best compression ratio street build?

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KramerSwinger70

KramerSwinger70

70 Dart Swinger360
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Sep 17, 2009
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Rohnert Park, CA
just curious what you think. i plan on buildin a '76 360 and drop it into my 70 dart (904 trans and prolly 3.23 gears). i want to run it on regular gas, get the best mileage and horsepower i can. its a daily driver so mileage is kiunda important in my build, though i dont expect miracles. i assume the higher the compression ration, the better power and mileage.....im not very tech savvy as far as motors go YET so bear with me, im prolly wrong in my assumtion (im only 18! give me some time to go to school for this stuff and ill know better!)

so what compression would ya suggest for a street car to run on regular and get round 300hp and 18mpg? 10-1 is what i hear most but idk if you can run regular with that
 
haha no clue. which ever will be cheaper is prolly wut ill go with. i just want the best bang for my very low buck. im assuming iron is cheaper?
 
stock bottom end and some kb107 pistons mated with cleaned up iron 360 heads will get you the 300 or more your looking for. Of course a good dual plane intake and a 4 barrel carb are a must. I think with standard 360 heads not milled you would be right around 9.7 cr. Then there is the exhaust to think about to.....then you need to figure out cam size, tire size, gear ratio, manual or auto tranny, converter......as far as gas mileage I would throw that out the window. You could just buy a 500 dollar 4cyl beater for that.
 
For what you describe, I wouldn't go to a 10-1 ratio so fast. A 9.0-1 - 9.5-1 max would be much more suiteable. If you want to run regular gas with the stock heads, a replacement 8.0-1 is what your after and from there, a slight adustment can be made on the heads for a higher ratio.

Confused? Let me suggest a combo for 8-1 -9.5-1 ratio. You'll just need a slug to get this ratio area with the head you have. KB probably has a slug for you and a calc to get a good idea of what the ratio will be with said slug.

From a grond level start, as if you have a 4bbl. allready, the OE iron intake and crb are good to go. A step up for the 360 would be a RPM or a Stealth and what ever carb you like in around 650 - 700 cfm.

Use the heads with the stock valves and a good angle valve job, just cleaned up and set up right. Use of headers are an option. And helpful in power.

With this OE looking package, I would use the Comp 265DEH (Without headers)
The Comp XE262H with headers.

these should be close to 300 HP and get very good mileage. 18-ish plus, maybe.

Going up the scale on performance parts, I would be looking at the intake and carb. (As money allows) and a cam choice would be ethier;

Comps XE268H or the OE 340 cam or the upgraded replacement which has a tighter centerline for a chop to the idle.

These will be over 300 HP and get decent mileage. 16 - 17-ish

The key's to mileage are mello cam timing, excellent breathing ablities and a great ignition. My last 318 had a similar cam to the XE262H, only from Crane Cams and the above basic add on parts, 600 Carb, intake and headers) in a less than 8.0-1 rato with a 904/3.21 gears and stock tire.

The car went low 15's with 18 miles to the gallon on a stock sized tire. More compresion and a larger valve would have been nice to get me into the mid 14's while retaining the mileage I believe.
Your 360 should do the same, only quicker considering it is a rebuilt engine and has more compresion to effectivly use the cam(s) above without a big comsumption of gas over the 318.
 
For a budget build that has to be reliable, I'd say don't worry about a horsepower figure... Just build a nice engine and it will be fine. Upgrade the pistons, internally balance it, have the heads done. Get the static ratio somewhere between 9-9.5:1. If you have power brakes run a cam with no more duration than 220°@.050. If you don't have power brakes, go no bigger than 230°@.050. With the usual headers and manifold/carb this should make around 350hp and be dead reliable, reusing many parts and keeping the cost down.
 
Like moper says, no more than 9.5:1 with a reasonable street cam in the 220 range and iron open chamber heads. With Magnum heads and quench set in the .040 range you can get away with 10:1-10.5:1 but you really need an experienced builder to guide you in regards to machine work.
 
What is the price range you are looking at spending?
 
right now i dont really know. i dont know what much of anything costs. im not goin to be building the motor for a while prolly, since i 1st have to get a paint job ect, on the car. all i know is i want my 360 and the 904 to be tanked, balanced and completly rebuilt. baiscally tryin to make whats old new again, but with better parts and there fore, better hp and mileage
 
FWIW, my 360 has 10.6:1 compression using KB107 pistons with the block machined to zero deck. I have magnum heads that have 64cc chamber volume. With a 0.040" head gasket that comes to 10.6:1. The cam is a Comp XE268H and I use a high rise dual plane intake with a 670 vfm vacuum secondary carb. I have 3.55 gears with an OD 4spd.

I get 16 to as high as 18 mph around town and 21 to 22 mpg on the highway. I run the car on 89 octane and have used 87 but it will ping under load at low rpms.

Oh yea the engine makes approx. 370HP based on 1/4 trap speeds.

The heads are the primary reason I can run as much compression and still use 89 octane gas. The cranking pressure is 190 to 200psi so a smaller cam would make more and require higher octane. Also, the OD tranny is responsible for a lot of the decent mileage but you could go with shorter gears.

