TPI Gas Mileage (longhorn man & any others)

[mocwon]

I went the TPI route for the engine in the 72 hoping to get the 18-20 miles per gallon that I have often heard comes with this swap. I'm getting 12-14 MPG so, to say the least I'm disappointed. I've got to believe I've done something wrong or have a bad combination of some sort. I'll spell out the particulars of the drive train and operating characteristics to see if to see if anyone see's any clues where I might look.

• New 350 GM long block #10067353
• .383 intake lift
• .401 exhaust lift
• .194 intake duration
• .202 exhaust duration
• 112 degree lobe separation
• 8.5:1 compression ratio
• 700R4 transmission
• Stock mass airflow TPI w/cold start injector
• Stock Bosch mass airflow sensor
• Electric cooling fans
• 24# injectors
• 1987 stock Z-28 Camaro ECM and prom
• Painless wiring harness for TPI
• 3.08 rear gears (open)
• 31” tall tires
• 2-1/2” exhaust w/X-pipe
• Stock exhaust manifolds
• 16 gallon under bed gas tank with in-tank electric fuel pump
• 40-43 PSI fuel pressure
• 3/8” fuel feed line and 3/8” fuel return line
• At 70MPH in OD the engine is turning about 2,000 RPM
• I tried running it in third gear and had a very small decrease in MPG
• Engine operates at 210-215 deg F

[Jerry Jones]

What size shoe do you wear?

[Musclerodz]

Looks like you are trying to push either to big of a tire, or to high of a gear. Engine is having to work extra hard just to keep the truck moving. Install a shorter tire or lower gear and recalibrate your speedometer. You might try installing a vaccum guage to help dial it in. Higher the vaccum the better the mileage.

Mike

[pjmoreland]

Maybe you need a higher ratio rear end, like 3.42 or 3.73.

[cdowns]

Originally Posted by pjmoreland

Maybe you need a higher ratio rear end, like 3.42 or 3.73.

a 3.08 which he has is a higher ratio than what you suggest// you've suggested lower ratios i doubt that will improve gas milage

[pjmoreland]

Is your TCC locking up in OD?

[cableguy0]

your lugging the engine. you need a lower gear with those tires and od. your at like a 2.73 gear or so before the overdrive and lockup come into play. i seriously doubt you will ever see 18-20 with that combo in a truck. a cam more suited to making torque would help a lot. that cam is way small to make the power you need to move a truck around and still get decent economy.

[qksilver]

I think your rear is a little high, factory they came with 3.42 or higher, plus check your timing that can make a big difference.
Plus the last thing is to probably have a new chip burned or what ever they do. Go with someone who knows what they are doing and give them all the details, that way you will have a computer more tuned for your combo, as far as timing, idle speed, a/f ratio rear end ratio etc

[71swb4x4]

Just for compairson for you, I have a 2006 Silverado 4x4, 5.3L, and 3:73 gears. I get 13.5 mpg at 80 mph. Last weekend I pulled an empty trailer at 60mph and still got about 13.5 mpg. I think it will be a challenge for you to get much above 15 mpg, but I am not saying it isn't possible.

[67ChevyRedneck]

Originally Posted by 71swb4x4

Just for compairson for you, I have a 2006 Silverado 4x4, 5.3L, and 3:73 gears. I get 13.5 mpg at 80 mph. Last weekend I pulled an empty trailer at 60mph and still got about 13.5 mpg. I think it will be a challenge for you to get much above 15 mpg, but I am not saying it isn't possible.

I think there is something wrong with your truck, unless you've done something to it to make it different than stock. I check my fuel mileage a lot and I consistently get 15 city and at least 18 highway. I too get around 13 towing, city or highway. I also drive 75-80 mph on the highway. My truck is the z71 5.3L extended cab. You must have one heck of a heavy foot


This may sound dumb, but are you SURE your speedometer is right? That could have the biggest affect on your numbers?

[72MARIO]

3.08 rear gears (open)

GET RID OF THE GEARS. you need 3:73's for sure. thoes gears are KILLING YOUR MPG.

[Boog]

It might be interesting to compare the present engine specs to the engine the TPI came off of. Compression ratio, valve size, intake runner size and the cam may be a good bit different than the current setup. Obviously the sleek shape of the Camaro will help in the mileage dept compared to a truck. 31" tall tires are a good bit taller than the Camaro tires though. 2,000 rpm @ 70 mph is right where it needs to be. Is that mileage checked around town or at 70? Speedo been checked for accuracy?

