What pistons/compresson rate to use..

[helgaairhead]

Some friends of mine are re-building an 6.5 Chevrolet - and they told me that the company selling them the marine conversion kit demand that they should use 18:1 pistons due to high boost.
I thought that IDI's needed like 21:1 in order to have an easy start and if we lowered the compression to much like 18:1 we would have an hard time getting these motor started.
When having more boost, yes I I'm familiar with studs, better head gasket and lowered compression rate, high compression will be an disadvantage.

Diesel Direct have Ceramic coated Pistons 18:1 for sale, advertised as "Marine Pistons". (http://www.diesel-direct.com/)
Are there any reason why they need such low compression when being sold as Marine parts?.
What is the lowest compression rate and still having "normal" starting procedure?.

This discussion are going high here in Norway . Please give me some good answers.

Helge

 

[FordGuy100]

Well my guess would be they need the lower compression pistons because they have a lot of boost because of alot of fuel. Marine diesel motors tend to be rated at more HP because they put more fuel into the motors, because they have an endless supply of water to keep the engine from overheating (unlike in a truck where you only have a couple of gallons of water that circulates to keep it cool). So anyways Marine motors tend to have the pumps turned way up, so they are putting in a bunch of fuel. And since boat operators/owners want to save money, they dont want to be throwing out a bunch of black smoke when they increase the rpms of the engine. So they would lower the compression ratio so that they could stuff a lot more boost than stock to burn more fuel than if they were running stock amounts of boost (save money because your getting a more complete burn, saving you on fuel). That would be my guess.

 

[helgaairhead]

Well my guess would be they need the lower compression pistons because they have a lot of boost because of alot of fuel. Marine diesel motors tend to be rated at more HP because they put more fuel into the motors, because they have an endless supply of water to keep the engine from overheating (unlike in a truck where you only have a couple of gallons of water that circulates to keep it cool). So anyways Marine motors tend to have the pumps turned way up, so they are putting in a bunch of fuel. And since boat operators/owners want to save money, they dont want to be throwing out a bunch of black smoke when they increase the rpms of the engine. So they would lower the compression ratio so that they could stuff a lot more boost than stock to burn more fuel than if they were running stock amounts of boost (save money because your getting a more complete burn, saving you on fuel). That would be my guess.

Maybe I put the question out wrong, the trouble is :
Due to low compression (21:1-18:1) shouldn't these guys have trouble starting an IDI motor, which demand on higher compression rate than DI motors do?.
Everyone keep telling me that IDI's with low compression need Etan fluid, just to get it started. When running there are no problem.
So, another question, what is the lowest compression rate on an IDI - Without any starting problem?.
20:1 ???

Anyone here with some experience?

Helge

 

[Diesel JD]

There is a place called peninsula diesel in Michigan that specializes in marine built 6.5s. Maybe you should dig them up and see what they have to say about starting problems. I know Ken had a low compression IDI on the DPS site which was indeed hard to start even at 50*F. The honest truth is I really don't know how low you can go without having to use ether all the time. I think if you have everything working and can plug the engine in you might have pretty good luck with at least moderately lowered compression. How low you can get away with is the 64 cent question around here.

 

[dieseldummy]

I run 17.4:1 and it still starts. (on 1 battery) It takes alot of glow plug time, but it's never let me down yet. Maybe someday I'll get an intake heater on to help out and a second battery, but until then I just try to plug it in when it's cold out. My rule of thumb is no build should be over 20:1 and if your looking at heavy use/more power no more than 18:1.

 

[Diesel JD]

17.4:1 are you kidding? I guess if I build a 6.9 at 19.5:1 just to make sure I can safely run a DPS or Moose Pump and boost at 13-15psi(no insane power grab) then it should start normally here in sunny Florida right?

 

[dieseldummy]

17.4:1 for real here... Started at 15 degrees this afternoon after setting for 1.5 weeks. A little more smoke than normal, but it started none the less. 19.5:1 is a good compromise between longevity and startability.

 

[Diesel JD]

So how do you lower the compression that much safely? Custom pistons? Shave the pistons? Shorten the rods? All of the above?

 

[dieseldummy]

I started with .010" shorter pistons and took around .040" off the tops on a lathe. Alot of people will offset bore the wrist pin bushings, but that's alot of work and you can only go .010" or so. 6.5 pistons are about .5" thick so there's alot of room to work with. Kennedy diesel offers a set of pistons with a custom dish done in a CNC machine, but they are yet to be proven more effective than just decking a stock piston.

On a race only ether motor you can delete the glow plugs for a small gain in cc's. I have it all written down somewhere in the shop, but can't remember the exact cc numbers off the top of my head.

Felpro offers a .010" thicker head gasket for 6.5's to compensate for head/block decking which can be used to lower compression. The only drawback to it is that it lacks the steel reinforcement that a standard gasket has. I'd be leary of running them with the lack of head bolts per cylinder...

IIRC, there was one person who shortened his rods to lower compression... I don't remember the whole story though. All that sticks out is that it was alot of machine work which costs alot of money for those of us who have to pay a shop to do it.

It's basicly limited by imagination at this point. There is nothing desirable about a 6.5 so everything has to be redone to hold much power. Heck, even stock they aren't very good! At least the 6.9 has a block that doesn't resemble a 350 gasser...

 

[Diesel JD]

In doing research for an Ag Operations Power machinery project for school this fall I noticed that CAT makes custom pistons...I wonder how cost prohibitive that would be....probably a lot of $$$$. Lack of headbolts? IDK about the 6.5TD but the 6.9 has 17 bolts per side....good holding power especially with studs.... IDK if felpro has that taller gasket for us or not. I have to keep my glows... its a DD. Even if it wasn't.......I'd not really want to be without it unless it was only a racer not even a weekend warrior or RV.

 

[helgaairhead]

So have we decided what do use yet?
Will an 7.3 IDI start easy enough on lets say 19:1?
Here in Norway its common having 10'C and more below zero, yes I have block heater and if things get really rough - ether.

Which company sells them together with head studs?????

Helge

 

[FordGuy100]

I would probably say yes, an IDI will start fairly easy at 19:1. There is nobody that sells a kit to do so though, it would all be a custom thing. I heard somewhere that Mahle offered low compression pistons for our IDI's, but I've never seen a link to it or anything.