New Motor for my Cobra

[STL79Coupe]

I'm using ET Streets with 26" x 11.5" sizing.

Yeah so that puts your gearing up in the 4.56 range guessing. If you go to a 28" tire you'll drop down to a 4.10 I assume. I don't know the formula to get the correct numbers.

[Zap's 85 GT]

Your right and i just noticed he has a Anderson N-91 camshaft(http://www.andersonfordmotorsport.co...-Camshaft.html)
The powerband is 2900-6900rpm!! So i guess you do need those 4.30's after all!! I should have looked at your combo more closely, your spot on going by Rick Anderson's spec's for the camshaft and supporting hardware. If you had a smaller camshaft that peaked at 5500-6000rpm then taller gears would have helped. Does your 347 pull to 6900rpm?

Is that 6900 rpm's with a 302 or a 347?

[306gt]

I believe that RPM is in a 302. I'm guessing in Pete's 347 it will peak around 6400-6500 rpm. Once I get some driving time in the Cobra I'll see where it runs out of steam. I'm sure Pete will hit up the dyno in the early fall to see where he stands.

[LilRed86coupe]

Is that 6900 rpm's with a 302 or a 347?

Well it clearly states in the link that the cam works best in a 347-408 size engine and a 6900rpm shift point using 4.10-4.30 gears.
But every car and engine combo acts different so he'll have to experiment with different shift points to see what the car likes.

http://www.andersonfordmotorsport.co...-Camshaft.html

[SVT1]

I truly feel that the 4.30's he already has are ideal for his combo.

I'd experiment with tire size at the track before I opted to change gears.

My guess is that he'll see around 6,800 rpm through the traps with the 4.30's and 26" tire, at 118-120 mph.

[Hissing Cobra]

Brian,

I'll meet up with you soon to give you a ride. I think you'll like it.

Even though the cam is good to 6,900, I'll be shifting it at 6,500 like my old motor as I would rather have longevity than faster time slips. My brother explained to me that if I shift this thing a 7,000 or 7,200, it may not last as long 'cause the crank's inertia will be pounding those main caps at the higher rpms and causing them to flex and move around. This will significantly reduce the longevity of everything inside. Since longevity is what I'm after, I have no problems with shifting it at 6,500 and losing a little bit of speed/et. Running down the track a 100 times at 118-120 is much better than running down the track 25 times at 122 before it blows up. It's all speculative at this point but if I can run what my brother's running, I'll be a happy camper. If it's faster, it's a bonus!

[varn82]

Only problem with the 4.30's and a 26" tire is possibly running out of rpm. I have 4.10's and with the 26" ET streets, the car trapped at 127, but I rode the 6800 rpm pill out the last few hundred feet or so. Just swapped to 28's, so I'm hoping for a better short time as well as mph.

[cjmuscle]

Pete if you wake up one morning and go to your garage and notice your Cobra gone and a 82 GT in its place....it wasn't me!

[306gt]

At this point with Pete's gears and tire combo it is like a double edged sword. My car peaks at 6400 rpm. I run it out through the traps at 6700 rpm in 4th gear with the same 26" tires and the same 4.33 gears. I am trapping 118-120 mph. I am using all of my power running down the 1/4 mile. I feel Pete will be around the same depending on where he makes peak power. I am guessing right now his will peak around 6400-6500 rpm. If it peaks lower the tires and gears will be fine. If it peaks higher like 6600-6700 rpm they will still be fine. If he goes to a 28" tire the traction will probably improve but it might not rpm at the other end of the track. It may or may not help at this point.The only way to find out is to try it. It is all speculation but we will figure it out. The chassis dyno will be a great tool for us to play with.

[Zap's 85 GT]

I wish I had a 1/4 mile track to play on. My car gets to the 1/8 mile at the top of 3rd w 3.73's w 26' tires. It's right where I should be shifting to 4th but if I do, I lose about a tenth.

[Hissing Cobra]

Pete if you wake up one morning and go to your garage and notice your Cobra gone and a 82 GT in its place....it wasn't me!

This one got me smiling a lot. LOL! I'd know who it was in a heartbeat. However, that might not be a bad trade off 'cause your car looks badass, sounds badass, and I'm sure it goes badass too. If I had lots of money it would definitely be on my list of cars to buy.

[PaceFever79]

If it were mine, I wouldn't rev it past 6000 except on special occasion (like track day)

It will probably take more, but why stress a motor that you want to last.

[Hissing Cobra]

I hear ya there but I used to shift my 306 at 6,500 rpms regularly and never had a problem. I think the 6,500 shifts are fine. It's the 7,000+ shifts that would concern me. At that rpm EVERYTHING is flexing/stretching/changing shape, etc...

[PaceFever79]

I hear ya there but I used to shift my 306 at 6,500 rpms regularly and never had a problem. I think the 6,500 shifts are fine. It's the 7,000+ shifts that would concern me. At that rpm EVERYTHING is flexing/stretching/changing shape, etc...

