How tight can you spin a 5.0?

[rd]

Watching the faster cars at the local eigth mile, the key to quick times seems to be rpm and gears.

I shift my 5.0, which is a 68 block with 78 crank and rods and forged TRW's, built in 1987, and now with 95k miles, at 6000.

Anyone regularly shift a stock short block setup at 6500 or 7000?

What would you build to go to 7000 regularly? Would going to a 331 or 347 make a difference?

Can I build a 347 and run it to 7k, with spray, regularly?

What is every one else doing, done, or had work, or fail?

 

[seijirou]

if the balance is good and the internals are strong, namely the rods and wristpins and bearings you can spin a 5.0 up to 8k without too much money in building it. A 331 should also be fine, I don't know about the 347 though because the rod ratio gets somewhat bad on these and with higher and higher rpms rod ratio really comes into play on the life of your motor. I've read of several de-stroked 302's down to the mid to upper 290's with long rods going into 12,000 rpm land.

 

[super302]

The rev limiter kicks in at 6250, stock cam shaft quicks making more power at about 5300...no reason to go past 5500

 

[65ShelbyClone]

Can I build a 347 and run it to 7k, with spray, regularly?
QUOTE]

Probably not. At that point eveything will have been upgraded except the block, which leaves it as the last weak link. The extra stress induced by the stoke increase, high rpm, and nitrous will likely split it. Dart block or R302.....yeah, provided all the rest is of good quality.

 

[pisgahhiker]

The rev limiter kicks in at 6250, stock cam shaft quicks making more power at about 5300...no reason to go past 5500

Thats why I threw all that crap away LOL

 

[Mavrick]

He has a 68 302 block.. i heard those are the toughest regular 302's made, with the thicker castings. It should be able to handle a lot more RPM's than an 79-present XXX or YYY block.

 

[dderek]

well. maybe i was the only one that didnt understand what 'how tight can a 302 spin' meant.
but, i would never take a stock internal 302 past 5,500. maaaybe 6k.

i dont see the need, since almost all powerbands are dropping off by then..

but id say if you ever spend the money to buy a new block. might as well put out for the best internals.
but i wouldnt really know how i would like driving a 331 with all forged aluminum internals that could spin 7-8k..
maybe i would like it.. maybe i wouldnt, anybody want to let me try?

 

[seijirou]

Uh, no aluminum internals for a high winding motor, they stretch too much.

I guess you could, but you'd have to allow for the stretchng in the build, the compression ratio would also actually increase as the rpms increasue due to the stretching.

 

[T_5.0_N'works]

I have picked up time in the 1/4 from switching from 6k shifts to 5400 RPM shifts and I have Ecam with stock heads also.

 

[rd]

Thanks guys.

I should have mentioned that this engine has some mods. This particular motor is a stock 68 block, 30 over with stock 78 rods on a 20 under stock 78 crank. It has forged TRW flattop pistons, and quality rings. It has a Rhodes Lifters 270 cam and Rhodes Lifters, Comp Cams springs, and iron GT40 heads, with an Edelbrock performer rpm and a Holley 600 vac sec carb. It was not balanced. Still with an old Duraspark.

It easily pulls hard to to 6000, and seems to give the best times if shifted at 6 using the stock tach. At least a tenth better than shifting a 5500, as I did for the 10 years this was my everyday driver. I have not shot more than the 100 shot so far, but more is coming next track visit.

I am either going to freshen this one and balance it, or build another block that I have. Hopefully I can afford the stroker kit in the spring.

So far it seems either a 331 or a 351 based 393 is the way to go.

Just trying to see what everyone else does. It does seem practical to use stock 302 crank and rod stuff to at least 6500 if you balance it.

I probably won't go the bucks to do the 351 with the new pan, etc, but if I could, would a stroked 351 also go to 6500 or 7000 easily and reliably?

 

[seijirou]

If it's done right yes. Again meanng a good balance and strong rods/wrist pins. 6500 really isn't that high.

 

[rd]

Thanks.

I like the sound of "6500 isn't that high"

 

[BlackFox5.0]

If theres no power up there, there is no point to rev that high.

 

[UMDSmith]

Sure there is, if the power that is left up there exceeds the power lower in the rpm range after the shift.

It is a power curve, draw a straight line across it. Even if you shift at peak HP everytime, you can still gain some time by staying a bit higher into the rpm's because for some charts, the drop off is more gradual on the high rpm side then on the lower. This would be easier to explain with a picture.

Imaginary number time.

Peak hp = 5500rpm
Dropoff of hp to 6k, then steep dropoff thereafter.

If you shift at 5500, and lets say your next gear starts at 4000, if the hp at where that next gear starts after the shift is LESS then the hp if you ride above the 5500 mark, then mathematically, riding out the gear is the optimum condition. Also, once you do shift from the 6k mark to the next gear, your engagement point will be a bit higher on the rpm band, lets say 4400, which keeps you closer to that magic 5500 level.

Feel free to tell me I'm a tool, this is just my little theory.

 

[seijirou]

No, you're right. The serious racers shift at different points for each gear.

 

[TheUser]

Sure there is, if the power that is left up there exceeds the power lower in the rpm range after the shift.

It is a power curve, draw a straight line across it. Even if you shift at peak HP everytime, you can still gain some time by staying a bit higher into the rpm's because for some charts, the drop off is more gradual on the high rpm side then on the lower. This would be easier to explain with a picture.

Wow, that makes perfect sense. I had never thought of that before and although I don't drag race, I'm glad I read that.