what's the ideal compression ratio?

staggers

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I'm wondering what the ideal compression ratio is for a 750cc performance streeter and a full racer? Also thoughts on an 880cc version.
Say I tune the streeter to run 98 octane only?
I know there's a lot of consider but just a guide would be fine.

Only reason I ask is that heiden tuning have an 880cc low comp kit with something like 8.5:1 I think? For a street kit. I thought that was a little low??

Cheers.

(Bet there's going to be some different opinions here haha)
 
Well...it's hot there. It's an air cooled engine. Not hard to figure out why lower compression is better for something you actually wanna use.

Well... FYI Heiden Tuning are based in the Netherlands and I really don't think they made their 880cc street kit that's suited to 533 rods only specifically for the Australian market just because it's hot!
 
You obviously didn't get that. It's hot in Australia. Once you start knocking out these air cooled engines and running high compression....they run HOT. Fine for racing where you might be tearing into it all the time...for any longevity on the street...or drive ability in hot weather and maybe slow traffic....your $$$$ money though.

The same reason some guys in southern usa run oil coolers. Hot....all the time. Cdn climate. Not really needed.

Hot rodding the shit out of an air cooled engine for street use in your climate seems silly.
 
You obviously didn't get that. It's hot in Australia. Once you start knocking out these air cooled engines and running high compression....they run HOT. Fine for racing where you might be tearing into it all the time...for any longevity on the street...or drive ability in hot weather and maybe slow traffic....your $$$$ money though.

The same reason some guys in southern usa run oil coolers. Hot....all the time. Cdn climate. Not really needed.

Hot rodding the shit out of an air cooled engine for street use in your climate seems silly.

I'm aware that increasing combustion pressure is going to increase temperatures.

On another note, just how "hot" do you think it is here?
My back yard isn't a desert waste land with camels running past me!
I'm in south east QLD and I'd say it's warm most of the year but certainly not hot! Although we have our moments it certainly does get HOT from time to time especially this time of year but you won't see me frying my arse on a bike at that time, I'll be drinking beer in a pub instead haha
 
Checking my highly detailed map of Australia:
Australia.jpg


Then checking Brisbane's weather charts:
wz_clim_annual_site_40842.png


It looks like you folks have some very pleasant, moderate temps there. Out here, we normally run 35°-40°C in the summer. I may have to move there.

Warning, the following regurgitation comes from the '70s, and may not apply nowadays.

When I built my 750 in the '70s, it was a 10.5:1 compression ratio. But, I broke it in on 130 octane avgas. Later, ran on 104-108 octane premium pump gas (easily found back then).

The fun thing about compression ratio is that there's multiple values.

The static compression ratio is the mechanical ratio of (displacement + chamber) / chamber.
I.E., a displacement of 9 and a head chamber of 1 gives a 10:1 C/R.

Then comes the fun part, dynamic compression ratio. Long duration cams can bleed off enough displacement to give manageable C/R during kickstart and idle. Then, as rpms come up, and the effects of intake/exhaust design come into play, the dynamic C/R comes up.

Another way to look at it is the Volumetric Efficiency (VE) of the engine at various rpms. Typical engines are lucky to get 95% VE, with full-race stuff getting above 110%.

On mine, the 10.5:1 was okay since I was running a long duration cam, reducing kickstart effort, and not achieving the higher dynamic C/R until into the 6000-8500 rpm zone, avoiding the 3000 rpm 'detonation prone' zone.

Well, that doesn't doesn't really answer your question. Just some fun stuff to ponder...
 
You obviously didn't get that. It's hot in Australia. Once you start knocking out these air cooled engines and running high compression....they run HOT. Fine for racing where you might be tearing into it all the time...for any longevity on the street...or drive ability in hot weather and maybe slow traffic....your $$$$ money though.

The same reason some guys in southern usa run oil coolers. Hot....all the time. Cdn climate. Not really needed.

Hot rodding the shit out of an air cooled engine for street use in your climate seems silly.


Round we go, feels safe to be condescending from so far away. You nor I own/manage this website. This is someone's else's endeavor, so cut the shit. You know how you know what you know? Someone took the time to explain it to you, and you asked questions along the way. Please stop treating people this way, for the sites sake. You're personal feelings should be left at the door, we're talking bikes here, not venting internal angst. Cool?
 
