High or Low Compression

Yellow Submarine · March 2nd, 2010, 05:00 PM

Hey guys, I just came into ownership of a 250 dragbike. It's only 90% finished, so I got a really good deal on it. Anyways, the thing has a turbo on it and nitrous bottles strapped to the side. I'm thinking that my stock pistons might want out of that situation. Would it be best to get high or low compression pistons? I know that for drag racing i.e. straight lines, low is best with a turbo, but I'm no drag racer and I'm thinking low might not be best for Nos either.

Snake · March 2nd, 2010, 05:35 PM

Those questions are a little too technical for me but I would like to say that is a great looking bike.

MKlein3004 · March 2nd, 2010, 06:01 PM

low compression works better with turbo regardless the situation

Yellow Submarine · March 2nd, 2010, 11:58 PM

Quote:

Originally Posted by MKlein3004

low compression works better with turbo regardless the situation

That's what I had heard, but I did some research and many people say that for the street, high compression is a better idea. I'm going to take it to GodSpeed and see what they think. They specialize in that kind of stuff for motorcycles

Racer x · March 3rd, 2010, 04:47 AM

Hey again
I love this thing. You need to check a lot of stuff before running that . I wounder if the whole engine is stock. since the bike is not finished.
Forget the nitrous for now . just try to get it running on the turbo. Pull the valve cover and try to see if the engine has head studs or a copper head gasket. Those thing will be critical to even make 1 full power pass with a 250 motor. What year is it ?
It looks first gen . But I have to wounder what the ignition system is made up of . If it is stock that willneed be to be addressed. 42 deg is to much timing for a turbo let alone nitrous.
So far I have not seen a 250 go faster than 15.0 in the 1/4 mile This looks like it may be the quickest 250 out there.

sharky nrk · March 3rd, 2010, 07:35 AM

The issue is cylinder pressure. Typically on a turbocharged motor, compression ratio is dropped as your charge volume is increased. Compression ratio in a turbocharged application can have alot of impact in how the power curve is shaped, and how you have to go about igniting the intake charge. In the past, fueling and ignition control was at a lesser state than it is today and turbocharged applications used lower compression and ignition retard to decrease cylinder pressure and limit the occurences of detonation. With modern fuel and ignition systems, better control allows for finer application of CR and charge volume. Where a early 90s turbocharged application would see 8.0:1 with only a few degrees of total advance, a newer application may make use of 9.5:1 and far more advance. Generally though as charge pressure increases, the envelope for mistake increases as well. You can run very high compression and forced induction, but you will be riding the edge at much lower charge pressures.

I am not as comfortable tuning nitrous applications as forced induction (I am very comfortable with forced induction) but in general theory, compression can be utilized more readily in chemical induction applications. Your total charge volume of a chemical induction application is still dictated by the VE of the engine. Your increased cylinder pressure comes from the chemical energy from the available oxygen in the nitrous oxide. Your compression volume is more tolerant of chemcial induction, but your actual flame propigation and ignition needs to be precise. Doing both forced and chemical induction results in both highly increased cylinder pressure and chemical energy release upon ignition.

ckholloway · March 3rd, 2010, 08:05 AM

You could start with low comp. pistons and has been said before but I would take the nitrous off. If it was me I would tear the motor down and start over so I knew what was in it.

Greg_E · March 3rd, 2010, 08:21 AM

The reason for the nitrous is to cool the intake charge, an alternative would be water injection. If you could fit a great big intercooler, then you could forget both water and nitrous.

Lower compression or you will have severe detonation, it's already a reasonably high compression motor, throw in the heat from the turbo which increases the chances of detonation, and the higher compression from the turbo (it is a compressor after all) and you could be looking at blowing the heads off.

I would probably ditch the turbo and just run a nitrous system for a while. Turbos take a lot of messing around to get them right. To get it right you also need to run different cams so that the overlap between intake opening and exhaust closing is about zero, otherwise you blow a lot of good air/fuel out the exhaust port. You can use stock cams, but you will never get the peak performance and depending on the amount of fuel expended might even damage the turbo.

In the current configuration I would not want to ride it on the street, doesn't look like it will turn as well as a normal machine.

Yellow Submarine · March 3rd, 2010, 10:03 AM

Alright guys, low compression pistons it is. I also checked the listing description and it has a K&N cone air filter, stock ignition, crankcase breather system with puke tank (plumbed) and mukini 40mm carbs. That's all that was done to the engine aside from the airshifter. I have no idea what half those mods are, but I'm assuming the puke tank will help me with the cam timing problem and the filter will help me keep it cooler?

