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RPM limit for my engine

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Canadian_CD9A

Supporting Member
1,077
833
Feb 10, 2012
Winnipeg, MB_Canada
My car is an autocrosser, basically a stock 2g motor with Tomei 260 cams (10.7/10.2 lift), Manley single valve springs and titanium retainers, ARP head studs, Cometic HG, all the power bolt ons. Since my gearing is short, I need as much speed out of 2nd gear as possible, and RPM is the only way to do it. I know my HTA68 is out of breath past 7000 RPM and it won't make any power, but is 8500-9000 RPM safe? I won't be shifting it at that RPM, it's just to have as much speed out of 2nd gear, then braking. Thanks guys.
 
Well I think usually the issue is how long you keep it at 9000RPM. You want the power there to get you out of that rev range as quick as possible. Seems pointless to go that high with no power and can imagine would hurt any autocross time as well as a stock bottom end.
 
It would be above 8000 RPM for maybe 1 or 2 seconds. There's no power there I know, but upshifting to 3rd and downshifting to 2nd hurts times substantially more than revving out 2nd with less power, plus the car unsettles when shifting gears, which is bad for setting up turns and braking. I've been using 3rd with my stock rev limit (7500 RPM) and it's hurting my times. Even 8500 would be great, but I'm not about to nuke a motor over it.
 
I don't know if it's possible for you but maybe you can put bigger tires on it so you could go faster at the same rpm. That way you won't have to rev your engine that high. It's just an idea.
 
Bigger tires won't work, I've already hammered my inner fenders in. Is 8500 RPM really out of the question for a motor with aftermarket valve springs and retainers?
 
It would be above 8000 RPM for maybe 1 or 2 seconds. There's no power there I know, but upshifting to 3rd and downshifting to 2nd hurts times substantially more than revving out 2nd with less power, plus the car unsettles when shifting gears, which is bad for setting up turns and braking. I've been using 3rd with my stock rev limit (7500 RPM) and it's hurting my times. Even 8500 would be great, but I'm not about to nuke a motor over it.

Well there's obviously an issue with the car then. There shouldn't be any unsettling in between gears. I think once in a while you should be OK but constantly doing it will eventually, how you said it, nuke your motor. NLTS (no lift to shift) I hear helps a bit in these situations but I'm not sure DSMlink has that feature.
 
Yes Link V3 has NLTS, at least for my 1G it is available.
 
Its not the valve train that is in question. It is the thin 7 bolt rods.

You may not be over 8k but a second or 2, but that abuse adds up and will take its toll on the rods till one pulls apart.

8500rpm x 5.905 inch =50192.5 feet a min /4= 12548.125 feet/min /60 = 209.135 feet/sec per rod

That is a bunch of stress on a rod, to me, not a question of if a rod stretches and snaps, but when

7500 x 5.905= 44287.5 /4 = 11071.875 /60= 184.53
 
I wouldn't recommend it if you've got balance shafts, spinning those up to 17,000rpm who knows what might let go. My build is very similar to yours and I limit my rpm to 8k and set my NLTS to 7k.

No BS's I'd say you're okay to 8.5k but you're running it to the edge so, is it worth it?


How's that 20ft/sec per rod make a difference to someone like me, Bogus? That's good info.
 
Average rod weight of a 7 bolt rod is 620-630g and on a 6 bolt it is 690-700g

So that is a fair amount more metal in the 6 bolt rods, and can take the stresses of the "blast off, stop, change directions" than the lighter thinner beam 7 bolt rods.

Aftermarket rods are designed with these stresses in mind for higher RPM.

@gofer, The same math applies to you.

It comes down to metal fatigue, take any rod out if its engineered range and it will fail.
 
NLS is out of the question, I'm not throwing my transmission under the bus for this.
Being at WOT at high RPM in 2nd, lifting and shifting to 3rd, then going WOT again will shift weight from the back to the front and then to the back again. If you lift mid-corner after being WOT, there's a huge chance of spinning the car from weight transfer if you're not ready for it. There's nothing wrong with the car, the suspension is set up quite well for rotation under throttle, but that's just the effect of torque. Same goes for downshifting - try shifting down twice for a 1st gear hairpin, while braking hard and the wheel isn't straight. It loses a lot of time, with a big possibility of spinning.

