Bending Rods and Walking Caps

[F-BIRD'88]

Yup, its your launch method that is spiking the connecting rods. The purge method does not help either.
Never hit the nitrous at part throttle. Ever.

[GARY C]

Yup, its your launch method that is spiking the connecting rods. The purge method does not help either.
Never hit the nitrous at part throttle. Ever.

Did it for 10 years and never so much as a scuffed bearing. Sometimes it's not what you do but how you do it that makes a difference.

[GARY C]

This is an inquiry about the 306 in my 1966 mustang that recently let go.
The engine consists of a half filled Mexican block (basically an early model block with full width main caps) Boss 302 steel crank, Boss 302 rods, ARP main studs and ARP black wavelock rod bolts. Engine also has a 200 hp nitrous plate.

Engine has 11:1 compression, 242/254 solid flat tappet, and is shifted at 6800 rpm.

Anyway, I tore it down several years ago for a freshen-up. The #5 rod was bent... (front driver side for you non-Ford guys) so it was replaced with another one, and put back into service.

Car is 3200 lbs w/driver, Toploader 4-Speed and 4.33 gear with 28x9 tires.

After about 12 or so total years of service, and 7-8 since the last freshen, a rod bolt broke and took the engine out. It was #4 (rear passenger side). Upon complete disassembly, I noticed TWO MORE bent connecting rods. One of the bent ones was #5... the same one that was bent on the first freshen, and the other was #3... third one back on the passenger side. There was also evidence of cap walk. I use C12 spiked with a little MMT. Total timing is 25 degrees on the nitrous with Twisted Wedge heads I ported.

Car is set up for 1/4 mile nitrous runs, and has done a best of 6.26 @ 109 in the 1/8, and 9.87 @ 135 in the 1/4. (probably with the bent rods. lol)

Question is, am I doing something wrong... or will the rods just not take that much power? I'm realistically guessing somewhere between 650-700 horsepower, based on the times. It still ran good ET's, but I think it should have shown some more mph in the 1/4. (could be the bent rods)

Anyway, here are the pics.

Thanks Guys!

I think you just found the wilting point of your rod beams...

[n2omike]

Yup, its your launch method that is spiking the connecting rods. The purge method does not help either.
Never hit the nitrous at part throttle. Ever.

Nitrous is activated by a throttle switch... so it does not come on until 90% throttle.
To launch, you need to lead with the clutch a little, as you don't want the nitrous to activate before the clutch is out. It takes technique, as even if It's at 5k rpm, the engine can drag down a tad before the nitrous comes on. It doesn't come down much, as it's really quick, but too low of an activation is possible.

In case activating the nitrous at too low of an engine speed is the issue, I'll likely install a window rpm switch so it won't come on until 4k rpm or so. I remember these being sold for the MSD 6AL back in the 90's... so hopefully they are still available. lol

Driving the car is an absolute blast! Launching on the bottle with a stock Toploader 4-speed on 28x9 tires is pretty intense. Car is a 3200 lb w/driver street car that goes to the track a 2-4 times per year. Best 60 ft with me in the car is 1.33 seconds. With my 160 pound son in the passenger seat, it has gone 1.31 and 1.32... so go figure. lol When it launches, sometimes it will stand straight in the air (have the damaged oil pan to prove it), it might carry the wheels most of the way through 1st gear 1-2 feet off the ground (most fun) or it will just skim them off the track a couple inches... so you've got to be ready and on your toes. Things happen FAST!

Thanks again for all your guys insight!

[Steve.k]

650-700 hp on stock c3ae rods. Hmm I’d say your lucky to have any parts left. The shock load on that stock rod is way over what it’s designed for. Thats the nice thing about factory rods. They bend instead of grenade.

[cjperformance]

Yup, its your launch method that is spiking the connecting rods. The purge method does not help either.
Never hit the nitrous at part throttle. Ever.

Did it for 10 years and never so much as a scuffed bearing. Sometimes it's not what you do but how you do it that makes a difference.

Absolutely 2nd this. I always ran thru the engine purge on sub 250 kits, never any problem.

And yes, just stronger parts needed, overpowered!

[Steve.k]

By the way. Excellent times for sure!!

[GARY C]

Yup, its your launch method that is spiking the connecting rods. The purge method does not help either.
Never hit the nitrous at part throttle. Ever.

Did it for 10 years and never so much as a scuffed bearing. Sometimes it's not what you do but how you do it that makes a difference.

Absolutely 2nd this. I always ran thru the engine purge on sub 250 kits, never any problem.

And yes, just stronger parts needed, overpowered!

I would usually go 2 stage so I could set up the system so when I purged the small primary stage all was good but yes a quick purge at 3000 does not increase rpm much more than a blip of the throttle.

In my budget 70 Nova with an SBC 306 I ran a primary 150 shot with the secondary 150 coming it 4 tenths out with the $50.00 delay timer I posted above. here is the time slip.

[Steve.k]

Gary stock rods??

[GARY C]

Gary stock rods??

No, stock crank, block and main bolts... it had old Micky Thomson aluminum rods and a burnt TRW piston when i bought it from a dirt tracker so I replaced them with the cheapest manlys I could find and a Speed Pro piston so I wouldn't have to rebalance (close enough), just did a garage hone and rings.

It was pretty lame straight motor but seemed to like nitrous, here is a comparison of times from my 377 vs 306 NA vs 150 shot and a pic of the old rods.

We did run 150/175 shots on stock rods with no problems, a cpl of friends ran 144 and 671 blowers with 75 shots on stock bottom ends.

[Steve.k]

Ahh yes. Thank god