Case for High Compression
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From: Delaware
Case for High Compression
If my vehicle runs regular gas and I get 8 MPG and gas costs $1.00/gal, then my fuel cost is 12.5 cents/gal. If I boost the compression and use High Octane gas at $1.20/gal and improve my gas mileage to 9 MPG, then my fuel cost is 13.33 cents/gal.
However if regular gas costs $2.00/gal and I get 8 MPG, my fuel cost is 25 cents/gal. Now If I boost my compression and use High Octane gas at $2.20/gal and improve my gas mileage to 9 MPG then my fuel cost is 24.44 cents/gal.
That is not really a large saving, but it depends on the actual improvement in fuel mileage that I would get and the number of miles that I drive.
I can't simulate gas mileage, but I did simulate the change in torque @ 2500 RPM in a 400 motor with stock CR and a 255DEH cam, and then with 9.0:1 CR. I got 12 ft-lbs increase in torque @2500 RPM. This would probably run on 89 Octane. It would also mean a lighter pedal to cruise at 2500 RPM and some mileage improvement.
If I boost the CR to 9.5:1, my motor is in 93 Octane territory, and my torque increase goes to 22 ft-lbs @2500 RPM higher than the first motor that uses regular gas.
However if regular gas costs $2.00/gal and I get 8 MPG, my fuel cost is 25 cents/gal. Now If I boost my compression and use High Octane gas at $2.20/gal and improve my gas mileage to 9 MPG then my fuel cost is 24.44 cents/gal.
That is not really a large saving, but it depends on the actual improvement in fuel mileage that I would get and the number of miles that I drive.
I can't simulate gas mileage, but I did simulate the change in torque @ 2500 RPM in a 400 motor with stock CR and a 255DEH cam, and then with 9.0:1 CR. I got 12 ft-lbs increase in torque @2500 RPM. This would probably run on 89 Octane. It would also mean a lighter pedal to cruise at 2500 RPM and some mileage improvement.
If I boost the CR to 9.5:1, my motor is in 93 Octane territory, and my torque increase goes to 22 ft-lbs @2500 RPM higher than the first motor that uses regular gas.
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From: Eastern Pa
You mean that your cost is 12.5 cents per mile in the first scenario not per gallon. With that in mind do you know what if any work around the valves is needed on an aussie head. When I worked on my stock 351m head I ended up with 82.3cc chambers after trying to give the valves a litte room on the exhaust side. If I were to put a set of aussies on my engine (with I believe 62cc chambers?) the way it sits I would have a cr of 10.6:1, a little higher than I want as this is an occasional driver. However if they also have a shrouded exhaust valve and after the work got them to say 68cc which wouldn't be hard if the exhaust is as close as it was on the stock head, I would end up at 10:1. Do you think I would srew up anything on the head by doing that? LIke the burn pattern?
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I think that Aussies come in at 58/59cc. I never heard that they had shrouded exhaust valves. I think that 62cc should work well in a 351M, with the right cam. I come up with 10.3:1 CR for 62cc in a 351M. What is the deck clearance and valve relief that you are using?
There is something published somewhere about opening up the chambers on Aussie heads. I think if you go more than a couple of cc's you are taking a chance. Maybe someone has a link to that article. I'll look myself.
There is something published somewhere about opening up the chambers on Aussie heads. I think if you go more than a couple of cc's you are taking a chance. Maybe someone has a link to that article. I'll look myself.
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From: Eastern Pa
I was looking at a tech article that had the aussies at 62cc. Maybe it's not right. Hear's what I have. Deck clearance of 16.8cc with a Felpro gasket at 8.4cc and chamber of 82.3. Badger flat tops which I believe have 2cc reliefs but I'm not positive on that. Running the Edelbrock performer with the 1406 600cfm carb, scorpion roller rockers, heads ported and exhaust matched,, bump removed, headers, stock electonic ignition at about 10 btdc timing. I had to put the dual cats on to get a shop to make me up an exhust for it and I certainly noticed a hit in performance there but once I'm through inspection they are coming back off as I will be able to get a mileage waiver. Its in a 79 f150 4x4. Originally a 351 but I used a new 400 crank and the Badgers.
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It's a known fact that higher compression means higher efficiency, thus more power made with the same amount of fuel used. It was a perfect formula back when there was high octane leaded gas and pinging was not an issue.
