Combat compression.

AussieCombat · Feb 16, 2011

G'day all... As some of you are aware my Combat motor is in a state of rebuild at the moment... I'm sure this topic has come up before, so I'm sorry for that, but do I rebuild, as is, New bottom end, new pistons etc, at 10.1 comp, or do I look to reduce compression a little just to try and help the bottom end. I read in another post about Copper Base gaskets, Aluminium base gaskets etc. I would really prefer not to go through this again for a while, however if we think the standard setup, (with Superblends) is good enough, then standard it is. P.S.... Has anyone come up with some magical Tappets yet... As you can tell I'm a bit gun shy now. Thanks.. A.C.

Hobot · Feb 16, 2011

I run Combats with flame ring gasket but no base plate on 91 octane at 500-2200 ft elevations in hi summer climbing steeps or lugging somewhat w/o detonation.
The more CR you can run w/o knock-ping the sweeter the response and not an issue to the bottom end, short of drive train jerking lugging. Cdo has an elastic engine so sand base plate is 1 less seam to seal or move around on and less bolt stretch slack to pop a gap oil weeps out.

Fuel it enough with float level set for best pilot screw idle 1.5 turn out and go from there on needles. Creep up on initial timing but usually fine if just don't backfire on starts. Boyers sluggish spark advance is less detonation prone than sharper points ignition rise. Others can be judged in between comparing their curves. Some wisdom on throttle use also goes a long ways on what ya can get away with.

shrugger · Feb 16, 2011

Going with the superblends and upgrading the breather (Old Brits or CNW can help with this) will give you a practically
bullet proof motor. Even at 10:1. Having the oil scavenge machined also couldn't hurt. :mrgreen:

AussieCombat · Feb 17, 2011

Thank you, That's all I wanted to hear. I'm not in the habit of high revving for long periods but I do have just the man to do the Scavenging mod as per the info suggested. And there seems to be several choices re the breather...Thanks again, A.C... P.S. Hobot, I am not familiar with..." flame ring gasket " is this a brand name over there ?

bill · Feb 17, 2011

I don;t know about gas in oz but with the poor fuel we are stuck with in the states I would drop the compression just a little. On my combat I have been stranded with nothing but 87 octane and it does like it one bit. I am NOT fond of thicker base gaskets or decompression plates. The way I do it is to machine the tops of the pistons

Hobot · Feb 17, 2011

There are like 4 options for head gaskets. Aluminum ones are absolutely taboo. Then there is the various thickness of copper gaskets. The standard for Combats is the "Flame Ring", which is slightly thinner than most copper ones and has steel rings around the bores and a composite construction that sorta self glues to seal.
Then theres a piss poor gasket that is paper with a very thin copper foil on both sides that can blow out on just lolly gagging easy running, so avoid as Al kind. I've experienced the down sides of all these and only use copper on flame ring now.

I have to use 87 octane in my Combats at times and works fine as long as I remember to use low throttle loads till better gas available. Might be worthwhile to spray water into carbs till the exhaust stopped belching gray crap, to de-cook and remove hot spots that tend to detonate like glow plugs.

2S cam works best with higher CR than lower, but just putting in a standard cam w/o lowering CR can lead to detonation as less over lap to bleed off compression till piston speed can out run the detonation potential.

Diddling the timing back usually is fine to run ok on lower octane with least loss of much performance.

rvich · Feb 17, 2011

What is the situation with gas in Oz? Can you still buy high octane fuel at the pump? As it becomes a thing of the past I am afraid those of us who want to keep Combats in original form will be forced to make some hard decisions just as you are contemplating. I realize my solution is not the direction that many might take. But I actually bought myself an 850 that if anything goes according to plan will become the touring bike that will be subjected to poor quality fuel and road adventures and my Combat will remain in its high compression state and live close to home where I can get high octane fuel. Once the fuel supply goes the way of most things I will be forced to consider the octane boost, avgas, etc etc.

