Compression ratio for daily rider panhead

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clunker1
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Compression ratio for daily rider panhead

#1

Post by clunker1 »

I know this topic has come up a million times. However, times change and so does fuel.

I have a very specific question about a 64 pan engine build.

Building a daily rider (5 gal fat bobs) with mild cam (Andrews J?), 4.25" stroke, stock pan heads, manual advance distributor. Had intended to buy stock 8:1 compression ratio, 80" shovelhead pistons to use with shovel rods (7.44 center to center) in 0.070" oversize bored pan cylinders. I accidently ordered the "Low compression" pistons which are 7.5:1. As I think about it, this may be a better option with the current/future fuel availability.

I would greqatly appreciate any comments or opinions on this topic. The cam matching then is the other concern as I know lower compression will have significant impact on cam performance. Futhermore, there is not a great selection on near stock cams out there.

Thanks
1951 adam
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Re: Compression ratio for daily rider panhead

#2

Post by 1951 adam »

J grinds are good cams , what's the current fuel issues your talking about? I have no problem finding high octane, ethanol is another issue . I love low compression engines , and small slugs.
Adam
clunker1
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Re: Compression ratio for daily rider panhead

#3

Post by clunker1 »

High octane is not always a choice at some stations. Also, higher octane means higher carbon requirement (more CO2 produced for making higher octane). I think it may not be long till the government starts limiting octane requirements.

Secondly, I was thinking the lower compression may be more forgiving with a manual advance???
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Re: Compression ratio for daily rider panhead

#4

Post by 58flh »

clunker-----7.5 to 1 & a 80-inch+,& J cam -easy starting & totally econmy on burning fuel!---I wouldn't be surprised if over 50mpg on the pike!,The added cubes will help pull up grades when riding 2-up!.If you get bored its easy to get a more aggressive cam?-But at 7.5 I think you will have a great daily runner!--I ran both the #1 & the J was in for 3yrs.Im at 8.5 to 1,& Its used DAILY!--I recently went back to a H cam last year/The first thing I noticed was the oomph under my foot when kicking her!--Its not bad I just got use to the J cam!---I think you will be happy with it!---Respectfully--RICHIE
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Re: Compression ratio for daily rider panhead

#5

Post by VPH-D »

As long as you use -74 style pistons to go with your -74 short rods, you'll maintain 7.5 compression ratio. I think you'll be happy with this set up
because of the easy starting. My 57 engine was just finished with -74 short rods and -55 pistons, which will be around 8.0 compression.
VPH-D
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Re: Compression ratio for daily rider panhead

#6

Post by Bosheff »

All other factors being equal, the difference in performance between 7.5:1 and 8.5:1 with todays fuels is not really as apparent as one would be lead to believe. You can pretty much ride up hill on a hot day with a really fat girl on the back with the timing fully advanced and not get spark knock. Over the years, I've learned to appreciate the lower compression issue. A cooler running engine lasts longer, and in this day and age, that's what it's all about in my book....bosheff
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Re: Compression ratio for daily rider panhead

#7

Post by Cotten »

Wow Bosheff.

A really fat girl?

Let me ask Folks for an opinion about "knock":
What did it sound like to you, and how do you know for sure?

One reason I ask is because often riders with 'pre-detonated' motors never heard it.

...Cotten
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Re: Compression ratio for daily rider panhead

#8

Post by awander »

I think that with 7.5:1 or even 8:1 compression, you would not get preignition with even the lowest-octane fuel available today.
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Re: Compression ratio for daily rider panhead

#9

Post by Cotten »

Andy!

You leave "you" wide open.
A heavy hack can make a 'stock' motor knock audibly, when it may not be noticeable at all on a 'stock' solo rig, (same fuel etc.,).

Motors of similar compression have had their issues in the past.
Considering that we have no control over the fuel, we can only prepare for the worst.

....Cotten
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Re: Compression ratio for daily rider panhead

#10

Post by kitabel »

Cylinder head temperature is more a function of load than of compression ratio.

The length difference between early and late OHV rods is 1/32":
1936-73 7.46875"
1974-* 7.4375"

Starting with 8.5:1 static CR, using late rods produces 8.08:1
Starting with 7.5:1 static CR, using early rods produces 7.85:1

It's only geometry, not trig!
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Re: Compression ratio for daily rider panhead

#11

Post by Huck »

I'm all ears..
We now need a cam. Performance at 5K is not a consideration as its never wound that tight. The load is 300lbs. 60 mph all day long is fast enough for me.
Clunker,
sorry I tried to stop myself.
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Re: Compression ratio for daily rider panhead

#12

Post by Huck »

Cotton,
I always thought of it as a "pinging" rather than a knock. Back before I could afford a timing light you'd advance until you heard it then back it down until it just stopped.
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Re: Compression ratio for daily rider panhead

#13

Post by Cotten »

Huck!

I never heard it timing, (Can't hear much of anything anymore..),
But lugging a sidehack away from a stopsign can go "clack, clack, clack", like hammers on top of the pistons.

....Cotten
Huck
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Re: Compression ratio for daily rider panhead

#14

Post by Huck »

no not like that.
I mean out in the road, with tools. set it, try it>>stop<< set it, try it stop again. Back roads are better, easy to get kilt ou there.
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Re: Compression ratio for daily rider panhead

#15

Post by Cotten »

Huck!

Been-there-tried-that
: Too subjective and time-consuming for me to make a living.

If the best timing isn't 'right on the line', you are on your own!

....Cotten
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