lowering compression: shaving pistons or shortening stroke, or thicker gaskets...

[Turbine Doc]

Not to disagree, well maybe a little; as goes up speed so does drag U know that, 150+ mph in a full size/full weighted GM truck Bonneville speed over a 3 mile course @ 5000' elevation IS a workout from reports of those that went there last year the time for the LSR to get to 100+ is pretty impressive .

I've not lowered CR myself I had thought about it at one time even have the 18:1 pistons out in the shop, I'm still at stock CR been towing that way for years though I self limit to 14 psi peak boost when moving my 18K# big load 2x a year mostly at seal level running to 300' or so at finish of my trips.

Bill specs re-mans he gets for folks to 22:1 and runs some higher than that in some of his shop trucks which are daily mountain drivers & tow rigs.

From what I recall when Bill was drag racing the 6.5 he was down to 14.5 CR or some such low CR, but running maybe 60 psi boost ??? on a special turbo, special IP & injectors & it took like 15 minutes to get it to get hot enough to make runs, but once hot it could make front wheels off ground launches.

As for cam Bill is back to a stock cam, as "special one" talked about in DP article didn't work as hoped, I don't remember what he said about valves & springs if he's going back stock there, probably running stiffer springs for 4000+ rpms he's running he did discuss that a while back.

Another member visited the shop that makes cams for Bill says according to the cam shop owner, he thinks that was about 90th purpose grind & valve combination's he's seen used on the 6.5 over the years, by different grind customers coming back to the stock grind when all is said & done as best cam overall mirroring Bill's comment last month when I revisited cams with him.

A purpose built cam/valve combo is probably out there waiting to be found with the magic combination for strip, more than a few have tried it the 6.5 has been around a while, sacrificing the vehicle utility to a sport driven vehicle; this is performance area of the forum bound to be someone that unlocks the key given enough resources and doggedness to make a point.

 

[Turbine Doc]

Back to OP topic; as far as peak pressure & CR, best thing as math will get you close, is to measure the dynamic pressure on a test stand but even that requires some sophisticated sensors as once fuel comes into play your pressure sensor has to handle the heat as well as pressure, if you had the equipment you could "listen" to the bang with an acoustic sensor and from that determine, but I doubt a specific calibration threshold has been established for the 6.5 at peak compression pressure as various CRs levels to give any usable data other than it makes a lot of noise in there.

Which put us back to do the basics bore x stroke to rough it like racers have done for years for CR or spend some time here if you wanna get deeper into the thermo calcs http://hyperphysics.phy-astr.gsu.edu...mo/diesel.html or plug in similarly into Buddy's spread sheet , pick a CR that suits your number crunching.

Then figure how you wanna get to your desired CR take to track and hope it stays together as math/science & dynamic forces don't always agree, all ways to change CR are open for conjecture/opinion which is best not really enough 6.5ers running various ways to get there to say which is best, talk to one guy your way is wrong,

I was gonna go shaved ones from Mahle until spending time with Bill on phone about what 18:1 would get me and negatives Slim Shady had with his 18:1 . Dummy me I should have bought them OS some vs std bores that way I could put in one of my used blocks. As far as what works methods of lowering CR best ??? Bill raised his CR which is 180 from others concepts.

Guys on Page did the 1st 18:1 engine anyone talked about feeling that was the way to go, several there swore my "oil squirter" at stock CR & way I tow would have been long done in by now, I'm almost 2x over the mileage now of predicted demise mileage.

Thicker gasket from what I've seen over the years seems to be the least preferred CR lowering method especially if upping boost levels over GM boost levels, wrist pin height changing was once an option from piston mfrs, but I think those would have to be custom pistons now; leaving Kennedys with opened up centers or reduced piston height if you can find them also now.

I have no personal experience other than purchasing but yet to install the "shave top" Mahles sitting on my shelf and the thicker gasket seems to get the most negatives for opinions offered for which is best way to go since I started studying up/visiting forums on the 6.5 in 2000 when I got mine.

 

[buddy]

Bill also has modifications to lessen drag, higher gearing and weight savings, so its not just his powerplant that gets him there. Ive run 130mph for about 5 miles on diesel alone, the engine was happy, ECTs werent high, EGTs werent high. I attribute that to my large exhaust, the ATT, and my old tired engine losing compression around the rings, its like a choo choo train out the oil spout.

Getting to 160mph is certainly more work, and the custom exhaust headers and twin turbo help quite a bit, he built a fast truck for sure. Any of the trucks he sent out of the shop for towing more than likely did not have the fueling capability and running 30psi of boost, and so at the power level the trucks are capable of, it would seem a lower CR could be quite adventageous. I doubt anyone would like seeing 1400F EGTs.

I'm not going to be concerned with anecdotal cam change theories, since we dont know what they tried, what science they prescribe to, and people have opinions on it. Heath using higher CR would show that his cam would not be anything close to what I think would help. Other than potentially lower dyanamic CR from BDC valve timing, but none of the gains I would expect at TDC valve timing, because he couldnt mess with it with higher CR. Pigeon holing the idea as possibly only being useful purpose built for the strip is the only beef I have with some starting opinions not based on known facts. There is likely a purpose built cam for better highway mileage and towing power waiting to be demonstrated. It might work at the strip too.

 

[Turbine Doc]

Pigeon holing the idea as possibly only being useful purpose built for the strip is the only beef I have with some starting opinions not based on known facts.

You are making some assumptions as well Tom about what is & isn't know to be fact/fiction I spend a lot of time with Bill learning what I can from folks that have been playing with the 6.5 since it's inception.

Naturally I can't share out of confidence assurances made to him what exactly what was done to get where he is and has been street-strip-and salt in quest for performance, I'm sure you aren't putting out all information with some of the things you do for quest for speed, no racer tells all their secrets even Bill to me and we've been conversing since about 2001.

As far as the cam pigeon hole if we wanna talk cams lets go to back to the cam thread I did not mean to infer better cams for strip only. Maybe I'm misunderstanding what I read from the previous posts but it reads to me like you are diminishing the accomplishment. We are really getting off topic so let's take this to PM or cam thread.

To OP I apologize for derailing thread with back & forth what is happening with the LSR.

 

[buddy]

DP claimed his LSR used a different cam, which could affect his dynamic CR, and negate the slight increase in static CR. So it would have been good to know if he uses that or not to know if he really has 22:1 compression, making a cam grind another option for lowering CR. With a stock cam the valve timing might lower CR by just 1% since it closes the valve fairly close to BDC. The loss around the rings is variable depending on RPM, since it will bleed out around the rings more with more time, like when cranking, which combined with less oil sealing is how you get down below 400psi. At 3000rpm you have much less time for it to leak. So the dynamic CR is quite difficult to determine.