mahle 18:1 pistons discontinued- how much will 10 thou drop my comp ratio?

[Tookie]

Discontinued - but there may still be sets lurking around?

dieselautoparts.com





staff edit direct hot links to non site vendors that compete directly with supporting vendors is not allowed, discussion is okay, that vendor also found as DieselDirect

 

[GJF]

Let's look at this......

$750 for 18:1 pistons

$425 for 22:1 pistons

You need to rebuild your rods when rebuilding your engine using either piston.

Matching all the rods to the same length is basic blueprinting and should be done regardless. Just have them shortened .015 - .025 and save yourself $325 by using stock pistons.

Bringing all parts to spec and matched weights add for a smooth, durable running engine.

 

[GJF]

And not all 383s have to have the skirts modified. Depends on what combination of parts you use.

Sorry AK,
I was just using the 383 as an example. It is nice now a days buying a 383 kit and simply building up your engine. Back in the day there was a few routes you could go to get the 383 combo and you had to machine the parts to fit.:thumbsup:

 

[ak diesel driver]

I've built about 4 383s and none of them were kits. Only had one that I had to clearance anything. And it was the block. I'd also be suprised if you would have to modify anything on the 6.5.

 

[Jasonsmack]

Many machine shops have, or have access to a Tobin Arp pin boring machine. They can put an undersized pin bushing into the top end of the connecting rod and bore the bushing out, setting the pin to pin length at the desired setting. The same proceedure could be done on a milling machine but it would require a jig or alot of time measuring and setting up each connecting rod. While the shortening of the connecting rod at the big end does work it is not something that most machine shops want to put the connecting rod through, nor the rod hone. I have done it on engines (different application) and it does work. I have never taken .030" off the length of a rod that way. That would require honing .060" or more out of the big end of the connecting rod, that is alot of work if you do not have a power stroking connecting rod hone.

 

[GJF]

If you have no 18:1 pistons this is another way to acheive your goal not just an option. ccing the chambers and deck to determine what you can do is a normal practice. While it can get into a expensive option depending on which way you have to go. As Packratt stated his combination would work well with .005 shorter rod for a 20:1 motor. Add a .010 thicker head gasket and you would probobly be arount 18-18.5:1. Also even though the piston might be listed as a specific compression ratio, by the time everything goes together it rarely is the compression ratio stated. Proper machining of all components and block are required to achieve exact ratios.

 

[Turbine Doc]

Add in the question of is it really necessary to go to 18:1, if staying with stock boosting levels there is no reason IMO to go there, it's only when going 15+ sustained non cooled boost where one needs to explore this option. IMO save the $$$ to splay and 18:1 piston, for a better crank, Fluid damper, cooling mods and better turbo like ATT and drive on.

 

[GJF]

Many machine shops have, or have access to a Tobin Arp pin boring machine. They can put an undersized pin bushing into the top end of the connecting rod and bore the bushing out, setting the pin to pin length at the desired setting. The same proceedure could be done on a milling machine but it would require a jig or alot of time measuring and setting up each connecting rod. While the shortening of the connecting rod at the big end does work it is not something that most machine shops want to put the connecting rod through, nor the rod hone. I have done it on engines (different application) and it does work. I have never taken .030" off the length of a rod that way. That would require honing .060" or more out of the big end of the connecting rod, that is alot of work if you do not have a power stroking connecting rod hone.

Absolutely, I have had the piston pin offset bored and pinned along with resizing the big end offset to get the length I needed. No never .030 off one end I think that would be too much for either end by itself.

 

[J.Peart]

Also keep in mind, offset grinding the crank is also an option, -that COULD yield another .010" -if needed.

Again, not a first choice, -but still a viable option.

In my opinion, -a 19:1 motor will still open a huge door for safe elevated boost levels.

The Mahle's and Zollners I've seen sure look like ceramic, -with carbon graphite skirts.

Sure is a shame we can't get these anymore.

I've got a brand new SCAT crank sitting in the garage- I dont want to mess with it other than for balancing.

