Comp XFI versus Extreme lobes

I'm about to order a custom grind cam from Comp and found out about the existance of the new XFI lobes. Since I'm a big proponent of the Extreme lobes, I was excited to see what XFI was all about. When looking at the spec's I was a little dissappointed in them. Let me list all the spec's:

224 @ .050:

Extreme 3314 lobe - 276 adv - 224 @ .050 - 145 @ .200 - .536" lift (1.6)

XFI 3015 lobe - 274 adv - 224 @ .050, 149 @ .200 - .573" lift (1.6)

Looks a little better, right? Well, take a look at this old Extreme high-lift lobe:

3192 lobe - 276 adv - 224 @ .050 - 152 @ .200 - .605" lift (1.6)

What would be the advantage of the XFI lobe over the high lift Extreme? 2 deg less advertised? Big deal - the Extreme 4X4 cams had even less advertised for the same @.050.

I was looking at lobes in the 230 to 236 range, but again, the Extreme high lift lobes are .622 and .624 respectively. The XFI 230 and 236 lobes are only .576 and .579 lift.

Can someone explain to me why the XFI lobes are better?

Mike

As I understand it, the XFI lobes are more aggresive (faster ramp) than the XE lobes even though the lift is not as high.. They work best with the beehive springs. Other than that, I dunno. By the way, thanks for the various bits of info I've picked up from some of your posts over the past several months. Very helpful.

Mike, How high you want to spin this thing???? There are some issues with the XFI lobes and RPM because you can't get enough spring in there to control them over the nose.

Bret

Bret, are you sure the XFI lobes are more aggressive over the nose than the high-lift Extreme's?

I'm hoping to make peak hp around 6200 and rev it to 6700. However, the cam events start looking really screwy when I go beyond 230 on the intake lobe. I know from a recent dyno test I did that the 230 will peak at or below 5800, though.

Perhaps you can comment on my conundrum. . .

I want to make max power on pump gas using my T-76GTS turbo on a 388 with AFR 210 heads. I've demonstrated 34 psi backpressure with 16-18 psi boost. My current cam is 224/236-114, +0 using Comp Extreme high lift lobes (3192/3196). I currently have 2 deg overlap at .050 and think that more will adversely affect me since my exhaust-intake pressure is 16 psi. It's sort of like a NA motor with a terrible exhaust system. So, we're stuck adding duration on the EVO and IVC ends. BUT, my IVC is already at 46 ABDC. Even full-race cams don't go much beyond that. So, we're limited to adding, say, 3 deg to IVC. At first, I thought my options were:

230/230-114, +1 or
236/236-116, +2

Then, after thinking about it a bit today, I came up with

230/236-115, +3

As compared to the current cam, it moves EVO 4 deg earlier (56) and IVC 1 deg later (47). The overlap is the same, but moved to just before TDC, rather than 2 deg after.

Thoughts?

Mike

Yes, the XFI cams are more aggressive over the nose and they also have more acceleration and jerk there as well which IMHO is a PITA to deal with in a hyd roller application.

With all the backpressure I don't know if a cam is what's going to fix your situation... I'd look more on reducing backpressure first then going to the camshaft. I'm sure I could figure something out but it would take a lot of time that I don't have right now for free.

I would think outside of the box in terms of advance and then.... aka work on the intake side of everything then monkey around with placing the exhaust events where they have to be to scavenge the system and get the overlap where you need it.

Bret

I'd look more on reducing backpressure first then going to the camshaft. . .

That's sort of hard to do when you have a turbocharger blocking things up. . . I actually had as much as 50 psi backpressure at one time before I installed the 3.5" Magnaflow.

I don't have much turbo gasser experiance, but a great deal of turbo diesel experiance....why is the backpressure soo much higher? We shoot for a 1:1 pressure ratio in the diesels by changing exhaust housings depending on how much fuel we are pushing at the time...

why is the backpressure soo much higher?

Yes, well, the gas engines are a bit different. It's a rare case that we can get less backpressure than boost. Every now and then, somebody with a large-frame T6 turbo can get there. However, I can only fit a T4 under MY hood, so I'm sort of stuck with 1.5x more backpressure than boost.

Just for example, I just built a 377 SBC with FastBurn heads, a 230/230-114 cam, Victor Jr. intake, carb, and T-76GTS turbo (same as mine). With a 3.5" downpipe extending about 2 ft out of the turbo, then open, it measured 26 psi backpressure at 18 psi boost. That's about as good as it gets for us. . .

Mike

screw shelf cams and talk to bret. well worth it for the extra 50 bucks or whatever.

screw shelf cams and talk to bret. well worth it for the extra 50 bucks or whatever.

:confused: Did you specifically see ANY 'shelf' cams in this thread? That's what this ENTIRE thread was about.

Ben T.

I would guess he does not understand that most "cam grinders" spec lobes they believe/know will work and let a larger company grind the actual cam, a good cam machine is not something cheap or easy to come by, when you pay someone for a "custom" cam you are paying them to help you select the right lobes and they will probably pick from a few cam companies as far as who has the lobes they want to use for your cam.

Seen some good results in b-bodies with the SMALL XFI lobes they were not boosted though.