Performance P 400 6.5 compressiom ratio poll

[SmithvilleD]

Thats good data to show how large changes in CR can affect efficiency. Why the diesels can be more efficient in general from gas engines with half the CR.

Add the turbo charger to the mix and wonder how it affects the same charts, since cylinder pressures will increase. The charts probably only consider actual engine losses without intake or exhaust losses. So the differences are probably even less when adding those conditions. The valve timing effects pumping losses as well.

We have to be careful to understand it's cylinder pressure during combustion that's making power. And then envision how effectively that pressure is transfered to the crank at say 10 degrees after TDC, then 20 degrees after TDC, etc.

The earlier example suggesting it takes almost twice the boost to achieve the same cyl pressure (PRE-combustion) can be misleading if one mistakenly interprets equal pressure (PRE-combustion) to mean these compared cylinders have the same mass of air in them. They don't. One has less air, mechanically compressed in the cylinder more; the other has more air, mechanically compressed in the cylinder less - both winding up at the same pre-comb cyl pressure, but with different amounts of air in each cyl.

These example cylinders (with different mechanical comp ratio's) with the same pre-combustion cyl pressure, almost certainly won't have similar cyl pressure after combustion has started. If there's adequate fuel, the one with more air - > more combustion -> more cyl pressure during the power stroke -> more power.

And back to the beginning, the cylinder that got more air, got the additional air by way of higher boost pressure putting more air into the cyl. But again, it's not so much that the small comp ratio difference made it appreciably easier for air to enter the cylinder, but a greater pressure difference (more boost) btwn intake & cylinder pushed more air in.

Practical experience from those running lower comp ratio 6.5's suggests it doesn't take twice the boost levels in a lower comp ratio 6.5 to make equivalent power to the higher comp ratio engine. It does take more boost, but not that much more.

Now this additional boost isn't free, as making it did impart some additional restriction on the exhaust. But the tradeoff with an appropriated sized turbo is such that the additional boost necessary to make comparable power in the lower comp engine doesn't hurt overall efficiency or power production dramatically.

 

[buddy]

interesting I'd always heard that when you lower the comp it allows the turbo to push in more air and thus maintain comp levels but with more air in the cylinder

I think what is being said is pretty much this is basically correct. Same cylinder pressure with more air, but as mentioned does take some additional work to achieve higher boost. This and the other information explained in this thread is why I do not recommend higher CR, and generally recommend slightly lower than stock. For the sake of efficiency and low end, off the line grunt, I do not recommend 18:1 typically, but maintain that 20:1 with a simple +.01" head gasket and diamond precups is the best bang for the buck, when installing the ARP head studs.

 

[dieselfuelonly]

So are diamond precups the best units to install in the 6.2/6.5 cylinder heads and how much roughly do they lower the compression? If I would want to get a said engine down to 18.5:1 I think the precups and probably a slightly thicker head gasket then .01" ought to get me there. How much would I be looking at to get diamond precups installed in a set of heads from the machine shop? Thanks.

 

[buddy]

I suppose the diamond cups lower the CR by 0.4, since I have seen the rated CR as 20.9 on newer engines and 21.3 on older ones. Although I have seen other numbers as well. Thats why I suggest the diamond cups and a +.01" gasket to get to 20:1 ratio. You'll probably be near an actual 18.5:1 dynamic CR with a +0.02" gasket from Cometic, but a set costs quite a bit, like $240. The good thing about the Cometic gaskets though is that they go to 5 layer of steel after about +.015", vice 3 layer before that. Or you could get a +0.025" gasket since they do custom whatever thickness, so that would be a 0.07" thick gasket.

I dont think any truck needs that low of CR, and the thicker the gasket I would presume the more likelihood it could fail, but with a set of head studs torqued down the steel gaskets should be alright.

The diamond precups are a good choice if desiring the most torque possible. Potentially sacrificing a little fuel efficiency. Although if planning to up the fuel output then you really need a larger mouthed precup to take most advantage of it.

 

[kimagine]

Not to sound stupid but, what are pre-cups and where are they? Ibuild houses not trucks, just drive them and try to look good in them. ;-)

 

[buddy]

precombustion chambers inside the heads. Its what makes it an indirect injection diesel engine. The injectors and glow plugs stick into this chamber where combustion starts and pushes out into the cylinder.

