lower compression?

[85idif250]

im new to the site, long time lurker first time posting, i have a 1985 6.9 idi, i have a lot of plans for the beast, but i was wondering if it would be safe to use 2 head gaskets to lower compression on these old high comp diesels, or would i have sealing problems? i plan on eventually running 20+psi so shaved pistons will be a must, and arp head studs, but would the 2 gaskets work?

 

[Agnem]

I'm going to say that catagorically the answer is NO! We have enough trouble just trying to keep one gasket in there. Not only that, but I'm pretty sure that will decrease your compression TOO much. I believe taking about .010 off the pistons is the accepted maximum, and a gasket is thicker than that.

 

[85idif250]

even with head studs clamping everything down? and what would the comp be at .010 of the piston?

 

[hesutton]

Yeah, even with studs, I would not trust two headgaskets stacked on top of each other. The compression ratio for 10 thousands off the crown will be close to 19 to 19.5:1. You can get that with just Mahle's lower compression height pistons. The pin is 10 thousands higher in the piston and thus 10 thousands lower compression height. If you mill the crown, going too far will transfer significantly more heat to the wrist pin and rings. How much it too much and how much is just enough????? There is no solid scientifically based cutoff. If I remember correctly, Mahle told Gary (icanfixall) 15 thousands was the max.

Heath

 

[85idif250]

alright thanks for the info, any tips you might give me for hitting the 200hp+ mark? already got a torque cam on the way and a turbo setup from a 93 7.3, any input would be greatly appreciated.

 

[icanfixall]

Welocme to the forum...
I'm the member that has talked to the original piston makers for these engines about milling them. That was Eric at Mahle pistons about 6 years ago. About 15 is the max but going to 20 is acceptable. Much more and you are looking for heating of the top ring. All the turbo and non turbo pistons have the same dome thickness at around 560 to 580 thousands in the center. Thats the thinnest area. The eadges are much thicker. Two head gaskets just wont work but if you try it please post what happens. Not noe person here will tell you I told you so or call you a dummy for doing it. Thats not how this forum works. About the best gasket for these engines is the Victor Reinz and then the Felpro. Supposedly the flepro is 10 thousands thicker than the oem gasket but... I have no idea what that was... I have done expensive measurements on complerred and non compressed flepro gaskets. Best I can come up with is around 62 thousands compressed and around 76 uncompressed. These gaskets spring back when the pressure it removed and its nearly impossible to get a measurement as the head is tightened up too. Taking a measurement for thickness at the fire ring is the wrong place to do that. It needs to be at an area with no metal around it like a bolt hole or collant passage thru the gasket. Some use only the fire ring for this measurement but it springs back when the pressure is removed some. Not much but it moves. This 200 hp you want... Where do you want it. I'm thinking at the rear wheels and thats easily possible with just a modified turbo. At the crank is done with any turbo out there. Boosting to 20 lbs will require an intercooler for best hp gins. Any boost past about 14 or 15 lbs is superheated air up around 300+ degrees going into the engine. Mostly a waste. An intercooler will drop this temp so you get all the hp from the higher boost. Remember compressed air heats up fast. Ever see a scuba tank being filled. They are in a bath of water so they don't heat up. Our turbo spins up around 120,000 rpm when its boosting good... Thats 2000 rpm per second...

 

[hesutton]

You need fuel to go along with the air provided by that turbo. I've had both DPS (turbo IP/stage 1's) and Moose (IP and injectors), and in my personal experience, Moose is hands down a better product for both the IP and the injectors. An intercooler would be a nice addition to the changes you are making.

Just my two cents...... if you are shooting for 200HP (+ or -), there is not benefit from shaving the pistons. You can accheive that power with mods listed above and stock compression. You mention 20+ psi. The factory ATS turbo will be out of it's effeciency range at 20 psi and may not live long with that kind of drive pressure. If you are wanting to stay in the 20+ psi range, a different turbo would be a better option than the Factory ATS.

 

[85idif250]

thanks again for all the input, and Ive had my eye on the moose setup for quite awhile now, everyone says there amazing and by far the best, and dps are, well, not the best. inter cooler is in the very near future, something that can flow at least 750cfm? how much hp do you think i could push from it without shaving the pistons? 300hp would be awesome but i haven't heard many people hit that. and does getting the headers and pipes leading up to the turbo ceramic coated worth it as in the gains and drop in egt's?

 

[85idif250]

and what turbo could i bolt up to the stock setup that could handle my wanted boost level?

 

[wmoguy]

A turbo w/ a T3 flange will bolt up to the up-pipe. I don't know of anything that would literally just pop back into place without modifications though.

