RAMM
Well-Known Member
This question seems to be coming up a lot more frequently probably due to ever rising fuel prices as well as being spoiled by modern muscle cars/trucks with their big HP/Fuel efficiency.
Now obviously electronic engine management along with variable valve timing, active intake manifolds, multiple displacement (MDS) and 8/9/10-100 speed transmissions aren't going to apply here, we are after all on an A-body forum in Small Block Tech. So here goes:
What if I told you an LA engine that was carb'd , solid flat tappet, vac distributor going down the highway @ 2800-3200 rpm could make 450-500HP and still knock down 18-20 mpg?
Don't believe me? Don't care, stop reading.
The key is understanding where (RPM) and why an engine is at its most efficiency. Modern cars are all about low RPM @ cruise but gear your overcammed LA this way and the engine will not be "happy". Happy = Efficiency
Do not build anything larger in displacement than required. Do not build anything with high internal friction. There is a reason almost ALL auto manufacturers have landed on the 340-370 cube displacement for best balance between HP AND MPG. Think about it. Why did GM debut the LS @ 346 ci? Mopar the G3 @ 346 ci? Its for a reason and by design it was NO accident. Moving on.
With old school iron we don't have all the electronic wizardry to work with so we must consider the entire driveline. I will outline what I believe and know to work.
1. Manual transmission is a big part in this. Why? With a 5 or 6 spd you will maintain acceleration (Fun) with no slip like an auto. A modern light synthetic oil in the box also helps to the end means of efficiency. Besides REAL musclecars have 3 pedals and I stand by that.
1.(b) Yes I mentioned the 5spd or 6 spd which obviously have OD gearing. Do not fall into the trap of under gearing the rearend with say 3.55's and a .50 OD gear. This will suck if you have any kind of cam duration. Instead you need to select a box with a relatively tall first gear so that it is useable with a 3.73-4.10 rear gear. If you don't you will have a first gear that is useable for about 10 feet which sucks.
Told you this would be longwinded-lol
2. Get your car low enough to reduce resistance and also get a proper alignment for the love of god.
Basically everything we can do to reduce rolling resistance helps. We all know this.
Moving on to the engine.
It takes about 60-80 hp to keep most cars such as an A-body rolling down the highway at highway speeds. Why build anything much bigger than what will get the job done?
So to make this even more convoluted here's what I would do and why I see it this way:
Build a true 340 with modern machining and extreme attention to detail and you will be rewarded handsomely. The awesome rod ratio is really part of the secret here. Use 6.2" rods and pick up a little more efficiency. Build the engine and gear the car to go down the highway as close to peak torque within reason. Here's where I'm going to ruffle some feathers because most would have you believe that low rpm is where its at. It is and it isn't, depends on the camshaft big time here.
So 340 block .030-.04" honed with plates, zero deck or slight positive straight and true. Stock stroke 3.31, with counterweights massaged and profiled-nitride it --less friction this way. Have your fave cranks shop turn the rods and mains .030-.040 under if the bearings are available. Heck do like I did in my 354 inch G3 and go Honda journals on the rods if you are serious.
Order a piston like a Mahle with their unbeatable 1mm/1mm/2mm ring pack<<This is another must IMO.
Gotta run , I will be back for Part 2. J.Rob
Now obviously electronic engine management along with variable valve timing, active intake manifolds, multiple displacement (MDS) and 8/9/10-100 speed transmissions aren't going to apply here, we are after all on an A-body forum in Small Block Tech. So here goes:
What if I told you an LA engine that was carb'd , solid flat tappet, vac distributor going down the highway @ 2800-3200 rpm could make 450-500HP and still knock down 18-20 mpg?
Don't believe me? Don't care, stop reading.
The key is understanding where (RPM) and why an engine is at its most efficiency. Modern cars are all about low RPM @ cruise but gear your overcammed LA this way and the engine will not be "happy". Happy = Efficiency
Do not build anything larger in displacement than required. Do not build anything with high internal friction. There is a reason almost ALL auto manufacturers have landed on the 340-370 cube displacement for best balance between HP AND MPG. Think about it. Why did GM debut the LS @ 346 ci? Mopar the G3 @ 346 ci? Its for a reason and by design it was NO accident. Moving on.
With old school iron we don't have all the electronic wizardry to work with so we must consider the entire driveline. I will outline what I believe and know to work.
1. Manual transmission is a big part in this. Why? With a 5 or 6 spd you will maintain acceleration (Fun) with no slip like an auto. A modern light synthetic oil in the box also helps to the end means of efficiency. Besides REAL musclecars have 3 pedals and I stand by that.
1.(b) Yes I mentioned the 5spd or 6 spd which obviously have OD gearing. Do not fall into the trap of under gearing the rearend with say 3.55's and a .50 OD gear. This will suck if you have any kind of cam duration. Instead you need to select a box with a relatively tall first gear so that it is useable with a 3.73-4.10 rear gear. If you don't you will have a first gear that is useable for about 10 feet which sucks.
Told you this would be longwinded-lol
2. Get your car low enough to reduce resistance and also get a proper alignment for the love of god.
Basically everything we can do to reduce rolling resistance helps. We all know this.
Moving on to the engine.
It takes about 60-80 hp to keep most cars such as an A-body rolling down the highway at highway speeds. Why build anything much bigger than what will get the job done?
So to make this even more convoluted here's what I would do and why I see it this way:
Build a true 340 with modern machining and extreme attention to detail and you will be rewarded handsomely. The awesome rod ratio is really part of the secret here. Use 6.2" rods and pick up a little more efficiency. Build the engine and gear the car to go down the highway as close to peak torque within reason. Here's where I'm going to ruffle some feathers because most would have you believe that low rpm is where its at. It is and it isn't, depends on the camshaft big time here.
So 340 block .030-.04" honed with plates, zero deck or slight positive straight and true. Stock stroke 3.31, with counterweights massaged and profiled-nitride it --less friction this way. Have your fave cranks shop turn the rods and mains .030-.040 under if the bearings are available. Heck do like I did in my 354 inch G3 and go Honda journals on the rods if you are serious.
Order a piston like a Mahle with their unbeatable 1mm/1mm/2mm ring pack<<This is another must IMO.
Gotta run , I will be back for Part 2. J.Rob
Bet this Bad Boy drags out 30 or 40 posts.