 

[Turbine Doc]

Not to sound stupid but, what are pre-cups and where are they? Ibuild houses not trucks, just drive them and try to look good in them. ;-)

Here are some visuals for you Mark

DI & IDI head cross sections and photos

 

[kimagine]

precombustion chambers inside the heads. Its what makes it an indirect injection diesel engine. The injectors and glow plugs stick into this chamber where combustion starts and pushes out into the cylinder.

Here are some visuals for you Mark

DI & IDI head cross sections and photos

Buddy and Tim,
Thanks! As usual a wealth of knowledge! It's tough trying to learn a whole new thing after spending years learning the older beast. Why couldn't these things be like houses... She'd have been built up long ago.:thumbsup:

 

[spdgofast]

My engine build came out ~ 19:1 & I've found this to be a good compromise. It certainly relies more on having a well maintained glow plug system & some extended glow time to start cleanly below say 35 degrees F. Have one gp go out, & that cylinder might take 5-10 seconds to build enough heat to begin firing cleanly below 35 degrees. It's started fine w/o engine heater (when elec not available) down to -7 so I trust it to start (although it's certainly better for it to use the heater when possible).

Keep in mind torque = cylinder pressure achieved & how that cyl pressure happens as the crank angle rotates thru the combustion stroke. Compression ratio is a part of the cyl pressure equation - but not the only factor.

Higher comp benefits are greatest at idle & taper down proportionally as more and more airflow is pushed thru the engine. At idle, comp ratio is a much bigger contributing factor to what cyl pressure is achieved. At idle, a higher comp ratio's extra squish significantly enhances how much turbulence is induced into the IDI chamber, thereby improving fuel air interaction, helping combustion efficiency.

With certainty, our IDI engines were designed w/ higher comp ratios to gain combustion efficiency. Among other things, IDI is/was a way to get adequate fuel/air interaction utilizing a relatively low pressure (cheaper to make) injection pump. These engines were engineered to make around/under 200 hp. As we try to make 300+ hp, we're quite a ways past original design limits.

If you never have a headgasket failure (which does happen often enough in turbo 6.5's), then the stock higher comp ratio is fine. The greater proportion of time you're going to run at high %'s of max power, the more a bit lower comp ratio is likely to show benefits in durability/reliability.

These relationships show are well established w/ performance diesel engine builders. In the continual improvement of max power, when they begin to run up against headgasket/combustion seal failures, they lower comp ratio, which lowers PEAK cyl pressure/temp for a given power level. Remember the failures occur around the peak spike of pressure/temp in the chamber. The higher the comp ratio, the higher this peak/spike is.

Everything else being equal, lowering comp ratio lessens how sharp the pressure/temp spike is. You've still got to make the same overall average cyl pressure to make the same amount of power with a lower comp ratio. (actually prolly slightly more cyl press, but with a lower peak/spike).

If you can avoid HG/comb seal failures, higher comp makes more power. So they use better gaskets, o-ring comb sealing surfaces, etc. When they get to the limits of maintaining the comb seal, they lower comp ratio & use a little more boost to gain durability/reliability & still make the same amt of power.

So the folks running performance 6.5's with stock comp ratio's & never having any headgasket issues prolly make their power a bit more efficiently. Head studs, optimized head & block deck sealing surfaces, & better head gaskets are all ways to strengthen the seal & enhance durability.

Thermal efficiency is directly related to comp ratio, so higher is more efficient. But up around the comp ratio's these engines run, the differences are less significant. If the difference in thermal efficiency was huge, then my 19:1 engine wouldn't be able to routinely do 19-20 mpg (it's averaged 19 for 20,000 + miles) on summer fuel in a 6000+ lbs 4x4 aerodynamic brick.

Summarized - I lowered my engine's comp ratio to gain a little peace of mind that my engine is less likely to suffer a HG failure when working it's hardest. Any gained durabilty is in probability of avoiding a HG or any other peak cyl press/temp spike related failure.

