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

Compression Ratio

Moderators: Curt!, Pa

Post Wed Feb 20, 2013 7:48 pm

Posts: 470
Location: Six miles East of Cheney, Wa.
I believe they both do, as the amount of air you take in increases, but as stated above the combustion chamber remains the same size.

I think that lift is more important for low end grunt/torque, and duration more useful for higher rpm/HP. But that's a whole other story!! However, a lot of overlap and duration will lower dynamic compression...the more cam you run the more static compression you can use. Did that make sense?

Post Wed Feb 20, 2013 11:45 pm

Posts: 1604
From past experience of the Morgo kits to increase Triumph twins to 750cc, that isn't a correct assumption because increasing the bore enough to produce a capacity increase in the 10% - 15% range ( and this means re-engineering the barrel, not simply going to +0.060" or whatever ) means you need a completely new combustion chamber to suit the new bore and provide a proper seal - a well-known problem with older Triumph twins in particular..

I have an engine somewhere in the depths of my "to do someday" cupboard which was built for grasstrack racing in the 1950s, using a 350cc Matchless engine fitted with a barrel and piston from a 500cc unit. It has the biggest inlet valve I have ever seen on a bike like that, CR around 12:1 ( on methanol ) and all round, it's a very interesting bike. I've ridden it in a couple of hillclimbs and it has tremendous low-rev pull but won't rev, so you just can't get any useful performance out of it. I can only deduce that the mis-shapen combustion chamber is the problem and I would probably be better off finding a 500cc head for it... at which point it wouldn't make any useful power anyway. Matchless tried to make a speedway bike, as did BSA and both found that they didn't have a workable design for a high-compression combustion chamber that would breathe well enough.

Flathead engines are a different case, because you have the basic engineering limitation of the valves alongside. There have been BSA M20s bored and stroked to as much 750cc for vintage racing in UK but they are very fragile
Shoot, a man could have a good weekend in Dallas with all that stuff...

Post Thu Feb 21, 2013 10:36 pm

Posts: 29
Location: Port Hardy British Columbia Canada
Do the math! I've got an extra 3/8 inch stroke my flathead and trust me it increases my compression ratio..

Post Sat Feb 23, 2013 4:46 am

Posts: 1642
Location: Interlaken, NY USA

Back to the BTSV, Paul Freibus is currently working on decreasing the size of the combustion chamber to increase compression on his engines, using techniques such as filling the current spark plug cavity and using a smaller plug, and careful filling around the valves on the back side, again to reduce head volume. Keep in mind that when working with both small and big HD flatheads that milling the heads for piston popup at longer strokes helps breathing by increasing the passage between bore and valves and allows you to dial in the desired amount of squish. It does, of course, increase head volume, and that was the conundrum that HD had with getting power from the KR's.
Frankenstein runs 9.4:1 compression with 4 13/16" stroke, .100 overbore, KHK cams, and "K" heads. It does have some issues with Pinging when hot under heavy load, so that might be the upper limit with somewhat stock components :)
This is Frankenstein if you're not already familiar with it:

Post Sat Feb 23, 2013 5:01 pm

Posts: 470
Location: Six miles East of Cheney, Wa.
Never would have guessed that you could go that high with CR on any flathead. !! That is impressive.

Post Sat Feb 23, 2013 11:58 pm

Posts: 84
A couple thoughts on the foregoing discussions.

Cam Considerations
Regarding a cam for the Knucklebolt project, if someone wants the torque/HP balance to favor torque rather than HP (higher rpm operation) then you'd want cams with shorter duration (225-235 deg at 0.050" lift) and relatively tight centerline. The torque will be high, build quickly, then be done. Lift really shouldn't be much of a consideration here since any flathead cam considered for the project should lift the valve to it's flow potential (usually ~ 20-25% of the valve dia - this is why all the K model cams, including KR, only lift ~ 0.380-0.395"). If you want an engine that is well behaved, yet makes some HP (has to rev to make HP - and of course "rev" is a relative term) you'd want cams that had at least 240 deg duration at 0.050" (KHK cams). Larger engines, e.g., 74-84 cu in can use more cam, so the KHK lobe in a UL would not be "big", as large flatheads can accommodate lots of cam.

