Head Swap

Ape Racing talks about a "two point" compression drop after swapping from old 77 1000 heads to the later J heads. What does "two points" amount to? 10:1 to 8:1 or 10.5:1 to 10.3:1 (example only number)??? Could someone clarify?

its like your 10:1 down to 8:1. perfect for a turbo build eh wireman?

my old motor has a 1075 high comp kit (I think 170-180 psi comp.) Do you think that would even it out?

your 1075 was a street bike motor right? running on pump gas? most likely was a 10.25:1 ratio. I think stock is around 9:1. the j head would drop your 10.25 down to 8.25 (this is a common combo for a turbo build) unless you were running about 12:1 (using race gas) you wont have the compression you need unless you find another way to bump compression, IE mill the head or get the proper pistons

The J motor valves are a tad bigger. Not sure if the loss in compression and resulting horsepower would make sense. Perhaps you could put a couple gaskets on to get compression back up but this would be a headache. If you want to go with bigger valves, buy the valves, get a valve job done on your old head as a decent performance shop could install the larger valves in the old head and you won't have to worry about adapting the cam chain idler arm and figuring out how to bump compression back up. Stock compression is 8.25:1 and that is about where you would end up.

kzmason wrote:

Ape Racing talks about a "two point" compression drop after swapping from old 77 1000 heads to the later J heads. What does "two points" amount to? 10:1 to 8:1 or 10.5:1 to 10.3:1 (example only number)??? Could someone clarify?

you might swap the pistons from an 1100 along with the head. they'll match the chamber ad you can increase compression a bit over stock with the right block base spacer.

Post edited by: Duck, at: 2007/03/08 21:34

wiredgeorge wrote:

The J motor valves are a tad bigger. Not sure if the loss in compression and resulting horsepower would make sense. Perhaps you could put a couple gaskets on to get compression back upadding gaskets lowers compression as its just like doing the opposite of milling the head or cylinders but this would be a headache. If you want to go with bigger valves, buy the valves, get a valve job done on your old head as a decent performance shop could install the larger valves in the old head and you won't have to worry about adapting the cam chain idler arm and figuring out how to bump compression back up. Stock compression is 8.25:1 and that is about where you would end up.

the j head has bigger ports to go with the bigger valves, so just putting j valves in the old head without porting wouldnt be as good as running the j head with the right compression.

i am enjoying this topic on the J head, thanks.

i'm still learning.

themachine wrote:

i am enjoying this topic on the J head, thanks.

i'm still learning.

you can learn a lot and quickly if you follow most of the tech posts. thats whats great about forums. you can learn from guys who have been there and done that. I learned a lot about the j stuff from wiremans posts. his current turbo project is using the j head on the older style bottom. his 1385 10.25:1 piston kit turns into a 8.25:1 piston kit with the j head and is in the correct compression area for a big nasty turbo:evil:

Some info on heads I've read before:

When swapping to a J head, a piston swap is manditory to adjust for lost compression. The J heads have a different valve angle in order to clear larger valves. To do this to a KZ head would take quite a bit of welding and machine work. It can be done and has been in the past but why??? A J head can take a 42mm intake valve without changing valve seats. Don't even try that with a KZ head. Really it's the intake runner of the J head that makes this head better then a KZ head. If you weld and grind a KZ head, you might get it to match a stock J intake BUT if you open up the stock J intake runner, you can go beyond what a heavily modified KZ head will go.

Look at it this way. ANYTHING larger then 1100cc's needs a larger head to fill and empty the increaced volume of the cylinder. A 900/1000 KZ head was simply made to work well with that size engine. Kawasaki knew the old heads were not going to cut it with the 1100 engines so they had to do something. It was a simple problem. KZ heads have too narrow of valve angle to run larger valves. Valves hit. So spreading the valve center-lines apart allows for larger valves. If you are going to redesign, might as well do it right so Kawasaki engineers gave the new-style head a larger intake volume as well. Th eexhaust port is very restrictive though, so that is where 90% power gains will be found when porting either a KZ or J head.

Way back in the day when these J engines came out. Street racer guys didn't waste to many seconds thinking about IF this new head was better. They KNEW it was just by stock horsepower figures on these new bikes. It wasn't just an 1100cc slug going up & down. Everyone already had big-bores and head work on their KZ's. Valve size, intake runner volume were unobtainable with a KZ head. That is, without 300 hours of welding and machine work. Kids couldn't afford that kind of cash layout. It had to be a simple deal, swap the head off a J onto a KZ. It's not REAL easy but I remember Star racing and several shops were getting buisness from kids in the neighborhood where I grew up. In fact I remember being able to identify those square cam covers half a block away. You can't hide those cam cover shapes. Everyone in that era knew what 900 cam covers looked like. Anyway kids back then weren't afraid to let go of the old and embrace the new. A 1075cc engine is the cap for a KZ head to flow air with cams & port work. After that, personally speaking, I would leave the KZ head on the shelf and go through whatever it takes to fit a J head with...maybe 1mm larger intake valves and .425 lift cams for the street and no more then 11-1 compression for a 1200cc engine. That's really a lot of engine and the guys that have them are very pleased with the horsepower. It's not the baddest thing ever BUT it will be around for 20 years or more when those 1385's didn't make it through one summer. One other item worth mention. ANYTHING larger then 1200cc sleeve in a stock block, will not be supported. You can see the sleeves though the fins. What happends is the sleeves warp. Machinists will warn you about this and not stand behind it when it smokes. Upper cases must be bored to fit 1200cc sleeves also. Whomever does the sleeves will tell you to send the upper case half with the block.