The truth is that the 850 in its stock state works better for my usual riding style anyway. But I always wanted a Combat just to try it out. Now that I have ridden one, I don't want to change it. It's too much fun!

Russ

batrider · Feb 17, 2011

Have had no trouble running regular gas in a pinch with my combat. I don't see much of a need to reduce the compression.

comnoz · Feb 17, 2011

If you are still running the Combat cam I would leave the compression alone. If you have a standard cam then I would consider reducing the compression by a point. Jim

ML · Feb 17, 2011

rvich said:
What is the situation with gas in Oz? Can you still buy high octane fuel at the pump?
Russ

At the pump, we can get 98 RON, known as Premium Unleaded, 95 RON and 92 RON which invariably has E10 in it. My 850 handles all of them, but the 98 is the go stuff. The most common availability brands are Shell, BP and Caltex for 98.

There are also Octane Boosters you can get in auto accessory stores. I found that they work OK with the crappy low RON stuff, just got to remember to carry it with you. Would definetly use that with a Combat if deprived of hi octane.

Mick

bill · Feb 17, 2011

comnoz said:
If you are still running the Combat cam I would leave the compression alone. If you have a standard cam then I would consider reducing the compression by a point. Jim

I was OK on the first tank as there was enough high test to help BUT on the second tank of low grade on a long mountain pull it was a serious problem. all this is with a combat cam and .020 decked off the pistons.

comnoz · Feb 17, 2011

bill said:
comnoz said:

If you are still running the Combat cam I would leave the compression alone. If you have a standard cam then I would consider reducing the compression by a point. Jim

Click to expand...

I was OK on the first tank as there was enough high test to help BUT on the second tank of low grade on a long mountain pull it was a serious problem. all this is with a combat cam and .020 decked off the pistons.

Well I certainly wouldn't recommend low grade in anything but a stock 850. Your asking for trouble in a 750. Combat or not. Jim

AussieCombat · Feb 17, 2011

Thanks again, Do the other Aussies know of Flame Ring gaskets. ??

bill · Feb 18, 2011

comnoz said:
Well I certainly wouldn't recommend low grade in anything but a stock 850. Your asking for trouble in a 750. Combat or not. Jim

That was the point I was trying to make. I was stuck with NO high test anywhere to be had. the next top end over haul I will probably deck another .020 off the pistons but I will carry octane booster if we have another gas shortage like the last one.

comnoz · Feb 18, 2011

bill said:
comnoz said:

Well I certainly wouldn't recommend low grade in anything but a stock 850. Your asking for trouble in a 750. Combat or not. Jim

Click to expand...

That was the point I was trying to make. I was stuck with NO high test anywhere to be had. the next top end over haul I will probably deck another .020 off the pistons but I will carry octane booster if we have another gas shortage like the last one.

And it's a good one. I had to put 4 gallons midgrade in my 67 Mercedes this week because that was the best available and baby it till the next fuel. It pings severely. Once in a while it pings severely on "high grade". It makes a good fuel tester.

But if you have a combat cam and lower the compression it makes a pretty big hole in the midrange power. Jim

bill · Feb 18, 2011

comnoz said:
But if you have a combat cam and lower the compression it makes a pretty big hole in the midrange power. Jim

That is why I only took .020 off the pistons. Next build it might get a GOOD quality stock grind and a bigger compression drop but maybe not. Just have to watch the fuel situation. The latest OK to boost the ethanol wont help . one of the local news stations did a story and found as much as 25% ethanol already so I will hate to where it goes from here.

Hobot · Feb 18, 2011

NOPE, E85 is 105 octane and burns cooler than pure gasoline so can bump up timing and enrichen some and make more power that on 93 octane safely.
The main issue is water separation in tanks and dissolving parts not made for alcohol. W/o adjusting tune you may just get worse mileage than gas is all.

A bit cooler plug may help anti-detonation also. Its a significant heat path out of chamber, besides not getting as hot an electrode into a glow plug.