Compression Ratio?

[69Rcode_Mach1]

Hey guys I have bought my Victor Jr.'s and I will be yanking out my motor this coming Saturday. Now I will be putting in brand new forged pistons and ARP Rod bolts along with taking the block to the machine shop to see what I will need done before I put my build together. Anyways I want to know what kind of Compression Ratios you guys are running. The previous owner put in 8:1 compression pistons when he rebuilt it so he could run it on regular pump gas. Stock from the factory these motors on the 69 with 4V run 10.7:1 compression ratio. I want to get 11:1 compression ratio and still run premium pump gas on it. I don't want to invest in mixing race gas whenever I fill up. I am at 5000ft elevation right now, but will probably be heading to San Diego in 4 years or so for med school and I want to be able to drive it down there. So if you guys could help me out on this and let me know what compression ratios you guys are running that would be great.

 

[jerry S]

So if you guys could help me out on this and let me know what compression ratios you guys are running that would be great.

My set up is pretty similar to yours. I am running 11:1 CR.

I'd make those new gears your next priority. Even with those new heads, I bet my old 1.0 liter geo metro would pull a faster 0-60.

 

[Max Power]

My set up is pretty similar to yours. I am running 11:1 CR.

I'd make those new gears your next priority. Even with those new heads, I bet my old 1.0 liter geo metro would pull a faster 0-60.

LOL, I bet it wont!!

I am running a little over 10:1, and that is plenty. The aluminum head can probably handle the 11:1, but you may not be able to run as much timing, and dyno tests have shown that each point in compression only gives you about 4% more horsepower. That's only 16hp on a 400hp motor, which you usually lose to timing curves anyway.

10:1 or close is plenty.

 

[D.Hearne]

I'm running 10.4 to 1 in my 331 stroker with Canfield heads. It'll run on 89 if I retard the timing some. Right now with 12 degrees initial it'll ping slightly with 89, never with 93. My heads have 57 cc chambers, a -.003 deck clearance, cam is a Ford Z303 with 1.7 rockers. Head gaskets are Felpro expanded graphite liberally coated with copper coat. Compressed thickness is .045 .

 

[69Rcode_Mach1]

Wow how did ford get those things to run 10.7:1 stock. O well what about 10.5:1 Would that be okay with premium pump gas?

 

[GaPonyFarm]

10.5:1 should be fine on pump gas. Just back off your timing a bit for cold weather... I'm sure you get winter grade gas in Utah.... not great for high compression engines.

 

[Great68]

Wow how did ford get those things to run 10.7:1 stock?

Leaded gas?

 

[jerry S]

LOL, I bet it wont!!

I am running a little over 10:1, and that is plenty. The aluminum head can probably handle the 11:1, but you may not be able to run as much timing, and dyno tests have shown that each point in compression only gives you about 4% more horsepower. That's only 16hp on a 400hp motor, which you usually lose to timing curves anyway.

10:1 or close is plenty.

the tech advice I got from Edelbrock was that alum heads allow you to run 1 full point compression higher than what you had stock. Now my car was orignally running 10.7:1. The 1970 4 bbl Mach Is were running 11.1:1 compression stock!!! I don't think I was being extravagent by going to 11:1 with alum heads. The tech guy whom I had emailed told me that I would have no problems whatsoever to go to 11:1 with my Performer RPMs.

 

[D.Hearne]

Back in the 60's and up to the late 70's premium gas was 98 octane ( leaded) regular was 92 octane. You can get away now with this much compression by using bigger cams to bleed off some of the compression. My 68 Merc's 390 is a case in point. When I bought it, the motor was completely stock with 10.5 to 1 comp. With the stock cam, it would ping on 93 octane. Now that I've rebuilt it ( overhauled it actually-- still has the original pistons) and by using a Crane 272 degree Energizer cam, it'll run on 89 octane. The compression is down 1/4 point with the thicker headgaskets I used, but that and the cam are the only changes.