Checking my highly detailed map of Australia:
View attachment 59582

Then checking Brisbane's weather charts:
wz_clim_annual_site_40842.png


It looks like you folks have some very pleasant, moderate temps there. Out here, we normally run 35°-40°C in the summer. I may have to move there.

Warning, the following regurgitation comes from the '70s, and may not apply nowadays.

When I built my 750 in the '70s, it was a 10.5:1 compression ratio. But, I broke it in on 130 octane avgas. Later, ran on 104-108 octane premium pump gas (easily found back then).

The fun thing about compression ratio is that there's multiple values.

The static compression ratio is the mechanical ratio of (displacement + chamber) / chamber.
I.E., a displacement of 9 and a head chamber of 1 gives a 10:1 C/R.

Then comes the fun part, dynamic compression ratio. Long duration cams can bleed off enough displacement to give manageable C/R during kickstart and idle. Then, as rpms come up, and the effects of intake/exhaust design come into play, the dynamic C/R comes up.

Another way to look at it is the Volumetric Efficiency (VE) of the engine at various rpms. Typical engines are lucky to get 95% VE, with full-race stuff getting above 110%.

On mine, the 10.5:1 was okay since I was running a long duration cam, reducing kickstart effort, and not achieving the higher dynamic C/R until into the 6000-8500 rpm zone, avoiding the 3000 rpm 'detonation prone' zone.

Well, that doesn't doesn't really answer your question. Just some fun stuff to ponder...

You're a good bloke mate!!

I've noticed and read a few of your posts and it's great to have people like you on this forum :thumbsup:

You my not have answered my question directly but you certainly open my mind on compression ratio. Cheers for that, I learn something new every day here :thumbsup:
 
fk I want a copy of that map ,but you forgot the areas for the spunky backpacker women,actually some good info as we can go to diferent climates here quickly ,eg cairns ..tropics ..1 1/2 ride west 1000 or so meters higher , low of -3-4-5 degs cel in winter normal ...lowest recordered -11 deg cel which all affects running of the bike and unfortunitly the clothes on the women and when you think about it you don't take a high compression bike from cairns to bris full out with out exspecting police tickets ,motor blowing up etc but unfortunitly except in the citys we don't build pub crawlers just practical well built bikes (well some of them):cheers::devil:
 
Staggers, you are right, everyone will have an opinion on this.

My thoughts are: For a hot street set up running pump gas 10:1 is about the limit. I bought an XS that the previous owner had done a big bore and rephase on. The compression ratio was near to 11:1 it was difficult to kick start and was prone to detonation unless it had 95 octane or better fuel. Reducing the compression ratio to around 10:1 made it much easier to live with.

As for racing, ratios 11:1 to 11.5:1 can be used with high octane fuel.With methanol higher compression ratios are used but I have no information on just how high you can go.

From this and other questions you have asked it looks like you are researching to build a monster XS engine. I suggest you buy the engine modification guide available from 650 Central as a PDF download.http://http://650central.com/
It is worthwhile and will save you money.
 
haha maybe I should of asked what everyone else is running?

You'll only see me riding in temperatures between 20-30C so I'll tune it to that temp range also I'm not going higher than 300m above sea level as around where I am that's about as high as it gets
I thought you could just jet the carbs to suit these climates and altitudes anyway as it's not too extreme!
 
Do you folks use oil temp gauges?