Thanks for the reply SmithvilleD, do you mind if I ask what kind of piston that you are running to get the 19:1 CR? I'm thinking of buying a new P 400 and putting 19:1 pistons in it because I feel the same as you as to the 19:1 CR being a lets say "happy medium" between the 18:1 and the 20.2:1 that the p400 comes with. kinda giving me a little more piece of mind relieving lower motor component stress and allowing for more boost if I want to put a higher output IP on in the future without pulling the motor just to change pistons. My thinking and experience is more fuel needs more boost and more boost is stress on the bottom end.

 

[SmithvilleD]

My engine got to that comp ratio via the Mahle 18:1 marine 6.5 pistons & the block deck lowering from cutting/machining the deck to provide a good square & flat sealing surface. I bought my truck w/ a blown head gasket & the block exhibited symptoms of the quality typical in GM engine products when the engine was built. It appeared the HG failure was related to less than impressive metallurgy, &/or the blocks not getting "dimensionally stable" until they'd been thru many - up to temp/then cooling - heat cycles.

There were still clearly visible original machining hatch marks in the depression where the OEM head gasket failed. I can't envision how those hatch marks got there unless that surface was originally level with the rest of the block deck when it was machined. Water/steam leakage erosion doesn't make hatch marks. It was clear where the leakage/failure happened (common 6.5 HG failure spot btwn cylinders) & that area's surface was low/below the rest of the deck surface.

It's probably logical to expect the head gasket seal with good machining & the seasoned block/heads is more durable than original to some degree.

If it was me, I'd have a hard time justifying buying different pistons, etc., to lower the comp on a P400 if the long-block or engine you buy comes as 20.2:1. I'd expect this later design block, etc., is better. But this is just my $0.02 & to each engine builder his own

Would be interesting to hear from anyone with military use P400 experience. Suppose Uncle Sam has bought & used significant numbers of the P400, in new Humvee's, &/or as engine replacements. They should have enough in service by now to have significant data on if/or how much improved the P400's durability is.

 

[spdgofast]

I was just wondering if Mahle made any 19:1 pistons, I guess I may take buddys advice and just put .010 head gaskets on when I install that ARP head studs and the gear drive and hope for the best. I just hope 20 to 25 LBS of boost won't hurt it. I plan on running a Hypermax intercooler out of one of my old 2002 7.3 Ford Powerstroke truck. I haven't decided on a turbo yet, still doing homework. I'm off to the Palm Beach International Raceway now, tonights big turbo night event, going to see what I can learn about turbos.
Thanks again for your reply.

 

[SmithvilleD]

I'm not certain if Mahle even sells/stocks the marine 18:1 pistons anymore? You might see if you can find out what peak boost levels the highest performing military application P400 runs. Seems like I'd read there is an intercooled military 6.5 application, but can't remember where I saw it. They ?might? have reliability stats & have evolved their design specs - not saying that's the case, but one could hope.

Be interesting to see if that Hypermax IC's inlet/outlet tubes are wide enough to go around our 6.5's radiator? I've only seen one IC mounted at the front of the cooling stack in our GMT-400 style trucks; believe he'd given up AC to eliminate the AC condensor & gain enough front to back space to fit another truck's OEM IC. If memory serves, he also wound up having the IC's end tanks refabricated/extended to move the inlet/outlet wide enough to get outside the radiator.

Another thing to consider is how much impact the additional airflow restriction & cooling air temp gain (from IC) will have on our IDI/6.5's engine cooling system that doesn't always have a lot of "headroom"/excess cooling capacity. Our IDI heads dump more heat into the engine cooling system than a comparable hp/torque DI diesel. I've read our 6.5 radiator is similar in capacity to the Duramax radiators cooling an engine making significantly more power.

 

[spdgofast]

Yea, I just measured the Hypermax IC and I would have to modify it to go around my radiator, I have a stock Dodge IC that I think is wide enough but it is alot thicker and I dont think it will fit between the grill and the condensor. I am definatly going to stay with an IC because when I installed my little Spearco IC it was like adding two more cylinders to the engine. I talked to a guy that makes an IC for the H1 with the P400 and an Allison transmission, I'll get some dimensions from him for his IC. I was thinking of getting a bigger alluminum radiator made to fit also, anyone heard of anyone making one for our Chevy trucks?