gingrass179
Full Access Member
im going ebay with the comp 910s
7.3 cylinder heads.......????
- Thread starter Nrliftedranger
- Start date
racer30
Full Access Member
The Hydraulic lifters will allow for some Decking without trouble. The flame swirl depressions on the piston tops are about .085 deep. If you cut .080 off the pistons you will have no swirl and only about 14 to 1 compression...Good Luck starting that without ether. My calculations show about 1.2 points reduction in compression with every .010 space added to the combustion area. Now when you cut the piston you are only removing metal from the surface minus the depression..so compression will not decrease at that rate.. So its kind of a gestamation...to get to that number.. I cut .030 off my pistons in my MH's 7.3 and it starts fine with glows now but its warm this time of year so we shall see when its 15* out at 6800 FT in AZ this winter.
bbjordan
Snow Monkey
Hmmm, .080 thou milled off...subscribed 
Done. It's in the FAQ now.
gingrass179
Full Access Member
well the engine already did 30,000 miles and some of that was in colorado this last winter...so i think ill be fine as start up stuff goes...im not the one who milled it or i wouldnt have done it that much im still going to be at 18-18.5:1 i would like to be closer to 19.5...but i dont think that that is going to happen...this is the block that was dynoed at 350 700 (not exact figures but close) from nmb2...pretty much i really have to plan out my turbo now cause im going to need some low end pull and that means im going to need low end boost since i dont have the static compression to be able to make the torque i want...he was diagnosing his oil cooler for the oil pressure problem in his driveway when he lost pressure all together...needless to say now i need to rering and bearing it....thats not bad if you ask me and the block came with main and head studs for a steal of a price ...lol cheaper than a set of studs pretty much lol...so im not worried about startup and he was starting his with one battery too...
freebird01
Post Turtle!
im not so sure on that compression ratio... i think 18-18.5:1 with pistons milled that far may be a bit optimistic. have you measured everything out and cc'ed the heads to verify? i would think you would be decking the block to the point the intake may not fit correctly to get back to that ratio. i think racer30 was closer.... not sure about 14:1...but it may be closer to 16:1
icanfixall
Official GMM hand model
A good point was made about milling the block to get the cr up but possible issues with the intake to head matchup will issue. The ports wont match up. What can be done is the bolt holes can be opened up to allow the manifold to fit and the manifold can be cleaned up to fit correctly too. Its a softer material to cut. But back to changing the block to match the pistons cr you want. There is a min demension from the centerline of the crankmain bearings to the top of the deck. Its 11.137 to 11.141. I have not found out why thats the demension but its probably something to do with piston protusion out of the cylinders and the hydraulic lifters action. As posted already we have some play in the lifters. Once you cut the block the metal is gone and thats not a block that can be used with other pistons.
jaluhn83
Full Access Member
By my math here's the CR for different piston decking:
0 - 21.5
0.010 - 20.5
0.015 - 20.1
0.020 - 19.7
0.025 - 19.2
0.030 - 18.9
0.040 - 18.1
0.050 - 17.5
0.060 - 16.8
0.070 - 16.2
0.080 - 15.5
This does not account for the existing flame cup volume which would make the actual CR slightly higher.
Also worth noting that the flame cup in the piston crown is an important part of the air dynamics and swirl pattern that help generate optimal combustion - shaving the piston changes that to some extent, probably (and almost certainly with that much removed) causing more inefficient mixing and combustion.
Honestly I can't see any reason to take more than about 0.025" off regardless of the other issues - you don't really gain anything with a overly low compression ratio.
0 - 21.5
0.010 - 20.5
0.015 - 20.1
0.020 - 19.7
0.025 - 19.2
0.030 - 18.9
0.040 - 18.1
0.050 - 17.5
0.060 - 16.8
0.070 - 16.2
0.080 - 15.5
This does not account for the existing flame cup volume which would make the actual CR slightly higher.
Also worth noting that the flame cup in the piston crown is an important part of the air dynamics and swirl pattern that help generate optimal combustion - shaving the piston changes that to some extent, probably (and almost certainly with that much removed) causing more inefficient mixing and combustion.
Honestly I can't see any reason to take more than about 0.025" off regardless of the other issues - you don't really gain anything with a overly low compression ratio.
gingrass179
Full Access Member
now add 30% for the for the swirl chambers not being machined...ill stop hijacking now!!haha
freebird01
Post Turtle!
I'm not really sure how much this does effect any kind of flame swirl. What I feel it would do more then effect mixing gases...is more importantly it will cause more of a localized hot spot in the top of the piston. if you look at the shape of the reliefs, you have simple valve cutouts and a notch for the pre-cup gasses.
I realize this is a bit of a stretch but if you look at a PSD piston they are a simple dish in the top. My theory is that this is to prevent the injector from overheating the center of the piston and to more evenly distribute the heat rather then having the injector try boring a hole through it. hold a garden hose on a stream right next to the concrete then pull it back by a foot or two. same principle.
With the IDI you have a notch right at the outward port of the pre-cup thats kind of doing the same thing. without this notch it would be possible you could be locally heating the piston crown more in that one spot then was previously done with a stock piston. Is it a concern? maybe...maybe not i don't know..
there really isnt much there on the top of a 6.9/7.3 piston to do any "swirling" so to speak... the valve chamber of the head is at a minimum too. its not like a gasser head/piston with large bowls and facets to the top of some performance pistons
again...my opinion and theory...
jaluhn83
Full Access Member
Look at the piston. Now look at a cylinder head. The "valve cutouts" do not align with the valves.
That geometry on the top of the piston has everything to do with combustion and is very critical and very carefully thought out.