These days, not only is the gas lower octane & no lead, but the quality of it can vary between stations and regions. That's why most people are building low compression street motors. It's just a safety factor. The difference between 8.5-1 and 9.5-1 isn't a whole lot as far as volumetric efficiency is concerned, but it's a huge difference if you get some bad gas, or accidentally pump the low octane by mistake. It could be the difference between ruining your motor or not. See the "Motor Noise - Lifters?" thread for a real life example.
These days, not only is the gas lower octane & no lead, but the quality of it can vary between stations and regions. That's why most people are building low compression street motors. It's just a safety factor. The difference between 8.5-1 and 9.5-1 isn't a whole lot as far as volumetric efficiency is concerned, but it's a huge difference if you get some bad gas, or accidentally pump the low octane by mistake. It could be the difference between ruining your motor or not. See the "Motor Noise - Lifters?" thread for a real life example.
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Posting Guru
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From: chicago burbs
there are two different aussie heads, 302C and 351C. they have different chamber volumes, i don't know what they are though?
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Postmaster
Joined: Aug 2001
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From: Delaware
OK, it's a 400 not a 351M.
I generally give 3cc for badger flattops.
I come up with 8.82:1 CR with 82.3cc heads. That is low for all that cam you have. You would be better off with >64cc. That would bring your static compression ratio to 10.4:1, and your dynamic compression ratio to 8.2:1. That is as much dynamic compression as you should go without quench.
The CHI-3V Aluminum heads that I have would work great on your motor. Other choices are 351C-4V closed chamber heads (62.8cc), but you would have to open them up a little, or Aussie 2V heads with a lot of chamber work done. Another choice is Aussie heads (58cc) with thicker copper gaskets. An 0.080" copper gasket will bring the CR down to 10.2:1 and the DCR down to 8:1. With that setup you will get Torque of 425 ft-lbs@2000 RPM, 441 ft-lbs @3500 RPM, Horsepower = 373@5000 RPM.
I generally give 3cc for badger flattops.
I come up with 8.82:1 CR with 82.3cc heads. That is low for all that cam you have. You would be better off with >64cc. That would bring your static compression ratio to 10.4:1, and your dynamic compression ratio to 8.2:1. That is as much dynamic compression as you should go without quench.
The CHI-3V Aluminum heads that I have would work great on your motor. Other choices are 351C-4V closed chamber heads (62.8cc), but you would have to open them up a little, or Aussie 2V heads with a lot of chamber work done. Another choice is Aussie heads (58cc) with thicker copper gaskets. An 0.080" copper gasket will bring the CR down to 10.2:1 and the DCR down to 8:1. With that setup you will get Torque of 425 ft-lbs@2000 RPM, 441 ft-lbs @3500 RPM, Horsepower = 373@5000 RPM.
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Postmaster
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From: Delaware
Scroob,
My point is that with Regular approaching $2.00/gal, and 93 Octane available in some areas for $0.20 more. It may be cost effective to use a higher compression motor with 93 Octane fuel. I know that most people will not want to do that for various reasons, but that doesn't change the economics.
My point is that with Regular approaching $2.00/gal, and 93 Octane available in some areas for $0.20 more. It may be cost effective to use a higher compression motor with 93 Octane fuel. I know that most people will not want to do that for various reasons, but that doesn't change the economics.
Posting Legend
Joined: Sep 2001
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What happens when gas price goes back down tho...
Remember that when you rebuild your engine or heads they usually mill .010 off of the faces to clean them up. You loose a few cc's of chamber volume that way and the piston comes closer to the deck.
My figures show 2 cc's for 0.010". Of course this does not work for closed chamber aussie heads or when the chamber shape changes. A SWAG for an aussie head might be half, or 1 cc for every .010".
Remember that when you rebuild your engine or heads they usually mill .010 off of the faces to clean them up. You loose a few cc's of chamber volume that way and the piston comes closer to the deck.
My figures show 2 cc's for 0.010". Of course this does not work for closed chamber aussie heads or when the chamber shape changes. A SWAG for an aussie head might be half, or 1 cc for every .010".
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From: Delaware
Here is the answer to tastyklair's and Torque1st's question.
http://www.flatlanderracing.com/husseygaskets_menu.html
These are not cheap, but you can change compression just by changing gaskets.
http://www.flatlanderracing.com/husseygaskets_menu.html
These are not cheap, but you can change compression just by changing gaskets.