I appreciate the math behind piston speed, but I'm more looking for real world results - such as "I tossed a 7-bolt rod at 8000 RPM" or "I've bagged on my 7-bolt at 8500 for years". My engine has maybe 30,000 miles on it, and I'm quite confident in its condition, for what it's worth.
 
stress reversal takes the fatigue limit of the material way down. Basically 50% of the yeild strength.

If you really want to know, go out to the interwebs and find the equation for piston acceleration interms of crank angle.
Solve it for peak acceleration. Take the weight of the piston, pin, and half the rod. Now you have the force pulling on it.

The area of max stress in the rod is in the middle of the beam, and the bolts. Now figure the cross sectional area of the two.

We don't know exactly what materials the rod and bolts are, but we can make an educated guess that the rods are a decent steel for for 100,000psi yield strength. We will assume that the bolts are at least a grade 10.9 type and good for 150,000psi+. (Pease note that the rod bolt calculations are more complicated, but this is simplified for your convienence) since we are reverse loading these joints the allowable stress is half of yeild strength. From the allowable stress and the cross sectional area you can calculate the maximum force that can be applied to the rod for infinite life.

I can do these calculations for you, but it is not going to be free.
 
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So would polishing the beam of the rod to get rid of the forging flash and rib, it would also change the surface gain of the metal from across it to length wise.

Makes the rod lighter and stronger.

Add shot peening, stronger still.

But by the time you do all that, you are past the cost of a set of H beam rods that are still stronger.
 
I'm basically just asking who's done it and what happened..it's a common platform so I can't be the first one to try it. I realize it's past what the OEM intended, and I'm not a special case. If enough people come forward saying they blew theirs up at 8000-8500, I won't try it. If enough people say they've done it with no ill effects, I might try it.
 
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So would polishing the beam of the rod to get rid of the forging flash and rib, it would also change the surface gain of the metal from across it to length wise.

Makes the rod lighter and stronger.

Add shot peening, stronger still.

But by the time you do all that, you are past the cost of a set of H beam rods that are still stronger.


Thats's if the beam is the weak point of the rod, but I don't think it is in the case of a 2g. At least for this case, if he starts making 500ft*lbs the rod is more likely to fail in a compressive failure of the beam. It should also be known that polishing the beams and shot peening only improve the fatigue strength. It does not improve the maximum load it can cary, it can just carry a load longer before failure.
 
I'm basically just asking who's done it and what happened..it's a common platform so I can't be the first one to try it. I realize it's past what the OEM intended, and I'm not a special case. If enough people come forward saying they blew theirs up at 8000-8500, I won't try it. If enough people say they've done it with no ill effects, I might try it.
I think there are some posts by english on this subject. 8500 is into the twilight zone of stock rod bolt failure, 1g or 2g.
 
option #1 If you upgrade the rods and pistons you will be able to more safely turn 9k rpms, I would have to agree with bogus here, I have personally seen bent stock oem 7 bolt rods more often then bent oem 6 bolt rods, the 7 bolt rods are way more weaker then the 6 bolt rods.

option #2 Get a 2.4 block and build a 2.1 11k rpm de-stroker motor on the side, and keep having fun with the 7 bolt 2.0 on track days and or for daily driving.

option #3 Get a custom transmission with gearing that better suits your auto racing course application.

it sounds like your having a good time with the car already, I can see where everyone else is going with this post tho, nobody here wants to see another destroyed 7 bolt motor, which is why they all are concerned about the general reliability of the motor... and I would have to agree with the consensus here, at those high rpm levels the added weight that the pistons will transmit to the rods is going to be more then the rods are designed to handle, think of a tooth pick supporting a brick now add extra gravity and heat to the mix and you now get a bigger picture of what they are all afraid of.

You did a good job with the upgraded springs retainers, and the cylinder head work, but the rods are going to be the next weak link in the chain, and worse yet they could ruin all the hard work you already put into the cylinder head, I guess that will be among the biggest sacrifices once the rods give up the ghost.
 
If you want real world results...

My HTA68 @ 30psi makes power all the way 8k, maybe more if I let it, so I bang it off of 8k when I race straight line and I've got a stock 7-bolt bottom end WITH balance shafts and 115'ish thousand miles on it. I've been doing it since 2010 and I think my car had ~95k miles on it at the time, so 20k miles and 5yrs of 30psi going to 8k.

Again though, I don't autocross so the stress my motor see's last less than 12 seconds... I do have a built 7-bolt 2.3l sitting on an engine stand just in case and my DSM isn't a daily so I don't NEED it to be running, which is why I run it like I do.
 
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