K75S HP & Compression

[burnszilla]

I've been reading and getting mixed messages on the horsepower of my bike.

HP (kW): 75.00 (54.7) @ 8500
Compression: 11.0:1

But I've heard that K75's built 1987 and earlier have a higher engine compression and therefore 5 HP more power.

Is this true?

[deilenberger]

Originally posted by Burnszilla
I've been reading and getting mixed messages on the horsepower of my bike.

HP (kW): 75.00 (54.7) @ 8500
Compression: 11.0:1

But I've heard that K75's built 1987 and earlier have a higher engine compression and therefore 5 HP more power.

Is this true?

You can see measured rear-wheel HP at:

http://www.eilenberger.net/K75S/dyno.htm

That's measuring an '87 against a '92. HP is as close to identical as I'd expect to see.

This set of dyno runs was taken to see if there was any effect to the claimed porting/polishing/oversize intake valves that my K75S had received at the hands of a former owner.

As you can see - no effect is visible. There may be a few reasons for this:

1. Brian's bike is running advanced timing - I believe 33 degrees. I haven't measured the timing on mine so I don't know if it was advanced or not. This is usually good for 5-8 more HP on a K100 engine:

http://www.ibmwr.org/ktech/dyno/index.shtml

I would expect the same percentage of increase on a K75 engine.

2. My bike was running lean. It had sat basically unused for 2-3 years, and I believe the injectors were partially blocked. This can be seen in the K75 dyno runs by the small features in the runs on my bike - ie - wiggles. These indicate the mixture is lean (according to the dyno manufacturer).

I've since attacked this problem (just recently) by running a full bottle of Techron through the system in two sequential tanks of fuel while on a long ride. It seems to have worked - the engine now idles smoother, and there is a bit of soot on the exhaust (before there never was) and it backfires a bit on decelleration (which it didn't before - another sign the mixture was lean).

I haven't gotten to running it on the dyno again- I might get to it sometime this summer.

I believe what you may be confusing with information on the K75 engines is that the K100 engine, 1985 and prior years had a different intake cam than 1986 and later years. This was called the "euro-cam" (and was used in all the US except California).

Starting in 1986 the cam was changed for emissions purposes and to get a bit of the vibration out of the engine. The change resulted in about 5 less HP in '86 and later years.

FWIW - I have never heard of a different cam being used on the K75 engines, and the only other way to change compression is with a different crankshaft or different pistons - the parts CD's list the same parts for all years of the K75... so I doubt that the compression ratio changed at all.

HTH,

[jdiaz]

There was a Euro-cam offered for the K75....I believe KBasa had a K75 equipped with one. I think there was a 5hp difference at the top of the rev band.

I believe all K75's had 11:1 compression ratios. Going from memory here.

FWIW, my wife owned two separate '87 K75S's, and both of her bikes had better snap than my '88 or '95 K75's, and the '95 even had advanced timing. Something was different about those bikes.

[deilenberger]

Originally posted by jdiaz
There was a Euro-cam offered for the K75....I believe KBasa had a K75 equipped with one. I think there was a 5hp difference at the top of the rev band.

I believe all K75's had 11:1 compression ratios. Going from memory here.

FWIW, my wife owned two separate '87 K75S's, and both of her bikes had better snap than my '88 or '95 K75's, and the '95 even had advanced timing. Something was different about those bikes.

I'd be VERY interested in the PN for the euro-cam. Just went through the parts CD from '86-95.. all years use the same PN's:

intake: 11 31 1 460 617
exhaust: 11 31 1 460 618

Looked under supercession - no other PN listed.

[jdiaz]

Originally posted by deilenberger
I'd be VERY interested in the PN for the euro-cam. Just went through the parts CD from '86-95.. all years use the same PN's:

intake: 11 31 1 460 617
exhaust: 11 31 1 460 618

Looked under supercession - no other PN listed.

May have been a Goofmeister or CCProducts part? Hopefully the sausage man or Phactory Phil will tell us.

[dbrick]

Don't hold your breath. I've been paying attention to K75 stuff since it appeared, and haven't ever heard of one. The only cam variation I ever saw was the one in the FYK. I don't ever remember seeing anything like this in a CC Products catalog, either. But who knows what the Goofmeister might have promised (and then not delivered)?

[kbasa]

My K75 had a set of Luftmeister #2 cams in it. It had the titanium valve keepers, high rate springs and a bunch of other stuff (fuel pressure regulator, spark unit, etc.). It would pull through redline in top gear, which was impressive.

However, it lost the midrange that makes K75s so much fun. It really didn't make much power until about 5500 rpms or so.

It also got horrible mileage.

But yeah, that bike would haul ass.

[blackfly]

I had a 1988 K75S with a 1992 shortblock (long story). At ~93k miles I took down the cylinder head for port and polish. What what learned from the flow bench is a bit different from what you have read. ACTUALLY, we found the stock [intake] valves to provide more air than the exhausts could capably dump on the otherside. In other words, in order to get "air flow" from the head, we had to fill in the exhaust ports by 30% to achieve flow.

Huh, you say? That's right. So, when someone tells you that bigger valves are the answer, they perhaps have some stock to get rid of.

The net result of the bench work resulted in a significantly increased mid-high range with a slight detriment to the low-end. Well worth the effort.

Except, if you want a balanced go-fast bike, buy something made to go fast - like a Yamakawazuki.

BFF

[VOLKMARE]

Oversized intake valves do work, on the top end.

If you simply put them in and do nothing else they will improve top end and decrease bottom end. The fact that your low end flow decreased on the flow bench is proof of that.

filling the exhaust does increase flow on the bottom end, by improving the low end port velocity, however it will give the engine asthma on the top end.

To every modivication there is a trade off. anyone who tells you different is lying. Even with a turbo charger and it's wheel standing horse power, there is a trade off. Loss of bottom end. But if you know how to get around it, there is no problem.