[MikeS]

I think you may be being a little unrealistic about your mileage expectations. 14-15 mpg sounds pretty reasonable to me. I'd be really happy to get that with my TPI Blazer.
Qksilver makes a good point about getting a custom chip burned. Was the ECM out of a 350 or a 305? You need to run some diagnostics on your system to check air/fuel ratios. The 24# injectors may be just a tad too big but that can be accounted for in the programming.

Where did you place your O2 sensor? If it's in the stock manifold you should be OK otherwise you'll need to use a heated sensor.

[red71cheyenne]

I'm getting the same mileage my friend. I don't know what all the specs are on my engine, but its a 355, .030 over 350, stock cam and all that jazz. I have a 3.73 rear end and at 70 mph I am turning just under or right on 2000 rpm. I very rarely get to drive on the highway, and most of my driving is in town and stop and go. My TCC is hydraulic and kicks in at 35 mph whether I like it or not. I am running 24# injectors because thats what is recommended for 350's, but I have a set of 19's I'd like to try to see if it makes a difference. I am also running a speed sensor on my speed density system, and it hasn't made one bit of difference. So who knows what is going to do it for us brother. Good luck and do like I do. Fill it up with gas, open the hood and admire the view, and forget about the mileage. Cheers, Jeff.

[CochinoFilipino]

I think your tach is off ,or your speedometer is off. If the speedo's off, maybe so are your MPG caculations. Did you get the right speedo gear for the 700R4 to run 31's?

According to the gear calculator at http://www.f-body.org/gears/ you are at 1700 RPMs on 31's doing 73 MPH.

You mentioned in another post that you are going into OD (.70 gear ratio) at around 40 MPH. At 43MPH in OD you'd be at 1000 RPM. I think your 3.08 gears (numerically low) have your engine lugging with he 31's.

You may want to drive in 3rd and manually shift into OD when you reach 70MPH. Or maybe try some shorter tires for a while if you have some available.

Your stock ECM /PROM are calibrated for 22# Injectors not 24#.

Fuel pressure on a 5.7L should be 35-39 psi. (42-46 is for 5.0L engine).

Maybe you're running rich from too high fuel pressure and too big injectors.

Another thing may be to check that the TV cable on the 700R4 is adjusted properly. Not only could maladjustment effect the when it shifts but will lead to premature death the tranny itself.

I'm new to TPI myself so the above info comes only from books and internet reading, not a lot of first-hand experience YET. But I'm learning alot chasing down a cold start issue (92% sure it's the cold start valve itself).

[formula_raider]

Mocown, I have the exact same good wrench engine that you have in my truck. I was goint to put a TPI on my engine much the same way you did yours. I decided to go with aftermarket fuel injection but just wanted to say that I did alot of research on that goodwrench engine. If you kept the complete longblock stock setup, which your cam is and you kept the heads, etc. This engine has cast dished pistons w/76cc heads. I know that GM claims 8.5:1 compression ratio; BUT I have followed a few articles where they built up that same engine and they really calculated the compression ratio at 7.8:1.!!!! Alot of the TPI engines were right at 9:1 and had a 1-piece rear main seal and ran a factory ROLLER cam that is bigger than your stock longblock which in itself cuts down on HP loss. And as most people have said, unless you are running exactly what the factory put in the engine the stock computer programming will not be exact. But I totally agree that the rear end needs to be in the 3.42 to 3.73 range, that will help alot. As will a custom tune to your stock computer if you dont want to change out the cam or increase compression ratio to stock TPI settings.

[Andy4639]

I have a stock 86 TPI camaro 305 HO engine in my 71 with stock 4:10 rear. I get 18 miles to the gallon. I'm running the street & performance Ferrio computer with a special chip to run the V-8 engine with a V-6 computer. I also have the 700-R4 trans. I went with the map senor instead of the Mass air. Just a better system to me and a lot cheaper at the time. Not to many people had this setup in 88-89 when I did mine.

[mocwon]

Thanks very much for the replies....

I've read a lot about the TPI engines and have been considering several points that have been made here.

