• Welcome to The Truck Stop! We see you haven't REGISTERED yet.

    Your truck knowledge is missing!
    • Registration is FREE , all we need is your birthday and email. (We don't share ANY data with ANYONE)
    • We have tons of knowledge here for your diesel truck!
    • Post your own topics and reply to existing threads to help others out!
    • NO ADS! The site is fully functional and ad free!
    CLICK HERE TO REGISTER!

    Problems registering? Click here to contact us!

    Already registered, but need a PASSWORD RESET? CLICK HERE TO RESET YOUR PASSWORD!

Lower compression pistons

6.5coalroller

6.5coalroller

New Member
Messages
184
Reaction score
0
Location
Farmland, IN
Does anyone sell lower compression pistons? Can they be custom made if i bought a new set could I have them cut down and recoated?
 
Mahle did make them but I think I heard they were discontinued, There may be old stock kicking around. Yes, they can also machine standard pistons, I think that is what Kennedy Diesel did.

Leo
 
You can have custom made pistons, the tops are fly cut. I have the ability to turn down piston tops on my lathe.

I have a low compression motor (18:1) and would have to think twice to do it again. If I was towing all of the time and running a lot of boost from the turbo I would do it. The 18 to 1 motor can be temperamental in the winter here in the north.

The motor likes high temp thermo stats to run efficiently ( I run A 195) had 180 stats and got about one more mile to the gallon of fuel by going to a 195 stat. The motors are great when pulling a load under boost at 2200 to 3000 rpm though. This is where I tow with 4:10 gears. I like it when towing but wouldn't spend the money to do it again. Just my two cents worth.
 
You shouldn't have a problem finding a machine shop that will cut down your stock pistons for you. It's not the best route to go in my opinion, but I haven't heard of any failures due to cutting down these pistons, just make sure you get a thermal barrier coating on the tops of the pistons after machining.
 
I'd suggest calling Diesel Depot or some other builder that does marine 6.5's & has a good relationship with Mahle Clevite. They should be able to confirm if you can still get the 18:1/marine 6.5 pistons from Mahle Clevite.

You occasionally see a set pop up on ebay or the 6.5 forum's parts ads.

Just an opinion, but I'd be inclined to think the flow pattern designed into the stock & 18:1 Mahle's would likely have some fuel/air interaction/combustion efficiency benefits in relation to the IDI pre-chambers.

Been a bit surprised at fuel mileage as theory & logic says lowering compression will give up efficiency. My truck's averaged 19 mpg over the first 5k miles on the fresh 18:1 engine. I still believe it impacts economy, but not to the degree I'd intuitively expected.

I agree on the tradeoff with cold weather starting. Benefitting from previous 6.5er's 18:1 reports, I added a manual glow extension/override, best batteries I could find, 1-0 batt cables, Powermaster starter, etc. Started OK a couple of ~15 degree F mornings last spring, fired OK but took longer to run cleanly. Sorta expected that with all new components providing good starting rpm. Will know more 3 months from now.
 
Diesel Depot offers about anything you want, 18, 19, 20, or stock. Several people probably have the thicker head gaskets and could comment on how that works.
 
I know about that stuff. what I want to know is has anyone documented anything for example has anyone built a 19 motor and verified how it starts in cold weather and what it did for power etc
 
ak diesel driver said:
also what would be safe boost levels to run with different ratios?
Click to expand...

That's a very open-ended question. As far as I know, as much as your internals can handle. I have yet to see ANY solid gains that people have gotten from an 18:1 engine. I will be keeping my compression ratio as high as I can until I find a reason to do otherwise.
 
I think if i was building one, i think i would shoot for around 19.2 to 19.5. Shouldn't be as tempermental to start and would feel a little safer about running higher boost levels. I would think that you would run into problems getting enough fuel out of a stanadyne pump before boost pressure would be an issue. What pump are you planning on running on your motor Inline?
 
Several 19:1 ish guys running around, unless towing big & often I question the need to lower CR myself, I once thought of going to 18:1 & have a new set of (8) 18:1 paper weights I can let go pretty cheap if anybody interested PM me.

The stock CR was chosen for emissions-economy-& ease of start, since we aren't being checked for emissions with a sniffer in most places we can get away functionally of trying some things that GM wasn't allowed to do, like fuel rich cold starts on a cold morning with a lowered CR.

Keep in mind Heath is running stock CR in all his, and IIRC he had/has a few running around with higher CR, I think that not sustaining boost & constantly peaking far above design spec., ARP studs vs bolts, and maintaining healthy cooling system are key to keeping the 6.5 alive.

We are much smarter with the 6.5 when 1st introduced, I think many of the failures that gave the engine it's bad reputation were as much the uninformed drivers fault as it was some poor design decisions made to keep the cost to produce them lower.

Now that we know to religiously watch for things & have installed gauges; like coolant temp, peak boost & EGT, heavy black smoke when boost is marginal, and the fact that it is a Diesel designed for light to moderate towing duty, just because it says Diesel under the hood, does not mean it's going to be able to pull big for long without some preparation for it 1st, early owners just hooked to anything & went with it then denegrated the engine when it came apart doing that for which it was never intended to do.
 
Rite now I jus have a ds4 pump but I plan on getting a chip made and doing the feed the beast. The marine injectors might help too. I've already got arp headstuds but i'd prolly need to upgrade to arp main studs as well.
 
I think the thicker head gasket and the ARP head studs will do you fine then.

Its not that the engine can't "handle" the boost, its that they would get too hot and the turbine drive pressure would be really high. Well with the Holset you probably won't have as big of issue with those.

You should be fine with 20psi sustained boost. Even with my GM4 for a year I ran sustained 15 to 17psi up long hills, going 85MPH and up over 100MPH.
 
chevyinlinesix said:
That's a very open-ended question. As far as I know, as much as your internals can handle. I have yet to see ANY solid gains that people have gotten from an 18:1 engine. I will be keeping my compression ratio as high as I can until I find a reason to do otherwise.
Click to expand...

The compression ratio tradeoff is btwn ease of start, power/emissions/economy - vs - durability/longevity. Little doubt some degree of efficiency is lost as comp ratio is lowered. The original 6.9 IDI Ford/IH diesels were 20.7:1. As the OEM HP/torque competition increased, they went to 7.3L & up'd compression to 21.5:1 (before going direct injection). Probably also fair to say those engines were a bit more structurally rugged than the 6.5.

Given how important emissions, power, economy, & startability are to an OEM, it's notable GM took the 6.5 from 21.5:1 to 20.2:1. Does anybody know for certain what the latest P400 6.5 comp ratio is? Would expect these guys to have good statistical data to understand the compromise.

The 18:1 marine 6.5's are designed to operate at a higher rpm & higher constant load, so it's a bit different situation than road going 6.5's. They run turbo's designed to have an appropriate pressure balance for their operation range & still went 18/19:1. The marine crowd is understandably more failure/risk averse; engine failure out to sea is a more serious & expensive issue than on the highway. It's fair to say the higher power marine 6.5 providers believe the tradeoff is worthwhile in their applications.
 
Last edited:
SmithvilleD, what I was meaning to say was I haven't seen any numbers, like say from a dyno, you know... that thing that every 6.5er seems to be afraid of :confused:

I can see how lowering the compression ratio reduces peak cylinder pressures, but when you pack in more air to make up for it, isn't that kind of counterproductive because you will be once again raising the peak cylinder pressures?

6.5coalroller, depending on what you plan on doing with the engine, how much time/money you have, and how long you want it last will dictate if a rebuild is necessary.
 
You must log in or register to reply here.
Back
Top