Compression Considerations
As has been noted, for an Otto cycle engine with all other things being equal, the higher the compression ratio the higher the efficiency and the higher the resulting torque/power. And of course since power is heat, the more power the more cooling required.

As we all know and as Dick already noted, too much of a good thing isn't always the best thing, i.e., Frankie, with CR of > 9:1, when hot and under heavy load detonates. Frankie is therefore not a bike to lug around town, and operating it in such a way that makes it detonate consistently would bring it's life to a premature end. I'm certain Dick will confirm that he makes good use of the gearbox and keeps the engine running free the majority of the time to circumvent detonation. The UL, being a considerably larger engine yet, has commensurately larger cooling demands and in the same configuration would be that much less forgiving.

Detonation, the product of heat and time, is not tolerable and if it occurs on a constant basis while riding, leads to only one place - a melt down. Thus, when considering the possible compression ratio for a flathead project, the owner must decide how he wants to use his bike. Is he going to lug it around town and roll it on at full temp in high gear at low rpm (these are the worst conditions for inducing detonation - lots of heat and time), or are you going to gear it a bit lower, and operate the engine at higher rpm in a more free-running state (opposite of lugging), where it will tend to run cooler, and only roll it on for brief bursts when the engine is operating at the torque peak or higher (this reduces the potential for detonation since higher engine speed shortens the combustion time in which detonation occurs).

That said, and in light of a given riding style/desired use, what sort of compression ratio can a flathead tolerate? For all out performance where the engine is run at WOT for sustained periods of time (KRs), racers settled somewhere in the range of ~ 5-6:1, which allowed the engines to survive without melting down. For drag racing or similar short excursions of 10-15 sec, where you start with a cold or lukewarm engine, this situation is much more forgiving and can tolerate higher compression (if you are willing to accept the potential for lost flow to gain compression) without melting down. But for a large displacement air cooled flathead, running on gas, at full operating temperature, with over 7:1 CR, I think that operating it at WOT for any length of time would result in overheating, detonation, and in turn engine melt down. For street use I personally wouldn't be looking at anything over 7:1. It is my opinion that ratios higher than 7:1, come at the expense of flow, and excessive heat generation.

Airflow Considerations
Relative to their OHV counterparts, flatheads by their very nature suffer from a compromised state of breathing, because ~ 40% of the circumference of a flathead valve is masked by the cylinder head. KR heads are generously relieved around the IN valve circumference to decrease such valve masking and increase air flow. So if an owner starts filling in the combustion chamber around the valve circumference and/or significantly decreases the height of the transfer port (by milling) to reduce combustion chamber volume, this may compromise flow to an extent that any benefit gained by increasing compression is more than offset by lost flow - thus the conundrum that Dick alluded to. Pop-up pistons provide a means of keeping the transfer slot proportioned appropriately while preserving compression ratio. However, I believe in severe use applications, such as the KR, the piston crown was modified by essentially removing the top of the piston on the valve side to shield it from the propagating flame front when it was fully exposed at TDC (of course this was another compression loss, but one that was lived with for the duration of the KR's reign). The cast KR pistons were typically fit at 0.007", which is really loose for a cast piston. Yet I think it speaks to the sort of heat encountered in the application, and likely at full operating temperature the piston fit was satisfactory.

KR Piston.jpg

The underlying tenet running through the majority of the above discussion is that flatheads are heat machines, that compression and power mean more heat, that more heat means increased potential for detonation, or need to retard ignition timing (which in turn adds more heat), all of which can lead to unhappy engines. Heat in a flathead is not only undesirable, it is by far our worst enemy. However, if we plan and execute our projects carefully, the results can be reliable and satisfying.