Another thing to keep in mind is there is about a 10 cent difference between 87 & 89 and 89 & 93 so on a 15 gallon fill you are only talking about $3 between 87 & 93 and 1/2 that between 87 & 89, hardly worth making that a high priority on your build.
 
FWIW, my 360 has 10.6:1 compression using KB107 pistons with the block machined to zero deck. I have magnum heads that have 64cc chamber volume. With a 0.040" head gasket that comes to 10.6:1. The cam is a Comp XE268H and I use a high rise dual plane intake with a 670 vfm vacuum secondary carb. I have 3.55 gears with an OD 4spd.

I get 16 to as high as 18 mph around town and 21 to 22 mpg on the highway. I run the car on 89 octane and have used 87 but it will ping under load at low rpms.

Oh yea the engine makes approx. 370HP based on 1/4 trap speeds.

The heads are the primary reason I can run as much compression and still use 89 octane gas. The cranking pressure is 190 to 200psi so a smaller cam would make more and require higher octane. Also, the OD tranny is responsible for a lot of the decent mileage but you could go with shorter gears.

Another thing to keep in mind is there is about a 10 cent difference between 87 & 89 and 89 & 93 so on a 15 gallon fill you are only talking about $3 between 87 & 93 and 1/2 that between 87 & 89, hardly worth making that a high priority on your build.
 
so if i were to build my motor like yours and run a 904 3spd wut should i expect? i cant afford to do the 518 tranny swap for OD and i definatly cant afford a Gearvendor OD ad-on.

if i go with my 904 (and i will), wut kind of gearing should iu be goin with. i just want its to have some good torque to play with at the bottom end and get the best mileage possible. i know with these cars you guys are thinkin "who cares? its a muscle car!" but its my daily driver (my ONLY car) and i need to commute to work and hopefully, college soon.
 
You won't get as much mileage with an auto as I do with the manual likely drop down to the 14 mpg around town and you won't likely break 20 on the hiway with the 904 and 3.23 gears.

If the car is going to be a daily driver for college I would stay with what ever is currently in the car and save the 360 for when you don't need to rely on it as daily transportation. The slant or 318 that is in the car will do much better.
 
My 318/904/3.21 drivetrain was from a '79 car. The tires were 225/70/14's. The car had a good bottom end. Though the combo lack a decent ratio, it wasn't a bad performer for what it was. Stock heads and comp ratio for '79 and all.

Keep the cam small. Mine was 218 @ .050 for the intake. (228 @ 050 exhaust) Do not exceed this IMO for best mileage possible. Following DGC's build with a cam like mine or a size smaller would be best with 3.23's. and still have the car move well and get good mileage.
 
dgc333 said:
FWIW, my 360 has 10.6:1 compression using KB107 pistons with the block machined to zero deck. I have magnum heads that have 64cc chamber volume. With a 0.040" head gasket that comes to 10.6:1. The cam is a Comp XE268H and I use a high rise dual plane intake with a 670 vfm vacuum secondary carb. I have 3.55 gears with an OD 4spd.

I get 16 to as high as 18 mph around town and 21 to 22 mpg on the highway. I run the car on 89 octane and have used 87 but it will ping under load at low rpms.

Oh yea the engine makes approx. 370HP based on 1/4 trap speeds.

The heads are the primary reason I can run as much compression and still use 89 octane gas. The cranking pressure is 190 to 200psi so a smaller cam would make more and require higher octane. Also, the OD tranny is responsible for a lot of the decent mileage but you could go with shorter gears.

Another thing to keep in mind is there is about a 10 cent difference between 87 & 89 and 89 & 93 so on a 15 gallon fill you are only talking about $3 between 87 & 93 and 1/2 that between 87 & 89, hardly worth making that a high priority on your build.
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But what if the motor were to run hot, around 200-210? What if the tune is a bit lean or the fuel system not up to par and the motor gets starve of fuel at 6,000+ rpm? In cold weather the carb leans out unless its re-jetted, and you could be out late on a cold fall night once in a while. By running 10.6, which I would do also, you leave little room for error. Why shops say 9.5 for iron heads.
 
LOL, but what if the moon breaks apart and falls to earth, what if a train falls of the track and your near by. LOL, Sorry, just had to do it. LOL


Seriously speaking, theres allways alot of what if's in many builds. He runs his as a daily driver.
 
