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Performance P 400 6.5 compressiom ratio poll

spdgofast

spdgofast

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Ok fellow diesel nuts, I am taking a pole to see what compression ratio pistons would be the most popularly used in a 300 too 350 HP obviously modified, upgraded injection pump, intercooled, 5" exhaust, all the latest GMT 800 cooling mods, ported heads with maybe like an ATT turbo ect. ect. ect. 6.5 P400 to be used for everyday driving and towing in the mountains. I'm interested in everyones opinion especially those that have tried different CR pistons in the 6.5 motor.

Thanks, Dave
 
1994 K2500 Suburban DS4 IP converted to DB2 340 HP IP, bosch marine injectors, with 4" lift, 33" tires, 4:10 gears, 4L80 trans with Compushift stand alone computer, gearvendors overdrive. I have some good used turbos laying on my garage floor like 2 HX 35s, GM 4 that was original, rebuilt GM8 and a Garrett off a 2000 7.3 Powerstroke. Presently running a Spearco intercooler but I have a Hypermax for a Ford Superduty that I can use or a stock 96 Dodge Ram 2500 12 Valve intercooler also.
 
Turbine Doc said:
Did you intend for a poll to be added to this thread so a tally could be kept ? If yes you'll need to push the add poll option or if you like let me know the choices & I can add it.
Click to expand...

I am just trying to see what has worked best for most. I am a liitle new @ negotiating sites like this so feel free to make the post keep track
 
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.
 
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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
 
If you look at where the piston is/what the range of cylinder volume is during the intake stroke, a compression ratio change doesn't change those cyl volumes to be filled on the intake stroke very much. Remember you're not trying to fill the chamber at TDC - where the different piston height/volume is significantly different w/ a lower comp ratio.

As the piston is going down towards BDC, the slightly diff piston top height doesn't change the overall volume to be filled much at all. So it's not so much that the turbo can more easily push in additional air. It's more that most turbo systems are easily capable of pushing a little more air (for a little more combustion), via a little more boost. So as long as your turbo can easily/efficiently supply the additional boost, any power lost from lower compression is easy to get back & with a relative gain in durability.

A little additional boost, pushes a little additional air mass into the cylinder (greater % fill of the volume) so that plus fuel & combustion achieves the same mean effective cyl pressure (that's the term they use to consolidate all the instantaneous cyl pressure points at each crank angle into an average cyl pressure value that can be compared on similar engines w/ diff comp ratios. Engines with the same mean effective cyl pressure produce the same amt of power. In essence, with lower comp ratio & the additional boost to make equivalent power, you're smoothing out the cylinder pressure over the power stroke a bit. Lower peak/spike - same average effective pressure.

If your engine is structurally limited (& I dare say our 6.5's fit that description), lowering comp ratio can add durability at the upper limits of power production.

Penisular's experience strongly suggests the 6.5 that runs under high loads for longer periods of time shows additional durability/reliability at a bit lower comp ratio. Granted most 6.5's aren't run that hard & will probably be better off with stock comp ratio.

I'm not saying that many individual 6.5 blocks/heads might be able to do this & never fail. But a pretty fair number have. And you really never know where the failure limit of each individual engine is - 'til you find it....which is usually expensive & time consuming.
 
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I have a nifty excel file in an anatomy of EGT thread that could show you the cylinder pressure when changing CR, boost and IAT.

Stock CR is a bit elusive too, because GM changed it from 6.2 to 6.5 then lowered it more with later model 6.5s, so when someone says stock, its not always the same, and you have to know what the P400 is starting with first. I think a 20:1 ratio is a good place to be. I dont think anything special needs to be done to simply get lower CR. No reason to spend extra bucks on new pistons just for that purpose. I think the P400 may actually have larger volume pre-cups and lower the CR a little more than stock truck motors. In any case, if it doesnt have head studs, IMO, the best thing to do is just throw a +.01" head gasket on with a set of head studs and be on your merry way for least amount of cost. They do hold up to high CR, but that doesnt mean its not harder on the whole thing when increasing CR.
 
It is my understanding that the GMT 800 is the 1999 HO water pump with the 21" plastic Duramax fan and dual thermostat housing. Someone correct me if I am wrong. Thanks for the replys guys. Let me give you a senario and buddy you know the specs on my IP. Lets say I have my Suburban loaded down with gear weighing 10,000 lbs and the 33" tires aired down to 15 PSI running long distances in 4wd High ,in soft sand in the summer with 90 to 100 degrees F or higher ambient temp @ sea level with a P400 motor tell me your opinions on CR then verses pulling the 10,000 lb Burb plus a 15000 LB travel trailer= 25000lb gross weight over the continential divide in the summer temps also, the same diferent? give me your opinions, Thanks.
 
For example if you take a 21:1 CR engine vs a 18:1 CR engine

You end up with about the same cylinder pressures if you run almost twice as much boost on the 18:1 CR.