Hopefully the above discussion provides a little food for thought as builders decide how they intend to operate their engine and which performance features might allow the engine to live happily ever after.

Post Tue Feb 26, 2013 6:25 pm

Posts: 470
Location: Six miles East of Cheney, Wa.
Wow WZ, that was quite a mouthful, as far as food for thought!

Well I think that also reinforces my basic instinct to keep compression at around 6.5:1. The bike I'm building will be light, and won't see a lot of city traffic, and will be geared a little bit on the tall side, although I'll have to experiment with gearing after she's up and running. I live outside of a small town, which is outside of a small city. (Spokane) Most of my riding is around the rural areas, and I can certainly avoid Spokane on those really really hot summer days.

At any rate, torque would be the best asset for me, high RPM, and especially periods of high RPM aren't going to happen.

Just got my engine, with a fresh rebuild and it's everything the seller advertised, (VERY nice engine) with the exception that the spark plug threads are shot in one hole. (9.3 heads) I could have that fixed easy enough, but it's probably well worth it, more cost effective in the long run to get some aluminum heads for all the other reasons pertaining to combustion chamber heat. The amount of work to get the 9.3's down to 6.5:1 could be a fool's errand, especially considering that when it's all done, they are still iron heads.

And that begs the question, can one still find the stock aluminum heads, or does one go the S&S or AAOK route?


krazy ken

Post Wed Feb 27, 2013 1:54 am

Posts: 1604
I'd say that the volume if metal acting as a heat sink in the castings, is a significant constraint for flathead engines. I think it's no coincidence that both HD and Indian's most successful and durable designs in this configuration, were 45" units.

HD BT flatheads have a reputation for rapid engine wear. BSA M series side valves would frequently show extreme wear and suffer exhaust valve seizures regularly. There is a Model X Matchless in my local VMCC section which has been rebuilt over the winter and the wear on the cylinders was very high, especially given its low mileage and generally easy life.
Shoot, a man could have a good weekend in Dallas with all that stuff...

Post Wed Feb 27, 2013 10:40 am

Posts: 470
Location: Six miles East of Cheney, Wa.
I believe I located some aluminum heads, so disregard my last question.

I wonder what normal cylinder temps are for a stock 74 or 80" flathead? I believe that putting a "big bore" kit on most engines increases engine temp...did HD drop the 80" after 41 because the 74" ran a little cooler? Although, not a huge difference between 74 and 78 cubes. ?

wonder if one could "nikesil" a BT cylinder, as in/like the BMW motor cycle cylinders.

Post Wed Feb 27, 2013 11:57 am

Posts: 766
Location: CA USA
Try Boretech. They have been doing WR race cylinders with some success.

Post Wed Feb 27, 2013 6:20 pm

Posts: 470
Location: Six miles East of Cheney, Wa.
Beachdog, do you mean that Boretech has been doing the BMW type nickel/silicone cylinder bore plating on WR cylinders?

I think it's called "nikelsil", and BMW claims that some cylinders will go 200,000. That's with a all-aluminum cylinder, no sleeve.

Could be an answer to premature cylinder wear 45Brit?

Post Wed Feb 27, 2013 10:23 pm

Posts: 84
There are many companies that apply Nikasil coatings to cylinders. Millenium Technologies is the one I am most familiar with.

This coating type, or a variant thereof, is literally on every unlined aluminum cylinder on every dirt bike, street bike, ATV, snowmobile, jet-ski, etc that has been made for decades. Unless you break the engine (blow it up or stick a piston), or run debris through it (no air cleaner or leaky air cleaner), the cylinder remains essentially unchanged for the life of the engine. You can go through several piston and ring sets and never have to touch the cylinder. Aluminum Harley XR's employ this type of coating on their cylinders.