Reply:

I sat here reading this and just about fell off my chair. Now, how to be diplomatic about a heavy dose of hogwash is the question...

That info is mix of fact and fiction....with a heavy dose of fiction on the Z vs J performance capabilities.

A stock J head doesn't flow but a CFM or two better than a stock KZ head even with it's 37mm Intake valve and larger runners. That's fact. I have the flow data to back it up.

Unless you are in a situation where you need to go with larger than 34 mm carbs, it's not really worth the effort to switch.

As for the gains being 90% from exhaust port work over intake port work.....NEVER IN A MILLION YEARS....that's the biggest load of I"ve ever read about a KZ motor.

The only J type head that's truly better than KZ in terms of bulk airflow and combustion chamber is the 83+ GPz11 head and it is also found on later 1100 LTD's. It's easily identified by the engine temp boss that is between intake port 1 & 2 but not drilled. Those heads out flow the KZ right out of the box.

A standard J head has larger runners and that means less work for the porter when the desired application uses 33mm and larger carbs....that's fact.

You can install a Suzuki GS1000 38mm Intake valve in a KZ head....that's fact.


It's a hell of a lot of work to get the KZ flowing what the J is capable of with relatively equal size valves...37 vs 37.5....that's fact.

Radical porting the exhaust on either and expecting a huge performance gain is a pipe dream...

Both heads flow in the high 60's to low 70's CFM @10" of water out of the box.....that's fact.

The 83 GPz11 flows 83 CFM at 10" of water out of the box....that's fact..

The J head motors came with 34mm CV carburetors because throttle response would have been horrible with standard throttle slide carbs of that size to matche the large runners.....that's fact.

DON"T EVER be mislead into thinking that just because a port is larger than another port that it's better.....You might be shocked to see how much clay can be laid in the floor of a 900 Ninja port and NOT hurt the CFM one bit but raise velocity over 50FPS...

Velocity is a very important part of the equation and is directly proportional to cross sectional area and flow....therefore with two ports flowing very close to the same volume in terms of CFM...which would yield the better velocity profile.....Hint....it's not generally the biggest one..

Acceptable port velocity is between 250 - 300 FPS down the port centerline. Less than 250 and you'll feel the result with a great big Waaaaaaaaaaaaaaahhhh when you open the throttle...Torque will fall off rapidly...

A stock KZ port is quite adequate for 90 -95 RWHP with a .360 lift cam and 34mm flat slide mikunis with the port runners not even being matched to the intake boots....I have the dyno sheet to prove that...

Are there benefits to installing a J head on a KZ bottom end? Absolutely there are. It's something that has to be thought over and applied to the right package though. And don't hand me that nonsense that you can't run a KZ head on a motor larger than 1075cc....that's more

Please excuse my rather blunt response to this but I have never read so much misinformation that was laid out there as fact.....

Post edited by: rstnick, at: 2007/03/09 13:57

rstnick wrote:

Some info on heads I've read before:

When swapping to a J head, a piston swap is manditory to adjust for lost compression. The J heads have a different valve angle in order to clear larger valves. To do this to a KZ head would take quite a bit of welding and machine work. It can be done and has been in the past but why??? A J head can take a 42mm intake valve without changing valve seats. Don't even try that with a KZ head. Really it's the intake runner of the J head that makes this head better then a KZ head. If you weld and grind a KZ head, you might get it to match a stock J intake BUT if you open up the stock J intake runner, you can go beyond what a heavily modified KZ head will go.

Look at it this way. ANYTHING larger then 1100cc's needs a larger head to fill and empty the increaced volume of the cylinder. A 900/1000 KZ head was simply made to work well with that size engine. Kawasaki knew the old heads were not going to cut it with the 1100 engines so they had to do something. It was a simple problem. KZ heads have too narrow of valve angle to run larger valves. Valves hit. So spreading the valve center-lines apart allows for larger valves. If you are going to redesign, might as well do it right so Kawasaki engineers gave the new-style head a larger intake volume as well. Th eexhaust port is very restrictive though, so that is where 90% power gains will be found when porting either a KZ or J head.