Yellow Submarine · March 3rd, 2010, 10:24 AM

Here's some pics of the setup if that helps

tapdiggy · March 3rd, 2010, 10:26 AM

Quote:

Originally Posted by Greg_E

The reason for the nitrous is to cool the intake charge, an alternative would be water injection. If you could fit a great big intercooler, then you could forget both water and nitrous.

Can you explain how that works? Are you saying the Nitrous is introduced into the air/fuel mix to draw off heat from the mix; or are you saying it works like a refrigerator, using the expansion of the compressed NO2 (with its resultant drop in temperature) to cool the air/fuel before it reaches the cylinder? Either way, your mistaken on the application of nitrous oxide in this situation.

sharky nrk · March 4th, 2010, 05:49 AM

Methanol/water injection would be AWESOME!!!!!! The only real concern I would have would be loss of intake charge volume, given the limited capacity from a 250cc motor. However - it would go a LOOONNGGG way to make more and more reliable power.

sharky nrk · March 4th, 2010, 05:51 AM

Also if this a drag bike only, a big air to air i/c is FAR from the idea form of charge cooling. Water/methanol injection or air to water would make much more sense.

Banzai · March 4th, 2010, 08:49 AM

Nitrous Oxide from Wiki

March 2nd, 2010, 05:00 PM	#1
Yellow Submarine ninjette.org member Name: Jake Location: San Francisco Join Date: Dec 2009 Motorcycle(s): Kawasaki Ninja 250 2008 Posts: 10	High or Low Compression Hey guys, I just came into ownership of a 250 dragbike. It's only 90% finished, so I got a really good deal on it. Anyways, the thing has a turbo on it and nitrous bottles strapped to the side. I'm thinking that my stock pistons might want out of that situation. Would it be best to get high or low compression pistons? I know that for drag racing i.e. straight lines, low is best with a turbo, but I'm no drag racer and I'm thinking low might not be best for Nos either.

March 3rd, 2010, 08:05 AM	#7
ckholloway ninjette.org member Name: Corey Location: Oregon Join Date: Feb 2010 Motorcycle(s): none yet Posts: 142	You could start with low comp. pistons and has been said before but I would take the nitrous off. If it was me I would tear the motor down and start over so I knew what was in it. __________________________________________________ Riding is not just something you do, It's a way of life!!

March 4th, 2010, 08:49 AM	#14
Banzai Psychic war veteran Name: Thomas Location: Norfolk, VA Join Date: May 2009 Motorcycle(s): Kawi Green '09 Ninja ZX6R Posts: 663	Nitrous Oxide from Wiki __________________________________________________ * If you're arguing with some idiot over the internet, chances are pretty good he's doing the same thing! Zodiac Mindwarp and the Love Reaction

March 2nd, 2010, 05:35 PM	#2
Snake ninjette.org certified postwhore Name: Rick Location: Alexandria, Louisiana Join Date: Jan 2009 Motorcycle(s): 05 Blue Ninja 250 Posts: Too much. MOTY - 2017, MOTM - Jan '19, Oct '16, May '14	Those questions are a little too technical for me but I would like to say that is a great looking bike.

March 2nd, 2010, 06:01 PM	#3
MKlein3004 Fighting Texas Aggie '08 Name: Mitchell Location: Allen, Tx Join Date: Feb 2009 Motorcycle(s): 2008 Ninja 250r (Race), 2005 Kawasaki Ninja ZX-6R 636 (Track), 2005 Ducati Monster 620 (Street) Posts: 541	low compression works better with turbo regardless the situation

March 3rd, 2010, 04:47 AM	#5
Racer x ninjette.org certified postwhore Name: Eric Location: Iowa City Join Date: May 2009 Motorcycle(s): 2008 Kawmeracchi 350 2010 Project X Posts: A lot. Blog Entries: 54 MOTM - Sep '18, Feb '16	Hey again I love this thing. You need to check a lot of stuff before running that . I wounder if the whole engine is stock. since the bike is not finished. Forget the nitrous for now . just try to get it running on the turbo. Pull the valve cover and try to see if the engine has head studs or a copper head gasket. Those thing will be critical to even make 1 full power pass with a 250 motor. What year is it ? It looks first gen . But I have to wounder what the ignition system is made up of . If it is stock that willneed be to be addressed. 42 deg is to much timing for a turbo let alone nitrous. So far I have not seen a 250 go faster than 15.0 in the 1/4 mile This looks like it may be the quickest 250 out there.