Awhile back, while researching Australia, I found these posted on an Australian tourism website, the answers are the actual responses by the website officials, who obviously have a great sense of humor (not to mention a low tolerance threshold for cretins!)
________________________________________________*
Q:*Does it ever get windy in* Australia? I have never seen it rain on TV, how do the plants grow? (UK).*
A: We import all plants fully grown, and then just sit around watching them die.*
__________________________________________________
Q:*Will I be able to see kangaroos in the street? (USA)*
A: Depends how much you've been drinking.
__________________________________________________
Q:*I want to walk from Perth to Sydney* - can I follow the railroad tracks? (Sweden)
A: Sure, it's only three thousand miles.* Take lots of water.*
__________________________________________________
Q:*Are there any ATMs (cash machines) in* Australia? Can you send me a list of them in Brisbane, Cairns, Townsville and Hervey Bay? (UK )*
A: What did your last slave die of?
__________________________________________________
Q:*Can you give me some information about hippo racing in* Australia* ? (USA)*
A: Af-ri-ca is the big triangle shaped continent south of* Europe* .
Aust-ra-lia is that big island in the middle of the Pacific which does not ..
Oh, forget it.* Sure, the hippo racing is every Tuesday night in Kings Cross. Come naked.
__________________________________________________
Q:*Which direction is North in* Australia* ? (USA)*
A: Face south, and then turn 180 degrees. Contact us when you get here and we'll send the rest of the directions.*
_________________________________________________
Q:*Can I bring cutlery into* Australia* ? (UK)*
A: Why? Just use your fingers like we do.
__________________________________________________
Q:*Can you send me the Vienna* Boys' Choir schedule? (USA)
A: Aus-tri-a is that quaint little country bordering Ger-man-y, which is ..**
Oh, forget it.* Sure, the* Vienna* Boys Choir plays every Tuesday night in Kings Cross, straight after the hippo races. Come naked.
__________________________________________________
Q:*Can I wear high heels in* Australia* ? (UK)*
A: You are a British politician, right?
__________________________________________________
Q:*Are there supermarkets in Sydney and is milk available all year round? (Germany)*
A: No, we are a peaceful civilization of vegan hunter/gatherers.* Milk is illegal.
__________________________________________________
Q:*Please send a list of all doctors in Australia who can Dispense rattlesnake serum. (USA)*
A: Rattlesnakes live in A-mer-ica, which is where YOU come from. All Australian snakes are perfectly harmless, can be safely handled, and make good pets.
__________________________________________________
Q:*I have a question about a famous animal in Australia, but I forget its name. It's a kind of bear and lives in trees. (USA)*
A: It's called a Drop Bear. They are so called because they drop out of gum trees and eat the brains of anyone walking underneath them.
You can scare them off by spraying yourself with human urine before you go out walking.
__________________________________________________
Q:*I have developed a new product that is the fountain of youth. Can you tell me where I can sell it in Australia? (USA)*
A: Anywhere significant numbers of Americans gather.
__________________________________________________*
Q:*Do you celebrate Christmas in Australia? (France)*
A: Only at Christmas.
__________________________________________________
Q:*Will I be able to speak English most places I go? (USA)*
A: Yes, but you'll have to learn it first.
 
Staggers, you are right, everyone will have an opinion on this.

My thoughts are: For a hot street set up running pump gas 10:1 is about the limit. I bought an XS that the previous owner had done a big bore and rephase on. The compression ratio was near to 11:1 it was difficult to kick start and was prone to detonation unless it had 95 octane or better fuel. Reducing the compression ratio to around 10:1 made it much easier to live with.

As for racing, ratios 11:1 to 11.5:1 can be used with high octane fuel.With methanol higher compression ratios are used but I have no information on just how high you can go.

From this and other questions you have asked it looks like you are researching to build a monster XS engine. I suggest you buy the engine modification guide available from 650 Central as a PDF download.http://http://650central.com/
It is worthwhile and will save you money.

Cheers for that mate.

Yeah I don't want to hold back on this build. I'm doing a tonne of research on this. I know it's an XS after all and not expecting massive power but would like to surprise unsuspecting sports bikes at the lights :wink2:
Yeah I'm in the city so the whole stop and go thing here works for me at the moment haha
 
Eig=TwoManyXS1Bs;440450]Do you folks use oil temp gauges?