Way back in the day when these J engines came out. Street racer guys didn't waste to many seconds thinking about IF this new head was better. They KNEW it was just by stock horsepower figures on these new bikes. It wasn't just an 1100cc slug going up & down. Everyone already had big-bores and head work on their KZ's. Valve size, intake runner volume were unobtainable with a KZ head. That is, without 300 hours of welding and machine work. Kids couldn't afford that kind of cash layout. It had to be a simple deal, swap the head off a J onto a KZ. It's not REAL easy but I remember Star racing and several shops were getting buisness from kids in the neighborhood where I grew up. In fact I remember being able to identify those square cam covers half a block away. You can't hide those cam cover shapes. Everyone in that era knew what 900 cam covers looked like. Anyway kids back then weren't afraid to let go of the old and embrace the new. A 1075cc engine is the cap for a KZ head to flow air with cams & port work. After that, personally speaking, I would leave the KZ head on the shelf and go through whatever it takes to fit a J head with...maybe 1mm larger intake valves and .425 lift cams for the street and no more then 11-1 compression for a 1200cc engine. That's really a lot of engine and the guys that have them are very pleased with the horsepower. It's not the baddest thing ever BUT it will be around for 20 years or more when those 1385's didn't make it through one summer. One other item worth mention. ANYTHING larger then 1200cc sleeve in a stock block, will not be supported. You can see the sleeves though the fins. What happends is the sleeves warp. Machinists will warn you about this and not stand behind it when it smokes. Upper cases must be bored to fit 1200cc sleeves also. Whomever does the sleeves will tell you to send the upper case half with the block.

Reply:

I sat here reading this and just about fell off my chair. Now, how to be diplomatic about a heavy dose of hogwash is the question...

That info is mix of fact and fiction....with a heavy dose of fiction on the Z vs J performance capabilities.

A stock J head doesn't flow but a CFM or two better than a stock KZ head even with it's 37mm Intake valve and larger runners. That's fact. I have the flow data to back it up.

Unless you are in a situation where you need to go with larger than 34 mm carbs, it's not really worth the effort to switch.

As for the gains being 90% from exhaust port work over intake port work.....NEVER IN A MILLION YEARS....that's the biggest load of I"ve ever read about a KZ motor.

The only J type head that's truly better than KZ in terms of bulk airflow and combustion chamber is the 83+ GPz11 head and it is also found on later 1100 LTD's. It's easily identified by the engine temp boss that is between intake port 1 & 2 but not drilled. Those heads out flow the KZ right out of the box.

A standard J head has larger runners and that means less work for the porter when the desired application uses 33mm and larger carbs....that's fact.

You can install a Suzuki GS1000 38mm Intake valve in a KZ head....that's fact.


It's a hell of a lot of work to get the KZ flowing what the J is capable of with relatively equal size valves...37 vs 37.5....that's fact.

Radical porting the exhaust on either and expecting a huge performance gain is a pipe dream...

Both heads flow in the high 60's to low 70's CFM @10" of water out of the box.....that's fact.

The 83 GPz11 flows 83 CFM at 10" of water out of the box....that's fact..

The J head motors came with 34mm CV carburetors because throttle response would have been horrible with standard throttle slide carbs of that size to matche the large runners.....that's fact.

DON"T EVER be mislead into thinking that just because a port is larger than another port that it's better.....You might be shocked to see how much clay can be laid in the floor of a 900 Ninja port and NOT hurt the CFM one bit but raise velocity over 50FPS...

Velocity is a very important part of the equation and is directly proportional to cross sectional area and flow....therefore with two ports flowing very close to the same volume in terms of CFM...which would yield the better velocity profile.....Hint....it's not generally the biggest one..

Acceptable port velocity is between 250 - 300 FPS down the port centerline. Less than 250 and you'll feel the result with a great big Waaaaaaaaaaaaaaahhhh when you open the throttle...Torque will fall off rapidly...

A stock KZ port is quite adequate for 90 -95 RWHP with a .360 lift cam and 34mm flat slide mikunis with the port runners not even being matched to the intake boots....I have the dyno sheet to prove that...

Are there benefits to installing a J head on a KZ bottom end? Absolutely there are. It's something that has to be thought over and applied to the right package though. And don't hand me that nonsense that you can't run a KZ head on a motor larger than 1075cc....that's more

Please excuse my rather blunt response to this but I have never read so much misinformation that was laid out there as fact.....<br><br>Post edited by: rstnick, at: 2007/03/09 13:57

wow what can i say to that? the second post sounds a lot like that bigheaded motormouth wireman!oh wait thats me!:blink: bigger ports and bigger valves are good for a motor that spends a lot of time at higher rpms but they arent always the best thing for a streetbike that spends very little time above 6500 rpms.ive run the j-heads on a couple motors one was a high strung 1170 and the other is a really healthy 1327 in the big inch motors the big ports and valves really help but ive also got an 1197 13:1 motor out of my z1 with an old style head with just a little port work and small valves that i have little doubt would have most looking for a box of depends and psychiatric help in short order :evil: you can build a lot of horsepower either way it just comes down to what you have to work with and how much money you want to spend.and yes you can make a j/gpz head look like something else that will have the average person scratching their head wondering what it is,just takes a lot of grinding filing and sanding!just ask solomrus!:evil: