Author Topic: Cam Choice  (Read 14363 times)

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Cobra1

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Re: Cam Choice
« Reply #15 on: November 18, 2012, 07:32:40 PM »
Jay,

Thanks for the suggestion! Interestingly thats roughly in the middle of all the cam suggestions I got from the other companies. One question though. The heads I've looked at all seem to flow in the 340-350cfm range (intake). Will that move the RPM higher with your cam suggestion vs a head that flowed about 300cfm? Or will it just result in more power? Not trying to be picky, just trying to drill down and optimize the cam/head combination. Thanks

afret

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Re: Cam Choice
« Reply #16 on: November 18, 2012, 07:36:49 PM »
Afret,

I agree regarding going with a solid roller. Like I said its a weekend car. I want it to be a little hard core. I'd like to run a DCR of at least 8.5. In Barry's FE book, he says Cobra's can get away with a little higher static compression due to there light weight. Say 11:1.

The 255/263 sounds a little big for my RPM range. What do you think??



If you go down one level to 249*/255* (279*/285* advertised) your DCR would be about 8.3 with 112* LSA and 108* ICL.  With 110* LSA and 106* ICL, the DCR would be about 8.45.  These numbers are with 10.5 to 1 static CR.  Just a thought if you already have your pistons.

jayb

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Re: Cam Choice
« Reply #17 on: November 18, 2012, 08:19:54 PM »
Jay,

Thanks for the suggestion! Interestingly thats roughly in the middle of all the cam suggestions I got from the other companies. One question though. The heads I've looked at all seem to flow in the 340-350cfm range (intake). Will that move the RPM higher with your cam suggestion vs a head that flowed about 300cfm? Or will it just result in more power? Not trying to be picky, just trying to drill down and optimize the cam/head combination. Thanks

It depends on what they did to the heads to get them to flow those numbers, and also whether you can believe those numbers.  A lot of times the ports are opened way up  to get the really big flow numbers.  In your case that would be a problem, because the throttle plates on the Weber carbs are fairly small, so as the flow goes from the intake manifold to the head, the cross sectional area of the intake tract would increase.  This is backwards from what you want; you generally want to see kind of a funnel shape for the intake tract, with the widest part at the carb.  So with those big ports and the fairly small throttle bodies on an individual runner intake like the Weber, the velocity of the intake charge will slow down as it goes into the head, and you may lose power. 

As a general rule of thumb an engine that is delivering about 2.1 HP per intake flow cfm is working really well.  If you are buying heads that flow 340 cfm, they are capable of supporting over 700 horsepower.  This is not really a good match for your induction system or your horsepower target.  I'd be looking for heads that deliver around 300 cfm on the intake, which would work well with your induction system and a cam in the neighborhood of what we've been discussing.  You can get that from a well ported set of heads that maintain the stock medium riser port cross section at the port entry, and that would be the best match to your induction system, IMO.

On final thought on intake flow cfm numbers.  You can't always trust the numbers that some engine builders present for their heads.  I bought a set of medium riser heads once from a very well known engine builder who advertised the heads flowed over 360 cfm.  On a calibrated flow bench at my local shop they only flowed 325, despite the fact that the ports were substantially enlarged.  I bought a set of high riser heads from another well known engine builder who claimed on his web site that they flowed 420 cfm, and they only flowed 360 when I tested them.  My advice would be to stick with heads from reputable engine builders like Barry at Survival, or Blair Patrick; there's a lot of BS out there about horsepower numbers and flow numbers from some pretty well known engine builders...
Jay Brown
- 1969 Mach 1, Drag Week 2005 Winner NA/BB, 511" FE (10.60s @ 129); Drag Week 2007 Runner-Up PA/BB, 490" Supercharged FE (9.35 @ 151)
- 1964 Ford Galaxie, Drag Week 2009 Winner Modified NA (9.50s @ 143), 585" SOHC
- 1969 Shelby Clone, Drag Week 2015 Winner Modified NA (Average 8.98 @ 149), 585" SOHC

   

Cobra1

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Re: Cam Choice
« Reply #18 on: November 18, 2012, 08:33:30 PM »
JayB,

Thanks for the head/intake suggestion. I never stopped to think about how an IR would affect the whole combo!

hotrodfeguy

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Re: Cam Choice
« Reply #19 on: November 19, 2012, 12:31:38 AM »
Still like Jays second post,
"If possible, I'd consider going to a higher compression ratio, or going to more cubes."
 I would go with the more cubes, less cam, Juice roller, less RPM 6-6500, and maybe a GV overdrive unit. And bottle feed. But thats why we all build out own cars. If they were all the same it would be boring at the cruise.  ;)

My427stang

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Re: Cam Choice
« Reply #20 on: November 19, 2012, 07:11:44 AM »
I also got bit by famous name heads.  Stage 2's that had a flow sheet for 324 cfm, flowed 280.  Sent back to the guy I bought them from for "personal" work, flow sheet came back 305 cfm  ::)