March 3rd, 2010, 07:35 AM	#6
sharky nrk Fighting Texas Aggie '05 Name: Neil Location: Hutto, TX Join Date: Feb 2009 Motorcycle(s): '07 ZX6R, '08 Versys, '09 250R Track, '93 F2/F3 Track Posts: A lot.	The issue is cylinder pressure. Typically on a turbocharged motor, compression ratio is dropped as your charge volume is increased. Compression ratio in a turbocharged application can have alot of impact in how the power curve is shaped, and how you have to go about igniting the intake charge. In the past, fueling and ignition control was at a lesser state than it is today and turbocharged applications used lower compression and ignition retard to decrease cylinder pressure and limit the occurences of detonation. With modern fuel and ignition systems, better control allows for finer application of CR and charge volume. Where a early 90s turbocharged application would see 8.0:1 with only a few degrees of total advance, a newer application may make use of 9.5:1 and far more advance. Generally though as charge pressure increases, the envelope for mistake increases as well. You can run very high compression and forced induction, but you will be riding the edge at much lower charge pressures. I am not as comfortable tuning nitrous applications as forced induction (I am very comfortable with forced induction) but in general theory, compression can be utilized more readily in chemical induction applications. Your total charge volume of a chemical induction application is still dictated by the VE of the engine. Your increased cylinder pressure comes from the chemical energy from the available oxygen in the nitrous oxide. Your compression volume is more tolerant of chemcial induction, but your actual flame propigation and ignition needs to be precise. Doing both forced and chemical induction results in both highly increased cylinder pressure and chemical energy release upon ignition.

March 3rd, 2010, 08:21 AM	#8
Greg_E ninjette.org sage Name: Greg Location: central new york Join Date: Aug 2009 Motorcycle(s): 90 Suzuki gsx600f Katana, 03 Buell XB9R Posts: 868	The reason for the nitrous is to cool the intake charge, an alternative would be water injection. If you could fit a great big intercooler, then you could forget both water and nitrous. Lower compression or you will have severe detonation, it's already a reasonably high compression motor, throw in the heat from the turbo which increases the chances of detonation, and the higher compression from the turbo (it is a compressor after all) and you could be looking at blowing the heads off. I would probably ditch the turbo and just run a nitrous system for a while. Turbos take a lot of messing around to get them right. To get it right you also need to run different cams so that the overlap between intake opening and exhaust closing is about zero, otherwise you blow a lot of good air/fuel out the exhaust port. You can use stock cams, but you will never get the peak performance and depending on the amount of fuel expended might even damage the turbo. In the current configuration I would not want to ride it on the street, doesn't look like it will turn as well as a normal machine.

March 3rd, 2010, 10:03 AM	#9
Yellow Submarine ninjette.org member Name: Jake Location: San Francisco Join Date: Dec 2009 Motorcycle(s): Kawasaki Ninja 250 2008 Posts: 10	Alright guys, low compression pistons it is. I also checked the listing description and it has a K&N cone air filter, stock ignition, crankcase breather system with puke tank (plumbed) and mukini 40mm carbs. That's all that was done to the engine aside from the airshifter. I have no idea what half those mods are, but I'm assuming the puke tank will help me with the cam timing problem and the filter will help me keep it cooler?

March 4th, 2010, 05:49 AM	#12
sharky nrk Fighting Texas Aggie '05 Name: Neil Location: Hutto, TX Join Date: Feb 2009 Motorcycle(s): '07 ZX6R, '08 Versys, '09 250R Track, '93 F2/F3 Track Posts: A lot.	Methanol/water injection would be AWESOME!!!!!! The only real concern I would have would be loss of intake charge volume, given the limited capacity from a 250cc motor. However - it would go a LOOONNGGG way to make more and more reliable power.

March 4th, 2010, 05:51 AM	#13
sharky nrk Fighting Texas Aggie '05 Name: Neil Location: Hutto, TX Join Date: Feb 2009 Motorcycle(s): '07 ZX6R, '08 Versys, '09 250R Track, '93 F2/F3 Track Posts: A lot.	Also if this a drag bike only, a big air to air i/c is FAR from the idea form of charge cooling. Water/methanol injection or air to water would make much more sense.