Compression ratio HP2S?

Joe the Viking · 06:24 AM

I need to know the compression ratio of the HP2S, has anyone actually measured the volume of the combustion chamber etc?

Probably not but it's worth a post anyway.

Guest24 · 12-17-2009, 06:24 AM

I thought it was 12.5:1 or maybe that's with SJBMW pistons.

Deans BMW · 09:58 AM

Just talked to Chris and he said the HP2S is exactly 13:1 CR. Here is them ccing the combustion chamber with a pipet to find exactly what it is. They have run with their own design pistons 13.5:1 and a full 14:1. Chris has settled on 13.5:1 whith his own design pistons, part of their reliable 145 RWHP.

Guest24 · 12-17-2009, 12:03 PM

Thanks Dean. I'm glad you have adirest line to SJ.

Joe the Viking · 12-17-2009, 11:20 PM

Thanks a million mate, saved me a top end tear down!
More often than not the manufacurers spec won't be all that accurate so I needed to know for sure. It's quite an important factor when gusstimating how much boost can be safely applied.

Any chance you could ask about the octane rating and ignition retardation for running 14:1?

Quote:

Originally Posted by Deans BMW

Just talked to Chris and he said the HP2S is exactly 13:1 CR. Here is them ccing the combustion chamber with a pipet to find exactly what it is. They have run with their own design pistons 13.5:1 and a full 14:1. Chris has settled on 13.5:1 whith his own design pistons, part of their reliable 145 RWHP.

Guest24 · 12-18-2009, 05:35 AM

What are you doing to boost the cr, Joe?

Joe the Viking · 12-18-2009, 06:03 AM

Quote:

Originally Posted by Nail24

What are you doing to boost the cr, Joe?

I'm not, but the dynamic compression will change.

I'm supercharging and converting to booze (E85).

Deans BMW · 12-18-2009, 06:40 AM

Haven't talked to Chris today, but I remember that he was running race gas and the results were a little hard on the big ends of his Ti rods, also a lot of heat was generated.

Dr. Curve · 04:56 PM

Another reason that the HP2 develops so much up top, beyond the higher 13 to 1 compression ratio is that BMW increased the RLR (rod length ratio) in the HP2 engine as compared to its older brother the R12S. As Chris, myself, and others, who have built long rod engines, know........every little bit that we can gain in RLR helps the HP up top. Even small gains are worth going for. The rods in the HP2 are 128 mm center to center as contrasted to the 125 mm rods in the R12S and some of the previous oil-head units. The RLR in the HP2 is 1.75 and may be contrasted to the R12S whose number is 1.71. Even this small 4 hundredths is significant however when one is talking about a contained explosion which is what combustion is all about.

If a given deck height is established and maintained...........then the higher the RLR of a engine...........the longer the piston stays near top dead center. When this happens, compression gas builds to higher pressures and maintains them longer as the piston shoots downward. This ability for the piston to hold max pressure slightly longer results in more HP at higher RPMs than the identical engine will offer with a lower RLR. BMW has been aware of this more and more over the past several decades and when possible they opt toward a longer rod ratio if the design allows it.

If pin heat is a problem, the small end of the rods need to be machined differently than Chris is doing it now I would think, but I have not seen the rods he is using. In the long rod air-head engines I built I had special rods made that increased the strength of the small end and increased the oiling in that areas as well. The pin should be give more surface area and multiple, cone cut, oil holes used. There are multiple ways of doing these "rod insurance" modifications and IMO when anyone goes to a compression above 12 to 1 and a RLR of above 1.7, special consideration should be given to the small end. Chris knows all this and has planned accordingly (I'm sure) in that direction, although perhaps not quite enough.

14 to 1 ratio is very high and a very flat "piston to chamber profile" must be used in conjunction with 105 or more octane gas. The 13.5 to 1 compromise is a maximum unless the expense of 12 dollar per gallon gas and possible overheating wishes to be encountered. Even at 13.5 to 1, Chris should be using 100 octane gas which is 7 dollars per gallon. With the HP2 engine, BMWs higher RLR, very flat combustion chamber, low inclined valve angles, and a radial valve arrangement of fill and release, allows the compression that BMW is offering stock (13 to 1)........a ratio that is higher than any other stock 4-cycle air-cooled motorcycle ever offered.

More is to come from this engine however and the next big step now in the works is a increase in bore to 102.5 mm and a stroke increase to 75 mm. This can be done without increasing the width of the engine and will result is more torque and HP all the way along the RPM scale.

Deans BMW · 12-23-2009, 04:26 PM

Curve, two points

Chris does use race gas in their race HP2S

Back in the day of the intro of the R1200CLC, there was a 1400 cc Boxer in reliability testing ready to slide in the C series, project was dropped because 1600 cc were the min in that type of bike market was the thinking to be sucessful.

Dr. Curve · 12-23-2009, 05:47 PM

1400 cc would be a lot Dean........too much for a high compression, over square twin. The bore and stroke increase I mentioned above (102.5/75) will offer up a displacement of about 1240 cc vs the 1170 the HP now has. I am sure Chris is using at least 100 octane in their current track bike. We will know soon if BMW decides to give us more displacement in the twin line.

Guest24 · 12-24-2009, 06:33 AM

Jim, given the longer dwell time at TDC, can we assume a more complete burn of the A/F mixture? Also, it would seem that ingiting the mixture at the proper moment becomes extra critical during the compression stroke with a higher RLR.

Guest24 · 12-24-2009, 06:40 AM

Jim, it is highly likely that BMW will move away from the requirement for premium fuel in the HP2 DOHC derivative engine for the GS/RT. A larger bore with reduced compression makes sense for a predominantly touring application. The GS/RT bikes have not really gotten any complaints about the 110hp that the hexhead comes with. With the afforementioned changes the DOHC would be the high miler that BMW boasts for those two bikes.

Deans BMW · 12-24-2009, 08:06 AM

Bubba, can't wait to find out, as I'll be riding one sometime in Febuary, the DOHC Boxer in a GS or Rt that is.

Guest24 · 12-24-2009, 08:20 AM

Quote:

Originally Posted by Deans BMW

Bubba, can't wait to find out, as I'll be riding one sometime in Febuary, the DOHC Boxer in a GS or Rt that is.

Are you getting one of those , also?

Deans BMW · 12-24-2009, 02:30 PM

Bubba, i am getting a new DOHC R1200GS, Ohlins, full Remus etc. Feb or march.

Guest24 · 12-26-2009, 05:03 AM

Quote:

Originally Posted by Deans BMW

Bubba, i am getting a new DOHC R1200GS, Ohlins, full Remus etc. Feb or march.

I wish I had your money and more importantly, your wife's understanding, Dean. Two bikes in the same month! WOW!!!!