I've got a 91 305 TPI automatic (originally peanut cammed) and a few months ago I put in a different cam. It was SLP .480 .487 lift, 206 212 duration. After the cam install I went to the track, and the car ran slower! My 60 ft time stayed the same, but it was downhill from there. I was shocked because I thougth my car was pretty well matched for this cam with a 2500 stall, 3.73 gears, headers, exhaust, and lots of other minor goodies. I've had several suggestions as to why this happened, the main suggestion being that I need to match the springs. I'm planning on getting new springs, I just haven't got around to it yet. But I've had a few people tell me that as long as I'm not floating the valves, that my springs are not the problem. I disagree with them, but I don't really know why. Can someone please explain to my why I need new springs when I'm not floating the valves? Also, does anyone have any other suggestions as to why the car went slower? Possibly the stock PROM? From everything I've read and heard, I expected this cam to make a world of difference, instead I lost a tenth and 1 mph.

 

Well, first off you could be coil binding w/ stock springs, which would will give no audible noise, but will make it run slower. After a little time, if this is the case, you could break one, possibly dropping a valve, and killing the whole motor. Secondly, Higher lift, higher duration cams need the higher spring pressure to work properly, esspecially critical on the closing side of the cam. The fact that the cam cannot close as quickly as it needs to, actually is the same as high rpm floating in terms of how it effects performance. On the circle/dirt track engines I have over the years, I've found that a #130 spring can be pulling #80-#90 in a very short time, due to the constant high rpm. Street/strip engines don't need the constant maintenance and changing that circle track engines do, but you DO need good springs. Your springs being stock and used, are not even close to having the pressure they did when they were new, and even then, they didn't have enough for the cam you are running now. You can put a set of good #100-#110 springs on a bone stock 305, 350 or whatever that has some miles on it, and you will see a dramatic difference from this alone. Hope this helps.

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82 Z28 350, Ported #882 Heads, Performer RPM cam and intake, hedman headers,650 Demon carb,
Trans: Turbo 350 w/ 4000 stall -- Rearend 7.5 w/ Richmond 4.10's, Auburn Minispool
Future plans: Dart headed, Roller cammed 383 in early '01


89 RS, L03 305, Hypertech Chip,cat delete, Dynomax exhaust,K&N open element Filter,160 stat, MSD coil --Trans:700R4 Corvette Servo -- Rearend: 7.5 GM 3.42 w/ posi-lock
http://www.bassettracing.cz28.com

 

Your stock springs probably don't handle that cam very well. I ALWAYS use the springs recommened by the cam manufacturer.

I really doubt you are binding the springs becuase that would result in damaged valvetrain parts in very short order.

You MIGHT be floating the valves, however. WHen you hit valve float it will feel like the motor is hitting an "RPM Wall" where it just won't rev any higher, even though the motor sounds like it working its guts out.

I think its more likely that your speed-density 91 TPI system isn't liking the cam with the stock programming. A fresh set of O2 sensors and a lot fo driving around time might give the computer some time to "learn" the new breathing characteristics of your engine, but I would think a chip would be the way to go.

After you check the simple stuff, that is. Did you retard the timing when you had the distributor out? Do you have a vacuum leak screwing everything up? Did you leave something somewhere disconnected or otherwise incorrectly installed?

 

I'd have to agree with Damon. The valve springs won't generally affect the power output of the engine especially at low RPM (less than 5000 or so), until they reach a point that something is being destroyed. However, even though they're not slowing you down, your stock springs aren't going to last long with that much lift; you're really taking a risk by leaving them in there. When the springs break, the valves fall into the cylinder, and let's just say that valve fragments are essentially uncompressible. A motor that has dropped a valve is not a pretty sight, and usually means you'll need to spend about 30 times what a set of valve springs would have cost before they broke. Comp 981s would be a good idea. Get the Teflon valve guide seals while you're at it.

I'd suggest looking into a custom chip. You've done a whole lot of things that change the engine's flow characteristics, which is basically what's burned into the chip. You're right, your combo should be good, it just needs some tuning.

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"So many Mustangs, so little time..."

 

Weak valve springs WILL affect performance throughout the entire rpm range. Now, I agree that it does have more effect on the top side, but it does effect the whole range. Watch a dyno pull with weak springs and you will see what I am talking about. This may not be your only problem, but regardless of what it is, your springs do need replacing and that is a good place to start.

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82 Z28 350, Ported #882 Heads, Performer RPM cam and intake, hedman headers,650 Demon carb,
Trans: Turbo 350 w/ 4000 stall -- Rearend 7.5 w/ Richmond 4.10's, Auburn Minispool
Future plans: Dart headed, Roller cammed 383 in early '01


89 RS, L03 305, Hypertech Chip,cat delete, Dynomax exhaust,K&N open element Filter,160 stat, MSD coil --Trans:700R4 Corvette Servo -- Rearend: 7.5 GM 3.42 w/ posi-lock http://www.bassettracing.cz28.com

[This message has been edited by Basett Racing (edited December 21, 2000).]