"Hot" 3.4 Build - Max safe RPM on stock rods w/9mm ARP Bolts?

[Jonny042]

I'm starting on a build for my autoX car (The "Rot Rod"). Most of the hard points are determined:

The 3.2 will end up a 3.4 with:

98mm re-plated stock 3.2 Cylinders (EDIT - sent to LN engineering)
98mm Pistons 10.5:1 (EDIT - JE 353287's on order)
46mm PMO ITB's
Heads - Stock 3.2 Ports/Valves/Twin plugged
DC43-102 Cams/Aasco ti/springs (EDIT - maybe DC44-106?) (EDIT 2 - DC43-106 on order)
1-5/8 exhaust (41mm SSI or 42mm Eisenmann)

The above specs should support a 7200 (or more) redline with peak power a bit lower than that. Street/track use.

Will the 3.2 rods be OK at those RPMS with ARP 9mm bolts? I'm not opposed to spending the money for better rods but seems like it would be a waste if not required.

Thanks in advance!!!

Jon

[Walt Fricke]

Lots of 3.2s racing with redlines at 7,000. Never heard (and my ear is fairly close to that ground) of rod strength being an issue. I assume all or almost all of them have replaced the stock bolts with the much superior ARP or other aftermarket 200,000 psi bolts.

I think it is when you look to upshifting routinely at 8,000 or more that you would start worrying, and that is pretty much governed by your cam and intake configuration. Stock rods themselves are pretty darn strong. "Better" rods often are better for racing because lighter. When used on the 74.4mm crank, hard to see how a larger diameter (as with all previous 911 rod bolts) could be accommodated.

Optimum upshift is governed by your gearing and torque curve. For straight line upshifting, you want to shift at the point where the rear wheel torque in the old gear equals the rear wheel torque in the higher gear - that maximizes the area under the torque curve, and thus accelleration. For track use, it is handy to have a mechanical limit higher than that, so you can stretch out a gear and avoid an upshift followed almost immediately with a brake zone requiring a downshift before track out for the next segment.

You know your gears (or can pick them), and your engine program probably can produce a torque curve, and you can use that info to see where your shift points are. If adept with spread sheets this is fairly easy, as you can end up with a graph showing rear wheel torque in lbs/ft or whatnot and speed as axes, and create curved graph lines for each gear. Where lines cross, that is the upshift point. You have to interpolate to see what the RPM is at those points. I use a program someone else wrote, and over the years I've corrupted some of its functionality (not good with the graphing function), but it is very useful and avoids rules of thumb like X rpm over max torque or max hp, which are not all that accurate.

[Jonny042]

Thank you, Walt, that's very helpful - and pretty much confirms what I thought to be true, in terms of beefiness and ability to stay together.

You bring up a great point about the ability to over-rev, saving a shift - I autoX the car and it's occasionally nice to be able to carry speed well past peak power to avoid having to shift.

I've installed a 2.00:1 second and a 1.381:1 third gear in the trans. This experiment was a huge success for street and autoX - I want to dig into the trans again to install a billet bearing retainer and side plate and LSD an am thinking of a lower 4th gear as well. 5th will stay stock for cruise.

I have similar ratios in my big-bore MFI 3.2SS Hot Rod (with that same cam) and it's an ideal combination. Ratios of that one are refined slightly, 1.933:1 2nd, 1.381:1 3rd, and 1.08:1 4th.

Thanks again for you reply! Jon

[Jonny042]

Walt, FYI - the ratios I chose for my "Project Heavy Metal" and the tractive force graph. For a street transmission using stock 1st gear it's pretty tough to get ideal ratios for race use, where the lines will intersect. The you can do is choose your compromise. It's still a huge improvement over stock.

Originally Posted by Jonny042

I'm not supposed to lift anything for a while, but I can at least work on transmission theory..... the prototype ratios on the Rot Rod were a success, but I think it's biased a bit towards the low end (which I expected and planned, given its autocross focus) so I'm trying on a few different ratios for size. Second and third a touch wider spaced but still close enough to keep the motor "on the boil":

[safe]

I rev to 6800 and slightly more (soft cut at 6800) sometimes. Totaly stock bottom end inluding rod bolts.

Edit: The above doesen't conclude that it's safe, even if it has been for me the last 10 years. I might have a bolt go loose on the next shift I do.

[Jeff Alton]

We have run the ones we have built to 7000 without issues. Not saying it will mean yours will be OK, just a reference point.

Cheers

[Jonny042]

Thanks, guys!!

I'll stick with the stock rods rebuilt with ARP of course, that will help the budget!

[stownsen914]

I'm sure you know this, but make sure to have your rods checked/resized with the new rod bolts. The higher torque value apparently can cause slightly different bearing clearances.

[Jonny042]

Originally Posted by stownsen914

I'm sure you know this, but make sure to have your rods checked/resized with the new rod bolts. The higher torque value apparently can cause slightly different bearing clearances.

Yes, I plan to have them re-sized and rebushed and balanced before re-using them, thanks!

[Rosco_NZ]

I know of a 3.5 MFI which died after <20 hours. Was a race engine being driven hard but exploded on decel / braking. The reason given was a (9mm ARP) rod bolt let go … I also suspect over rev on downshift .. everything has it’s limits ..

[Jeff Alton]

^^^^ likely not the fault of the bolt... Fault of the over rev....

Cheers

[Jonny042]

Originally Posted by Rosco_NZ

I know of a 3.5 MFI which died after <20 hours. Was a race engine being driven hard but exploded on decel / braking. The reason given was a (9mm ARP) rod bolt let go … I also suspect over rev on downshift .. everything has it’s limits ..

