18:1 pistons

[Salemone]

Is there any difference in fuel mileage with 18:1
pistons?

[grape]

the brake specific fuel consumption is going to go up with less compression. So in theory the lower compression is less efficient, and yes mileage will suffer.

[Salemone]

Thought so. Thanks for the reply.

[gmctd]

17.5:1 is usually considered optimim compression ratio for a Diesel. GM went with 21.5cr for driveability\warranty purposes - cold starting, cold drive-away, etc.

The 18:1 motors suffer reduced fuel economy due to increased boost pressures, turned up fuel, and the driver gets 'happy feet' at the first taste of 'super power'.

It's mostly incurable, so be very cautious! ;)

[Salemone]

So what you are really saying is ----Do to the added power capability I would use it and it takes more fuel to make more power. The reason I
asked the question was the article that told of
dimishing return on comp ratio. Thought that if
one drove exactly the same the mileage would be
almost imperceptfully close.

[gmctd]

A number of DP members have upgraded to the 18:1 works, but may not monitor these forums daily, and so response may be slow.

If you are not pressed for time,I would prefer you wait and read their responses for actual driveablilty data.

However, most drivers that upgrade crave the power increases.

But, esssentially, you are correct.

Also, you might want to try the 'search' function, above, for more data on any subject.

[ 04-16-2004, 06:55 PM: Message edited by: gmctd ]

[dstoops]

Hey Salemone,
I recently R/Built my 6.5 with the 18:1's. Fuel mileage is definatly better but I made some other changes as well: R/B injectors, went from 3" to 4" exhaust, '97 cooling system. I had installed a BD computer chip, K+N filter, and the 3" exhaust prior to the rebuild. Recently I recorded 14 mpg while hauling/towing 15K lbs, and 17 mpg empty at 70 mph. Final drive ratio is 3.73/1. Power increase is impressive and I love hearing that turbo "spool up"! I searched for theorys on why the 18:1 C/R is better and the best explanation I can find is that there is more cylinder volume at TDC which allows more oxygen in the burn. On our old gas engines we would increase C/R to increase power but with a TD the turbo boost is the same at either C/R so the burn efficiency is not compromised by the lower C/R.
Well I've already told you "more than I know" :D Hopefully some one else will weigh-in. I'd be interested to learn more myself.

[tanker]

I have 18:1 in my "Avant" engine that I have used for just over 2 years. I have not seen a decrease in fuel mpg. The drawbacks are a lot of smoke when I start up in cold weather, but after warm up its fine. I have increased boost so it really pulls the hills well when towing. Its slow off the line until the turbo spools up, but my drag racing days were over many moons ago. :D
I guess like any modification, or new ride, we must always see what it'll do! :rolleyes: So then yes fuel mpg will suffer. My best was 18 mpg solo at 60-65 mph. I haven't seen that in a long time. I get about 15-16 mpg solo, 14-15 mpg back and forth locla running. Towing of course varies with terrain, and of course speed. I would say about 12 mph, but in the western rockies, I have dropped below 10 mpg.
Hope this helps answer some of your questions. ;)

[More Power]

Our performance 18:1 engine has delivered slightly better fuel economy than a typical factory original 6.5. With 4.10 gears, Chris is seeing 17-19 mpg on summer fuel, with an alltime best of 21. Having an intercooler, DSG gear drive timing set, balanced, and port matched probably play a part in all this.

MP

[Kennedy]

In time we will be working with the timing tables to tweak them up a bit as the 6.5 likes more timing than available on the factory tuning in many cases, ESPECIALLY 97 and newer.

Also working with extended glow times!


YES this means OBD2 reflash is coming!

[JoeyD]

Is this going to be your own design?
I have been talking with a guy who does this, acually supplying a few that market them, and there just does not seem to be much more than the 40-50 hp gain available in the pump unless you were to create all new fuel maps which will drive up the $$$$

[Salemone]

The motor will be used in a pusher motorhome.
GVW 15K--towing a 5k trailer. Trans will be a 700R-4 --rear end 4.60:1. 1993 block, 18:1 pistons
GM-8 turbo, intercooled, mechanical IP--I have a
21' class C motorhome with a 6.5 crate motor running nonwastegated turbo 10+ lbs boost 4911
pump ,not intercooled EGT nevergoes above 1050.
Plenty of power --towing 5k trailer 12mpg towing
not much better not towing (70+ mph with 4.11 gears) All my diesels are non computer.