As you fill the cylinder through the intake valve a swirling effect in generated in the cylinder due to the offset of the valve from the center. Through the compression stroke this swirling is continued and intensified as the air is compressed and the geometry of the cutout in the piston crown and the throat/inside of the prechamber creates a very vigorous turbulence inside the prechamber. Once injection occurs, this aids in the mixing of the fuel with air and also in the propagation of the flame front out into the cylinder proper. This turbulent flow of the air and propigation of the flame front out into the cylinder is critical to complete and efficient combustion.
Consequently, when you mess with the height of the piston you're doing 2 detrimental things. First off you're increasing the ration of the prechamber volume to the cylinder volume. This decreases the turbulence in the prechamber to some extent (less gas compression into the chamber and lower velocity of gas entering) and also means more of the combustion is going to occur in the cylinder proper. This is more important since all of the fuel is injected into the prechamber. Essentially what happens is you wind up with is incomplete combustion within the prechamber and unburnt and partially burnt fuel flowing out into the main chamber. The more of the combustion volume is in the cylinder, the more of this flow there is and the less efficient combustion is due to the less effective mixing in the cylinder as well as the fact that combustion is going to rapidly cool once it gets into the much cooler chamber.
This is why a DI engine runs with a much higher injection pressure - they don't have the highly compressed turbulent flow within a small volume in the prechamber. The DI system is fundamentally more efficient, but harder to get to work.
You're essentially right about the DI piston shape - it's also designed to maximize swirl and combustion efficiency as well as maximize the exposed volume of air for injection and combustion.
That geometry on the top of the piston has everything to do with combustion and is very critical and very carefully thought out.
As you fill the cylinder through the intake valve a swirling effect in generated in the cylinder due to the offset of the valve from the center. Through the compression stroke this swirling is continued and intensified as the air is compressed and the geometry of the cutout in the piston crown and the throat/inside of the prechamber creates a very vigorous turbulence inside the prechamber. Once injection occurs, this aids in the mixing of the fuel with air and also in the propagation of the flame front out into the cylinder proper. This turbulent flow of the air and propigation of the flame front out into the cylinder is critical to complete and efficient combustion.
Consequently, when you mess with the height of the piston you're doing 2 detrimental things. First off you're increasing the ration of the prechamber volume to the cylinder volume. This decreases the turbulence in the prechamber to some extent (less gas compression into the chamber and lower velocity of gas entering) and also means more of the combustion is going to occur in the cylinder proper. This is more important since all of the fuel is injected into the prechamber. Essentially what happens is you wind up with is incomplete combustion within the prechamber and unburnt and partially burnt fuel flowing out into the main chamber. The more of the combustion volume is in the cylinder, the more of this flow there is and the less efficient combustion is due to the less effective mixing in the cylinder as well as the fact that combustion is going to rapidly cool once it gets into the much cooler chamber.
This is why a DI engine runs with a much higher injection pressure - they don't have the highly compressed turbulent flow within a small volume in the prechamber. The DI system is fundamentally more efficient, but harder to get to work.
You're essentially right about the DI piston shape - it's also designed to maximize swirl and combustion efficiency as well as maximize the exposed volume of air for injection and combustion.
icanfixall
Official GMM hand model
This posts continues to amaze me with some of the funny responces. What we need is an apple and an orange. Now cut them both open and compare.. Gees.. They are completely differant. Same with Direct or indirect injection. A psd piston or as its refered to here direct injection is completely differant from the indirect injection puston. The idi piston has a swirl chamber Called a Recardo Cup. I realize many "THINK" it valve reliefs but its not as posted here. The reasons the direct injection puston looks like it does is because the fuel being sprayed on to it is at a much highr pressure. 20,000 psi is the minum it can be. This pressure is 10 times higher than what we have. So the direct injection injecters are all electronic and not mechanical. thats so they can open and close faster than a spring can close them. Indy car tried experimenting with electronic valves and was getting some high rpm.. Something over 20,000 rpm but not reliable. I have heard it works to a lower rpm though. The direct injection injecter nozzles have several opening in them. Usually 5. Sometimes those multi port nozzles are nearly impossible to see the holes they are so small. Higher pressures pushing thru very small holes in 20,000th of a second creates a very fine atomized mist that better for ignition and combustion. Higher pressures equals a better complete burn of the fuel. I'm not trying to show up anyone nor put someone down. This is free information and should be used that way. Others from around the world can read this and they can fly the flag if they choose too. I enjoy helping others to understand so they can make better calls.
freebird01
Post Turtle!
ok...well i apologize... im seeing some old posts that better describe this...
again...i did not try and throw it out there as fact...was simply my observation...wrong or not....
with that said then... why do so many people decompress these engines when looking for more power and bigger turbo's? are you saying to properly do this you would mill less off the pistons and re-machine the recardo cups in the piston crowns?
again...i did not try and throw it out there as fact...was simply my observation...wrong or not....
with that said then... why do so many people decompress these engines when looking for more power and bigger turbo's? are you saying to properly do this you would mill less off the pistons and re-machine the recardo cups in the piston crowns?
jaluhn83
Full Access Member
Lowering the compression ratio allows higher boost because it lower the compression pressure a bit - you see the same thing in gassers. Lower overall compression ratio lets you run higher boost which means more air which ultimately makes for more power.
i understand waht you are saying Jahlue but "lowering compression alows higher boost because it lowers teh compression" makes no sense lol. cat chaing its tail.
You need lower CR for higher boost because of heat and pressure. There will simply be too much heat built up, therefore PRESSURE and power, at which point somethings gotta give.
You need lower CR for higher boost because of heat and pressure. There will simply be too much heat built up, therefore PRESSURE and power, at which point somethings gotta give.