The mahles i ordered are hard anodized- doesn't say anything bout ceramic.

And yeah its too bad the are discontinued- i had them search high and low for these and they couldn't even find any in stock in any of their stores or warehouses- and i checked a couple companies.

As Packratt stated his combination would work well with .005 shorter rod for a 20:1 motor. Add a .010 thicker head gasket and you would probobly be arount 18-18.5:1. Also even though the piston might be listed as a specific compression ratio, by the time everything goes together it rarely is the compression ratio stated. Proper machining of all components and block are required to achieve exact ratios.

yeah i think i will get the .010 thicker gasket to go with my pistons. And i realize the numbers aren't exact- however, i am having a machine shop machine the block and balance the rotating assembly etc. this shop has a good reputation and has done many 6.2 and 6.5l's.

Add in the question of is it really necessary to go to 18:1, if staying with stock boosting levels there is no reason IMO to go there, it's only when going 15+ sustained non cooled boost where one needs to explore this option. IMO save the $$$ to splay and 18:1 piston, for a better crank, Fluid damper, cooling mods and better turbo like ATT and drive on.

Well, i don't need to go all the way down to 18:1- 19 or 20:1 is alright by me. I already have a lot of mods to this truck and i don't plan to intercool. - however the GM-4 is going on the '87. i have an ATT sitting in the garage:thumbsup:.

Also upgrading to '97 cooling and '99 heads, SCAT crank, main studs, and head studs amoung other things, i was gonna go fluid damper but just ended up getting new stock balancer and pulley (figured this should be fine with the SCAT and main studs and saves me about 400 CAD-stupid exchange rates :mad2.

finally got a pyrometer too- when it blew the boost levels were fine (about 12 sustained, 15-16 peak) but i guess the EGT's could have been to high- stupid on my part i geuss.

Jpeart: where in Kelowna are you? I have probably seen your truck around town.

I live in westbank now, but i used to live and work in kelowna and still get across the bridge quite often. I stopped driving the '95 in september, so if you've seen is since then its not me.

Ive been driving the '87 since. its rusty and white and silver on 31"s with white rally rims. barn doors, cab marker lights, sagging rear springs. loud.):h

before the '95 it was the black/grey and red 86 blazer. lifted 3" on 33" BFG mud terrians, big winch bumper w/ lights, boat rack.

If you check my garage on the 'Place there are pics of the '95 and the '86.

 

[J.Peart]

I called mahle myself today. it seems that certain 18.5:1 pistons have been discontinued, but they still make them. Looks like i can still get them serves me right for listening to the guy at Lordco. :rolleyes5:


For your reference they still make the 027060ha model number but the wr (with rings installed) option has been discontinued or something- he wasn't too clear.

the 027070HA can still be had with the wr option. the 027070HA isnt listed in the catalogue descriptions but its also an 18.5:1 6.5l piston, the mahle guy gave me this number.

 

[Jasonsmack]

I talked to the Mahle reps at the PRI show in december and they said they were going to send me a catalog that applies to the diesels, still waiting....

 

[chevyinlinesix]

Changing the connecting rod length will not affect your compression ratio.
Factory pistons are hard anodized (I believe only for turbo engines)
Zollner pistons are still made for the 6.5, but I have heard bad things about them, not just in 6.5 applications)
You can cut down the stock type pistons and then have them either hard anodized or have a thermal barrier coating applied to the tops.
Bringing the pistons down .030" there should be enough clearance (I didn't measure) but I can tell you it's damn close factory. Trimming the counterweights is an option, but the whole rotating assembly would have to be balanced then.

 

[turbovanman]

There IS a shop I use in Seattle that is fully capable of doing ceramic coating (titanium dioxide). I was under the impression that the piston tops were ceramic, not anodized, -am I wrong?

Performance coatings? if so, they are awesome, I use them too, :thumbsup:

 

[Matt Bachand]

Changing the connecting rod length will not affect your compression ratio.Factory pistons are hard anodized (I believe only for turbo engines)
Zollner pistons are still made for the 6.5, but I have heard bad things about them, not just in 6.5 applications)
You can cut down the stock type pistons and then have them either hard anodized or have a thermal barrier coating applied to the tops.
Bringing the pistons down .030" there should be enough clearance (I didn't measure) but I can tell you it's damn close factory. Trimming the counterweights is an option, but the whole rotating assembly would have to be balanced then.

I thought we resolved in the performace section that changing connecting rod length will slightly change compression because you are changing the 'squish size'...

??

 

[buddy]

Yes, .01" off the pistons or the rods, or increasing the gasket thickeness would bring it down to about 20:1 comp ratio.

Doing two of those would be about 19:1 ratio.

Some of the math is in THIS POST

 

[chevyinlinesix]

I thought we resolved in the performace section that changing connecting rod length will slightly change compression because you are changing the 'squish size'...

??

Go to this site for example (that is Keith Black's Silvolite website, they do not make competing products with any vendor here that I am aware of) or any other compression ratio calculator will tell you that connecting rod length will not affect static compression ratio (which is how engines are normally spec'd)

IIRC, I already said before that it WILL affect dynamic compression ratio, because changing the connecting rod length, will in effect change the location of the piston in relation to camshaft timing. However, dynamic compression ratio is an educated guess in itself.

Here are the correct numbers to enter in that calculator:

Cylinder head volume: 26cc
Piston head volume: 5cc
Gasket thickness: .040"
Gasket bore: 4.100"
Cylinder bore: 4.030"
Deck clearance: -.003"
Stroke: 3.820"
Rod length: 6.280"
Intake closing point: 31 deg. (that's calculated as per common instructions)

As you can see that gives you:
Static compression ratio: 21.465
Dynamic compression ratio: 20.420

Change the rod length, and it does not effect static compression ratio.

You COULD drop your stroke to about 3.5" to get a 19.7:1 static compression ratio. However, shortening the stroke by .010" will change static compression ratio from 21.465 to 21.411 (not very much at all) Take note of how many decimal places back the change is, you would get a similar drop just from thermal expansion and RPM change.

Note: These numbers are from my engine, my block, my heads, my precups, my pistons ect... every engine is slightly different. I can show an VERY simple way to explain it, but figured I'd try and give you guys some insight on it first

 

[JiFaire]

Need to keep eyes on the basics...

Changing the rod length will not change piston aspect vis-a-vis cam timing position... TDC is still TDC for each piston, just that the new TDC is 1/10,000" shorter than the last one (stroke length is changed)

It indeed won't affect static compression ratio, which is calculated as a function of Cylinder max. volume (bottom of intake stroke) to cylinder min volume (top of compression stroke, becuase the difference in rod length is taken off both ends (smaller intake volume as well as larger compression volume).

As Buddy indicates, bringing TDC down by 1/10,000" will increase static compressed cylinder volume, which will decrease the compression ratio - trimming the pistons will affect only one end of the volume calculation, changing rod length affects both ends.

Increasing head gasket thickness increases both ends.

 

[davo727]

Cometic makes head gaskets that can be specified .030 thicker than stock. Missy had to use one on her last build to make up for a block that had one deck milled twice.

 

[chevyinlinesix]

James,

It's very hard to figure exactly, as topping the pistons will change the volume of the "Comet Riccardo" dish in the piston.

If you were to use .010" thicker head gaskets, it would drop you to 20.39:1 static compression ratio, and a dynamic compression ratio of 19.4:1.

If you use .010" topped pistons, the compression ratio would be slightly higher than the above numbers.

 

[SmithvilleD]

As always my goal is to correctly understand things, & spatial orientation isn't my strong suit, so I draw things out.

Here's a drawing of how I understand changing rod length, piston pin height, or piston crown height affects compression ratio. Granted I pictured cyl area instead of volume, but as the bore stays the same.....

I believe the comp ratio calculator on the Silvo-lite site only includes rod length in the dynamic comp ratio calculation.