You can have them finish the part to a nominal dimension or you can provide a piston and tell them how you want it fit. Because of the hardness of the coating, a diamond hone is required to finish the cylinder to size, so unless you've got diamond stones for your hone, you can't alter the size. If you need diamond honing done locally, look for a shop that does lots of late style ATV, snowmobile and dirt bike engine repair/building and they will either have diamond stones or know who does.

I suspect the mainstay of their cylinder coating business is likely recoating factory Al cylinders (snowmobiles, ATVs, dirt bikes, street bikes, etc), which is a good sized market. However they can and do coat cast iron cylinders and cast iron blocks.
Last edited by WZ507 on Thu Feb 28, 2013 6:30 am, edited 1 time in total.

Post Thu Feb 28, 2013 3:27 am

Posts: 1604
Would improved metallurgy leading to improved cooling and lower overall running temperatures, improve the cylinder life on big flatheads? Very likely it would.

The other problem is lubrication. No-one would seriously dispute that big flatheads need better lubrication, see all the various detail on Sportster pump conversions etc for this subject. I came to the conclusion long ago that the WL and Scout represented the optimum size for an engine f this sort, in the same way that 500cc represented the optimum size for the Britsh "big single" and 650cc, the optimum for the parallel twin, at least of the British type ( makers like Kawasaki went in to show that with better technology, the limit was 750cc).
Shoot, a man could have a good weekend in Dallas with all that stuff...

Post Thu Feb 28, 2013 11:11 am

Posts: 766
Location: CA USA

Post Thu Feb 28, 2013 7:13 pm

Posts: 470
Location: Six miles East of Cheney, Wa.
That's all very interesting stuff. I didn't try to look up any pricing on that stuff, but it seems like looking into having one's BTSV cylinders nikasil'd might be an idea...unless the cost is sky high.

On optimum size, I might agree, although how much HP you intend to squeeze out of an engine would blur the line a bit. In the motorcycle world, too much never seems to be enough!

Post Sat Mar 02, 2013 4:52 pm

Posts: 1642
Location: Interlaken, NY USA

Nikisil might be an answer to some problems concerning wear, but here's been my experience.
My flathead 84 and 88 run about 350 cylinder temps, depending on where you "shoot" it.
Not excessive, I'm thinking
Bore wear has been minimized with good lubrication, to date no virtually no bore wear at 10k miles and counting.
Also, Paul Freibus has done ceramic coating of exhaust port and combustion chamber and reduced heat absorption to a remarkable degree. I think if going the coatings route, that might be money better spent vs the nikisil route.
Just my opinion, of course, "your results may vary"

Post Sat Mar 02, 2013 8:36 pm

Posts: 216
Location: Georgia
I also had the ceramic coatings done. Tops of pistons, underside of heads, inside exhaust ports and inside head pipes. Coatings were not all that expensive but only have about 2000 miles on the engine so I can't judge the advantages yet.

Post Sun Mar 03, 2013 12:00 am

Posts: 1604
It's often remarked that the BT flatheads suffered from having basically the same lubrication system as the 45" engines.

Reducing heat absorption by using ceramic coatings would be an entirely logical approach. It doesn't matter his much heat goes straight out the exhaust; its the heat that is absorbed into the castings that causes te problems
Shoot, a man could have a good weekend in Dallas with all that stuff...

Post Sun Mar 03, 2013 7:36 am
Pa Site Admin

Posts: 5545
Location: Ohio USA

Ceramic ?? Have you folks put any thought into the other uses of the material ? Though there are many, I chose the following to make a point. I bought the wife some skillets coated with ceramic. She has not been able to make any hot or burnt foods stick to the ceramic coated pans yet. Even without cooking oil used !! The thought I am referring to is how this same coating may prevent rings from ever seating properly. Just another point to discuss.

Post Mon Mar 04, 2013 2:06 am

Posts: 1604
Ceramics are seriously wear resistant. They stick them on the outside if the space shuttle, and on the insides of turbines, after all.
Shoot, a man could have a good weekend in Dallas with all that stuff...


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