Mike,

      Get Ted or John to see if they can grind you something with 112 degree centers. I have around 230 degrees on the intake and a 236 exhaust on the red car. But, factory manifolds, and 471's with flattops. So it's about 8.5 or so. I'd think you might get away with 9.5 on the 'Bird. As far as doing it right, gimme a break, when DIDN'T you do it right. Anybody wants to understand that comment, look a Mike's profile and check out his rides. I'd kill for that Roadster!

Frank/Rebop

Bristol, In ( by Elkhart) 

You're probably right in most cases. We've been running it at full load on the dyno, so the heads are hot when we make the the pulls. But even in traffic, a sudden pull ( small hill) will let it rattle a bit, before it gets the revs up. The ignition curve (two of them, based on manifold pressure) is in the computer, so it may be the crossover point is wrong. The blower was an old Paxton, running at 4 lbs @ 5000 rpm, so it was a bit low, but the static was 9:1. It ran well. The 2.97 with the 3.31 rear is part of the problem with the new set up, as you put a lot more load against the cylinder fill than it did before ( the Paxton at that ratio made no boost till 3000 rpm). But I'll sort it out. But I think the lobe center/overlap/cylinder fill is still part of the answer. And it's not a racer, it's a driver for varied conditions and temperatures. Pulling the motor to change the cam is not something I need more practice at. I'd like to get it right.



Getting a bit off my original post, sorry for the diversion.

miker
55 bird, 32 cabrio F code
Kent, WA
Tucson, AZ

You should be able to get it to work fine. Mine's a copy of some blower grind with 112 lobe separation and I'm currently running naturally aspirated. Our compression probably works out to be nearly the same when the requirements for an iron headed vs aluminum headed engine are considered. I have mine set for 20 crank degrees by 3000rpm with 14 initial.


Lawrenceville, GA

Actually, Frank, that's what got me to thinking about cams again. I took the blower off the bird and put a set of John's heads on, added the FPA headers, and ran a 2.25 exhaust. And a proformance throttle body, hooked to an Acell computer. It calc's at 10.2 to 1, and with the 110 lobe centers on the cam, we had to pull a lot of advance out down low to stop detonating. (It's a variant on the 274 cam, widened for the blower.). I don't want to do two cam changes, so I'm trying to remember all this stuff and do something right the first time. For a change.

miker
55 bird, 32 cabrio F code
Kent, WA
Tucson, AZ

Mike, Add to the conversation that with a blown motor you don't want a lot of Overlap. The centers need to be wider for a pressurized motor to allow pressure to build in the cyl without blowing it right back out through the exhaust valve before it closes.   

Frank/Rebop

Bristol, In ( by Elkhart) 

One reason for a cam with the same basic lift, duration and different LSA is some people want a car with the sound they associate with power so they can impress all the guys in the Good Guys parking lot. Somewhat like " if it won`t go, chrome it". Pete

Download one of those freebie degree wheels.Then mark the intake and exhaust events with some tape or a marker.

That will give you a visual idea of the overlap zone.

Oldmics

Well, the above is a lot to think about, but it helped me get my head around it. The relationship between overlap and lobe centers was wrong in my head, and this gets me started. I Never get anything till I re read it, and let it sink in. Thanks, all.

miker
55 bird, 32 cabrio F code
Kent, WA
Tucson, AZ

I've always liked some of this guy's thinking about engines - even if he does prefer to build Chebbies, he gets results and makes sense.

 http://www.popularhotrodding.com/tech/0607phr_camshaft_basics/viewall.html

He makes some really good points about choosing the lobe centers and overlap. 

 
 Steve Metzger       Tucson, Arizona

Ted, you are one of the very few people i,ve met that can explain clearly how things work in easy to understand terms and is great help to the forum. Best regards bill.

PS this is 3rd attempt at posting this .

  AussieBill            YYYY    Forever Y Block     YYYY

 Down Under, Australia