Thanks Rosco,

I can't imagine why anyone building a 3.5 MFI race motor would think even for a second about using stock 3.2 rods, even with ARP bolts...... probably would have been subject to a lot of forces in that application!!! No doubt was an expensive learning experience.

It does give a valuable data point, though - like you say everything has limits! I'll make sure to keep the limiter set realistically.

All the best, Jon

[Jonny042]

Originally Posted by Jeff Alton

We have run the ones we have built to 7000 without issues. Not saying it will mean yours will be OK, just a reference point.

Cheers

Hi Jeff, seeing as how you've got 100 times more experience than me in this regard, what do you think about the engine combo proposed in the original post?

Besides the limiting size of the 41mm SSI's, Biggest question at this point is the cam - it's the same one I put in my 3.2SS MFI (with 41mm throttle bodies and bored plastic stacks). I love that motor and it's character, it's practically a religious experience to drive, and keep coming back to the same cam for this build.

Most people would choose a DC60 for a similar build but for street and AutoX I think the higher lift and lower overlap of the DC43-102 is a better choice. I have yet to touch base with Camgrinder on this but I'm sure he'll steer me in the right direction.

Thanks and all the best, Jon

[MST0118]

I may be in the minority but if you’re not opposed to spending the extra money, maybe consider getting a set of reasonably priced aftermarket rods which can be a lot lighter and you can even consider a longer rod (perhaps 130mm?) since you’ll be going with CP pistons anyways.

Why? If you later decide to go with a more aggressive cam, you’ll have the rods to support it. Also, better rod to stroke ratio. You’ll probably spend another $2k but sounds like the next logical upgrade for your new hot rod engine. Nothing wrong with stock rods but if it were me and had the cash to invest, I would highly consider doing that.

[Ed911S]

Not that there is anything wrong with Porsche rods but, by the time you have used rods resized, small end re-bushed, balanced and buy ARP rod bolts you might be half way to a lighter stronger aftermarket rod and shift at 8000

[Jonny042]

Originally Posted by Ed911S

Not that there is anything wrong with Porsche rods but, by the time you have used rods resized, small end re-bushed, balanced and buy ARP rod bolts you might be half way to a lighter stronger aftermarket rod and shift at 8000

Yes that's occurred to me..... $400.00 for machine work and balance and $250 for ARP bolts and the 150 or so I could sell off my cores for brings me within $1000 of a set of new Carrilo or Pauter machine rods.

Of course (like MST0118 says) that would open the door directly to the slippery slope of getting custom pistons and longer rods..... not a huge step in expense in itself but then.... you think to yourself "ive gone to all that trouble I may as well do (insert expensive nice-to-have here)......

Oh well. Seems to be the way it goes. Better keep it simple with off the shelf parts and save the fancy stuff for the engine project waiting in the wings.

[mikedsilva]

How about a 3.6 crank for some stroke and extra cc?

[Jonny042]

Originally Posted by mikedsilva

How about a 3.6 crank for some stroke and extra cc?

Haha! See...... slippery slope. In fact I have a new in box GT3 crank but I'm saving it for a 3.5L MFI build with a bit more vintage flair than the current project which is really just supposed to be a top end and cams to take advantage of the ITBs....

[Jeff Alton]

Originally Posted by Jonny042

Hi Jeff, seeing as how you've got 100 times more experience than me in this regard, what do you think about the engine combo proposed in the original post?

Besides the limiting size of the 41mm SSI's, Biggest question at this point is the cam - it's the same one I put in my 3.2SS MFI (with 41mm throttle bodies and bored plastic stacks). I love that motor and it's character, it's practically a religious experience to drive, and keep coming back to the same cam for this build.

Most people would choose a DC60 for a similar build but for street and AutoX I think the higher lift and lower overlap of the DC43-102 is a better choice. I have yet to touch base with Camgrinder on this but I'm sure he'll steer me in the right direction.

Thanks and all the best, Jon

Jon,

From and expert as every motor we build we learn. I like the spec sheet you posted. On a 3.4 with 10.5 I do like the DC44-108 as it has a quite wide power band and is still pulling at 7200 rpm. But we have only run those on ITB motors. I see your DC43-102 has the same lift specs, slightly different exhaust duration and narrower lobe sep. That said, for every motor we build I always discuss with John D about what his thoughts are for our application.

41mm won't be horrible exhaust size at all. What muffler will you run?

Cheers

[Jonny042]

Originally Posted by Jeff Alton

Jon,

From and expert as every motor we build we learn. I like the spec sheet you posted. On a 3.4 with 10.5 I do like the DC44-108 as it has a quite wide power band and is still pulling at 7200 rpm. But we have only run those on ITB motors. I see your DC43-102 has the same lift specs, slightly different exhaust duration and narrower lobe sep. That said, for every motor we build I always discuss with John D about what his thoughts are for our application.

41mm won't be horrible exhaust size at all. What muffler will you run?

Cheers

Thanks, Jeff, that sounds like something I should look at (this engine will also be on ITBs and I plan to run full sequential alpha-n only) I'll talk with John about it - I suspect it will be dependent on the exhaust configuration. I'm getting too old for loud cars and plan to use a 2 in 1 out Dansk sport with the slightly larger tip. I know that will cost some Hp at top end so it might be a matter of taking advantage of the situation and choosing a cam with a little more grunt, or maybe a little more exhaust duration to help out in that regard.

It does bring up the point that John can (I assume) dial in a different lobe separations for the various grinds - he doesn't list the DC44-108 as an option on his website but it sounds interesting. Would bias towards the bottom end a little more for autoX and street.

I found John to be very helpful in the past. He's my next stop for advice.

Thanks again!