Bad mileage help
Thread Starter
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Laughing Gas
Joined: Aug 2010
Posts: 896
Likes: 55
From: Bothell
Unfortunately this is an open chamber head motor with Badger flat top pistons that are way below deck height.... there is no squish.
Dynamic compression is very useful when choosing parts and when building a motor. It doesn't mean anything if opening up the motor is not an option.
Static compression is very helpful in an assembled motor, consistent compression across all 8 cylinders can help show that all cylinders should mechanically be healthy.
Dynamic compression is very useful when choosing parts and when building a motor. It doesn't mean anything if opening up the motor is not an option.
Static compression is very helpful in an assembled motor, consistent compression across all 8 cylinders can help show that all cylinders should mechanically be healthy.
Logistics Pro
Joined: May 2010
Posts: 4,436
Likes: 19
From: Regina, Saskatchewan
Unfortunately this is an open chamber head motor with Badger flat top pistons that are way below deck height.... there is no squish.
Dynamic compression is very useful when choosing parts and when building a motor. It doesn't mean anything if opening up the motor is not an option.
Static compression is very helpful in an assembled motor, consistent compression across all 8 cylinders can help show that all cylinders should mechanically be healthy.
Dynamic compression is very useful when choosing parts and when building a motor. It doesn't mean anything if opening up the motor is not an option.
Static compression is very helpful in an assembled motor, consistent compression across all 8 cylinders can help show that all cylinders should mechanically be healthy.
You calculate a lot of mileage properties which are based off of the dynamic compression ratio.
I'm not trying to throw stones here - just saying that the language needs to be clear so we can see what you're dealing with.
If you had 8:1 dynamic compression ratio, that's actually pretty decent for a mild street build (in my opinion, others may say differently). However, if your static compression ratio is 8:1, then your dynamic is always less than that, which isn't as good.
Just trying to be clear as mud lol.
Elder User
Joined: Aug 2016
Posts: 723
Likes: 3
From: Cape Town, South Africa
@willowbilly3
Good call on the cylinder balance test.
I agree with you, it was sad that you couldn't get to determine the cause.
@buckin69bronco
''Badger flat top pistons that are way below deck height.... there is no squish.''
Therein lies the answer to the poor gas mileage.
Good call on the cylinder balance test.
I agree with you, it was sad that you couldn't get to determine the cause.
@buckin69bronco
''Badger flat top pistons that are way below deck height.... there is no squish.''
Therein lies the answer to the poor gas mileage.
Tuned
Joined: Sep 2003
Posts: 348
Likes: 2
From: Northern Michigan
Unfortunately this is an open chamber head motor with Badger flat top pistons that are way below deck height.... there is no squish.
Dynamic compression is very useful when choosing parts and when building a motor. It doesn't mean anything if opening up the motor is not an option.
Static compression is very helpful in an assembled motor, consistent compression across all 8 cylinders can help show that all cylinders should mechanically be healthy.
Dynamic compression is very useful when choosing parts and when building a motor. It doesn't mean anything if opening up the motor is not an option.
Static compression is very helpful in an assembled motor, consistent compression across all 8 cylinders can help show that all cylinders should mechanically be healthy.
I have a stock bottom end on my 351M with less stroke. I figured I could get away with advancing the same cam 2 degrees to get higher DCR. I did and I did. If I struggled mightily trying to get rid of spark knock and the external fixes were tried and failed, I would certainly try retarding the cam before I gave up on the engine. 91 octane is pricey, and 4.10 gears have a tendency to spin an engine. Retarding timing increases the rpm of peak torque.
Thread Starter
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Laughing Gas
Joined: Aug 2010
Posts: 896
Likes: 55
From: Bothell
Well it took me forever but I got the wideband O2 sensor/gauge installed.
The readings jump around a bit but it looks like I'm running at about 15.5 or so at freeway cruise and full throttle is about 14.3 with 13.0 for idle. Idle will be easy to tune out and it looks like I'm running lean for cruise and WOT. One thing I noticed was that when the metering rods come up to their rich setting (have it set 5" Hg with the orange springs but it seems like they actually open at 9 or 10" Hg on the vacuum gauge) the truck pulls really well power wise and the gauge reads 14.7ish so maybe I'm so lean I'm leaving quite a bit of power on the table. I'll change some jets/rods tonight, fill up the tank and see what happens over the next week or so.
The readings jump around a bit but it looks like I'm running at about 15.5 or so at freeway cruise and full throttle is about 14.3 with 13.0 for idle. Idle will be easy to tune out and it looks like I'm running lean for cruise and WOT. One thing I noticed was that when the metering rods come up to their rich setting (have it set 5" Hg with the orange springs but it seems like they actually open at 9 or 10" Hg on the vacuum gauge) the truck pulls really well power wise and the gauge reads 14.7ish so maybe I'm so lean I'm leaving quite a bit of power on the table. I'll change some jets/rods tonight, fill up the tank and see what happens over the next week or so.
Tuned
Joined: Oct 2010
Posts: 357
Likes: 11
From: Roanoke, VA
Well it took me forever but I got the wideband O2 sensor/gauge installed.
The readings jump around a bit but it looks like I'm running at about 15.5 or so at freeway cruise and full throttle is about 14.3 with 13.0 for idle. Idle will be easy to tune out and it looks like I'm running lean for cruise and WOT. One thing I noticed was that when the metering rods come up to their rich setting (have it set 5" Hg with the orange springs but it seems like they actually open at 9 or 10" Hg on the vacuum gauge) the truck pulls really well power wise and the gauge reads 14.7ish so maybe I'm so lean I'm leaving quite a bit of power on the table. I'll change some jets/rods tonight, fill up the tank and see what happens over the next week or so.
The readings jump around a bit but it looks like I'm running at about 15.5 or so at freeway cruise and full throttle is about 14.3 with 13.0 for idle. Idle will be easy to tune out and it looks like I'm running lean for cruise and WOT. One thing I noticed was that when the metering rods come up to their rich setting (have it set 5" Hg with the orange springs but it seems like they actually open at 9 or 10" Hg on the vacuum gauge) the truck pulls really well power wise and the gauge reads 14.7ish so maybe I'm so lean I'm leaving quite a bit of power on the table. I'll change some jets/rods tonight, fill up the tank and see what happens over the next week or so.
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Trailering
Joined: Jul 2017
Posts: 18
Likes: 3
I just read this thread and thought i would answer post #32.What caused low compression when the engine was running but showed good compression cranking with starter?Wore out valve guide while the engine is running the valve will flop around and not seat but cranking with starter will seat.I have pulled heads before and 15 guides would be in spec and one would be so wore out that you could grab the face of the valve and move it a 1/4" back and forth.Engineers say it's caused from bad metallurgy at the foundry bad mixture in that area and it's softer than it should be.
Freshman User
Joined: Apr 2018
Posts: 25
Likes: 0
Perhaps you can play with tire pressure. Get the tires warm and run chalk line across the tire from sidewall to sidewall .Drive a block or so and see where the chalk goes away .You changed tire size so factory settings wont work and even then ive consistently run 5-10 psi higher than factory on my vehicles for increased MPG . you could have too low pressure and that will kill mileage. Too high and you can have controll issues so be carefull.
If the chalk goes away on the sides you're low, in the center you'rer high .There will be a range of 5-10 psi where the lines wear evenly .
If the chalk goes away on the sides you're low, in the center you'rer high .There will be a range of 5-10 psi where the lines wear evenly .
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