any way to lower the CR on an IDI 6.9 7.3?

[aaron1976]

Hey everyone,

I have made my first post any forum earlier today and here on Oil Burners and so far it has been great. Good response and lots of help.

Wondering if any one has found a way to lower the CR on these IDI diesel engines to enable higher turbo boost pressures? Don't think you can mill the pistons. You need that thickness to hold up to being a diesel engine.

Anything you can do? Would have to be done with at least the heads off but probably during an engine rebuild.

Thanks

 

[FordGuy100]

Actually some people have shaved the pistons down to lower the compression some. You cant take much off though, as like you said it still needs its strength, but you can take enough off to lower the compression. I think .060" is the limit.

 

[aaron1976]

Cool! I may do that if I ever have to rebuild the engine. Not sure it is worth it though as I would loose some of the low end that I love about the engine.

 

[Alex S]

is there a thicker head gasket you can get?

 

[Black dawg]

I wonder how much it would drop with no glow plug

 

[dyoung14]

i have herd some were that ken at dps cut .125 off some and didnt have any problems

 

[FordGuy100]

I've asked the same thing about the glow plug. To me you could take it out, put a plug in there...advance the timing more, and have a little fun for the night at truck pulls, or the drag strip. I would think it would drop it more than shaving that little bit off the pistons.

 

[The Warden]

is there a thicker head gasket you can get?

I don't think so (I've certainly never heard of one), and I rather doubt that a thicker head gasket would be able to hold up in this application.

i have herd some were that ken at dps cut .125 off some and didnt have any problems

Yes, Ken cut the pistons on a 6.9l to get it down to 17:1. I would NOT call that truck streetable, however. Here's a sound clip of him working to light it off...IIRC that was with glow plugs, ether, AND a warm day. Also, IIRC, he ran the truck on the track once and had a failure on the bottom end of the engine. I think he was planning to strengthen the main bearings, but I never heard anything beyond that.

IMHO the way to lower the compression on one of these and still keep it streetable would be to find a way to get more space on the insides of the precups. But, then, you have to deal with swirl patterns and such...

In short, it's not impossible to lower the compression ratio on one of these, but IMHO it's more trouble than it's worth unless you're only using your truck for racing or tractor-pulling.

Just my $.02...

 

[hesutton]

If you are planning on running a "standard" IDI turbo (Banks, ATS, Hypermax), then lowering the compression ratio is not going to help you much. Those turbo don't make boost effiecently over 11-12 psi. If that is all you are going to run, then lowering the compression ratio will likely cause a power loss. If you are going to use a PSD or a DT466 turbo, that can effiecently and easily make 20-30 psi, and you have a heavily moddified fuel system to deliver a lot of fuel, then their might be some benefit to lowering the compression ratio.

What ratio, what turbo, what fuel volume is best is not known. Build it, dyno it, change it, dyno it, change it......and so on.

Heath

 

[ameristar1]

The right way to lower the compression is either altering the cam timing, getting custom pistons made or a combination of both.

 

[aaron1976]

Well, that was a good post and a lot of replys.

I think I will be happy with the stock engine. I do like the low end on these, way more than most newer diesel engines. The truck has plenty of power for what I do.

My little brother has a 2008 Duramax, 4.5 inch straight pipe, no soot filter and I can't remember which programmer he has but we did a 4.87 second 0-60...... in 4 wheel drive and at 4500 ft above see level. Way fun but way expensive. You can't keep the rear tires behind you when you are in two wheel drive. It wants to come around avery time you put the fuel to it.

 

[icanfixall]

About milling the Mahle piston tops.... An engineer at Mahle Corp. named Eric told me 20 thousands is all he would allow off the pistons. Any more and the top ring will be damaged from the expousre to heat. All the Mahle pistons are 560 to 580 thousands thick on top. I used the newer and the 89 older pistons for this measurement They offer a lo-comp piston thats 10 thous lower in the cylinders. They mill the pins bores that much closer to the top of the pistons. It drops them in the cylinder. I'm running those lo-comps and I milled around 14 thous off the tops. Then ceramic coated them with Tech Line CBX material. I still have 530 lbs of compression. Yes.. You can actually mill the pistons as much as you want but... Thats not adviseable... Milling an 1/8th inch off a piston is like "testing" your pyro... Just how far did you want to go on the pyro.... 1200....1400... Or oh dammit....

 

[aaron1976]

Very interesting information. I appreciate that. Sounds like Mahle is the way to go on a rebuild. I would like to lower the comp a bit and send a little more boost into the engine.

 

[DeepRoots]

Getting different pistons made with either deeper valve cutouts or the wrist pin in a different place would leave more options to tweek the engine. Or custom rods that are shorter.
All of this would cost a tremendous amount. Even if they were already in production it'd be an easy 5-6hundred bucks.
As it stands now there cannot be any improvement in the valvetrain due to the clearances. Turbo performance would be tremendously enhanced even with larger valve overlap.

I guess someone could always contact Probe, Silvolite, or someone of that nature and give them the specs on what they want out of a piston.
Just remember changing the engine to make more high end power costs something.
Great, now we have alot of power 3500-4500rpms, but guess what, the engine is crap below 1500rpms. There is always a tradeoff.

 

[aaron1976]

Yes, I agree. If I were to build an engine with great high end power most likely I would loose a lot of low end.

I use this truck in the hills a lot and haul wood and a camp trailer up to the mountains so I would not want that.

I think I may at some time look into the ATS aurora turbo system. I hear it would be a much better turbo than the old non waste gated turbo I have. It could move a lot more air at 10 psi than the one I have, or so that is the word. That along with an intercooler should be plenty for me.