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Old 12-18-2017, 09:42 AM   #1
chap52
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Default 5.5:1 or 6.1:1 High Compression Heads?

Working with a basically stock motor. Other than the obvious increase in horse power with a new high compression head, what are the pros and cons of the addition? Secondly 5.5 vs 6.1 pros and cons.
Just getting into these engines and would appreciate your in sites.
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Old 12-18-2017, 10:25 AM   #2
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Default Re: 5.5:1 or 6.1:1 High Compression Heads?

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Originally Posted by chap52 View Post
Working with a basically stock motor. Other than the obvious increase in horse power with a new high compression head, what are the pros and cons of the addition? Secondly 5.5 vs 6.1 pros and cons.
Just getting into these engines and would appreciate your in sites.
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Old 12-18-2017, 12:20 PM   #3
BRENT in 10-uh-C
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Default Re: 5.5:1 or 6.1:1 High Compression Heads?

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Working with a basically stock motor. Other than the obvious increase in horse power with a new high compression head, what are the pros and cons of the addition? Secondly 5.5 vs 6.1 pros and cons.
Just getting into these engines and would appreciate your in sites.

My personal opinion is you telling us that your engine is "basically stock" really does not tell us what we need to know to help you determine the best advice for you. If the babbitt bearing material in your 'basically stock' engine was incorrectly done, --or was poor quality babbitt, it will likely fail using either head. On the flip side, good quality babbitt work done on a 'basically stock' engine will last a long time with either of the heads you referenced, and also do equally well with heads that have a higher compression ratio too.
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Old 12-18-2017, 01:27 PM   #4
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Default Re: 5.5:1 or 6.1:1 High Compression Heads?

I have a 5.5 head on my tudor, with babbitt bearings, and it has done fine so far, at about 12,000 miles.
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Old 12-18-2017, 01:43 PM   #5
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Default Re: 5.5:1 or 6.1:1 High Compression Heads?

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Originally Posted by Jim Brierley View Post
It's not high compression that causes babbitt problems, it is detonation. I ran a 7.4:1 Cyclone head for years with no problems.


Yesterday we heard compression isn’t a problem with Babbitt?
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Old 12-18-2017, 01:56 PM   #6
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Default Re: 5.5:1 or 6.1:1 High Compression Heads?

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Yesterday we heard compression isn’t a problem with Babbitt?
Hm, so whom do you believe ?
If you know WHO posted that comment, you would be inclined to believe it. You could also ask him , about any reservations that you may have. He is a very helpful /very experienced / older member !
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Old 12-18-2017, 03:24 PM   #7
Ron in Quincy
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Default Re: 5.5:1 or 6.1:1 High Compression Heads?

I am partial to the 5.5 Snyder Head.

A high compression head would be the most notable change you could make.

It might be a good time to drop the pan and tune up the main and rod bearings, depending on the miles you have put on the engine.

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Old 12-18-2017, 03:59 PM   #8
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Default Re: 5.5:1 or 6.1:1 High Compression Heads?

This is all "brain picking" for me as I have not started the project yet.
Thanks to all.
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Old 12-18-2017, 05:02 PM   #9
Bruce of MN
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Default Re: 5.5:1 or 6.1:1 High Compression Heads?

My motor has insert mains, the stock cam, and a Tilly. I'm planning on getting a 6.0 head, but no other changes, at least not right away. Will the carb need any more than adjusting?
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Old 12-18-2017, 05:17 PM   #10
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Default Re: 5.5:1 or 6.1:1 High Compression Heads?

I had a Snyder 5.5 head installed on my Town Sedan, shortly after I bought it about 11-12 years ago. Probably the single most effective power gain for the cost and ease of installation, and with prudent use of your spark control (not over advancing), should not be harmful to your bearings.....assuming you have good quality to begin with. I have over 12,000 miles on mine with no issues.
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Old 12-18-2017, 05:53 PM   #11
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Default Re: 5.5:1 or 6.1:1 High Compression Heads?

