when picking a new cam how does a guy know how much duration spread to grind in a cam? what determines it, head flow, compression, rpm, CI?
Ryan spec'ed a cam for my indy headed 408 and it has a 4* spread. bullet also recommended a 4* spread. i talked to a guy from another forum last night and he's going real fast (8's)with a w8 motor. he tells me the guys in his crowd run a 11* or 12* spread to make big power. Uratchko does this guys heads and seems to be on board with the 12* spread. brett millers 395 had an 11* spread. so what determines the spread needed?

Oh, I'll add this comment before seeing how others respond. You don't really NEED any additional duration on the exhaust side... and I just saw a dyno sheet for an 800+ HP N-A engine that actually had a NEGATIVE (i.e. shorter) exhaust duration spread.

Back to the .

I`ve been told and have read it has to do w/poor exhaust ports and flow that the OLDER heads and even some of the newer have so you hung the exhaust open a bit longer to scavenge better..... I don`t remember the other reasons as my S.W.A.T. goggles take a bit of adjusting this early in the morning.

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brett millers 395 had an 11* spread. so what determines the spread needed?

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You will also notice that the engine made peak power of 662 HP @ 7300 RPM
on Rider's 902 with 255º @ .050" cam..........

So again I say, put the one you have in @ 104º and run it.

A bracket type motor works just fine with no spread, in fact, a lot of cams use the same lobe on both intake and exhaust. As engine speed goes up there can be a benefit to changing the intake lobe and making it different from the exhaust lobe.

Typically on a high speed design, the exhaust lobe has more duration and a lot less lift than the intake but that isn't always the case. Sometimes the exhaust side requires less duration than on the intake.

Ryan should have a pretty good handle on that type of combination. If you don't trust him then drain $10,000 from the bank account to buy a pallet of camshafts and a month of dyno time and then you'll be an expert. At least you'll be an expert on your particular engine combination.

it just seems these guys that are making 800 and 900hp small blocks and do things differently than the guys in my leage(600hp). they have this anchient chineese secreat voodoo thing going on with how they build thier engines.
i'm not talking about where to set the intake centerline angle on installation. i'm talking about the degree spread between the intake lobe and the exhaust lobe, for instance this cam bullets grinding for me is 268/272@50, thats a 4* spread.

Bingo, to pick up that extra 200 hp takes some serious research.

Ever go to the track and look at the Pro Stock truck motors with their super tiny header tubes? If you asked on this forum what size of header tubes you needed for 900 hp people would tell you something like 2 1/4 or maybe bigger. If you asked on here how big of a cam it takes to make 800 hp people will tell you something huge. Heck, people on here recommend the Comp .650 grind for bracket motors sometimes and have you looked at the specs on it? There are pleny of guys making 800+ hp with cams that are 20 degrees smaller than that cam.

Moral of the story is that there is a lot of different combinations that can be put together to build a 600 hp motor. Almost any random collection of parts can be used at that point. At 800 hp some of those combinations just aren't going to get it done. At 900 hp you'll need to be pretty darn sharp on which parts are working together and which ones aren't.

yeah, and my theory on that 12* is maybe a screaming small block turning 8000-9000rpm would need the large spread to help the scavenging keep up in order to feed that beast to make that kind of big power.

I was just trying to help.

no problem, i appreciate all opions. i was just trying to be more clear. i thought you may have misunderstood.

Hey Dave, My W5s don't need too much spread like 2*, heads like Js need a ton , I think that RJ and Vic are both correct, We stayed with a smaller cam and maxed out intake lift which netted great results< thanks Vic > RJ spec ed us a cam that will make more power than our current one but at the cost of some more RPM which on this block I dont want to do, So my point is its a balance on how much power vs how much piston speed

thats kind of what i figured. these 8-900hp small blocks are built using a different recipe than what we would use.

The 48* w8 stuff is a major step up from the 59* we run, its very easy to max out a factory block with a good set of heads, Lots of opinions on how much will it take,

282/298 @ .050 and 314/334 @ .020
900hp @ 9500 truck motor

Sounds like the same cam I just picked up from Bullet, Dave. We must be pretty close on builds......

All depends on the heads and flow....The trend in cup motors has been get the intake Valve as big as you can get it and hang the exhaust open longer to compensate for the lack of size.

2/3rds (at least, some say) of the POWER a motor makes is on the INTAKE side.

What Andy says in terms of Bracket motors and single patterns is essentially true

Generally with a Bracket Wedge motor if you want a 260 degree exhaust lobe and take a 254/260 against a 260/260 and the bigger intake will almost always make a bit more power but it may be softer on torque somewhere in the band. It's always harder to Fill the hole than it is to empty it, air has to Fall in but it is essentially pushed out....i.e. it's basically already trying (gases flow from higher pressures to lower) to get out.

MAFO's 900hp engine has a 16* spead on his cam.
i understand the theory of more duration on poor flowing exhaust ports, but when it comes to 900hp small blocks and large degree spreads on duration it does'nt jive with this theory.
my indy 360-1's have a 2.100 intake valve and a 1.650 exhaust valve.

IMO 0 Spread is fine and a duration spread is not needed in most cases for a bracket car.

With 0 spread like the MP cams have you can run deep into the 9,s with them.

Most of the power is made from the intakes lobe Duration and Lobe sep. The exhausts lobes were just a duplicate of that in the past and that theory works out just Fine

The only reason to deviate from that would be IMO

Or other wize add more Exhaust duration would be;

For a street car;

Poor flowing head exhaust ports
Full exhausts system
Stock manifolds.


For a race car;

Nitrous motor
Blown motor
Turbo motor

And High RPM motors. A higher RPM motor can be compared to a poor flowing lower Head motor.

Its a function of Time. They both would want and can be helped with more time to get the exhaust out.


With your typical bracket motor with good heads and headers a same lobe duration cam will run just as good as a dual lobe cam. Maybe even better because overlap will be reduced. mike

my motor have 2.25 / 1.55 valves maybe it has something to do with it..

Something else to consider that may be enlightening... If you are making 600hp at say, 6800rpm, what is that 900hp at 9500rpm engine doing at 6800? Part of the reason higher rpms make more power is more power strokes in the same amount of time. Horsepower is force over time. Does anyone have a dyno sheet that shows the power output of one of these 900hp engines at the same rpm peak of a typical multi-use 408? Yes, all parts need to work together, but having rounded power peaks rather than peaky ones is a lot harder than targeting a small rpm window to maximize output in. The PS truck mentioned does just that. The power curve of it is very high rpm and not very flat. It's not that hard to get a good idea of what the perfect setup is for a given rpm point. Its the packaging and execution, and trying to get that same perfect setup for a wider rpm band that is extremely difficult.