Board index Flathead Power-Technical Questions, Answers, and Suggestions Big Twin Flatties Compression Ratio

Compression Ratio

Post Wed Feb 13, 2013 6:27 pm

Posts: 497
Location: Six miles East of Cheney, Wa.
Sorry to ask this question, but I've searched through all the threads on Big Twin Flatties and didn't find a good answer. I probably missed something.

I have a 1937 UL on the way with iron heads, which I believe are supposed to be a 5.5:1 compression ratio, assuming they are UL heads and have not been previously modified. I realize that head gasket thickness, overbore, base gasket thickness, deck height etc. all effect CR. Even so, my question is, what volume, in CC or ML, generally speaking, equals that 5.5:1 ration, and what would it be to obtain a 6.5:1 ration.

And, how much or a cut would be milled off the head to achieve that?

Also, what is the highest compression these will run at for normal riding type running. The bike will be light, and I weigh 180. I have a '48 Dodge with a flathead 230 inch six that I have at 7.5:1, (which required a pretty big cut) and it's right "on the edge", if you know what I mean...requires premium for sure. My guess would be that 6.5-7.0:1 would work, especially as I can retard the timing with the left hand grip when lugging/pulling. I had very high compression in my knuck, and the left handgrip worked great, but I realize the flathead is a different animal.


knucklebolt/krazy ken

Post Wed Feb 13, 2013 9:05 pm

Posts: 29
Location: Port Hardy British Columbia Canada
I run a 4 5/8 stroke in my 48 u and when I was running 50 over 80 Pistons I best calculation for compression ratio was 7.21. Now I had only shaved the aluminum heads slightly to cleanup the surface. At this compression ratio running regular gas is not an option because when I got warmed up and I was climbing the hill the pinging was deafening. So I run 94 octane and pack around some octane boost in case I can't find it. I think to achieve a 7.2 compression ratio with just shaving their heads, your valves were probably collide with the head.
You see with my 3/8 of an inch extra stroke I would normally have half of that, 3/16 pop-up on my pistons. By running corvair pistons the distance from top of wrist pin the top of piston is 3/16 less than a standard Harley piston so the 3/8 of an inch stroke is all in the bottom of the barrel. This might be a little bit more work than that most people are willing to do to get some extra compression out of their flatheads that I quite enjoyed it.

Post Wed Feb 13, 2013 9:36 pm

Posts: 497
Location: Six miles East of Cheney, Wa.
You have plenty of stroke, that's for sure! I'm sure that mixes things up, as far as determining CR.

From my experience with the Dodge flathead, (which being water cooled probably tolerates a little more CR than an air cooled engine) and your experience at 7.2:1, I think that 7.0:1 is probably about tops for any/most flatheads if you don't want to pack the octane booster I don't want to go over 7.0:1, and could live with a little under that...6.6, 6.7, 6.8:1, etc. It just strikes me that 5.5:1 is much/way lower than it needs to be with the much better gas we have compared to the flathead era. Perhaps just coming up to 6.5+ might be the best thing.

I was considering either shaving the heads, or welding some material to the edge of the squish area above the piston, and then re-shaping it, and taking a small cut just to square and clean things up. But I really need to know what volume will produce what compression ratio, generally speaking and all things considered or not considered. !!! The engine I have is a stock rebuild, .060 over....which also effects the CR.

Okay thanks for the reply. Again I'm sure this has all been hashed over before, but I can't find much, and can't seem to find any kind of chart online that addresses the issue.

Post Fri Feb 15, 2013 12:39 am

Posts: 159
Just fill your heads up with mig and dremel to a KH or KR shape! Image

Post Fri Feb 15, 2013 7:11 pm

Posts: 497
Location: Six miles East of Cheney, Wa.
Yes I was thinking about adding material, and then shaping.

Those are interesting heads. I did have a K once, but never took the heads off. Just bought, got running, and sold for the funds to buy a knuckle basket. Also had a 45 with aluminum heads, but didn't really pay attention to the finer points of combustion chamber shape in those days. But that engine ran good.

I hate to ask, but which head has been re-shaped? My guess would be the left one, but the right one seems to have dremel marks on it. ?? Or is one a KH and the other a KR?

Looking at the add for S&S heads, if I read it right, they state that the head at 5.5:1 has a 126cc chamber, and that a .070 cut will bring it (the 5.5:1) to 7.1:1. As the 7.1:1 head is the highest compression head they offer, that kind of reinforces, to my mind at least, that 7:1 comp is about as high as one would want to go with stock cams and pistons. ?? I'm just spit balling here. But I don't want to go as high as possible, still thinking 6.6, 6.7, 6.8:1 would be fine for my purposes. However, they offer four different compression ratios, but they don't give the CC's for each one, just the 5.5:1.

I'm also thinking that S&S knows a little bit about it, and perhaps their combustion chamber shape might be something to strive for. Or does the KH/KR shape really "work"?

Again, just spit-balling, any comments, ideas, or rock solid knowledge appreciated. I realize there are a lot of variables, and that "X" amount of CC's don't always equal "X" amount of compression. I also understand static vs. dynamic compression. But again again, my motor will be pretty much stock.

P.S.okay..ha ha, just did a search and found a lot of stuff. Sorry to bother.

Post Fri Feb 15, 2013 8:31 pm

Posts: 112
if i am not mistaken, each cylinder displaces 39.4 cu. in. if the piston top is flush with the deck. divide the displacement by whatever compression ratio you want (6.5) and that will be the desired volume of the head and gasket. 39.4 / 6.5 = 6.06 convert that to a your measure of choice.
where is Panic when we needum ?

Post Fri Feb 15, 2013 9:38 pm

Posts: 497
Location: Six miles East of Cheney, Wa.
Very interesting indeed. I will remember that. Although, with a .060 overbore, (that's very cool that I can take that variable into consideration) I believe my 74 now displaces 76 c.i. But I get the point. !! Doing that math, I get 5.67 c.i. (which is 92.91cc's I think) for a 6.7:1 compression ratio.

Does that sound right?

And that kind of begs another question, which would be "best", to shave the heads, or add material and reshape to get the desired volume. When valve to head clearance is reduced, does that tend to "shroud" the valves, reduce flow? Or does most of the flow occur under the valve head?


Post Fri Feb 15, 2013 10:20 pm

Posts: 1676
Location: Interlaken, NY USA

O.K, compression ratio = displacement(1 cyl) plus combustion chamber, divided by combustion chamber,
displacement: 39.422 Cu in
combustion chamber volume: 126cc + volume contained by valve pockets in head, unswept portion of cylinder, etc
126cc= 7.686 cu in
So figure out combustion chamber total volume first,
total volume of combustion chamber CRvol
we know 126CC(head vol) plus non head volume, gives 5.5 on stock bore 80
To Solve:
CR=5.5= (39.422+CRvol)/CRvol. CRvol equals total combustion chamber volume.
solving for CRvol,
CRvol=39.422/4.5=8.76Cu in
This is the total volume of the combustion chamber
8.76cu in -7.686 cu in, volume of head, leaves 1.07cu in combustion chamber volume not in head.
So, to find a volume for the head for any particular compression ratio
go through the same calculation
7.1:1 ratio, for example:
CRvol=39.422/6.1=6.462cu in
head volume=6.462 cu in minus 1.07 cu in=5.338 cu in. This equals 88.33 cc's
Make sense?
As an aside,
I found some data I came up with a few years ago when my flathead was 84 inch
I milled the aftermarket heads from Kurt .120" 120 thou, and lowered the volume from 128cc's to 112 cc's
that only got me up to about 6:1 ratio with 4.5" flywheels.
Have Fun,

Post Fri Feb 15, 2013 10:27 pm

Posts: 1676
Location: Interlaken, NY USA

Just read your post,
If you're serious, reform the chamber to look like the "K" heads with shelf. Then, yes, fill in the back sides of the chamber around and between the valves, make your own head gasket to properly mask the new chamber and leave no pockets. minimize the relieving between valves and cylinder bore to help minimize chamber size. While you're playing around do NOT chamfer the relieving where it meets the cylinder bore. This can cause undue bore wear.
By the by, I just spent the big bucks and had Paul Freibus rework a set of cylinders for me, incorporating inlet ports to mate to a "K" model intake manifold. They are a work of art, far beyond what my poor hands could have accomplished. He pointed out the above suggestions to me.

Post Fri Feb 15, 2013 10:37 pm

Posts: 497
Location: Six miles East of Cheney, Wa.
To that I say WOW and thank you. I believe I'm getting a good/better grasp on it now. Thanks again.

krazy ken

Post Sun Feb 17, 2013 9:16 am

Posts: 1654
As an aside, all flatheads suffer from breathing issues caused by the port shapes. HD discovered long ago, that there was a definite limit to what you could achieve by raising the CR. The last KRs had compression ratios around the 6:1 mark because the factory had long discovered that maximum flow rate was more important.
Shoot, a man could have a good weekend in Dallas with all that stuff...

Post Sun Feb 17, 2013 11:45 am

Posts: 497
Location: Six miles East of Cheney, Wa.
I would agree with that...and the more I think about it, the closer to 6.5:1 I plan to shoot for. I understand what you are saying.

Correct me if I'm wrong, let's just say for the sake of discussion that 6:1 is ideal, but with a racing KR, or a KH, or K on the street where you are trying to get as much RPM as possible, and trying to beat your friends to the next café, and trying to keep up with the Nortons and Triumphs, (I used to have a AJS) then you might trade off more CR for flow. Or should.

In my case, where I'll be riding the UL mostly on Saturdays or Sunday afternoons, sometimes to work, the bike is a stripper, of minimal weight, and just hauling my tired 180 pound butt around, no passengers, not trying to keep up with my son's Victory or Sporter, I'm thinking RPM's will be much lower, so I would trade off my high RPM flow, which my motor will rarely if ever see, for a little more CR and low end grunt.

Does that sound like reasonable reasoning? Is there a point (ha good pun) where you are trading off a little of one for the other? I would think so, but I could be wrong.