As far as the tranny goes, there is no lockup. The guy that rebuilt it for me said he could add that if I wanted but I'm not sure what the point is since it goes into OD fine and stays there unless I'm going up a steep grade or accelerate to make it down shift.

I think my next step is to swap out the gears for 3.42's or 3.73's

[67ChevyRedneck]

Originally Posted by mocwon

Thanks very much for the replies....

I've read a lot about the TPI engines and have been considering several points that have been made here.

As far as the tranny goes, there is no lockup. The guy that rebuilt it for me said he could add that if I wanted but I'm not sure what the point is since it goes into OD fine and stays there unless I'm going up a steep grade or accelerate to make it down shift.

I think my next step is to swap out the gears for 3.42's or 3.73's

You also may want to consider a lock-up kit from somewhere like bowtie overdrives (of course, that would be if your TC is a locking TC, which it may not be). I think its around 75 bucks. When the trans is not locking up even though you are in OD there is unnecessary slippage which will take a little away from fuel mileage. WHen the TC locks up it makes the trans and engine a "direct drive." I can feel when my TC locks in my Z71 and in my 67 before the trans blew. My 67 had a carbed 350/700R4 with lock-up/3.73 rear and 235/70R15 tires and I got 16 mpg at about 2400 RPM at about 80 mph. I agree on the computer set up too... may need to be tweaked?

[smokekiki]

I also have a 350/700r4 with 3.73 gears and 29 inch tires.I average 17 mpg.Speed has a big effect on fuel mileage with our aerodynamic trucks.

[mplex2000]

You'll definitely need to put it on a dyno to get your A/F ratio measured to you can burn your own eeprom. Also, a roller cam and rockers wouldn't hurt if you don't have those already...

[CochinoFilipino]

I think the non-locking trans is hurting your MPG big-time. And not only is it hurting your MPG, it's also wearing down your tranny.

http://www.700r4.com/faq/whatlock.shtml

About that speedo gear, I don't even see that a speedo gear for 3.08 and 31s exists. Do you know what's in there? http://www.transmissioncenter.net/sp...n_______va.htm

[Russell]

I'm with everyone else here, there are a number of things contributing to your poor milage numbers.

First is those 24 lb injectors with a stock chip. That right there is going to cause problems. The ECM doesn't have the adjustment capability to account for the richness of the extra flow of your injectors. Your fuel pressure is perfect BTW. You want to be running about 3 bar of fuel pressure, which roughly translates to 43 PSI.

Next thing that could possibly be an issue could be your cold start injector. If the cold start switch has failed, and has the cold start injector open all the time, it will cause some serious milage issues too.

Third is your camshaft. You could pick up a lot of torque (which helps with the engine lugging) by switching to something more like 212 intake and 218 exhaust duration @.050" with valve lift (1.5 rockers) of .449 intake and .456 exhaust, and finally a 112 lobe seperation angle. A cam like that will make power starting at 1200, and will pull hard all the way up until you run out of breath.

Fourth, as far as your TPI related stuff goes, you need to get a scanner, a dyno, and get some tuning done on that engine. You will pick up several mpg by getting that thing tuned properly. Also, if you have access to a scanner, I'd suggest that you hook it up and make sure that all of your sensors are working properly using WinALDL or something.

Finally, I agree with the others. Your gearing is simply terrible for an overdrive transmission. I'd bump up to 3.73 gears, and if possible go for some smaller tires. You want that engine pulling about 2200 rpm in overdrive, with lockup engaged at your most common cruising speed. That speed is just a general rule of thumb, but is pretty close to most 350's sweet spot.

You also need to get lockup installed. That not only is very important for your tranny to survive (slippage = heat = death of autos), but also eliminates a ton of slipping when your engine is in OD.

My 77 (C10, short step) is running a poorly tuned carb (I'm more of an EFI guy, carbs scare me, lol) with a HO 350 from a 83 1 ton truck (lower revving, higher torque), built (shift kit, boost valve on gear one, shifts out much sooner than stock, reducing the big loss of rpm shifting from 1st to 2nd, kevlar bands etc) 700r4, 4.10 open rear diff, and 28" tires. I'm pulling 2100 rpm at 60 mph in OD, under lockup, where much of my driving is done. If I am cruising at like 50 mph, my engine does work a little bit, but becuase it is so torquey, I don't really notice any significant loss of fuel milage.

