I'll try to shed a little light on the choices: The stock cams measure out to 209 degrees duration at 0.050" lifter rise. Lift was somewhere around 0.440". Avail in 114 degree and 109 degree lobe separation. I measured a bit over a 1.6 rocker arm ratio at max lift on my last '66 build. The TA 20 gives a little more lift and duration over the stock cam at 0.470" and 218 degrees. The TA 25 adds quite a bit more exhaust duration which the Nailheads seem to like.... 218 deg intake/ 228 deg exhaust. More lift too. Schneiders has several comparable cams. Comp Cams can make you anything you want if they have the cam cores in stock. All you have to do is pick which lobes you want ground on it! For example, Their 'High Energy 260H' grind is close to Buicks '091 2x4 cam with 212 degrees duration, 0.447" lift and 110 degree lobe centers. It most likely has faster opening/closing ramps than the Buick cam which will increase performance. Duration...as you add more and more duration, low rpm power will drop off, and high-rpm power will increase. The split duration cams like the TA25 are a great compromise! Lobe separation... 114 degrees for a smoother idle and more vacuum for power brakes. 110 degree +/- seem to give more low end and overall good power. It's tough to make real comparisons as there haven't been any published dyno-tested cam comparisons. However, you can play around with engine simulation software like Dyno 2003 to compare cams.
Yeah definitely difficult to compare with no real dyno numbers, mostly just butt dyno's or a et here or there, but so much more goes into et's... wkillgs, what do you mean a rocker ratio a little over 1.6? I thought the nailheads were rumored to be around 1.4??? A custom grind is one of my top choices right now cause I can copmrimise between my other two choices, the wild side Thumpr and milder side TA25 (or similar Schneider)... What's the fastest a Dynaflow has ever gone down the quarter?
The rocker ratio is claimed to be 1.6 by Buick and is probably close to that with a stock cam. But with a more aggressive cam, with additional lobe lift, the design of the rockers is such that it only has a 1.4 ratio.
So I spent all night at work looking at Comp's lobe profiles, see what you guys think about what I came up with... The idea was something similar to the Thumpr but tamed a bit (The Thumper is - int 279 dur and .478 lift (1.5 rockers), ex 297 dur and .465 lift (1.5 rocker))... Lobe Ad Dur .050 Dur .200 Dur Lobe Lift 1.5 Lift 1.6 Lift Int lobe #5239 (Magnum) 276 228 140 .316 .474 .506 Ex Lobe #5204 (Magnum) 292 244 151 .323 .485 .518 LSA 109 Degrees (Similar to 091 cam) 104 Int Lobe Centerline (5 degrees advance) Work in progress... Too big? Muahahahah
Not sure if you can call that tame compared to the Thumpr, since the Thumpr has less @ .050 duration. The Thumpr is 227/241 @ .050 on 107* LSA. I'd shoot for something like 224/236 @ .050 on 108* LSA (that's what I was going to go with). It'll still sound really mean, and be a little more tame. If you want it even tamer, go with a 109* or 110* LSA. Also I don't think I'd advance it more than 4*. If you call TA or Schneider you can get whatever you want - you wouldn't be limited to the lobes in Comp's catalog. If you wanted to stick with Comp, check out the Xtreme XFI lobes (5086 & 5088). They are both intake lobes, but it doesn't matter. Providing it would fit without valve to piston clearance issues, I think that would be the hot ticket.
When I said tamed I just meant by a little, yes the valves are open at the higher lifts longer, but they are also shorter overall by a tad. In the real world there's prob not much of a difference in the lobes. The thought was between the shorter overall openings and wider LSA that it would have less overlap, and hence be slightly more tame. If someone said the cam is too big, I prob couldn't argue with them hahaha. Work in progress... One of the things I like about the Comp Cams lobes is they seem to be slightly more aggressive on the ramps, holding the valve open more longer, compared to others I have seen. But on the other hand I was worried about using the Xtreme or XFI lobes in case they were too aggressive for the rockers to handle. Funny you mention a 224/236 108* LSA, my initial selection was a 226/236 @ 0.050 .514/.506 (1.6 rocker) on a 109* lsa (also threw a 228/236 @ 0.050 up but didn't feel there was enough split between the profiles). My only concern was that the intake lobe was a Xtreme one and I wasn't sure about that. If we feel the ol' Nailhead is up for some super aggressive ramp rates that opens up a lot more possibilities.
i run the xfi you need good valve springs. 125-130 seat and 275-300 open. dont think or know if the stock rocker arms and push rods will handle it.
I don't necessarily think it's too big, but if you were trying to go milder than a Thumpr, that cam really wasn't it. I think if you went with the Xtreme or Xtreme XFI lobes you'd definitely want to address the valve springs. Those lobes have a much more aggressive ramp rate. Realistically, the Magnum lobes are also a good choice. Check out the 5201 & 5241/5208 lobes. Like I said in my PM, when you start looking at lobes for a custom cam it's hard to know what's going to hit and what's going to fit inside a stock motor. It's not all a function of lobe and valve lift - overlap and intake centerline have a lot to do with it as well. So if you do decide to go this route, pulling the heads to check clearance would be a smart choice. That was one of my big motivators in going with a Thumpr - it was the most aggressive cam that I knew would fit. And the only way I knew it would fit was because 2 people before me were guinea pigs. When the Thumpr first came out, the consensus around here was that it wouldn't fit in a stock motor. We were wrong - "bench racing" only gets you so far.
I profiled my stock '66 401 cam (with degree wheel and dial indicator) using a solid lifter. The max lobe lift on the Intake was 0.261" and at the valve it was 0.425"...... giving a 1.628 ratio. The exhaust was 0.270" at the lobe and 0.448" at the valve... a 1.66 ratio. Using Tom's roller rockers, the lift at the intake valve was 0.498" and 0.514" on the exhaust.... ratios of 1.908 and 1.904:Brow: I read somewhere cam companies may compute rocker ratio at mid lift and not at max lift. Speaking of roller rockers, they are a nice option, but an expensive one. If going this route, you won't need a fast ramp cam since the rockers will do that for you.
Has anyone looked into using COmp Cam's Beehive springs? Seems you could run less spring pressure with having a smaller/lighter spring retainer which would have less inertia which would help keep the valve from floating and the lifter on the lobe with the more agressive lobes, plus be easier on the stock rockers... And my old man and myself use to check valve clearence with the heads on, might not be the best way but it's always worked for us... we intall the lifter/pushrod/rocker on #1 intake along with a weak valve spring (just enough to hold the valve up) then using your hand on the valve and rocker slowly rotate the engine through the valve motion while periodically pushing down on the valve with your hand making sure it can move down some, eventually you'll start to feel the piston when pushing the valve, slowdown and feel for either the min clearance or where the clearance gets so small your going to run into interference and stop (dont want to bend the valve haha). Then if needed you can advance/retard the cam to get clearance. Measure the gap between the valve (while pusing it with your hand into the piston) and the rocker and that's your clearance. If you can't advance/retard the cam an accepable amount to clearance the cam then it's too big. Repeat for the exhaust, if the cam is advanced or retarded more you recheck the intake... Ghetto and un-scientific but it has worked for us haha... Cause if I pull the heads that opens a whole new can of worms!!! It's another one of those "while your there" things ou:
