Absolutely... I am with you on not wanting to push the engine to failure. I feel my engine now at 13psi with the IC and stock compression is pretty decent for what it is. I'd like to try 20psi with 18:1 but only as long as durability and longevity isn't compromised. I can't imagine trying to run 30+ psi on a 6.5. But just to ask does your engine run substantially different with the low compression and 15psi than before the build?
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HX-35 holset
- Thread starter Samyguy
- Start date
Absolutely... I am with you on not wanting to push the engine to failure. I feel my engine now at 13psi with the IC and stock compression is pretty decent for what it is. I'd like to try 20psi with 18:1 but only as long as durability and longevity isn't compromised. I can't imagine trying to run 30+ psi on a 6.5. But just to ask does your engine run substantially different with the low compression and 15psi than before the build?
SmithvilleD
Active Member
I can't really say, as I bought the truck w/ a failed head gasket, so I never drove it w/ stock compression beyond a short test drive & on/off the trailer getting it home.
In comparison to other stock 6.5's I've driven, I suspect there is a little loss/difference in throttle response.
And in theory, there should be some thermal efficiency loss associated w/ lower compression. But that relationship in not linear, and as comp ratio's go higher than 17/18/19:1, the law of diminishing returns is kicking in.
I keep pretty complete fuel fill/mileage records, and it has averaged well into the 18's for the first 5k miles w/ 180 degree thermostats. Obviously, I can't say it wouldn't be doing better w/ 21:1, but I'm happy with it's mileage. Swapped to 195's & the first few tanks looked to trend going up about 1 mpg. I need to put my latest fuel receipts into my mileage spreadsheet to see how the recent cold snap & any winter fuel blends have impacted things.
The lower compression does definitely impact starting as temps go below say 45 degrees F. I've done a bunch of things to enhance the starting systems. It wouldn't be very tolerant of bad glow plugs, etc. But the stuff I did works as it starts reliably w/o excess white smoke, etc. A bit over a week ago it started after an overnight low at 0 F, with plenty of extra/manual glow time, but no block heater. Only did it once, to know if I could trust it to do so, if necessary. Otherwise, it'll get plugged in when I can.
Samyguy
New Member
Hey SmithvilleD could you elaborate on your cold start mods for a 17/18-1 comp
motor? As I'm thinking of going this way, but it does get cold here -15 last week.
motor? As I'm thinking of going this way, but it does get cold here -15 last week.
dieselfuelonly
Member
Speaking of lowering compression on these engines you can also do it with thicker head gaskets from Cometic, but how well it would handle under 25+ psi boost pressure I don't know. Being that they are thicker you wouldn' get that closer head(s) to block mating surface as usually seen with the stock thin gaskets. I'd really much rather do it with the pistons as it would be the best way to do it, but I've heard and seen prices that the lower compression pistons aren't cheap. How much does a set of new 18.5 pistons run and is there a full engine rebuild kit you can buy that has the lower comp pistons included to replace the stock comp pistons? Thanks.
When I rebuild my engine I will deck the pistons and coat the crowns. Let's see how much the block needs to machine off to straighten out the deck. The cost of Marine pistons are ridiculous, and it only requires a little machining to get the numbers needed for 18:1. Also while machining the rods I will see how much I can shorten them. I know some of the board members running 18:1 and a DB2 are running 20psi but I don't know what the maximum boost pressures people have run successfully running the lower compression. I do think it is better to run stock 6.5 gaskets or a gasket that is stronger than stock but not thicker. I think the thinner the gasket the less area for failure especially under higher boost. Anybody run 25psi or greater?
SmithvilleD
Active Member
I can only comment from experience on my engine, which should be right around 19:1. chevydiesel has said there's a significant difference btwn 18:1 & 19:1, & I believe him to know this from experience.
First, it's good to understand the somewhat unique challenge IDI diesels face in cold weather starting. Diesels are compression ignition. The air in the cylinder must get hot enough upon compression to ignite the diesel. When the engine's all warmed up-n-toasty, all the cast iron around the combustion chamber is warm. When the engine is cold, all that metal is cold. So upon cold cranking, some of the heat built by compression is being absorbed by all the cold metal surrounding the chamber. All diesels face this.