That time they flowed 295 on the local bench, close enough for bench variance, but still far from what I purchased.  I ended up having them done a 3rd time by a local porter before ever being bolted to a motor

340 is a lot of flow for an Edel head if that is what you are looking at, be sure you get what you are paying for.
« Last Edit: November 19, 2012, 07:14:02 AM by My427stang »
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Ross
Bullock's Power Service, LLC
- 70 Fastback Mustang, 489 cid FE, Victor, SEFI, Erson SFT cam, TKO-600 5 speed, 4.11 9 inch.
- 71 F100 shortbed 4x4, 461 cid FE, headers, Victor Pro-flo EFI, Comp Custom HFT cam, 3.50 9 inch

TorinoBP88

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Example of too tight a LSA w/ IR webber intake
« Reply #21 on: November 20, 2012, 02:33:14 AM »
A local check-book racer built a supposed 289 hipo but he would not divulge the specs or what he paid for the engine.  It should have been a 350+ HP engine.  It sounded good... But I noticed staining on the underside of the hood.

When that thing ran on the chassis dyno it I would generate a revertion fuel fog cloud 12 thick 6 inches over the top of the velocity stacks!!!!!! This would happen probably 500 to 800 rpm below peak torque.  Not good.  This is where you drive on the street. The engine was maybe 320 HP (just barly over a stock hipo 289 and down on torque from where it should be. Webbers like wider cams which is actually nice for a widemore manageable power ( a good thing in only 2500 pounds!!!)

I did not understand what Ross said about smaller engines wanting tighter lobes... Tighter lobes will allow a smaller engine to make up some lost power but by sacrificing wide smooth power and getting more peaky power.  Actually it's my understanding that as displacement (peticularly stroke) goes up the head flow becomes more restrictive to the larger CID.  More overlap and a tighter LSA will alow for more scavanging effect on the carb/intake to maximize HP.  But in a webber cobra not only do you not have any intake manifold volume, but the exhaust systems are terrible and don't scavange well. 

I like jays recommendation, it should be very drivable and have  bodatious acceleration in any gear from 3000 to 3500 rpm, and pull evenly and hard right on to 6500.

I know solid rollers will make you more power because of love profiles, but I like the flat tappet rattle so I would use a "traditional" cam but bump the duration up 2-4 degrees each  if you get my meaning.

My427stang

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Re: Cam Choice
« Reply #22 on: November 20, 2012, 07:25:01 AM »
In my experience, the smaller the displacement, the more it likes a narrow LSA.  That doesn't mean the motor becomes milder by any means, but the increased cylinder fill from overlap helps torque, and although it may not be as flat of a curve, it is more powerful. 

I look at the reversion problem differently.  To me, reversion is more due to intake lobe timing than overlap, although you are 100% correct that individual runners and poor header design will make it tougher to solve.

Matter of fact, with a 300 adv/250 @ .050, 108 LSA and ICL, single plane 427, I was coloring the runners black, and the intake bowls looked like exhaust bowls, by merely rolling the cam forward to 104 ICL, the entire intake port cleaned up.  Certainly LSA didn't change, but how the cylinder pulled on the port did.

Regardless, all of these numbers are pretty close.  I still would like an earlier timed intake lobe with an IR induction, but its all technique, any of these cams will do well.  If the 108 ICL is sloppy down low, all he has to do is pull the timing cover and crank it forward.

With that in mind, if clearance is close, recommend you check intake valve clearance at 104 ICL and 108 ICL, so if you decide to later move it, you know nothing will hit.



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Ross
Bullock's Power Service, LLC
- 70 Fastback Mustang, 489 cid FE, Victor, SEFI, Erson SFT cam, TKO-600 5 speed, 4.11 9 inch.
- 71 F100 shortbed 4x4, 461 cid FE, headers, Victor Pro-flo EFI, Comp Custom HFT cam, 3.50 9 inch

Cobra1

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Re: Cam Choice
« Reply #23 on: November 20, 2012, 07:35:46 AM »
After hearing and reading so much about him I decided to call Blair Patrick to buy the heads from him. We went back and forth discussing the issues with Webers. He made some suggestions and offered up some tricks to help with the reversion issue. Jay, I can tell you, that you were pretty much spot on with the cam selection! Although Blair did feel the head can flow alot better, say 360cfm with a small port that matches up pretty well with the throttle bore. We both agreed its best if I send him the manifold with one Weber to design the port around the carb....

Torino, your spot on about the reversion issue. It can be a real problem if not addressed early. Webers are by no means a bolt on system. They require careful thought and one of the biggest areas is with the cam. Too many guys just bolt them on and think they will work ok. Its things like this that give them grief and the carbs a bad name. They work great if you give some thought to them.