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Discussion Starter · #1 ·
hello everyone I was wondering if anyone is running 18:1 compression in their 6.5 diesel for towing or just for having fun, is it good to run a 6.5 with that low of compression with more boost? how much boost can I run? what wound the mpg be? and is it better for longevity? and any other helpful information I need to know? P.S. I know low compression is bad for cold starts.

thanks!
 

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18:1 engines were mainly used in marine applications. They are good, economical, reliable boat engines. Designed to run and make best power/ economy in the 2400-2800 RPM cruise speed.
Some have built them for street use with good results both for towing and Fun. They build good power and perform well. . But it takes a lot more than pistons and boost to make a good reliable 18:1 engine. You have to build the whole engine around the 18:1 compression ratio.
What are your build plans?
 

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Discussion Starter · #3 ·
18:1 engines were mainly used in marine applications. They are good, economical, reliable boat engines. Designed to run and make best power/ economy in the 2400-2800 RPM cruise speed.
Some have built them for street use with good results both for towing and Fun. They build good power and perform well. . But it takes a lot more than pistons and boost to make a good reliable 18:1 engine. You have to build the whole engine around the 18:1 compression ratio.
What are your build plans?
oh ok good to know. the plan would be main studs and girdle, 18:1 pistons, and head studs, and I'm not sure what turbo yet. would there be any disadvantage to running 18:1 with more boost around town and on the freeway?
 

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Discussion Starter · #4 ·
18:1 engines were mainly used in marine applications. They are good, economical, reliable boat engines. Designed to run and make best power/ economy in the 2400-2800 RPM cruise speed.
Some have built them for street use with good results both for towing and Fun. They build good power and perform well. . But it takes a lot more than pistons and boost to make a good reliable 18:1 engine. You have to build the whole engine around the 18:1 compression ratio.
What are your build plans?
what do you mean to build the engine around 18:1? all i can think of doing is getting the crank rebalanced.
 

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I just wanted to see what you are doing to the block. Seems you are on the right track. If you plan on making a lot more power. I recommend splayed mains over the crank girdle for the most strength. But that is mucho $$$$. With studs and girdle you should be able to support a moderate build.
I am sure you know more boost = more fuel needed. You will need a IP that deliver the needed fuel to support the higher boost.
In the marine apps. They usually used a reworked DB2 pump for higher fuel flow, and marine injectors.
DS4 pumps deliver more fuel than stock DB2. They can be custom tuned to work with a moderate build up to a point. But they have their limits.
 
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18:1 is a very common modification, it lessons the cylinder pressures while allowing room for more air and fuel that in turn takes a lot of stress off of a already weak platform. A site vendor Unique Diesel knows where to get them machined ... Welcome to Unique Diesel - GM Truck - High Quality 6.2 6.5 Parts Source

They are hard to get a hold of, they stay busy, if you want more info message me ....
 

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Discussion Starter · #7 ·
I just wanted to see what you are doing to the block. Seems you are on the right track. If you plan on making a lot more power. I recommend splayed mains over the crank girdle for the most strength. But that is mucho $$$$. With studs and girdle you should be able to support a moderate build.
I am sure you know more boost = more fuel needed. You will need a IP that deliver the needed fuel to support the higher boost.
In the marine apps. They usually used a reworked DB2 pump for higher fuel flow, and marine injectors.
DS4 pumps deliver more fuel than stock DB2. They can be custom tuned to work with a moderate build up to a point. But they have their limits.
I was thinking about doing splayed manes but I heard for the 6.2 and 6.5 diesel‘s that it’s just a gimmick because the machining process would weaken the sides of the block so they would crack overtime. In fact I was thinking of buying the main stud girdle and putting that on top of the splayed mains just for a little added security even if it’s not needed. I’m just worried about durability because I heard that when you have lower compression pistons it lessens the chance of having a crank failure but I’m also worried that the engine is going to run weird because IDA dieselI’m just worried about durability because I heard that when you have lower compression pistons it lessens the chance of having a crank failure but I’m also worried that the engine is going to run weird because IDI diesels are supposed to run at high compression.
 

