Max Reasonnable Cutoff Tune for the gen 3

[Aevus]

Correct me if I'm wrong but 6,100rpm is the stock cutoff limit and about 6,300rpm would be the absolute max limit for the stock gen 3 engine?

Would a more aggressive camshaft + higher compression and/or forged pistons change that, without the risk of premature engine failure?

[Steve M]

In general, the valve train is the limiting factor. Still on the stock valve train? Best to keep the stock rev limit.

Upgrade the valve train, and you can spin it higher. It doesn't make sense to do that on a stock cam since it will run out of steam at high RPMs, but a more aggressive cam could certainly take advantage of it.

Everything has to work together though, so this isn't something where you can just slap in some stronger valve springs and call it good. Weight matters (the valves, retainers, etc.), lifters matter, pushrods matter...if you want to do it right, you need to either go with a proven combo that has been thoroughly tested, or if you are going outside the box, you'll want someone that really knows what they are doing. Bonus points if they put it on a SpinTron to see exactly when a setup will run into valve float issues.

By doing it right, you'll make good power, and you'll end up with a motor that will last. The trade off is that you'll need to keep a closer eye on things (like the valve springs). You might also find other weaknesses you didn't know were there (like the pistons).

Everything you do will be for nothing without a solid tune, so that also needs to be factored in. Need to pass emissions? That's something else you need to decide up front. Keep in mind that while you might not have to pass emissions, someone else might need to down the road should you decide to sell your car.

I remember back in my LS1 days where guys would throw in the biggest donkey dick cam they could fit and zing their motor to the moon. Yeah, it sounded cool and made good power at high RPMs, but they drove like shit on the street; soggy on the bottom end, surging/bucking at low RPMs, low vacuum at idle, etc. Some broke valve springs, some sent valves through their pistons.

Just beware that at some point you could end up making the car not so much fun to drive anymore, and try not to cross that line.

My $0.00.

[Aevus]

Thanks for the information Steve, very appreciated. Makes all sense.

Will be Roe Racing's 710r cam + a valve spring upgrade, 3/8″ 0.080″ wall pushrods and Mopar Slow Bleed lifters from the 92′-99′ Vipers.
Swing arm oil pan + probably a 2/3 quarts accusump kit for further safety and better cold start (running thicker oil).

Just got 3.73 gears with a Wavetrac and while I do enjoy the gears, shifting comes pretty fast. Cutoff around 6,300-6,500rpm would help a little

[Steve M]

Sounds like you have a solid plan. Are you thinking about having the heads ported since they'll be off to swap the lifters?

3.73s are a bit steep, so I can see the motivation for the extra revs.

FWIW, I spin my stock long block Gen 4 to 6,600...obviously not the exact same engine, so probably not worth much other than a random data point.

[Aevus]

Are you thinking about having the heads ported since they'll be off to swap the lifters?

Yes, but I'm not yet convinced of the benefits in doing so considering the long wait (bad time of the year) and cost.
I've read that only a mere 5whp is to be expected...

But got pretty much all other bolt ons: 67mm TB, K&N air filters, UD pulley, Headers, hiflo cats, race muffler and will most probably be dyno tuned.

[Aevus]

Also, is the lighter cast pistons will be in advantage compared to forged pistons, for higher rev?

[Steve M]

I wouldn't get too caught up in the weight of the pistons...max effort NA build? Sure, but I don't think that's what you are after here.

My rough understanding of hypereutectic (cast) pistons vs. forged goes something like this:

Hypereutectic (cast): don't have as much thermal expansion as forged pistons, so can be made to tighter tolerances. This keeps blow-by, oil consumption, and cold start scuffing to a minimum - good for emissions, good for your oil, and good for your cylinder walls. The down side is that they are more brittle, so are prone to cracking under stress (e.g. detonation, big HP power adders).

Forged: have more thermal expansion, so they'll allow more blow by, possibly more oil consumption, and scuffing during a cold start until the engine is warmed up. Once up to operating temps, they should seal just as well. The upside is that they are more durable, especially if detonation occurs (to an extent). The down side is that they are likely more expensive, and can be heavier, although I'm sure a well designed, properly machined example could possibly be lighter.