At any rate, I think I've learned through my '48 Dodge that going over 7:1 certainly isn't good, she will ping if I go full throttle if the RPM's aren't up there, and I have to keep good gas in the tank and some booster handy. Not a big problem, as she sure runs sweet, and I'm not in any hurry to get anywhere, or race anyone, in a 48 Dodge with a flathead six! It's the journey, not the destination. !!?! And she does have incredible low end really don't have to shift gears just does out of habit.

Thanks for the additional info...did not know that.

Post Sun Feb 17, 2013 3:00 pm

Posts: 1676
Location: Interlaken, NY USA

On a street 80, I'd suggest the following to have max fun cafe to cafe.
Consider upping displacement to 84" with 4 1/2 wheels. You gain 2 things:
1: more displacement, (duh)
2: the opportunity to obtain an adequate squish distance from head to piston. Stock 80's have way too much.
You can use the regular aftermarket pistons avail. for 80's and mill about 35 thou popup recess in the head to get around 30 thou head to piston clearance.
Next, relieve the cylinder between valves and bore, remembering not to chamfer the cylinder edge. Don't go hog wild on the relieving process.
Mill the heads as much as you can to reduce chamber size.
Add the Carb of your choice, 38mm is a good venturi size for this motor. I like VM Mikuni's, cheap and easy to tune for good results. It's up to you, before the VM's I used DC linkerts, and they also worked well. Be sure to use PEEK seals on the manifold and pressure test for leaks.

The end result, you'll get plenty of torque at low speeds, and you're friends on modern day HD's may still pass you, but you'll enjoy cranking on at 30 in fourth and feeling that low end torque.
I would also advise improving the oiling system, these motors really appreciate it.
By the By, I've been riding BTSV's and blowing them up for 40 yrs. Still love'em.

As to the KR's, the mods they made basically increased relieving and incurred the resultant lowering of compression.

My present BTSV, a 37ULH is my test bed for all my pipe dreams. It's currently 88" ,some mild cams from Paul Freibus, and standard ports and nipples. This year it gets the jugs with larger ports and manifold.
Increasing the displacement from 84 to 88 last year didn't net me the power increase I had hoped for, which said to me that it was time to increase port size so the engine could breathe in more mixture. Stay tuned :)

Post Sun Feb 17, 2013 8:02 pm

Posts: 497
Location: Six miles East of Cheney, Wa.
I hear you, and believe you. However, I have a fresh, rebuilt UL, 76 inch with the overbore, (1937 too) so I won't be taking it apart anytime soon. Now I need the rest of the bike. I'm sure I'd love an 84 or 86", but that will be further down the road. But anything I can do with my heads will be a plus, or a win-win situation, so I really appreciate all the input.

Now on that 88, will you be able to increase intake valve size, or are you limited to the port?

Thanks for all the good information.

krazy ken

Post Mon Feb 18, 2013 5:29 am

Posts: 1676
Location: Interlaken, NY USA

Yes, the valves are 1/8" oversize and Paul flowed the increased opening into the larger ports. Too many projects, probably won't get the top end swapped out until May. Hope to have it ready for the run to Rhinebeck in June though.

Post Mon Feb 18, 2013 1:08 pm

Posts: 93
Location: Norway
Hi. On my 74" with old Flathead Power heads I milled off 0.1". Then I reshapet the area close to the bore to resemble KR-heads. I had to grind some clarence for one of the exhaust-valves. No welding or reshaping of the roof of the chamber over the valves. Total CR is 6.8-1. It goes just good as stock, but with at bit more power all over the range.

Post Mon Feb 18, 2013 2:49 pm

Posts: 497
Location: Six miles East of Cheney, Wa.
That's a pretty big cut. On the Dodge .090" didn't require making room for the valves, but head to valve clearance is now close. I don't remember now exactly what CR that head was to begin with, but I ended up at over 7:1. Good to know you can cut the Harley heads that deep if you have to. I'm hoping that after adding some material the cut will be much less than that. S&S says that a .060"-.070" cut will/should bring their 5.5:1 head to 7:1, so I guess no two heads are exactly the same, those heads seem to have a much different shape than either the stock heads, or the KR/KH. ?

Post Tue Feb 19, 2013 10:58 am

Posts: 93
Location: Norway
If you only mill your heads (with stock shape) to get 7:1 CR, the opening close to the bore will be much smaller. This is why I reshapet this area and is also why I had to cut 0.1" to get 6.8:1 CR.

Post Wed Feb 20, 2013 12:01 am

Posts: 1654
One thing I had never thought about before, until I got involved with my stroked WR motor, is that stroking increases the CR automatically; the swept volume increases while the combustion chamber volume remains the same.

For my engine, using #6 heads, CR would seem to be 6:1 x ( final swept volume / original swept volume ) = 6.8:1 or so. This would fit with what I was told long ago, that the useful limit on stroking for increased capacity for a streetable motor was about 15% and the result would be an increase of around half a point on CR. I was also told that inlet valve size was important, exhaust much less so, and that duration was more valuable than lift.
Shoot, a man could have a good weekend in Dallas with all that stuff...

Post Wed Feb 20, 2013 7:20 pm

Posts: 159
Overboring increases compression, not stroking?


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