I have a Comp XE268 cam and Brodix IK180s. Brodix advertised 100lbs seat, and 300lbs open, however the heads came marked with 85lbs seat and 290lbs open. Comp recommends a 100lbs seat and 300lbs open with a minimum of around 90lbs seat. What problems will/could I run into with a light seat pressure? Leaky valves?

You could shim the springs to reduce the installed height and bring up the seat pressure as long as you don't get close to coil bind. The XE268 is pretty mild lift though so it probably won't be a problem.

Consider all of those numbers you are posting as the MINIMUM recommended.

There's probably also a maximum; not on the seat, but open. The cam doesn't much care what the seat pressure is, but it can only tolerate JUST SO MUCH over the nose. On the other hand, the rest of the valve train doesn't care so much what the open pressure is, but will want enough applied on the seat to keep the valve from bouncing when it closes ("valve float").

If you set up a hyd flat tappet for about 125-130 on the seat and 325-350 over the nose, you'll generally get the best results.

I would not even attempt to run anything hotter than a 929 with 85 lbs on the seat.

I would not just randomly "shim up" weak springs, without measuring their ACTUAL installed height.

The installed height on the I is 1830 and the E is 1815. Comp said 290-300 for the open pressure and 100lbs on the seat.

Again, those numbers Comp gave you are the MINIMUM.

What springs are they? What's their coil bind height?

I'll call Brodix tomorrow to ask them what brand of springs they used. I don't mind shimming the springs so I guess my question would be how to calculate how much I would have to shim the springs to get the desired seat pressure.

The spring they used to list as their "premium" choice for that cam, the 986 (back before they put beehives in as all the "premium" recommendations), has a seat pressure of 132 @ 1.750" and open pressure of 293 @ 1.250".

That's the kind of numbers you need to be aiming for, for best results.

I cannot stress strongly enough that 100 lbs on the seat is the ABSOLUTE BARE MINIMUM for an agressive cam profile like that, to get best results. This is different from optimum, desired, goal, etc. etc. etc.

The heads have a Comp Cams 980 spring. I wouldnt call .477 I/.48 E, 224/230 @.050 an aggresive cam shaft IMO. Comp cams said a .030 shim would bring the seat up to around 95 and the open to around 300 which they said is fine. The recommended springs for my cam are the 981's.

Yea, the 981 is the basic plane jane 1.25" spring. I'm running the xe268he with a brand X version of the 981 spring (100/300).

A comp 980 spring isn't something I know offhand, but I imagine it has a .600" coil bind (1.800-1.200"). Since you're using 1.5" rockers (right?)
you could shim it up .060" or so, since you're above 1.8" installed height. If you've got a drill press, caliper and a bathroom scale you can get an idea of the installed height spring force.
You want ~.050" of breathing room though, so don't overshim.

The catalog says 92 lbs @ 1.7", 230 @ 1.25". It sounds like your springs are installed even taller than 1.7", though.


It's not those numbers that make it aggressive, it's the ramp design.

I have 1.6 roller tip rocker arms, but still that would put me at a .512 max lift on the E. I'm going to purchase a set of .030" shim to install to bring up the seat and open pressures. The 980's just have a lower spring rate than the 981's.

A .030" shim is only going to bring your installed height down to around 1.8", which will still be well under 90 lbs of seat pressure.

To get the minimum acceptable 100 lbs on the seat you'd need an installed height of about 1.675", which would have an open height of 1.163" with a .512" lift cam. Coil bind height of that spring though is 1.150.

You need new springs.

My installed height on my springs are 1.830 I and E 1.815.

Yes, and at 308 lbs/in for the 980 spring, from the recommended installed height of 1.7" that loses you about 40 lbs of seat pressure on the intake and about 35 lbs on the exhaust.

You need new springs.

So if I install the 981's (470lbs/in) wouldn't I have too much open and seat pressure?

981s are 370 lb/in... 105lbs @ 1.7", 273lbs @ 1.25", 1.15" coil bind. But .512" lift though still has you at 1.188" open height, which is too close for comfort to coil bind.

I'm still trying to understand why my springs have 85lbs seat @ 1.830 and 290 open. when the specs say that it should have 95lbs seat @1.7. Thats .13 x 302lb/in = 40lbs.

The spring rate is only really usable over the range from the recommended installed height to coil bind. Suffice it to say that you don't have enough spring pressure, especially while they're installed incorrectly.

Something just doesnt add up right. 85lbs @ seat and the 290lbs. The max valve lift for the spring is .550 (so the card said). If I had 85lbs plus (.550 x 302lbs/in) = 251lbs should be my open pressure correct?



So I guess my options are shims or new valve springs. So which springs would be correct though? Especially if I have an install height of 1.83? Or will I have to get a valve spring mic

For the 980, (230 lbs open - 95 lbs closed) / (1.7" closed - 1.25" open) =~ 300 lbs/in, pretty close to the 308 lbs listed in the catalog. Don't worry too much about what your spring rate is outside of the recommended installed height. It doesn't matter anyway, since it's not nearly enough.

You'll have to check the catalogs to see if there's a spring that will fit inside the spring pockets of your heads, that are close to the recommended spring forces for the cam, that still have enough room to avoid coil bind. They might be difficult to find.

Coil bind isn't the only thing limiting the lift of the cam. You're also going to need to check the clearance between the top of the valve seal and the bottom of the retainer.

I talked to Brodix once again about the big gap in seat and open pressures. They measure every spring for seat and open pressure. I guess the reason why my install height is so high is because they cut the valve spring seats a little deep. This being said, I probably just can't buy a set of springs and go by what the catalog says. My open pressure is fine, but the seat needs to come up 15lbs or so. Does this math sound right, .030 shim x 302lbs/in =9lbs. That would bump up the seat pressure 9lbs correct?

More or less, but you still haven't changed the fact that you're not going to be able to shim those springs enough to bring the seat pressure up to a minimally acceptable amount without getting too close to coil bind at full lift.

If I shim the springs .045 that would give me 1.785 I and 1.77 E on the install heights would increase the spring rate (302*.045) roughly 13-14lbs which would work out to 98-99lbs on the seat. The exhaust would be my closest to coil bind which would be 1.77-.512(lift w/ 1.6 rr) -.060 (standard buffer) = 1.198 - 1.115= .083 of extra before coil bind.

If I shim the springs .060 that would give me 1.77 I and 1.755 on the E which works out to increase of 18lbs which would be 103 on the seat and 308 on the open with plenty of coil bind clearance.

Does that all sound correct?

I think I'm going to just install the heads as is and then change the springs out later. My cam is brand new so the lower spring pressures will help out with cam break in.