But those "what if's" are not far out maybe's. Fuel filters get plugged up, weather changes. "Daily drivers" aren't all the same. My daily driver back when I was in college was a 1970 Cutlass in the late 80's. I just threw a cover over it and walked-or hitch a ride to class when a snow storm struck. But it was on the road pretty much every month of the year. Up there they used no salt, just sand. Wasn't much different then driving after it rained. Same in the summer heat, no matter how hot it was I was on the road driving it hard. I racked the mileage up to, running round trips of 400 miles every 3-4 weeks and driving up in college. Got like 70,000 miles on it in 3 years before I smashed it up, lol

Some "daily drivers" get driven to the local McD's and drive inns and rack 3,000 miles a year. Big difference.
 
dodge freak said:
But what if the motor were to run hot, around 200-210? What if the tune is a bit lean or the fuel system not up to par and the motor gets starve of fuel at 6,000+ rpm? In cold weather the carb leans out unless its re-jetted, and you could be out late on a cold fall night once in a while. By running 10.6, which I would do also, you leave little room for error. Why shops say 9.5 for iron heads.
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Well if you build it right it won't run hot. I run a 180 t-stat and it runs at the same temp whether it is 20 or 100 outside. In my 30+ years of wrenching on cars I have never felt the need to re-jet for the winter, adjust the choke maybe. The fuel system supports the engine to well past 6000 rpm. You need to build everything right or it will fail regardless of the compression ratio. I will be passing 40,000 miles before fall on my engine so I think I got it right.

With modern closed chamber heads like the magnums the old rules of thumb are just that, old. I am quite sure I could run my engine at over 11:1 on pump gas.

I would not be a bit surprised if you set the piston down in the hole to get the compression down to 9.5:1 it would have more detonation issues that at 10.6:1 with a nice tight quench.
 
True, I heard of 11 to one motors before. If the motor is built right and the timing is set right the motor will run cool, I run no t-stat in the summer and find 170 the motor runs at, with 185 max. That was last week in 92 degree weather and just over 100 miles on the new motor.

Its just some guys run late timing, old plug wires, etc. If I ran a shop I would rather error on the caution side. Hear of so much dumb stuff from shop bosses on speedtalk and the like. One shop says OIL IS NOT IN THE MOTOR, if you want to run a motor with no oil, please buy your motor someplace else!
 
73charger said:
right now i dont really know. i dont know what much of anything costs. im not goin to be building the motor for a while prolly, since i 1st have to get a paint job ect, on the car. all i know is i want my 360 and the 904 to be tanked, balanced and completly rebuilt. baiscally tryin to make whats old new again, but with better parts and there fore, better hp and mileage
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you are on the right track, spending time doing research BEFORE spending money on parts is the key to any successful build, a 360 is an excellent engine with tons of potential, clean up the stock heads, good valve job, shoot for around 9.5:1 CR, and fuel mileage is more about HOW you drive than WHAT you drive
 
I'm not so sure, I drive a 1992 Olds with the 3.3 v6 with a heavy foot and get 20+ mpg, only burn 93 octane gas since it was brand new-yep, gm did make some good cars back then..had other gm's with 200,000+ miles. Now I can't take off very fast but it gets up to 50 mph ok and even with 165,000 miles can past new Buick SUVs, Ford Focus, etc. I limit my speed on the E ways to like 75 mph max cause its like unsafe in a crash and has 14 inch rubber.

I can NEVER get 20 mpg in my 3.91 gear car with the 340. Now true, I could pass my little "hot rod 3.3 car" without even trying, ha ha.
 
dgc333 said:
Well if you build it right it won't run hot.


I would not be a bit surprised if you set the piston down in the hole to get the compression down to 9.5:1 it would have more detonation issues that at 10.6:1 with a nice tight quench.
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Danm right build it right. He's such a domes days sayer. What if's abound in daily life and he's worried about jetting issues. Funny, I never ever did or had a problem. Cloged fuel filter? Don't you change them out? Regular service! Lean tune? Come on now. Learn. better yet, purchase a lil'known device called a 02 sensor and learn from that. It's actually amazing how lean a engine can be and still run.

Your acting like a worry wart over this.


The low comp. ratio pinging issue is so soooo soooooooooo true. My 400 is in that boat now. What a pig of a smog engine it is.
 
For gearing, run 3.23s max. 2.74s would be fine with the small factory tires too. What kills mpg is two things... The rpm at which the engine runs most efficient and the combination of torque convertor and rear axle ratio. A good running 360, with a tight (factory stall) convertor, and 3.23s or higher gearing should get at least 16mpg mixed driving and if it was mine I'd bet I could get 20 out of it with careful tuning.
 
I'm wondering about gearing for my 360/904 combo also; how much of a difference in acceleration and highway gas mileage would be between 2.94:1 and 3.21:1? I'm considering taking a road trip to Pennsylvania with this car and it would be awesome if I could get over 20 MPG.
 
MOPEkidD-3 said:
I'm wondering about gearing for my 360/904 combo also; how much of a difference in acceleration and highway gas mileage would be between 2.94:1 and 3.21:1? I'm considering taking a road trip to Pennsylvania with this car and it would be awesome if I could get over 20 MPG.
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A 2.94 would help. Alot depends on tire height. The 3.23 works great with two sets of tires and speedo gears for the trans. Like a 26 inch for around town and a 29 inch on long road trips. 25 to 28 inch might work better depend on the car body.

A 2.94 gear with 28 inch tires in back could maybe get 20 mpg @ 60 mph. Back in the 55 mph days we heard all the time how 70-75 mph was so wasteful compared to 55 mph. Today they just gear the heck down on the new cars. Like 2.94 gears WITH overdrive!
 
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