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I build 4-5 of these a year for customers, ALL get machined and coated pistons, not a single complaint from anyone... as for the splayed caps, they go into a thicker part of the block, done a bunch of those as well, not a crack yet... those that say otherwise have never seen a cracked block with splayed caps so they just repeat something some kid said, and that kid heard it from another kid... I've been building these motors for 25+ years, but I don't know as much as those kids do... the girdle is a joke, and has been shown to cause more harm...
 

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Re the splayed mains. PM sctrailrider as he said. He has more experience than me and can be helpful in 6.5 18:1 builds.
I do know with the proper build. They can be a good durable high performing diesel engine.
Re crank failure. That is more of an issue with bad harmonic balancer and pulley causing crank failure. Rather than a weak crankshaft.
For a performance build I would use new crankshaft. If you want Forged. They are hard to find. Unique diesel,as linked in post 6, has them along with most other 18:1 parts you will need. They also do 18:1 marine engines Welcome to Unique Diesel - Marine - Peninsular Diesel - The Beast 6.2L - 6.5L Marine So they have a lot of experience in 18:1 builds.
 
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Discussion Starter · #10 ·
I build 4-5 of these a year for customers, ALL get machined and coated pistons, not a single complaint from anyone... as for the splayed caps, they go into a thicker part of the block, done a bunch of those as well, not a crack yet... those that say otherwise have never seen a cracked block with splayed caps so they just repeat something some kid said, and that kid heard it from another kid... I've been building these motors for 25+ years, but I don't know as much as those kids do... the girdle is a joke, and has been shown to cause more harm...
Re the splayed mains. PM sctrailrider as he said. He has more experience than me and can be helpful in 6.5 18:1 builds.
I do know with the proper build. They can be a good durable high performing diesel engine.
Re crank failure. That is more of an issue with bad harmonic balancer and pulley causing crank failure. Rather than a weak crankshaft.
For a performance build I would use new crankshaft. If you want Forged. They are hard to find. Unique diesel,as linked in post 6, has them along with most other 18:1 parts you will need. They also do 18:1 marine engines Welcome to Unique Diesel - Marine - Peninsular Diesel - The Beast 6.2L - 6.5L Marine So they have a lot of experience in 18:1 builds.
Thanks guys for the help, I was going to spend the money and buy a fluid damper to help hopefully solve the crank failure and I was wondering if you guys also knew anything about the “scat” crankshaft i’ve heard good things and I’ve heard bad things such as they break easier than stock cranks. Also if I do 18:1 compression pistons should I have the crank balanced to the new pistons????? I just want to make the bottom end as bulletproof as possible.

thanks guys!
 

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Discussion Starter · #11 ·
644485

I saw this and I’m not sure if it’s true but this is what turned me off from thinking about doing splayed main caps at first
 

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Ole Steaksauce raises his ugly head again. Is he still in business? Lol
 

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Ole Steaksauce raises his ugly head again. Is he still in business? Lol
It's been a while.. The last time I booted steaky for trying to offer up parts for bitcoin
 
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LOL Steaksauce! .....................Getting back to your question about balancing. Yes, if you are making more power. Get the complete rotating assembly balanced. By a competent, qualified, shop that knows what they are doing.
As far as crank shaft. It would be nice if you tell us what your plans for the engine you are building. Heavy towing, Fun, Grocery getter? What kind of vehicle is it going to be used in?
 

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Discussion Starter · #16 ·
LOL Steaksauce! .....................Getting back to your question about balancing. Yes, if you are making more power. Get the complete rotating assembly balanced. By a competent, qualified, shop that knows what they are doing.
As far as crank shaft. It would be nice if you tell us what your plans for the engine you are building. Heavy towing, Fun, Grocery getter? What kind of vehicle is it going to be used in?
The plans are kind of a do everything truck not necessarily heavy towinThe plans are kind of a do everything truck. I want to do 18 to 1 compression because I hear having 21 to 1 even for the 6.2 as part of the reason why the mains crack and I want to make a little more horsepower and torque than a stock 6.5 reliably if that’s possible I’m shooting for around 450 to 500 torque.
 