5psi on 21:1 vs 10psi on 18:1 1400psi at TDC before combustion

as boost grows the differences increase, like

10psi on 21:1 vs 16psi on 18:1 1750psi at TDC before combustion
15psi on 21:1 vs 22psi on 18:1 2100psi at TDC before combustion

IATs have the largest factor in determining cylinder temp, independent of boost/cylinder pressure
if youre at about 100F IAT then before combustion you would be looking at about 1400F, but 1700F if at 200F IAT, 3 to 1 change, which the lower CR is also a little more tolerant to. And if the air is hotter then EGTs may be impacted more than 3 to 1 if all the fuel isnt combusted from less dense air. Although I havent had any issues running up to 200F IATs in the 6.5 with crazy boost. It burns fuel pretty well with the precombustion chambers and a good amount of injection timing advance. The DMax actually has injection just after TDC at idle in some cases, since its direct, but a good running 6.5 has idle injection of 18 to 22 crank degress before TDC, and up to 40-45 degrees before TDC at higher fuel output and RPM.
 
Wish we had some actual cyl pressure data from a running/pulling engine to compare. Empirical data shows this type of thing most clearly. There are some Duramax actual cyl pressure curves floating around the 'Net, but I've never seen anything on our IDI 6.5's. Would expect the inj timing differences relate to DI - vs - IDI flame path & the overall higher cyl pressures/hp conditions which promote faster burn rates. Would be interesting to see any differences in the cyl pressure curves relative to crank angle/TDC.

Regarding comp ratio in the latest design/metalurgy P400, I'd prolly run the stock comp ratio. I would have to believe that engine is going to be tougher/more reliable than the older 6.5's & will tolerate higher peak cyl pressures & temps before failures. And higher comp ratio is certainly better in an IDI engine - right up until the point of having a failure. The point where failures become more & more likely ought to be higher in the P400.
 
buddy said:
I have a nifty excel file in an anatomy of EGT thread that could show you the cylinder pressure when changing CR, boost and IAT.

Stock CR is a bit elusive too, because GM changed it from 6.2 to 6.5 then lowered it more with later model 6.5s, so when someone says stock, its not always the same, and you have to know what the P400 is starting with first. I think a 20:1 ratio is a good place to be. I dont think anything special needs to be done to simply get lower CR. No reason to spend extra bucks on new pistons just for that purpose. I think the P400 may actually have larger volume pre-cups and lower the CR a little more than stock truck motors. In any case, if it doesnt have head studs, IMO, the best thing to do is just throw a +.01" head gasket on with a set of head studs and be on your merry way for least amount of cost. They do hold up to high CR, but that doesnt mean its not harder on the whole thing when increasing CR.
Click to expand...

Buddy, could you either re-post this spreadsheet, or point me to where I might find it? I'm curious to see if, or how it includes cam/valve opening/closing timing to calculate projected cyl pressures.
 
Excel Calculators for CR, TDC temp and pressure

[/ATTACH]
SmithvilleD said:
Buddy, could you either re-post this spreadsheet, or point me to where I might find it? I'm curious to see if, or how it includes cam/valve opening/closing timing to calculate projected cyl pressures.
Click to expand...

I dont consider the valve timing to be very significant, since the valves are both closed at TDC and all the way through the compression stroke, aside from intake at bottom dead center, and its less than 20 degrees. Although, I account for it in the CR calculator file so it can be used to be more precise. It does drop the effective CR a little. And bleeding pressure around the rings obviously does too, so numbers are most accurate at high RPMs when there is less time for pressure to bleed. I guess I should just combine the two files into one.

Edit: I fixed some locked cells that didnt allow changing the valve timing in case modifying the camshaft or rockers. Added some info about dynamic CR.
 

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  • TDC Air Temp & Pressure.xls
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  • 6.5_CompressionRatioCalculator.xls
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View attachment Comp ratio impact - CI - A.pdfView attachment Comp ratio impact - CI - B.pdf

Took me a while to recall where I had a reference that describes the impact of changing compression ratio in compression ignition engines. This text below describes things far more concisely than I can. Sorry for the horizontal pics, but rotating them chopped off the top of each page. If you've got Adobe Reader, click each pic to open the file, then rotate it on your screen.

Here's the accompanying text -

Compression Ratio (r) Effects, in compression ignition engines

Increasing the compression ratio exercises the following effects:

- The cylinder compression pressure & temperature increase: the ignition time lag between the point of fuel injection to the instant when ignition first commences reduces

- The density & turbulence of the charge increase, and this increases the rate of burning, & accordingly the rate of pressure rise,& the magnitude of the peak cylinder pressure reached. The characteristics of the pressure rise relative to the piston stroke or crank-angle movement is shown in Fig. 6.3 & Fig. 6.4

- Thermal efficiency & the specific fuel consumption are improved. (Fig. 6.5)

- Raising compression ratio results in reduction in the mechanical efficiency as shown in Fig. 6.6 (since the higher cylinder pressures increase the pumping losses, friction losses, and compression & expansion losses, as more work is done in squeezing together the trapped air charge).

When interpreting the cyl pressure curves, remember the pressure (& temp) peak is the most likely point of headgasket failure. Maybe GM was lowering the 6.5's comp ratio as they increased it's power output to keep warranty claim rates in check. Maybe it was to lower NOx emissions. Would have to hear from both their engineers & bean counters to know.

Also look at how the thermal efficiency & specific fuel consumption curves are behaving around the comp ratio's we're considering. Yes, lower comp ratio is giving up both thermal & fuel efficiency - look at the % difference btwn 18 & 20:1 - above 18:1, the differences are fast approaching insignificant. I would wholeheartedly agree that there's efficiency to be gained going from 19:1 to 21:1. But not a great deal. If you gain 2% on a typical modded 275 engine hp 6.5, you'd gain 5 hp.
 
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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.
 
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