking when its 50-60*, double that when its cold, granted a few GPs are out.

As for the more efficient CC... not buying it. only thing a PSD has over us as far as cold starts are concerned is higher injection pressure, and the fact that the injector does the atomizing instead of the precup.

They cant be more efficient if we make more power per cc/fuel.

 

I'm not following... If you limit the intake stroke, the same thing could be accomplished using a butterfly valve on the intake, reducing air intake... but that doesn't make any sense when you want to get more air in for power?

There is no way of changing the compression ratio short of changing the bore or stroke, limiting the amount of air entering the cylinder is simply going to limit the air IN the cylinder, which is the opposite of what you want for making power.

Are we actually talking about compressing air (turbocharger) , and then restricting that air from entering the cylinders?

I'm not getting it...

 

So you don't think that the lower compression will be a problem with the cold starting?

 

I shaved .050 off my pistons and my truck does not start very good at all.

 

Have you ever considered other problems? Mine has those same symptoms, at stock compression with 229000

 

As far as the cam thing, valve overlap is how its done, and duration. You can buy "compression building" cams and cams that bleed compression. Basically the stock cam in our engine is a compression building cam. Has zero overlap and short duration (as i understand it). The overlap is OK for NA but hurts turbo engines. By having some overlap, you drop static compression / cylinder pressure, but the air in the cylinder is cooler and fresher (the overlap alows intake air to blow through "cleaning out" burnt / hot exhaust" thats the theory, or at least how i understand it.)

 

Mike, Actually, the big thing to bleeding off compression is the intake valve being open longer. Overlap has very little to do with "bleeding" compression, because 9 times out of 10 (thats conservative) exhaust pressure is going to be more than intake pressure, and that works the opposite way of bleeding compression off... A cam with very little overlap and an intake event with decent duration and a late ICL is going to bleed compression off. Ill compare the Stock cam, The R&D, and The J2 below (Id toss the typ4 in there as well, but russ hasent given me the specs yet):

The stock cam has 37* of total overlap (-27*@.050"), and the intake valve stays open for 42* after bottom dead center. It effectively stays open for 7* after BDC at .050", which is a good lift for dynamic measurement.

The R&D cam is 56* of total overlap (-24*@.050"), and the intake valve stays open for 52* after bottom dead center. It effectively stays open for 21* after BDC at .050"...

The J2 cam is 56* of total overlap (-12*@.050"), and the intake valve stays open for 74* after bottom dead center. It effectively stays open for 32* after BDC at .050"...

Putting these together... The further the intake stays open after BDC, the more compression is bled off. Reversion happens when there is too much overlap at too little flow (read:RPM or boost)... The stock cam doesnt have much total overlap, but has as much dynamic overlap as the R&D (which is negative anyway, meaning both valves overlap for less than 0* of duration when the valves are open to .050" of lift...)... The J2 cam has about as much total overlap as the R&D, but has more dynamic overlap at .050", meaning more chance for reversion at lower RPM's, but when higher RPM's and boost are achieved, there is more cylinder filling, and therefore more power.... This just covers the intake side though, there is more as far as the exhaust sides are concerned...

As far as the TYP4 cam is concerned, the best I can do with the knowledge I have of it, is that it increases lower end cylinder filling by increasing duration on the intake side, but maintaining stockish lifts and timing events as much as possible... This is why it works great in low boost and N/A situations, there is no exhaust pressure inhibiting cylinder filling, and the extra filling it gets from the intake event makes a little more torque lower in the RPM range.

Basically it coems down to where you want your cam to perform... The stocker is a good all around cam for everyhing, the TYP4 is better than the stocker at low RPM, the R&D is great for engines that spend their time boosting 10-20psi while doing work, and the J2 is great for WOT/Higher RPM/higher boost situations while doing work... They all have their place...

 

i only thing i would shave off the the pistons would be for balancing only. due to the heat these things make i want all the meat i can have. yum

when a truck is hard to start gets annoying fast, and once you change something you cant get to easily your kinda screwed.

what they used to use on the old cummins in the old semi truck is have a compresion start. this held the valves closed, and you crank and cranked ont the motor until you built enough heat, and then you released the compressor to open the valves and let the fuel in and boom. we dont have but crappy glow plugs, so i would keep these top notch, or carry a can of ether.

 

Deere used to do the exact opposite, they would spin the engine over while have the decompression lever held open, and they started very, very well. As far as shaving the pistons, they will be so I can push in as much boost as I can. I think I will push it up to around 17:1.

 

Yeep... thought i was backwards lol

 

I think thats probably what he meant. Take the compression away so the starter can get going and build some heat in the engine. Electronic pony motor is all a start was in those days... and werent that great, but got the job done.