Sooo, all this said, if you address all these issues (yeah, I know, its more money, but what can you say?) there is no reason why your 350, in a more aerodynamic truck, can't net 17 - 18 mpg. The biggest things is that everything needs to work together, and work properly to get that kind of milage, and you need to drive for milage. I drive like an old man unless I am spanking some import off the line, lol, and that helps a ton with milage as well. No need to have the skinny pedal to the floor all the time.

[mocwon]

I really appreciate everyone's input which is quite valuable to me... Russell you have summed it up very well.

I think I'm going to take this process one step at a time; 3.73 gears (like Longhorn Man said quite some time ago) and install proper speedo gears since swapping out the 20's is not an option. Have my tranny guy make the lockup functional. Then, if necessary, have a custom chip burned to the parameters of the truck and driving conditions. If I end up having a chip burned, it will work like the speed density setups where the cold start injector goes away and all primary injectors are pulsed at startup.

Being born in the mid-50's, I'm a product of the muscle car era. So, dealing with fuel injection is an interesting learning experience for me. I must say, I do believe fuel injection is the best way to go plus it looks really cool. Although, the looks of a Holley, Barry Grant or Carter four-barrel, dual-feed, double pumper is a sight to behold (like the one on my Mach 1; yeah, I know it's a Ford).

Thanks again for everyone's help!!!

Originally Posted by Russell

I'm with everyone else here, there are a number of things contributing to your poor milage numbers.

First is those 24 lb injectors with a stock chip. That right there is going to cause problems. The ECM doesn't have the adjustment capability to account for the richness of the extra flow of your injectors. Your fuel pressure is perfect BTW. You want to be running about 3 bar of fuel pressure, which roughly translates to 43 PSI.

Next thing that could possibly be an issue could be your cold start injector. If the cold start switch has failed, and has the cold start injector open all the time, it will cause some serious milage issues too.

Third is your camshaft. You could pick up a lot of torque (which helps with the engine lugging) by switching to something more like 212 intake and 218 exhaust duration @.050" with valve lift (1.5 rockers) of .449 intake and .456 exhaust, and finally a 112 lobe seperation angle. A cam like that will make power starting at 1200, and will pull hard all the way up until you run out of breath.

Fourth, as far as your TPI related stuff goes, you need to get a scanner, a dyno, and get some tuning done on that engine. You will pick up several mpg by getting that thing tuned properly. Also, if you have access to a scanner, I'd suggest that you hook it up and make sure that all of your sensors are working properly using WinALDL or something.

Finally, I agree with the others. Your gearing is simply terrible for an overdrive transmission. I'd bump up to 3.73 gears, and if possible go for some smaller tires. You want that engine pulling about 2200 rpm in overdrive, with lockup engaged at your most common cruising speed. That speed is just a general rule of thumb, but is pretty close to most 350's sweet spot.

You also need to get lockup installed. That not only is very important for your tranny to survive (slippage = heat = death of autos), but also eliminates a ton of slipping when your engine is in OD.

My 77 (C10, short step) is running a poorly tuned carb (I'm more of an EFI guy, carbs scare me, lol) with a HO 350 from a 83 1 ton truck (lower revving, higher torque), built (shift kit, boost valve on gear one, shifts out much sooner than stock, reducing the big loss of rpm shifting from 1st to 2nd, kevlar bands etc) 700r4, 4.10 open rear diff, and 28" tires. I'm pulling 2100 rpm at 60 mph in OD, under lockup, where much of my driving is done. If I am cruising at like 50 mph, my engine does work a little bit, but becuase it is so torquey, I don't really notice any significant loss of fuel milage.

Sooo, all this said, if you address all these issues (yeah, I know, its more money, but what can you say?) there is no reason why your 350, in a more aerodynamic truck, can't net 17 - 18 mpg. The biggest things is that everything needs to work together, and work properly to get that kind of milage, and you need to drive for milage. I drive like an old man unless I am spanking some import off the line, lol, and that helps a ton with milage as well. No need to have the skinny pedal to the floor all the time.

[cableguy0]

you have 20 inch wheels? if so more than likely those are eating more mileage than anything else. they are generally incredibly heavy compared to other wheels which means more rotating mass and unsprung weight which will kill mileage in a hurry