203 VS 255 VS 251

[roofeditor]

On a 07 with 103 Bb Kit I know you said you preferred the 251. however there are some that say that the 103 BB is to low compression for a 251 can in that they are only slightly over 9:1. They are also saying the 251 has too high lift and needs other head work and springs.

This is an excerpt from v-twinforum

That's right. First of all, I think stock heads can only handle a lift of around .550, I don't know, someone chime in on that please. You'd probably have to buy stiffer springs to keep the lifters from bitch-slapping the $hit out of the cams. To use the 251s with stock heads (even if the lift could be handled), you'll need new pushrods. None of that sounds like "bolt-in" to me. The 203? Now that's a bolt-in. You just, well, put 'em in. Nothing else is needed.

The 251 closes at 46, and, therefore, bleeds off quite a bit of compression. The normal 103 BB kit (stock heads, no high-compr pistons) has a compression in the 9s, maybe even low 9s. With the bleed-off of the 251s, it will suck in low RPMs. A late intake close takes advantage of momentum in the higher RPMs to continue to pull fuel even during part of the compression stroke. In order to take advantage of this and not kill your bottom end, you need the increased compression. Period. I'm not saying you can't put the 251 in the standard 103 kit, I'm just saying the bottom end will hurt.


Both you and Doc say the 251 is one of the better company cams.

1. So is it actually a drop in cam or do you have to do other modifications.
2. Is the compression high enough?
3. How much does the compression come up as you add the 103 BB and is it worth it to use the thinner comedic gaskets.

Dealer obviously says go with the legal 255 cam but also admits the 251 is the better cam but is vague on compression and additional modifications.

I'm sure you must have this many times and know the real story?

Thank you again for your help and opinions.
RE

[sexyharleyguy]

stock gaskets 251 cam for 95 a 103 consider 257 maybe even 260

[Mike]

Individual engine builders will have their own prefered choices. I believe the 251's will work well in a 103ci motor with 9.5 compression due to the larger engine size vs. a 95. It would work even better with higher compression but the increased ci's of a 103 will carry the low end tq better than a 95 and thus somewhat compensate for the lower compression. The exhaust system will likely have a larger affect on low end performance than the cam diferences.

Last, many do not realize that high lift is not really about the max flow numbers the heads can handle at max valve lift. I.E., why run a .650 lift cam with heads that stop flowing at .550 valve lift? The answer is that the higher lift will move the valve farther off the seat as the valve is opening vs a lower lift cam. This has the affect of making the motor think it has bigger valves allowing it to take a deeper breath, increasing cylinder fill at all rpms. This holds true for stock heads and more so as motor size increases. Your forum guy is not looking at the whole picture.

The 251's do require longer or adjustable pushrods due to the smaller base circle. To put 203's in requires removing the pushrods so you just substitute. Some engine builders use the quick install pushrods when installing even the 203's to eleminate the need to remove the rocker covers (they cut the stock pushrods out). It is a "bolt in" for 05 and later heads with the ovat coil (beehive) valve springs. We define "bolt in" as not requiring the heads to be removed or other clearancing or machining. The addition of complementry required parts does not change this definition. The higher lift with the stock springs will not "bitch slap" the cams unless the motor is routinely reved above 6k rpm. The 251' have similar TDC lifts as the 203's and so valve to piston clearances should not be a problem.

The computer says the compression will be 9.8 with the 103, but in reality it will be closer to 9.6. This is enough to support the 251 cams in a motor of this size. With the motor appart for the 103 installation, then using the Cometic .030 head gaskets would be a good choice and should bring the compression up to around 9.9 (with stock heads). Given the alternative choices, we would recommend the 251's vs the 255's or 203's for 103 motors.
Mike

[sexyharleyguy]

Mike thats why your the expert. Great explanation. Dont misunderstand me anyone the 251 is a fabulous cam. I was thrilled with it in my 95 inch Had I realized it was also a great choice for a 103 I may have done a few things different in my last build. The 251's in my 95 kicked in hard at 3100 and pulled right past 5 k. I ran 10.25 / 10.5 compression.

[roofeditor]

Thanks for clearing that up mike. Experience IS knowledge! I assume you do recommend the Comedic gaskets bumping the compression up a bit? And lastly do you prefer longer pushrods or adjustable? If longer is the answer is there a HD or SE part number?

[Mike]

I like the longer "perfect fits" as they tend to run quiter and are stiffer. The HD SE part number to use with the 251/257/258 cams is 18401-03 when used with stock heads. Using the Cometic .030 gasket (recommended) will not adversely affect the pushrod choice.
Mike