Awhile back, while researching Australia, I found these posted on an Australian tourism website, the answers are the actual responses by the website officials, who obviously have a great sense of humor (not to mention a low tolerance threshold for cretins!)
________________________________________________*
Q:*Does it ever get windy in* Australia? I have never seen it rain on TV, how do the plants grow? (UK).*
A: We import all plants fully grown, and then just sit around watching them die.*
__________________________________________________
Q:*Will I be able to see kangaroos in the street? (USA)*
A: Depends how much you've been drinking.
__________________________________________________
Q:*I want to walk from Perth to Sydney* - can I follow the railroad tracks? (Sweden)
A: Sure, it's only three thousand miles.* Take lots of water.*
__________________________________________________
Q:*Are there any ATMs (cash machines) in* Australia? Can you send me a list of them in Brisbane, Cairns, Townsville and Hervey Bay? (UK )*
A: What did your last slave die of?
__________________________________________________
Q:*Can you give me some information about hippo racing in* Australia* ? (USA)*
A: Af-ri-ca is the big triangle shaped continent south of* Europe* .
Aust-ra-lia is that big island in the middle of the Pacific which does not ..
Oh, forget it.* Sure, the hippo racing is every Tuesday night in Kings Cross. Come naked.
__________________________________________________
Q:*Which direction is North in* Australia* ? (USA)*
A: Face south, and then turn 180 degrees. Contact us when you get here and we'll send the rest of the directions.*
_________________________________________________
Q:*Can I bring cutlery into* Australia* ? (UK)*
A: Why? Just use your fingers like we do.
__________________________________________________
Q:*Can you send me the Vienna* Boys' Choir schedule? (USA)
A: Aus-tri-a is that quaint little country bordering Ger-man-y, which is ..**
Oh, forget it.* Sure, the* Vienna* Boys Choir plays every Tuesday night in Kings Cross, straight after the hippo races. Come naked.
__________________________________________________
Q:*Can I wear high heels in* Australia* ? (UK)*
A: You are a British politician, right?
__________________________________________________
Q:*Are there supermarkets in Sydney and is milk available all year round? (Germany)*
A: No, we are a peaceful civilization of vegan hunter/gatherers.* Milk is illegal.
__________________________________________________
Q:*Please send a list of all doctors in Australia who can Dispense rattlesnake serum. (USA)*
A: Rattlesnakes live in A-mer-ica, which is where YOU come from. All Australian snakes are perfectly harmless, can be safely handled, and make good pets.
__________________________________________________
Q:*I have a question about a famous animal in Australia, but I forget its name. It's a kind of bear and lives in trees. (USA)*
A: It's called a Drop Bear. They are so called because they drop out of gum trees and eat the brains of anyone walking underneath them.
You can scare them off by spraying yourself with human urine before you go out walking.
__________________________________________________
Q:*I have developed a new product that is the fountain of youth. Can you tell me where I can sell it in Australia? (USA)*
A: Anywhere significant numbers of Americans gather.
__________________________________________________*
Q:*Do you celebrate Christmas in Australia? (France)*
A: Only at Christmas.
__________________________________________________
Q:*Will I be able to speak English most places I go? (USA)*
A: Yes, but you'll have to learn it first.[/QUOTE]

I'm not surpised with some off the responses lol
We sometimes get some random questions from foreigners haha.

Oil temp gauge? Don't you just stick your tongue on the case? Haha just kidding :laugh: I haven't thought about that yet. Although what you think about the dip stick temp gauge idea?
I'm going to run an external cooler and thought about teeing off the flow line (before cooler) for a temp gauge?
 
Round we go, feels safe to be condescending from so far away. You nor I own/manage this website. This is someone's else's endeavor, so cut the shit. You know how you know what you know? Someone took the time to explain it to you, and you asked questions along the way. Please stop treating people this way, for the sites sake. You're personal feelings should be left at the door, we're talking bikes here, not venting internal angst. Cool?

as I recall the man asked for opinions. So cut the shit. And it's a valid one. Not saying don't build am engine at all. Close to 900 with high c.r. will be a warm one that's all. I'd love an 860 kit myself.

I'd imagine with our not hot all the time summers and 6 month riding season it s the reason I can always find good charging rotors here. Less heat combined with riding time.

My apologies for assuming since you are alot further south than me. Assumed your summers would be pretty hot.
 
...Oil temp gauge? Don't you just stick your tongue on the case?

Yes. If it sticks, it's too cold to ride.

...I haven't thought about that yet. Although what you think about the dip stick temp gauge idea?
I'm going to run an external cooler and thought about teeing off the flow line (before cooler) for a temp gauge?

Here's some oil temp threads. Seems that 230°F (110°C) is typical during summertime here. I'd prefer it to be more like 95°-100°C. Member Weekendrider has hit highs of 280°F (138°C)!!!

http://www.xs650.com/forum/showthread.php?t=17479
http://www.xs650.com/forum/showthread.php?t=14997
http://www.xs650.com/forum/showthread.php?t=42650
 
the temp gauge is a good cheap idea ,I run one of bunnys(?) good investment...mine runs at constant 200f road speed except going up slow dirt tracks then up to 225 f , have just bought temp gun and when new motors in the bike will take different readings over different parts of motor during running period just to see if any changes out of curiosity
 
When I parked the other afternoon (low 50's), after a 30 mile 75-80 mph ride, the oil temp was 250.
 
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