air gap intake poor fuel milage ???
03-10-2013, 07:41 PM
#1
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air gap intake poor fuel milage ???
After doing the heads and cam in my mustang (twisted wedge with 170cc intake runners + comp cam roller cam with 218 intake and 224 exhaust) I drove around for a while with my old performer 289 intake and my carter afb 600 mech secondary carb.
Everything ran good and I made one fuel mileage calculation for a single trip of 80 miles and got about 22 to 24 mpg.
The dyno shop talked me into a performer air gap intake and holley 670 street avenger. after adding this combo I noticed what appeared to be an immediate drop in fuel mileage. I thought it was because of the bigger carb so I started switching between my holley and my carter (with and without 1" 4 hole phenolic spacer)
The spacer made no difference (as far as I can tell so far)
With holley 670 im getting about 13.2 to 14 mpg for the same single 80 mile trip
With afb 600 same as above is unknown
With afb 600 around town + freeway daily driving multiple trips is 11.7 to 11.95 mpg consistent
With holley 670 around town + freeway as above + 1 extended 80 mile freeway trip was 15.33 MPG.
Im still working on collecting multiple results for the same type of driving using the different combos but its looking like the holley provides better fuel mileage in all situations but Im way down from where I was with my carter 600 and performer 289 intake.
Its looking the intake is causing a significant decrease in fuel mileage at this point. Ive checked for vac leaks and have replaced the intake gasket once after doing a lifter replace.
Is this intake too large for my cam? what the hell is going on?
Thanks
-Gun
Everything ran good and I made one fuel mileage calculation for a single trip of 80 miles and got about 22 to 24 mpg.
The dyno shop talked me into a performer air gap intake and holley 670 street avenger. after adding this combo I noticed what appeared to be an immediate drop in fuel mileage. I thought it was because of the bigger carb so I started switching between my holley and my carter (with and without 1" 4 hole phenolic spacer)
The spacer made no difference (as far as I can tell so far)
With holley 670 im getting about 13.2 to 14 mpg for the same single 80 mile trip
With afb 600 same as above is unknown
With afb 600 around town + freeway daily driving multiple trips is 11.7 to 11.95 mpg consistent
With holley 670 around town + freeway as above + 1 extended 80 mile freeway trip was 15.33 MPG.
Im still working on collecting multiple results for the same type of driving using the different combos but its looking like the holley provides better fuel mileage in all situations but Im way down from where I was with my carter 600 and performer 289 intake.
Its looking the intake is causing a significant decrease in fuel mileage at this point. Ive checked for vac leaks and have replaced the intake gasket once after doing a lifter replace.
Is this intake too large for my cam? what the hell is going on?
Thanks
-Gun
03-11-2013, 04:55 PM
#2
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Join Date: Feb 2006
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You changed your powerband with the new intake.If you would have dynoed before and after you would see the torque powerband probably move up. They probably told you to change it for more power and MAYBE mpg
03-11-2013, 07:33 PM
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haha not yet...Im not convinced switching from a performer 289 to air gap alone would cut fuel mileage in half...If that is the case then I bet there is a more balanced intake option especially since it may have scored me 15 hp vs the performer 289...
At 13 mpg freeway I should be making 425 + rwhp.
At 13 mpg freeway I should be making 425 + rwhp.
03-11-2013, 08:14 PM
#5
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If you switched to an intake that was engineered for a higher rpm power band, you won't have enough intake velocity at lower rpms to keep air/fuel mixture in proper suspension, and some fuel droplets might "condense" or fall out of suspension leading to the engine needing to suck down more gas for the same output...in other words, poor efficiency. At least that's my understanding of it, and why... for a more extreme example... you shouldn't try to run a tunnel-ram intake on a strictly street cruiser. For what it's worth...I'm running a Weiand Stealth with a 600 Holley on my 289, and am getting about 18-20 mpg.
03-11-2013, 10:12 PM
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I looked into the stealth intake that maybe an option I know there a more than a few members here that run them. The design looks identical but edelbrock wont tell me the runner volume of the air gap.
It certainly seems like the intake is not matched at this point. The performer 289 was restrictive evident by dyno results but it seems like the air gap went way past the sweet spot.
im wondering how much not having an exhaust crossover in the intake factors into this....