In addition, our 6.5 IDI diesel has a little pre-chamber (spherical space) that's above the cylinder & connected into the cylinder by a little slot. Upon compression, a good bit of the entire cylinder's air charge is compressed up into that pre-chamber. The injector squirts the fuel into this pre-chamber & that's where ignition must start. In essence, the pre-chamber walls are additional metal surface area (that direct inj diesels don't have) that absorbs additional heat (from compression) out of the comb chamber during a cold start.
Lower static compression = less compression heat being built during cranking. As cold start temps get colder, at some point, you approach where glow plug & compression heat just isn't enough to get clean compression ignition & a good start.
I bought what I believe is one of the better battery sets available & easily fit into our trucks. They're the Sears Diehard Platinum P-4, AGM style, 880 CCA, 130 min reserve. Quite expensive at $180 each. But they've soundly beaten the redtop Optima's in Consumer Reports testing for several years now. 4 year free replacement, 100 month pro-rated warranty. They're the same battery sold under the Odyssey name & used in some military apps. I dared spend that much on batteries because I have some friends that have had this type Odyssey battery last 8,9, even 10 years in their hobby cars.
Next, I built 1-0 gauge batt cables utilizing the top-post mount (saves the sidemounts for stereo amps, other power connections). The cables go to a Powermaster starter that really spins a 6.5.
I also run Mobil 1 5w-40 synthetic oil.
This stuff was done to spin the engine as fast as possible during starting. More compression strokes per unit time = putting heat into the chamber at a faster rate. With these batteries, cables, & starter - a recent start at -8 F still spun the engine over quite speedily. In short, this thing really cranks!
I recently put in the Bosch Duraterm glow plugs. They're self regulating & should tolerate the additional glow time I do manually, before turning the key & letting the PCM do it's WTS glow cycle.
As said before, all this allows it to start pretty well, w/o undue hassles. I'd still emphasize, this engine isn't going to be very tolerant of bad glow plugs, etc. Had 1 plug/connection go bad before the Duraterm's. First time it was started at ~ 10-15 F w/ the bad plug, it fired up fine - but the cylinder w/o glow took prolly 5-10 seconds to begin firing cleaning. Put on a bit of a smoke show in those 5-10 seconds. Besides not being optimal, it also makes one feel pretty conspicuous setting in a truck running roughly spitting smoke.
6.5 head gaskets are horribly designed,If you compare it to a stock cummins head gasket its very obvious . The felpro gaskets are perforated core, like a 3400 chevy venture every body knows how great they are. Ive seen some gas motor head gaskets that look 100 times better than felpros for the 6.5 I actualy bought a brand new set .10 thicker for my 6.2 they they looked very flimsy I didnt even use them,I tried the cometic from summit they started leaking badly on the first test drive, my fault,over advanced timing,took the heads back off and cut the rivits on the cometics and put aviation sealer on both sides of every layer, indian head shellac might have bin a better choice, torqued the head studs to 150 ft.lbs and test drove again not so much timing this time, they stated leaking promptly at probably 60 psi boost. So far the best best gasket I found is the victor rienze solicor for the 6.2 its not perforated still not greatest looking but better.So far they held up to a few 50 psi passes ,I also roughend the head and block with an engraving pen,in hope for more grab,no sealer, I think they might seep a little, over flow gurgles after I shut it of. It looks like the 6.2 head gasket will work on the 6.5 on stock bore only.cometics would work better with more layers ,and nix the center one?Also head and block must be perfect for mls, mine isnt.I talked direct to cometic and he said they make the head gaskets for the amg optimizer of, course that a brand new motor it should seal, probably very little boost also. Mine al ways blew at the end cyls front and rear my felpros where bent there and the 6.5 victor reinze, the cometic is just burned the viton coating ,same place at least cometic blocked the front water ports, they have a left and right. I wonder if factory gm head gaskets are good? Dont torque to 150 it lifts the block around the bolt holes I torqued to 135 with mistic red grease last time the 150 was with moly arp recamends 130 with 30wt oil, I think with moly its 110 . If you would put 6.5 pistons in a 6.2 would they be.020 reduced compression height, account of the 6.5 having a .020 longer stroke.
