Triumphrat

SCCTrim · 10-15-2006

There have been posting's here and elsewhere regarding a exhaust cam mod that increase's low end / midrange torque.

Friday I had to run some errand's and was in Frederick and decided to drop by HD of Frederick and check them out...I've seen dyno report's of another bonnie done there and wanted to drop by for a price on a baseline test.

Much to my suprise...32.00 So I figure wth :-)

I have not had the chance to dyno the bike with the stock headers and predators, and while I wont beable to directly compare this dyno run to my prior ones, it will serve as a baseline to compare to when I advance my exhaust cam.

I also have ordered a pair of 42mm Mikuni HSR carbs for my bike from Bonneville Performance. I was able to get the price knocked down some as I didnt need a airbox elim kit nor billet intakes...( my bike has both of these installed already )...still, they were expensive...

I rationalized that if I was going to go the carb route that it wasnt going to be for the same size carb, which left the choice of either FCR's or Bill's 42mm mikuni's. The mikuni's have the accel pump disabled, while the fcr's do not. The mikuni's have produced some very good numbers, hence my decision.

The exhaust cam mod will happen this coming week, as soon as I get a chance to get the bike back to the dyno, I'll post that sheet here as well.

Here is the dyno sheet from Saturday.

HiVel · 10-15-2006

Looks like a healthy motor-what do you do to the exhaust cam? Advance it how much??
Thanks

SCCTrim · 10-15-2006

Sorry, I should have included that info in my original posting. The mod is to advance the exhaust cam 1 tooth on the 790cc cams, this decrease's the overlap and is said to make a difference that can be felt in low end and midrange torque.

Jojje has done this to his bike, and while he has spacer's between his cylinder and the case's to keep the cr to 8:1...my motor with the bigbore kit hold's 10.5:1

It will be intresting to see if the added compression make's a difference in whether or not the motor lose's any top end hp as a result of this.

Agro....are you watching this thread?

:wink:

Rip · 10-15-2006

I'll be doing this mod if the results are good :clap:

johnyC · 10-15-2006

Rip,

I'm doing it anyway when the lid's off next at 12,000. Jojje said a bloke he knows races a 270' bike with the exhaust cam advanced 1tooth (exhaust valve closes earlier) Here's the link to a thread on the subject from another forum:

Jojje's ex' cam advance thread

[ This message was edited by: johnyC on 2006-10-15 14:16 ]

monkeyboy · 10-15-2006

Quote:

On 2006-10-15 10:18, SCCTrim wrote:
Sorry, I should have included that info in my original posting. The mod is to advance the exhaust cam 1 tooth on the 790cc cams, this decrease's the overlap and is said to make a difference that can be felt in low end and midrange torque.

Jojje has done this to his bike, and while he has spacer's between his cylinder and the case's to keep the cr to 8:1...my motor with the bigbore kit hold's 10.5:1

It will be intresting to see if the added compression make's a difference in whether or not the motor lose's any top end hp as a result of this.

Agro....are you watching this thread?

:wink:

The added compression should not concearn you as for lost top end as much as a lost top end.

Is there sufficient piston to valve clearance to start playing with valve timing like this?

I know some bikes have it, and some don't.

Devious2XS · 10-16-2006

How would you compare the driveability of the 42mm Mikunis over the stock carbs? Usually the FCRs have fantastic throttle response compared to Mikunis, but the 42mm option looks very interesting.

SCCTrim · 10-16-2006

Quote:

The added compression should not concearn you as for lost top end as much as a lost top end.

Is there sufficient piston to valve clearance to start playing with valve timing like this?

I know some bikes have it, and some don't.

My knowledge is limited, but here is what I understand. The stock 790cc cams have alot of "overlap"....especially on the exhaust side. Advancing the exhaust cam...( stock 790cc cam )...merely open's and close's the valve sooner, decreasing the "overlap" considerably. There is no added "lift".

The 865 cam's have the same lift but not the amount of overlap as the 790's, hence they make more torque, and have a little less "pull" on topend.

There are other's whom have done this mod...Jojje as well as Agro..( whaleoilbeefhookt )....specifically. Jojje claimed a loss of hp on the topend somewhere between 3 to 5, but his motor was also shimmed between the cylinder and the case's to lower the compression....( turbo-addict ) :-)

It will be intresting to see if the added compression ratio will negate the topend loss.

I cannot comment on the minkuni's driveability as I do not yet have them....expect more details and result's on this thread as this come's together.

sweatmachine · 10-16-2006

Interesting stuff, basically what you create is an 865 cam with more lift, so it makes the motor have more power sooner.

That will the carbs and big bore should be fun.

I'm interested in the carbs too.

monkeyboy · 10-16-2006

Quote:

On 2006-10-16 05:03, SCCTrim wrote:

Quote:

The added compression should not concearn you as for lost top end as much as a lost top end.

Is there sufficient piston to valve clearance to start playing with valve timing like this?

I know some bikes have it, and some don't.

My knowledge is limited, but here is what I understand. The stock 790cc cams have alot of "overlap"....especially on the exhaust side. Advancing the exhaust cam...( stock 790cc cam )...merely open's and close's the valve sooner, decreasing the "overlap" considerably. There is no added "lift".

The 865 cam's have the same lift but not the amount of overlap as the 790's, hence they make more torque, and have a little less "pull" on topend.

There are other's whom have done this mod...Jojje as well as Agro..( whaleoilbeefhookt )....specifically. Jojje claimed a loss of hp on the topend somewhere between 3 to 5, but his motor was also shimmed between the cylinder and the case's to lower the compression....( turbo-addict ) :-)

It will be intresting to see if the added compression ratio will negate the topend loss.

I cannot comment on the minkuni's driveability as I do not yet have them....expect more details and result's on this thread as this come's together.

You stated exactly why I am concearned.

If the exhaust valves open sooner, that means that the piston is gonna be in a different spot when they open, possibly causing interference issues.

Also raising compression, usually done by raising the top of the piston, can cause clearance issues.

Do the math, test fit the pieces, and roll the motor over by hand, slowly when you assemble it.