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Old 06-07-2014, 10:12 AM   #1
gilitos
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Default What's the best compression ratio for everyday driving

Trying to choose between installing a 5.2 or 7:1 head. Anybody use a 7:1 for 'normal' driving? What are the pros and cons in your experience?
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Old 06-07-2014, 10:55 AM   #2
Jim Brierley
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Default Re: What's the best compression ratio for everyday driving

it depends on the combustion chamber design. i ran a cyclone head at 7.4:1 for years, with no problems. do not let it detonate (ping) as that is hard on rod bearings.
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Old 06-07-2014, 10:57 AM   #3
PC/SR
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Default Re: What's the best compression ratio for everyday driving

A 7-1 head is totally unnecessary for "normal" driving. The full benefit of that ratio cannot be realized anyway unless other modifications are made as well. There is just no reason for it. IMHO.
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Old 06-07-2014, 11:16 AM   #4
Tom Wesenberg
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Default Re: What's the best compression ratio for everyday driving

I do fine with my stock head, but at some point I will install a 5.5 head to see if it helps mileage.
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Old 06-07-2014, 12:40 PM   #5
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Default Re: What's the best compression ratio for everyday driving

If you're not going fast, the lower compression-
if you want to cruise highways, the higher compression with other modifications as well.
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Old 06-07-2014, 01:36 PM   #6
MikeK
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Default Re: What's the best compression ratio for everyday driving

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I'm running this Super Brumfield on one of my A's. At 117cc it is way over 7:1, probably 7.8:1 or so. Cranking compression with a Stipe SS340 cam is 160psi. The higher compression makes cold starts much easier and the plugs do not carbon foul at low speeds. It is much peppier than the 5.9:1 Brumfield it replaced and has a wider power band than the Lion III I also tried. The engine runs cooler than it did with the 5.9, a thermostat is necessary to keep it above 160 in cooler weather. With the 5.9 mileage averaged 17 in a 160B. With the head shown below I get 20 mpg at ~50mph with no overdrive. You do need to be much more active in minding the advance. I had a Lion Speed Head III that was 124CC and cranked @153 psi, about 7.2:1. That head was very touchy about advance and did not yield any advantage for me over the Brum 5.9. The power band was much narrower with a peak you could not stay on top of with the stock trans and 3:78 rear. The Yapp/Lion III did not look like a stock head so I sold it because it thought it was UGLY on a stock A.
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Old 06-07-2014, 01:37 PM   #7
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Default Re: What's the best compression ratio for everyday driving

Thought this was interesting...check out the ad from Pierre Bertrand (top right) in the May 15, 1937 Muroc speed trials program;


"Winfield flat head job with Bertrand cam and 6:1 compression ratio holds the record here at the Lakes; 121.62 M.P.H.".

Pretty surprising given that Winfield did, at that time, offer a 7:1 head as well; not to mention the Cragar and Riley heads that were popular at the time as well...

I suspect that cars with higher CR heads or OHVs were not configured in such a way to take advantage of their 'superior' designs, at least on that day. As has been mentioned, I think the 7.0 head is probably overkill on a typical touring engine.
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Old 06-07-2014, 02:46 PM   #8
Kevin in NJ
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Default Re: What's the best compression ratio for everyday driving

Any head works fine for everyday driving. The original Ford head would give you the 60 MPH.

The real need for an everyday driver is all the rotating parts being well balanced. You also need a great condition cam with properly adjusted valves. Many cars are no wheres near Ford factory specs and this will have substantial effect on how well the car runs and drives.

Mix in any of the mellow modern chamber heads and you will have a sweet driver good for 50,000 miles of hard running.
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Old 06-07-2014, 03:25 PM   #9
Mike V. Florida
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Default Re: What's the best compression ratio for everyday driving

Quote:
Originally Posted by PC/SR View Post
A 7-1 head is totally unnecessary for "normal" driving. The full benefit of that ratio cannot be realized anyway unless other modifications are made as well. There is just no reason for it. IMHO.

I agree.
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Old 06-07-2014, 03:47 PM   #10
Terry,NJ
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Default Re: What's the best compression ratio for everyday driving

Remember, raising the compression makes it go faster, but it also raises bearing load. I believe in moderation. I went with a Snyder's 5.5 on my T.S. and on my coupe, the head was cut down about .090, for what that's worth (4.6/7?). A 40 HP ain't never going to match that 2013, V12 vomit comet your neighbor has in his driveway. All you can do is make it run a little better.
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Old 06-07-2014, 03:57 PM   #11
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Default Re: What's the best compression ratio for everyday driving

"The higher compression makes cold starts much easier and the plugs do not carbon foul at low speeds."
I never had any problems with cold starts or plug fouling with a standard head. Easiest car I ever had to start at 30 below zero, and that was with the 6 volt system.
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Old 06-07-2014, 04:44 PM   #12
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Default Re: What's the best compression ratio for everyday driving

I think it also somewhat depends on what you are driving. The 4 doors, such as the Town Sedan, weigh significantly more than say a 2 door Coupe or Roadster. The increased power of a 5.5 HC head will help acceleration and hill climbing both in these heavier cars.0
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