I am encouraged by the testimonies. I guess that I was wondering, if all else is is OK is it worth going with the 6.1 over the 5.5?
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Old 12-18-2017, 06:22 PM   #12
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Default Re: 5.5:1 or 6.1:1 High Compression Heads?

5.5 is a good compromise and heed Terry's post #10.
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Old 12-18-2017, 06:43 PM   #13
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Default Re: 5.5:1 or 6.1:1 High Compression Heads?

5.5:1 works well for me.
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Old 12-18-2017, 07:05 PM   #14
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Default Re: 5.5:1 or 6.1:1 High Compression Heads?

On mine I have insert bearings and a 5.5 Synders.
Works fine and I drive mine a lot. It will cruise at 50mph for hours on end and I have over 15000 miles on the head with no problems. I don't find any increase in top end speed with the 5.5 Head. (about 60mph on the rare occasions I open it right out) but pick up is definitely better -Karl
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Old 12-18-2017, 07:56 PM   #15
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Default Re: 5.5:1 or 6.1:1 High Compression Heads?

With a babbitt engine, I would only consider a 5.5 head. As previously stated, just watch the spark advance and do not lug the engine.

If the bearings are in good condition, with a little prudence in your driving technique, you should get very good service out of your engine.

You will really like the increase in compression, it really improves performance of the engine.

My opinion,

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Old 12-19-2017, 08:28 AM   #16
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Default Re: 5.5:1 or 6.1:1 High Compression Heads?

Thanks to all for the input.
Have a Blessed Christmas
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Old 12-19-2017, 09:05 AM   #17
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Default Re: 5.5:1 or 6.1:1 High Compression Heads?

When I was a pilot and wanting to buy my own plane, I had a discussion about power requirements. Many small plane manufacturers were not inclined to "overpower" their machines. The original Piper/Taylor Cubs when first built, were only 36HP. That's hardly enough to get a heavily loaded Cub out of ground effect. You have no reserve power to prevent a stall. Put-Put airplanes are widow makers. It's a similar case with a put put car. Take a stock A out on a interstate. In rush hour you'll get to know what real fear is. Some will argue that the A is not a "put put" automobile. I will argue that compared to a modern car, It is. My Transit has over three times the HP and the rush hour traffic on I-25 in Denver, this year, kinda scared the sweat out of me and I wish I had more. The most effective way to to boost the HP to more modern levels is a H.C. head. Being conservative myself and always conscious of the bearing load, I advise a 5 1/2-1, but even a 6-1 isn't even that high. My diesel Jetta was 19-1. Word to the wise, Watch your timing!
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Old 12-19-2017, 12:46 PM   #18
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Default Re: 5.5:1 or 6.1:1 High Compression Heads?

my banger is a 7-1 lion3 head . inserts & full pressurized . daily driver with no problems
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Old 12-19-2017, 12:57 PM   #19
chap52
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Default Re: 5.5:1 or 6.1:1 High Compression Heads?

Thanks Terry, I remember back in the 80's when I could afford to fly, and flying a Cessna 150 into a strong Southern Oregon headwind, speed indicating 70 kts, ground speed calculated at 10 kts. . That's how I feel about driving old cars on todays freeways.I cruise to enjoy the trip, not white knuckle through the whole thing.
As I said I am just gathering info right now and apparently the difference between the two HC heads is not much of a gain.
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Old 12-19-2017, 01:59 PM   #20
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Default Re: 5.5:1 or 6.1:1 High Compression Heads?

Just a couple of comments
Raising the compression ratio that little is not going to cause any problems or raise the horsepower that much.
I build Duesenberg and Cadillac engines with Babbitt bearings and I run them with 8.5:1 with no problems
Like one other person said it the detonation that takes out a Babbitt bearing or an insert bearing
So just get a good bearing person to do the work and hold the tolerance .
Terry Harkin did mine until he got sick now someone took over his business and they are doing good work now


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