- Staff
- #29
There needs to be a disclaimer here:
Torque specs for the heads on a 6.5 are in 3-steps (as the bolts are TTY), with the second step being 75 ft-lb. Putting 150 ft-lb on studs is asking for issues, as is running 60 psi boost.
Nothing you are reading here should be attempted on a GM 6.x engine that you want to run for more than a block.
That being said, it's just a disclaimer... anybody who wants to do this just to find out what happens does so entirely at their own risk.
Torque specs for the heads on a 6.5 are in 3-steps (as the bolts are TTY), with the second step being 75 ft-lb. Putting 150 ft-lb on studs is asking for issues, as is running 60 psi boost.
Nothing you are reading here should be attempted on a GM 6.x engine that you want to run for more than a block.
That being said, it's just a disclaimer... anybody who wants to do this just to find out what happens does so entirely at their own risk.
ak diesel driver
6.5 driver
Aw come on Jim we don't need a disclaimer just videos
NVW
Well-Known Member
Yeah Jim, quit acting your age!):h
- Staff
- #32
ROFL... yeah, I just don't want somebody trying this with his stocker and saying 'But I read all about it on...(insert other Diesel site here) - THEY didn't tell me it was gonna explode!'
I asked about reliable boost................. If one were to try (and I am floored on this one) 30psi I would think they would o-ring the block and heads and use a copper head gasket. I can't even comprehend 60psi on a 6.5. Hats off to you though!
BigBlueChevy
Compression Ignition Addict
Awsome thread so far guys :thumbsup: Its always interesting when threads derail cause someone starts sharing there wealth of knowledge on a subject.
Reguard 6.2 Diesels post about pushing 60psi, or hell enven 50 psi of boost on a 6.2... I'm not going to ruin to ruin the thread or smash what your saying, but is there by chance a video you could put up of the truck running this level of boost? Most guys around here are lucky of they hit 20 psi without blowing something up.
However if you are actually doing this by all means please share!
Reguard 6.2 Diesels post about pushing 60psi, or hell enven 50 psi of boost on a 6.2... I'm not going to ruin to ruin the thread or smash what your saying, but is there by chance a video you could put up of the truck running this level of boost? Most guys around here are lucky of they hit 20 psi without blowing something up.
However if you are actually doing this by all means please share!
HighSierra86
Active Member
All that I have to say is at 50psi, the last of my worries would be head gaskets!!
But back to the holsets......is there someone who can explain the difference between an HX and a HY-35 holset? I believe someone on here had posted their experiences on using both turbos, and had noticed different performance and economic charachteristics of both.
Fun reading so far!
But back to the holsets......is there someone who can explain the difference between an HX and a HY-35 holset? I believe someone on here had posted their experiences on using both turbos, and had noticed different performance and economic charachteristics of both.
Fun reading so far!
gulfyankee
New Member
SmithvilleD
Active Member
The HY is a little tighter turbine section (9cm^2). Most HX-35's came w/ the divided 12cm^2 housing.
The HY's tended to come on the 5.9 Cummins trucks w/ automatics. Before the torque convertor locks, torque convertor slippage allows engine rpm to "flash" up quicker (than w/ a manual tranny) when you first put the pedal down. During that transition, the tighter turbined HY-35 has a lower boost threshold, so it catches up w/ air demand a bit quicker than the HX-35. So on the auto tranny trucks, the HY helped meet emissions regs by cleaning things up in that transitional rpm jump.
I've heard the HY-35W from the 305 hp '03 & '04 5.9 Cummins had a turbine section still called 9 cm^2, but the turbine was differently shaped & flowed better than previous 9 cm^2 housings.
Forgot to mention, kids, dont try this at home, this stunt was done by trained professionals. I just wanted to share my cometic story eventually I will post pics and video on my own thread . Not to worry, 60 psi is not possible with out lots of fuel. I recently found out my h1c seized from high drive pressure, so hx 35 will have the same problem, and their waste gate is small , I would recommend checking drive pressure especially pay attention to your readings at higher rpm .
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SmithvilleD
Active Member