-Gun
It certainly seems like the intake is not matched at this point. The performer 289 was restrictive evident by dyno results but it seems like the air gap went way past the sweet spot.
im wondering how much not having an exhaust crossover in the intake factors into this....
-Gun
03-11-2013, 10:38 PM
#7
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It would affect fuel vaporization before the engine reaches operating temp, especially in cold weather, but if anything it would help prevent boiling of the fuel in the carb in hot weather. I blocked off the exhaust crossover on my Stealth and don't see any harmful effects.
I'm not convinced it's entirely an efficiency problem relating to the intake. Yes, I'm sure there is some efficiency lost in the large runner volume and naturally colder intake, but I don't know if that alone is enough to justify the decrease in mileage that you're recording.
I have a Stealth on my 331 and I don't have any problems with it, but I also don't know if it's going to fix all of your problems. I think you should retune your carb before changing the intake. A good tune kit is always handy, and it's a lot cheaper and easier than swapping the intake.
I'm not convinced it's entirely an efficiency problem relating to the intake. Yes, I'm sure there is some efficiency lost in the large runner volume and naturally colder intake, but I don't know if that alone is enough to justify the decrease in mileage that you're recording.
I have a Stealth on my 331 and I don't have any problems with it, but I also don't know if it's going to fix all of your problems. I think you should retune your carb before changing the intake. A good tune kit is always handy, and it's a lot cheaper and easier than swapping the intake.
03-11-2013, 11:09 PM
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I agree...Im not sure a 100% of the issue is the intake as well.
The holly is feels great with #70 jets front and back and that gets me about 13.8 to 14.3 indicated AFR on the freeway at 70 mph. Any leaner on the jets #68 for example runs like total crap with afrs in the 15 to 16.8 so It not like I can put any leaner jet in it...
The carter runs great with the rods that are in it now with 14.5 indicated on the freeway and city streets. Under power WOT It actually needs more fuel as it indicates about 13.
My timing is about 36 total with about 14 advance...I really dont know what to do timing or carb tuning wise to make any real improvements in fuel mileage
The holly is feels great with #70 jets front and back and that gets me about 13.8 to 14.3 indicated AFR on the freeway at 70 mph. Any leaner on the jets #68 for example runs like total crap with afrs in the 15 to 16.8 so It not like I can put any leaner jet in it...
The carter runs great with the rods that are in it now with 14.5 indicated on the freeway and city streets. Under power WOT It actually needs more fuel as it indicates about 13.
My timing is about 36 total with about 14 advance...I really dont know what to do timing or carb tuning wise to make any real improvements in fuel mileage
03-12-2013, 07:41 AM
#9
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Hi,
I've read through this thread and may have missed this. Did you swap the 670 with the 600 and keep the air-gap and checked the results? At one point, I ran a custom tuned 680 Holly on a Performer RPM. My best ever mileage on an extended "roadie" was 16-17, all the while running a T5. My average Rs were in the 2K range. Running around the "hood" I was in the 13-14 range. MY WOT AFR was in the high 12s -low 13s.
I would try installing the 600 then re-tuning for best AFR. Your AFR numbers reported earlier, looked pretty good. Good luck!
I've read through this thread and may have missed this. Did you swap the 670 with the 600 and keep the air-gap and checked the results? At one point, I ran a custom tuned 680 Holly on a Performer RPM. My best ever mileage on an extended "roadie" was 16-17, all the while running a T5. My average Rs were in the 2K range. Running around the "hood" I was in the 13-14 range. MY WOT AFR was in the high 12s -low 13s.
I would try installing the 600 then re-tuning for best AFR. Your AFR numbers reported earlier, looked pretty good. Good luck!
03-12-2013, 11:06 AM
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With the performer 289 (small intake ) and 600 I got 24 mpg on a single trip
after switching intakes im getting about 13 mpg for the same trip. Switching between carbs (to attempt to eliminate them as the issue) actually seems to be suggesting the holly 670 provides better mileage but neither carb can get close to the 24 mpg mark on the freeway.
Thanks
-Gun
after switching intakes im getting about 13 mpg for the same trip. Switching between carbs (to attempt to eliminate them as the issue) actually seems to be suggesting the holly 670 provides better mileage but neither carb can get close to the 24 mpg mark on the freeway.
Thanks
-Gun