Question about Cam LSA

itsell

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I have a 94GT that I’m putting together parts for a rebuild with GT40P heads with 1.72 SVO rockers, and cobra GT40 intake. I’m doing a lot of research into cams for my combo. I’d like it to run at least without a tune initially and I know people say stick to Lobe separation angles 114 and above generally. Since I already have 1.72 rockers I was looking specifically at cams designed for 1.7’s. The Comp XE264hr or XE270hr fit the bill, but the versions of those cams when designed for a 1.7 rocker drop down from a 114 to a 112 LSA. The LSA is litally the only change comp makes to them when designed for 1.6(114LSA) or 1.7(112LSA) My question is why is this? And will that affect the ability of the sn95 computer to run it? maybe someone with more valvetrain knowledge can explain why you need less LSA with a higher ratio rocker?
 

96blak54

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I like cams!

What other mods will be associated with this cam swap? Intake swap, headers, valves springs?
 

itsell

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BBK cold air, 24lb tuned Pr0-M, 70MM TB, 24lb injectors, 190lph pump, 94-95 Cobra gt40 intake manifold(unported), gt40P heads(unported), 1.72 SVO pedestal rockers, Ford performance 15/8 gt40P specific shorty headers, BBK off-road H, Flowmaster American thunder.
the gt40p heads will be getting valve springs yes, but those will be determined by what cam I end up choosing. I'm probably going to do a drop-in spring kit without machining the gt40p valve seats, so that and the pedestal rockers put a limit on spring pressure I can run.
it's a bit annoying there's not a lot of cam options for 1.72 rockers, a lot of the popular grinds end up being too high lift for my combo. I might end up having to sell the 1.72's and get 1.6's.
 

95opal

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94-95 ecm friendly cams
Steeda. #19
Crane 2031
Ford F cam
 

itsell

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The Steeda #19 is one I’m strongly considering for sure. With 1.72 rockers it will be at .510/.510 lift. A nice place to be I think with the spring pressure I’ll likely use to keep my pedestal rockers reliable. The duration on that cam is high though 280/286 advertised, 220/[email protected]. I do wonder how my piston to valve clearance will be as I’m using stock pistons . Also I wonder what effect 1.72 rockers will have on how that cam behaves, quite a few people say it’s a little soft below 3k rpm when used naturally aspirated. It was also designed for use with 1.6 rockers I believe, but I’m unsure what makes a cam designed for one rocker ratio to begin with. That was kind of my initial question.

The Crower 15511 is also high on my list 278/282 advertised, 218/[email protected], .503/.522 with 1.72 114LSA
Seems similar to the Steeda #19, but perhaps less soft down low.
 
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96blak54

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Think of the rockers more of a tool than as a performance part. Yeah sure, they are performance parts snd can be used as performance parts, but the question needs to be asked "how do you want your valves to respond at high rpm"?.

In a perfect world, the valves would react in perfect sync with the cam lobe at all rpm, but thats just not the case. When in the upper rpms, valve float happens. Meaning after being smacked by the cam lobe, the weight of the valve plus all associated parts that are directly controlled by the valve spring go into a suspension state untill the spring can react. Once the spring comes into play and begins contolling associated parts, shutting the valve and keeping it shut becomes another issue due to spring reverberation. This is when the valve bounces off the seat. All of this gets amplified with the use of higher ratio rockers. Upgraded springs reduce this alot, but still happens to a certain degree. Keep all this in mind.

So now, .. not only does the higher ratio rocker cause the valves stay open longer than desired. It also opens the valves sooner than desired. In short, higher ratio rockers amplify the camshaft pattern. This is where we use the ratio of the rockers as tools.

My suggestion, do the cam swap 1st along with spring upgrade, but hold off on the rocker swap regardless of cam choice....use the stock rockers. Get a feel for the car. Nine times out of 10, keeping the cam upgrade/rocker ratio moderate, becomes the better performer. I know your thinking that more valve lift means more air in and you are right, but valve open and closing events far out weigh a slight improvement in air flow.