This is true, it will probably take it, and many people do it. But a 331 stroke exerts more rotational force than a 302 or a 289 crank. I used to wing my 289 to 7000 with 1970s technology. I have never blew the bottom end out of some pretty stout NA small blocks. But if I wanted my stroker to last in my street car I would keep the recreational revs to 6000 and the track revs to 6500. I know it's a bit conservative, but you won't win any prize for wringing out those few extra HP, but it might cost you your motor.

[Hissing Cobra]

This is true, it will probably take it, and many people do it. But a 331 stroke exerts more rotational force than a 302 or a 289 crank. I used to wing my 289 to 7000 with 1970s technology. I have never blew the bottom end out of some pretty stout NA small blocks. But if I wanted my stroker to last in my street car I would keep the recreational revs to 6000 and the track revs to 6500. I know it's a bit conservative, but you won't win any prize for wringing out those few extra HP, but it might cost you your motor.

Very true. On the street I'm rarely up in that rpm range unless I want to play around. At the track, the 6,500 shifts are good!

[SVT1]

Paul, and others, I hear what you're saying regarding where the motor makes peak HP in the RPM band. This is a fun point to argue, and has been the topic of debate of bench racers online since the inception of car forums. Lol.

Having said that, in the real world of exhaust fumes, burning rubber smell, and the art of the powershift, all this theory means little in the real world of actual drag racing. The only issue I see is will Pete have enough RPM to make it through the traps in high gear, which I resoundingly say he will, with the 4.33 final drive and 26" tires. Of course it's speculation right now, but I say he'll be fine.

His cam is a high revving cam and the 4.33's will help pull the car efficiently to 7000 rpm (even if he stays at his self imposed 6800 rpm limit). If the motor produced more torque than HP (it could but I doubt it), then gearing down to a 3.73 or 3.90 would make sense as you'd want to spend more time in the meat of the torque band especially on recovery after shifts, but it is a peaky motor and wants to scream. If it runs out at 7k through the traps and makes peak HP at 6,500 rpm, it will not be advantageous to re-gear or re-tire it to a 28" tire as you will be relying more on the total torque output of the motor versus the HP, and through the traps you want the highest engine speed the motor is capable of safely turning since HP is a result of rpm. HP peak be damned!

When I had my Boss 302 I did a ton of research on this issue as well as real world testing and concluded that yes, RPM'S are our friend. That motor revved to 8300 rpm and trapped right around 7200 rpm in high gear at 116-118 mph.

On the other hand, my GT500 with 4.10's and 635 lb ft at the tire was slower than when I geared it down to 3.73, although trap speed and ET remained the same, DA affected the test results negatively, but the car was clearly stronger throughout the rev range with the 3.73 gear, since it made 635 lb ft from 2k rpm through 6500 rpm.

High revving NA motors love the gear is my long winded point.

And yes Pete, I am dying for that ride!

[cjmuscle]

Ya all this mumbo jumbo about gears,tires and rpms. All I care about is how it sounds and looks going sidewards down the street in all four gears! lol Now thats what gets me smiling!


I cant't wait for that ride!!!!!!

[Hissing Cobra]

I just hope that I can be in the mid 11's. Who knows, I could blow it up on the first pass and never even make it down the track. I certainly hope not! Like I said, if I can run what my brother's running and be reliable, that's the end goal.

[varn82]

Absolutely Pete, reliability is the number 1 issues for real world cars like ours. Fun, reliable, fast, ect. I have no doubt you will have the reliability factor, fun, hell yeah. fast also no doubt. That bottom end will handle some serious abuse. I run the same. With close to 18k miles on it, and dozens of bottles run through it. You have cruised with me, so you know it gets hammered. Just enjoy it. I know you will.

On a side note, If you do run it, and run out of rpm, dont buy tires. You can borrow mine for a trip to the track. 28" M/T drag radials, already mounted on wheels. They will dry rot before I use them up, and they are only 3 onths old. Id rather see them hammered on, and save you 500 bucks, if it turns out they are not the best size for you to run.

[Hissing Cobra]

Thanks Chris, I really appreciate that. Any plans to hit up Tasca Day?

[varn82]

Not a bad idea. I was hoping to have a trailer by then, want to get a 16 foot open, to tow with the Lightning. Things just have not come together like I wanted them to. Just got the valves re-adjusted after the new cam was installed, and ran about 1000 miles, all even, about 1/8 to 1/4 turn out of adjustment, so I'm happy. I just need to get a good clutch in it before the track. And the small pile I have got on the back seat to install.

[Hissing Cobra]

I think I'll be going. As long as it goes straight and doesn't break, that's the most important thing.

[SVT1]

Not a bad idea. I was hoping to have a trailer by then, want to get a 16 foot open, to tow with the Lightning. Things just have not come together like I wanted them to. Just got the valves re-adjusted after the new cam was installed, and ran about 1000 miles, all even, about 1/8 to 1/4 turn out of adjustment, so I'm happy. I just need to get a good clutch in it before the track. And the small pile I have got on the back seat to install.

What "small pile"? Blower?

[varn82]

HA! In due time for the Lightning. Want 8-10 lbs on that, intercooled. The small pile I have ammased is actually that. New gauges and solid motor mounts, and an oil pan.