Too much compression for these cams?

[Adam76]

I'm looking at the MR103 cam option for a softail build 96" to 103" using oversize pistons.

KB flat top 103" + .030" pistons
Stock heads 85cc
S&S intake and Rush slip on mufflers
.030" head gasket
Puts me at 10.2 static comp / corrected comp 9.65 and ccp of 203...

Main goal is good midrange power without losing low end TQ. S&S MR103 cams say they're good to 10-1 compression or even a bit more.

Haven't seen anyone use these cams other than a bolt in 103" cam. Looking for advice on these cams as part of a big bore build at higher compression?

Thanks.

[JSD]

What is the inlet closing at on that cam

[Adam76]

What is the inlet closing at on that cam

33* IVC

[sfmichael]

You'd be at the upper threshold of compression for that cam but with a really good tuner and the availabilty of premium fuel you should be fine and have a very strong running engine

[Adam76]

You'd be at the upper threshold of compression for that cam but with a really good tuner and the availabilty of premium fuel you should be fine and have a very strong running engine

Thanks. Not many people seem to run these cams so I'd be interested in how they turn out.

[sfmichael]

That cam will make great power, I'm pretty sure you'll love it. At .585 lift you will be right at the safe limit for stock heads / valve springs. Have you considered having the heads freshened up while they are off?

[Adam76]

That cam will make great power, I'm pretty sure you'll love it. At .585 lift you will be right at the safe limit for stock heads / valve springs. Have you considered having the heads freshened up while they are off?

This particular motor only has 15K miles on it and I'm wanting cams that will work best with stock heads, but I could probably stretch the budget to a simple valve job.

Are you recommending upgrading the springs? S&S say it's a "bolt in cam for 103".

Thanks

[JSD]

A 570 may be a better choice

[sfmichael]

That cam will make great power, I'm pretty sure you'll love it. At .585 lift you will be right at the safe limit for stock heads / valve springs. Have you considered having the heads freshened up while they are off?

This particular motor only has 15K miles on it and I'm wanting cams that will work best with stock heads, but I could probably stretch the budget to a simple valve job.

Are you recommending upgrading the springs? S&S say it's a "bolt in cam for 103".

Thanks

If a competition valve job is in the budget, then do it. A multiple angle valve job by a savvy cylinder head specialist will yield additional airflow over the stock cylinder heads at a relatively modest cost, adding more power. Some bowl work by a competent head porter will yield even more. keywords: savvy / competent
I would never claim to know more (or even half as much:) as S&S, so if they say it's a bolt-in, I say go for it. I've kept .580 lift as the safe limit on stock springs but we're basically splitting hairs with an additional .005 lift. If you've already purchased them I would proceed as planned. If not, I'd take a look at the CycleRama CR-575. But the MR103 is undoubtedly a really good cam and definitely makes my short list of cams for a lightly modded twin cam

[rigidthumper]

S&S states the MR103EZ are suitable up to 10.5:1, so you should be fine.
Looks like 112-113 TQ and mid 90s HP, your results "should" be slightly better, with your compression bump.

Here is a chart.

Here is another.

[Adam76]

S&S states the MR103EZ are suitable up to 10.5:1, so you should be fine.
Looks like 112-113 TQ and mid 90s HP, your results "should" be slightly better, with your compression bump.

Thanks for your feedback. Yes I was going with the EZ start version so I didn't have to install compression releases.

[Adam76]

A 570 may be a better choice

Hey JSD, I have considered the 570s for this build. I've run then in a 88" - 95" build but wasn't really happy with the overall performance. They seemed to be lacking, even at the right compression.
Cheers

[speedzter]

So many Cam's to choose from !!
https://university.fuelmotousa.com/article/h-d-twin-cam-103-camshaft-shootout/

[Adam76]

So many Cam's to choose from !!
https://university.fuelmotousa.com/article/h-d-twin-cam-103-camshaft-shootout/

Yes there are....

That cam comparison is really just a comparison of "bolt in" cams for the 103" motor with no other modifications.

My build is 96" - 106" at much higher compression, so a different animal altogether.

I have not seen anyone use the MR103 cams with high compression, hence my initial concerns about creating too much detonation with an IVC of 33*.... With a good tune it will probably be fine.

The other cam I have heard good things about is the Comp Cams TC-3103 when set at 10.3-1 which is close to where I am anyway. With an IVC of 41.5* I would not have to worry so much about pinging issues...  Yes, too many choices. 

[speedzter]

I didn't see 106 mentioned ?
If it me , from past experience I'd be using Wood TW7 .