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Discussion Starter · #17 ·
I just need some advice on what block and crank I should use as a base I read on another forum that the 92 through 93 599 Blocks are the best to use without spending the money to find a optimizer block, that brings up another question if I find a 6.5 block and it has main web cracks is there anyway to get them repaired when I do splayed main caps or do I just leave it be and roll with it?
 

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I just need some advice on what block and crank I should use as a base I read on another forum that the 92 through 93 599 Blocks are the best to use without spending the money to find a optimizer block, that brings up another question if I find a 6.5 block and it has main web cracks is there anyway to get them repaired when I do splayed main caps or do I just leave it be and roll with it?
Use the "Community Search" feature located at the top of your screen to find threads on topics to help bring you up to speed on things.
Take your time and do some research on the 6.5L and it can answer a lot of your questions

The GM 6.5L and 6.2L Blocks are all prone to cracking due to the thin metal casting. This was especially true for the early years.
If the blocks are cracked they are not worth repairing..

If you are wanting a HP monster then I would advise looking for a Duramax motor and build it up the way you want. It can withstand the Builds better.
If you are looking for a daily driver that can provide you a good day's work then the 6.5L can work for you.
You cannot get Duramax or Cummins power from a 6.5L block
 

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IMO If all you are looking for is 450-500 TQ you can pretty much get there reliably with a stock 6.5, 4" exhaust, a good performance tune, GM8 turbo with better impeller, and K47 intake. A little minor gasket matching and some polishing work on the heads,intake and turbo mating surfaces. Would be nice icing on the cake. With any mods done, IGT and boost gauges are a must have.
18:1 is better to keep heat down in the combustion chamber/piston. But IMO. If you are not pulling hard at steady high RPM. It really is not needed.
As for crank. IMO The stock nodular iron is just as good as anything out there except for forged. Main thing is have rotating assembly balanced together with a good harmonic balancer/pulley.
 

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Discussion Starter · #20 ·
Use the "Community Search" feature located at the top of your screen to find threads on topics to help bring you up to speed on things.
Take your time and do some research on the 6.5L and it can answer a lot of your questions

The GM 6.5L and 6.2L Blocks are all prone to cracking due to the thin metal casting. This was especially true for the early years.
If the blocks are cracked they are not worth repairing..

If you are wanting a HP monster then I would advise looking for a Duramax motor and build it up the way you want. It can withstand the Builds better.
If you are looking for a daily driver that can provide you a good day's work then the 6.5L can work for you.
You cannot get Duramax or Cummins power from a 6.5L block
IMO If all you are looking for is 450-500 TQ you can pretty much get there reliably with a stock 6.5, 4" exhaust, a good performance tune, GM8 turbo with better impeller, and K47 intake. A little minor gasket matching and some polishing work on the heads,intake and turbo mating surfaces. Would be nice icing on the cake. With any mods done, IGT and boost gauges are a must have.
18:1 is better to keep heat down in the combustion chamber/piston. But IMO. If you are not pulling hard at steady high RPM. It really is not needed.
As for crank. IMO The stock nodular iron is just as good as anything out there except for forged. Main thing is have rotating assembly balanced together with a good harmonic balancer/pulley.
I don’t plan on doing anything extreme I don’t plan on getting the RPMs over 3K at any point I want to build a 6.5 to go into my m1028 cucv To be unique and I feel like with splayed mains and a fluidamper it will greatly reduce the danger of a crank failure if I am correct. And I don’t have the money to do a Cummins swap at the moment. I just don’t know if all of this would be worth it I’m just scared to have fun with stock 6.5 because I feel like if I put my foot down a little bit The crank is it going to get out.
 
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