 

[85idif250]

alright so i just picked up the turbo kit, got the turbo with wastegate set to 18psi, two pipes, y pipe, air box, and a assortment of nuts and bolt, does anyone know of a good picture of the motor with the kit on it? or install instructions so i can figure where errything goes

 

[racer30]

There are as many ideas about compression calculations as there are M&M's in a bag. The most accurate is to cc the engines cumbustion chamber. The IDI dosen't have a "chamber" it has an area. The IDI has a depression on top of the piston that is about 8cc's the valve pockets are aprox 6cc's for the intake and 3cc's for the exhaust. the 7.3 turbo pre cup is 20cc's with the glowplug and a injector installed. The accepted gasket thickness compressed is about .062 with this info put into a computer engine dyno the piston had to come .014 above the deck to get 21.5 to 1 compression. with this info you can adjust the gasket thickness on the computer to see what happens to the compression. what I found was the compression changed 1.2 to 1 for every .010 thicker the gasket was. I also calculated the adding of space in the combustion area by cc's. what I found was every 1cc of space was .47 to 1 compression. This is what I found from running the numbers. Hope it helps with your build Ideas.

 

[icanfixall]

Our engines run with the piston riding up out of the block as much as 31 thousands. With a felpro head gasket that means the piston to flat head has a clearance of approx 31 thousands given the above demensions. We still do not know if the pistons are chasing the valves or are the valves chaseing the pistons. Time to degree a cam and piston movement.

 

[racer30]

The pistons move much faster than the valves. The piston moves 8.36 inches total (4.18 stroke) up and down in 1 revolution, the valves move only .760 inches per revolution. (.380 valve lift) the piston completes its movement in 360 degrees. the valve completes its movement seat to seat in 230 degrees or so on the exhaust. I think the piston clearance comes from the fact that the stock cam starts opening the intake valve slowly so soon that its at .050 open at 15 degrees after top dead center. With the valves Head surface set into the heads about .050 intake and .060 exhaust. (on one of my turbo heads) That means the .030 to .050 of space above the piston created from the gasket thickness minus the above deck piston dosen't leave much room for head cutting or valve seat replacement unless your machinest is aware of this tight clearance. When you change cam profile you have to recalculate again. Thats why most people building a more performace IDI want to cut the pistons to lower compression, It makes more room for turbo boost and a bigger cam without worrying about having had there block decked or the heads shaved just a bit past spec. seems like the easiest way to kill the clearance problem is to cut the pistons. Its been done as much as .100 way to much in my opinon. I think .030 is a good number if you ceramic coat them. should get you to about 18.9 to 1 unless you think about the fact that cutting them dosen't remove the total surface, only the area not already involved in the flame path depression on the top of the piston. with this taken in to the math I think .030 cut will net you about 2.5 points not the 3.6 the computer sim calculated. But who knows for shure without measuring the cc's lost in the depression. I do have a extra piston or 8 laying around to play with so I might get that number also.

 

[88 Ford]

thanks again for all the input, and Ive had my eye on the moose setup for quite awhile now, everyone says there amazing and by far the best, and dps are, well, not the best. inter cooler is in the very near future, something that can flow at least 750cfm? how much hp do you think i could push from it without shaving the pistons? 300hp would be awesome but i haven't heard many people hit that. and does getting the headers and pipes leading up to the turbo ceramic coated worth it as in the gains and drop in egt's?

There is only one person that I know of that is close to 300hp. That is NMB2 and he hit 292/614. Also for the record I really like my Dps injectors. I have a Moose Pump providing fuel to them and my truck seems to make decent power. But anyway to make 300hp you will need close to 150cc of fuel. Justin is running a Db4 pushing that much and after he does his compressor upgrade to put his compressor map in its proper efiiciency range he will easily be over 300hp on fuel only. I can't wait to see what his water/methanol adds too. As for shaving the pistons, I think an engine would be alright up to 25-30psi without shaving. Justin hits 40psi with his water/methanol on without blowing up but he is running about 18.5:1. From what I have read they say that for every point in compression taken off, you can add 2psi. So since his is at 18.5:1 that is a 3point drop which means that he can easily add 6psi. Of course he has cryo treated his internals for added strength and I would do that too even if you didn't shave your pistons. But anyway this is all my opinion and I think 30psi is definitely doable with cryoed internals on stock compression. It definitely would be interesting to see what our engines can handle for boost just as long as the head gasket holds. Also Hypermax is working on a fire ring head gasket that will allow it to hold. It should be really good for idi performance in the very near future...