[Adam76]

I didn't see 106 mentioned ?
If it me , from past experience I'd be using Wood TW7 .

Yeah, 103 pistons .040" oversize gets me close to around 106 cubes.

[Adam76]

S&S states the MR103EZ are suitable up to 10.5:1, so you should be fine.
Looks like 112-113 TQ and mid 90s HP, your results "should" be slightly better, with your compression bump.

Hey rigid, I was going with the .040" oversize pistons to get me to effectively to 106" cubes.

At 106" and around 10.3 compression would you still run the MR103 cam? Or would you choose something else?


Thanks.

[Adam76]

I didn't see 106 mentioned ?
If it me , from past experience I'd be using Wood TW7 .

No doubt the Woods TW7 would get me great results. But for the budget I've got I'm thinking Comp cams 3103 or S&S both of which I can get downunder much cheaper.

[rigidthumper]

What cams/brands are available to you, at a reasonable price?

I have not personally ridden anything with an MR103 cam, so I can't say. Riding anything with the 570 always felt like a "downshift first" cam to me.
Fuelmoto's testing showed the MR103 & 570 had similar curves, with the MR doing everything a couple hundred RPM sooner. Of the 2, I'd prolly prefer the MR.


I would measure/verify the #s, as almost all of the 2006 and newer 96/103 TC heads I've checked were 86 to 86.5CC. What part number are the +.030 KB pistons you're using? They call the KB908LCA pistons flattop, and they have a 1.9 cc relief, and .005" down deck height on standard cylinders. That combo puts you @ ~10.1:1

[Adam76]

What cams/brands are available to you, at a reasonable price?

I have not personally ridden anything with an MR103 cam, so I can't say. Riding anything with the 570 always felt like a "downshift first" cam to me.
Fuelmoto's testing showed the MR103 & 570 had similar curves, with the MR doing everything a couple hundred RPM sooner. Of the 2, I'd prolly prefer the MR.


I would measure/verify the #s, as almost all of the 2006 and newer 96/103 TC heads I've checked were 86 to 86.5CC. What part number are the +.030 KB pistons you're using? They call the KB908LCA pistons flattop, and they have a 1.9 cc relief, and .005" down deck height on standard cylinders. That combo puts you @ ~10.1:1

Yes you're right they are listed at 10-1 compression. KB908C-040 are the pistons I'm looking at and with the stock 86cc heads and 3.915" bore it does come out at 10.1:1 comp which would possibly be a good compression to run the MR103 and keep it a budget build.

But if I'm taking the heads off it's not that much more to get them cc'd to 83/84cc with a simple valve job. Then I'm at 10.3 / 10.4:1 and it opens things up to many more cam options....

Basically, the hardest part of this 106" build is coming up with the right recipe. 

[Adam76]

KB oversize Flat tops = 106" cubes

These are two of my options

~ 86.5cc stock heads
.030 HG
10.15:1 static comp
MR103 easy start camshafts
Corrected comp 9.6
CCP - 200

OR

Heads at 84cc + simple valve job
Manual comp releases
Comp cams TC-3103
Static comp 10.35
Corrected comp 9.46
CCP - 198

Looking for power at 2500 - 5000

Any advice on the best option?
Thanks

[Adam76]

Finally ordered the parts today.

Went with
KB 3.915 flat tops
85cc stock heads with comp releases and basic valve job
Comp 3102 cams (almost identical to Andrews 57H)
Puts me at 10.2 comp
Corrected 9.5
CCP 199

Hopefully should be a good combo for a mild 106" build.

[sfmichael]

Come back and tell us how you like it 

[Adam76]

Come back and tell us how you like it 

Will do. Just got started pulling the tank and exhaust. Will be waiting on parts for a few weeks.

[JSD]

3102 cam IVC ?

[Adam76]

3102 cam IVC ?

This one is 37* very similar to the Andrews 57H.
234/242 Duration

It was a coin toss between the 3102 and the 3103 which has
41.5 IVC
242/250
and would need more compression, like 10.3 / 10.4

I don't think either one would be a bad choice.

[Adam76]

Come back and tell us how you like it 

It was a coin toss between the 3102 and the 3103 which has a later 41.5* IVC and a bit more duration
242/250.
I would have this one set up at  10.3 / 10.4 compression to keep the low end TQ.

[Hilly13]

Hey Adam, she done yet?

[Adam76]

Hey Adam, she done yet?

Hilly, not done yet. Stuck a set of Andrews 48H cams in for the time being until I get enough funds to finish off the build.... These cams are actually quite good in a 96" motor. 

[Hossamania]

If I remember right, those cams were made specifically for the 96" motor.