Now if you were to build an high compression 10000 rpm single plane intake long rod 302 for the quarter mile, cam choice and rocker section would be different and more close related to the scenario of parts you have your eyes on.
 

tinnocker

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I am not an expert but have read a lot about what you are about to do. Some cams will work ok with the 1.72 rockers and some will give too much lift on the valves and they will hit your stock bottom end pistons so you may need to go to 1.6 rockers. The cam I bought for my stock bottom end 94 gt was the Comp Cam XE-266-HR and I am using Scorpian 1.6 rockers. I will be having it installed next week. This is a good youtube video of a ride along with this cam, how it handles day to day driving.
Also, what do you want out of your Mustang? Lots of strip times or mostly ever day driver? This video will get into that a little bit. My build is almost exactly like yours, but I went with AFR 165 heads and a ported GT 40 intake. I waited 4 months on ported GT40 heads but gave up and bite the bullet on the AFRs.
 

itsell

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Hey, I did check out that video. That's a sweet Fox, makes me miss my old 89 LX.

My 94 is street only. I want it to idle fine, no stalling or surging, and have power down low and midrange but rev out to 5500+ better than the stock setup did. I was going to get the AFR165's too, but these GT40P's came up at a price I couldn't say no to and they'll do the job.

I'm currently thinking I have a few options
1. Stock cam and 1.72's. Not a bad option, should run perfectly and give a bit more lift than the GT cam came with on the friendly 115 stock LSA. Ford did this with the 94-95 cobra, though maybe the Cobra cam was slightly different than the GT's
2. sell the 1.72's and go 1.6 with any of the good cams designed for a mild build (TFS1, XE264HR-14)
3. use the 1.72's with a cam designed for 1.7...close enough. (XE264HR-12, Anderson B-21) I know I'll have to check PTV clearance here

4. I know the 4th option is custom. The FTI Gt-40 cam packages sound appealing, but man they show a 108LSA, defiantly necessary to have a tune for that to work.

...there's another thing. there may be a supercharger in the future(that Vortech heritage gear blower sound...mmmmm). For that, I'll want the wider LSA and a blower-friendly grind. that's where the Steeda #19, Anderson B-21, Crower 15511 would be good I think.

so many options it's mind bottling.
 

95opal

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To be honest i would just sell the 1.7 rockers and pick up some 1.6s
 

Chip66

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I have a 94GT that I’m putting together parts for a rebuild with GT40P heads with 1.72 SVO rockers, and cobra GT40 intake. I’m doing a lot of research into cams for my combo. I’d like it to run at least without a tune initially and I know people say stick to Lobe separation angles 114 and above generally. Since I already have 1.72 rockers I was looking specifically at cams designed for 1.7’s. The Comp XE264hr or XE270hr fit the bill, but the versions of those cams when designed for a 1.7 rocker drop down from a 114 to a 112 LSA. The LSA is litally the only change comp makes to them when designed for 1.6(114LSA) or 1.7(112LSA) My question is why is this? And will that affect the ability of the sn95 computer to run it? maybe someone with more valvetrain knowledge can explain why you need less LSA with a higher ratio rocker?

The best lobe separation angle is a product of displacement, compression ratio, and intake valve size. The best LSA has nothing to do with duration or lift. If you go too big on LSA you are wasting power across the board. As compression ratio goes up on an engine so should LSA. Add boost, LSA should go up.

I higher ratio rocker changes the valve event timing slightly. You don't need to change the LSA, you may need to change the cam timing to compensate. Odds are high that straight up on any cam out of the box is not the best timing for the cam.

I haven't seen anyone explain this better than David Vizard. He's tested 1000s of cams on the dyno.


For a small block ford at 10.5 CR the formula would be:
LSA = 127-(CID/# cyl/V dia x 0.91)

If you want the best cam spec for your engine, get David's Torque Master program.

http://davidvizardperformanceseminars.com/torque-master-software.php
 
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