 

[Max Power]

Back in the 60's and up to the late 70's premium gas was 98 octane ( leaded) regular was 92 octane. You can get away now with this much compression by using bigger cams to bleed off some of the compression. My 68 Merc's 390 is a case in point. When I bought it, the motor was completely stock with 10.5 to 1 comp. With the stock cam, it would ping on 93 octane. Now that I've rebuilt it ( overhauled it actually-- still has the original pistons) and by using a Crane 272 degree Energizer cam, it'll run on 89 octane. The compression is down 1/4 point with the thicker headgaskets I used, but that and the cam are the only changes.

Using the cam to run a little higher compression is an old trick. It doesn't actually bleed off compression as it does cylinder pressure with larger amounts of valve overlap. Can make the bottom end a little soft if you aren't careful.

Even with the aluminum cylinder heads, I think 11:1 is as high as you could go on pump gas and is still pushing it. It depends on the quality of fuel in your region, of course. You are right at the point of giving up all your HP gains to timing issues.

 

[jerry S]

It depends on the quality of fuel in your region, of course. You are right at the point of giving up all your HP gains to timing issues.

what kind of octane can you get? I run 98 RON (94 PON) and have no issues with 11:1.

 

[brianj5600]

Compairing max cr at 5000 feet elevation to near sea level is not possible. The thinner air where you live will allow extra compression, even with the lower octane usually available in those areas. You need to talk to someone that is familiar with high elevation performance engines. It will be tough however to build one that is happy there and when you go to San Diego. You will have to build it for one place and deal with it in the other. I don't think there is any magic combo for both.

 

[D.Hearne]

Using the cam to run a little higher compression is an old trick. It doesn't actually bleed off compression as it does cylinder pressure with larger amounts of valve overlap. Can make the bottom end a little soft if you aren't careful.

The Energizer really isn't what I'd call a big cam, more like a middle of the road grind as big blocks go. Not too much wilder than the CJ/GT cam. Adertised duration is 272 at .050 its like 214-216 ( if I recall) lift with the stock rockers is .523. it's got lots of bottom end too. This 4200 lb car runs mid 14's with this mildly warmed up 390, the addition of the 3.70's out back didn't hurt either.

 

[jerry S]

The Energizer really isn't what I'd call a big cam, more like a middle of the road grind as big blocks go. Not too much wilder than the CJ/GT cam. Adertised duration is 272 at .050 its like 214-216 ( if I recall) lift with the stock rockers is .523.

May I inquire what you would opine to be a big cam on a SBF, in particular, a 351W?

Inquiring minds want to know.

 

[D.Hearne]

Anything with the advertised duration over say about 280 and the lift over.525. That Energizer I've got in my 390 idles like a stock cam. It's got just a hint of lope at idle when it's cold.

 

[Max Power]

I used to use 280H cams on small blocks to get around compression issues. Those specs for a big block are pretty tame from an overlap standpoint, but stout enough for good grunt.

As I have gotten older and seen dyno tests comparing different compression ratios, I'v realized that I would rather run one point less and be safe than run the risk for a 4% gain.

If you are building a motor for the street, you will barely notice the difference between 11:1 and 10:1, from a performance standpoint. Seems like the only reason to do it is bragging rights.

 

[jerry S]

=

If you are building a motor for the street, you will barely notice the difference between 11:1 and 10:1, from a performance standpoint. Seems like the only reason to do it is bragging rights.

my cam has an adv. dur of 292. Truth be told, I wanted a cr of 9:1 so I could eventually add a hair dryer to my car. When I got it, it was 11:1. I haven't run it yet but the mechanic who did my suspension for me told me he drove it for 300 miles before shipping it and he ran it against a car with time slips in the high 12s and my car was nose to nose with it. the high CR isn't what I asked for nor what I wanted but what's done is done so now I am bragging about it. I am not dissatisfied with how the car runs.