compression question for possible 6.9 build

[SPR1NK]

Lowering the compression would allow for a higher boost level correct?
Do with that in mind would it possible to lower the compression of a 6.9 from the 20:1 to say 17.5:1 to make it closerish to a psd and therefore be able to put more boost to it safely?

 

[SPR1NK]

And if possible how would I attain that compression level? The engine I will hopefully be buying is already bored .030 over

 

[riotwarrior]

These engines require a very high compression ratio to actually light off, so in all honesty lowering to that level would kill the starter and make it pretty much not worth the effort. You would require custom machining of pistons and so forth.

DIG here on search there is tons of info.

These are NOT PS engines and should not be considered anything like them other than the are fueled by oil.

Buy an IDI if you want an IDI, buy a PS if you want a PS plain and simple.

What is the purpose of the build? Why do you require high boost levels?

The head bolts are 7/16" and a weak link in the 6.9 engines and studding for higher boost levels is required.

Lots of info to be had at the tip of the search button!

 

[SPR1NK]

My intent would be just try to make it as powerful as I can
I do intend to stud the heads, and I know they aren't PS but I was mainly just curious if it was possible, I didn't realize they needed that much compression to pop off, why do they need such high compression to pop?
I did try to use the search but I want finding what I was looking for as far as lowering compression

 

[SPR1NK]

And what would it take to machine pistons? Cuz I have access to mills and lathes both cnc and manually operated so I might be able to do it myself

 

[icanfixall]

Fly cutting the pistons on a mill will work fine. Also chucking them up in a lathe will work too. All idi pistons are about 560 to 580 thousands thick in the center thinnest part of the piston crown. The Mahle engineers recommend cutting 15 thousands as the top limit because any more gets the flame front too close to the top ring. Many have cut much more without trouble. But like some say. Why take the chance ruining pistons and blocks. I don't believe lo comp pistons are available for the 6.9. Those are the pistons that are 10 thous below normal stroke. It means the wrist pin is cut 10 thous higher up in the skirt to drop the piston that much in the cylinder stroke. Buy a typ4 cam from tpy4. Port the heads. Add a turbo. Add an set of Moose Mister injectors and a Bull Moose pump. You will squeeze the wheaties out of the engine. Making sure the rotating parts are balanced gets more hp out as does line boring the cylinder and the main webs. Look at the wear on any rod or main. Its usually worn thru on one side or the other. that a bad bore job.

 

[SPR1NK]

Awesome, thanks
I was planning on getting Russ's cam already and putting the turbo I have on my current engine on the new one and going with the moose injection stuff

Does anybody know how to figure compression ratio so I could figure out what it would be if I machined the pistons?
And if I lower the compression will it make my truck a PITA to start?

 

[riotwarrior]

You may also want to contact Jason at R n D IDI performance he does some interesting things with IDI's

 

[PwrSmoke]

The issue is Indirect Injection, which hasn't been used in the industry for quite some time. Most everything new is Direct Injection and in that world, lowering the compression is part of the performance game. Not in the IDI world.

Step one for anyone contemplating an IDI build is to fully understand the differences in Direct versus Indirect Injection, learn about the Ricardo Comet V combustion chamber and the limitations IDI places on power density (ratio of displacement to power output).

 

[SPR1NK]

I might have to contact him, thanks

I didn't realize indirect injection made such a difference vs direct injection

 

[icanfixall]

Our fuel is injected into the precups at 1850 lbs where a psd its directly injected into the piston bowl ar as high as 40,000 lbs.

 

[SPR1NK]

So lowering the compression would be more detrimental than anything?

 

[BrandonMag]

I'm not an engineer. But, the answer to your question is essentially yes.
The engineers who designed this engine spent a lot of time and brainpower on making it as good as it is at what it does. There are some tweaks you can do to squeeze some more power out of the engine, but you're talking about lowering the CR too much.
Do some research: Google, Wikipedia and some time reading around on here and you'll have a better understanding of the questions you're asking.

 

[icanfixall]

A member no longer on this forum did lower a 6.9 to something like 17.5 to 1. He had a sound clip of it staring. It cranked for probably 1 minute and nearly 1 can of ether was used. Then when it finally caught and started it ran ruff till it warmed up some. Ken eventually pushed the crank out the bottom of that engine it had so much hp. Nobody has built an engine that can do that yet. You might goggle the Hypermax idi that was built too. It made so much hp that the freeze plugs would pop out of the block from the twisting it did. So the owner used the rubber freeze plugs and it worked well. It had a twin or triple turbo on it and a custom huge intake that wrapped around the top of the engine. It was direct injection too.

 

[SPR1NK]

A direct injection idi? I didn't know that was possible