Engine/Transmission (1994 - 1998) Question on injectors and timing for fuel efficiency

[CEdwards]

I have been reading old posts all week on injectors and timing and lots of tuning threads that I still don't fully understand. I have no plans to do any mods for power. I am fully content with the towing power I have, but I am interested in mods that would be more efficient and still stay basically stock. From what I gather from reading timing can be bumped to 15. 5 on a stock truck. They were set lower to meet emission standards. My truck has over 300,000 miles and worn injectors could cause you to lose a degree or so of timing. Most people report that after advancing timing on a stock truck there is no great change in economy unless their timing had slipped.

Now lets talk injectors, this subject is over my head. It seems from most posts that people with larger injectors seem to get better mileage and they tend to bump their timing up more. Is this because they want to run in the higher rpm's and have 3000 gsk's, etc? I realize that you want your timing to be optimum for the rpm you mostly drive in.

The next subject is the fuel plate. If your not at full throttle does the plate really matter? Do delivery valves matter? If your not racing does governor springs really matter?

I guess what I'm really asking is what is the best setup to drive a stock truck at 1900 to 2000 rpm, which is about 70 mph on the highway. The best relationship between timing, injectors, etc. for cruising speed. Feel free to correct any statement I made. I know there are thousands of posts on the subject but I couldn't find any with setups just for cruising and I'm still confused. Just trying to learn a little and maybe get all the info in one post. Sometimes it's hard to search and find the answer.

 

[GAmes]

I can tell you from personal experience that bumping the timing to 15. 5 or so WILL increase your mileage, everthing else staying constant. Replacing high mileage (570k) stock injectors for new stock injectors does not make any increase. The best mpg gains are made slowing down to 67 (or less) rather than 70. BIG DIFFERENCE!!

 

[CEdwards]

Games, It looks like from your signature that you are mostly stock. What kind of mileage do you get? Stock injectors? What were they in 97? My mileage hasn't really dropped off, It's always gotten around 17 highway. I've just kind of gotten the bug here lately for working on my truck and am wanting to delve a little deeper in. Been fixing it up lately. I kind of enjoy working on it when it's not actually broke down. Would you say that 20-21 is attainable mileage at 65mph?

 

[GAmes]

Games, It looks like from your signature that you are mostly stock. What kind of mileage do you get? Stock injectors? What were they in 97? My mileage hasn't really dropped off, It's always gotten around 17 highway. I've just kind of gotten the bug here lately for working on my truck and am wanting to delve a little deeper in. Been fixing it up lately. I kind of enjoy working on it when it's not actually broke down. Would you say that 20-21 is attainable mileage at 65mph?

If I drive 70 (empty) my mileage is 18-19, at 67 it increases to 20-21, at 65 (the speed limit in Oregon) I have been up to 22. Stock injectors in 97 with a 5-speed were 215s. Even with ULSD 20 to 21 is attainable at 65 for you without much work. My mileage was better when the truck was new, but I didn't haul 1000 pounds of tools and fuel everywhere I went either. The combination for me currently is advanced timing, a PDR cam and a lighter right foot.

 

[CEdwards]

I noticed a lot of people with 215's getting good mileage. Can anyone chime in explain the relationship between injectors and mileage? I don't know anything about it. How do bigger injectors still get good mileage?

 

[K5IP]

I noticed a lot of people with 215's getting good mileage. Can anyone chime in explain the relationship between injectors and mileage? I don't know anything about it. How do bigger injectors still get good mileage?

I changed the original 175 or 180's (not sure which) in my 95 5 spd to 215 injectors. I did not see any change in MPG. I still get about 20 or 21 MPG at 65 MPH unloaded.

The 215 injectors did help reduce the stunble at around 1200 RPM

Rog

 

[Bob4x4]

In my 94 I got the best fuel economy at about the 300hp level which came from pump mods and 230 hp injectors. I was also running the Horton fan. with the 5 speed it was easy getting low to mid 20's depending on cruise speed. With a stock set-up I found the best timing on most trucks to be about a 1 degree bump from stock.



Bob

 

[SKnapp]

97 auto here mpg is 16-17 hwy. 4:10's 265 7516 tires. i have not recalculated for my newer 32" tires. and yes why do the 5spd get better milage? 4-5 mpg just isn't fair.

 

[Vaughn MacKenzie]

CEdwards for better mileage and a better running truck I'd look for some low-mile 215 injectors and bump your timing to 15. 5 - 15. 8 degrees. Don't bother with a GSK. You might consider a different fuel plate, such as a #8 run forward a bit or a #6 in the stock location. . . these supposedly help MPG a bit.

If you haven't yet you might put a boost elbow on your turbo so when you're pulling hard you can run higher than stock boost and get a little more air.

For the exhaust if you still have the stock system and only looking for MPG then lose the stock muffler and cat & replace with Walker 3" muffler. Have you done anything to improve the intake? Even on stock trucks the stock airbox is a little restrictive.

 

[JGK]

My only comment is about your injectors. With that many miles on them I would bet they aren't working anywhere near optimum.



Injectors will stay closed and not allow fuel to flow due to an internal spring until the fuel pressure reaches a certain point and then they will "pop" open/close to let the fuel spray out properly. Injectors will eventually loose their ability to "pop" - the spring wears out. They can easily be tested by removing them and taking them to a proper diesel shop. All 6 can be tested in about 6 or 8 minutes. If any are weak you're not getting proper fueling which will hurt mileage.



Whether you choose to have yours rebuilt or get new ones (stock or 215s or whatever) is your choice, of course. But I'll bet doing one or the other will improve your mileage and performance.



-Jay

 

[hsmith]

PM me if you want some real low mileage 215's, PDR 215's that is.

 

[CEdwards]

I appreciate all the feedback. I am thinking of possibly some 215's. I know I don't really need them for mileage gain, but it seems like a mod that won't hurt mpg either. What will I notice with 215's? Will it basically be more fuel for take off, Maybe a better spray pattern.

To answer the question of exhaust, It is straight piped. I will research the boost elbow some more. I have seen them mentioned before but didn't really understand what it does. I thought the waste gate was the only way to increase boost. I am also getting ready to rebuild my turbo. It has quite a bit of end play. I have printed off the instructions from holset and I plan on taking some pics and maybe post it just so others considering it can see what is involved.

Bear with me on this thread as I am just trying to put together a simple mod package that will be economic and a little better than stock. I only know what i've read on here and I don't have any experience on this subject. Thanks for the advice. Let me know what you think.

 

[CEdwards]

also, plan on bumping timing to 15. 5. I have always wanted to do it myself to learn a little more and at least then I will know where it was and where it's going to be. Anything else?