As long as your tune is in line, the cast pistons should work just fine up to a certain power level. Too much timing and/or a bad batch of gas could spell disaster though, so there's always a risk.

Gen 4s with cast pistons seem to do okay up to around ~650-700 HP...beyond that, and they are on borrowed time. From what I've seen of Gen 3s, they are probably about the same. Gen 5s with forged pistons can support substantially more power - I'm not sure what the current record is, but I'm pretty sure it is well over 1,000 HP. Many Gen 5 owners, however, have reported pretty excessive oil consumption, especially during the break in period. That + not regularly checking oil levels likely led to more than a few of those motors popping.

There's always a trade.

[z20ateU]

Be warned that the stock crank will have oiling issues at high rpms, due to the way the oil passages are drilled.

There are several threads around about said problem.

Just a FYI.

[Aevus]

Great info again, thanks Steve.

Do you know by any chance if the stock piston from gen 3: Mopar part 5189426AA is using the very same manufacturing process (and alloy) as the gen 4 pistons (Mopar part 68031890AD) ?

They look different on the pictures, but it doesnt mean anything of course..

https://www.moparpartscanada.ca/oem-...ton-5189426aa?

https://www.moparpartscanada.ca/oem-...on-68031890ad?

[Aevus]

Yes. And my stock crankshaft is being modified for that (along with the swing arm oil pan and accusump solution)

I really want to avoid the dry sump upgrade though. Anyway, I'm not even sure I'll ever bring my car to a track again...

[Steve M]

Great info again, thanks Steve.

Do you know by any chance if the stock piston from gen 3: Mopar part 5189426AA is using the very same manufacturing process (and alloy) as the gen 4 pistons (Mopar part 68031890AD) ?

They look different on the pictures, but it doesnt mean anything of course..

https://www.moparpartscanada.ca/oem-...ton-5189426aa?

https://www.moparpartscanada.ca/oem-...on-68031890ad?

I don't, but if I had to guess, I would say it was likely the same process used for both.

The biggest difference is that the Gen 4 pistons were a shared part with the hemi V8s, which I'm sure the bean counters appreciated. The Gen 3 pistons only fit SRT-10 Rams and Vipers.

[Aevus]

I'm in a dilemma now, because my engine builder says 6,500 rev limit would be fine while Scott from Roe Racing says otherwise: that it's best to keep stock rev limit to 6,100.

I know for sure that I'll keep my 3.73 gears, but 6,100 rpm cutoff is coming very quickly. Especially if the race-like cam kicks in from 3,500rpm...

Let's say, worst case scenario with 6,500 rev limit..? Will need to forget any drag or track use, only street? Or doomed for a premature engine failure, say 20,000 miles engine lifetime instead of 40,000 miles ? (just throwing numbers here for the sake of the discussion)

Basically, I'm just trying to understand the risks implied with that +/- 400rpm extra rev limit

[Steve M]

In general, lower RPMs are always going to be preferred, but as you've already pointed out, a more aggressive cam is most beneficial if you can wind it out to take full advantage of the high RPM gains they usually provide.

In the end, your intended usage is going to matter most. Road racing where you are living at high RPMs for extended periods of time + high G turns would favor a lower cutoff. For the occasional blast on the street or at the drag strip, the higher rev limit isn't as big of a deal because the amount of time you spend there is generally very low. That's why I've bumped mine up...if I was going to run Mid-Ohio or something, I'd likely set it back down to stock.

You can always step back down to 3.55s, but that isn't exactly cheap to do. For the drag strip, you can go to a 28" tire on an 18" rim (M&H or MT) versus the stock 27" tire on a 19" rim if you want to keep the 3.73s, but that only matters if you are banging the rev limiter before the traps.

I guess what you need to figure out is where you want to run, and what speeds you expect to achieve. For drag racing, you want to cross the traps as high in the RPMs as possible - if you can guess what you might trap, you can do the math to figure out what gearing/tire combo will work best to put you in a sweet spot. With 3.73s and 3.55s, you'll be crossing the traps in 4th gear. You can do something similar for a road course - figure out how fast other Viper owners are going at the end of the longest straight, and that should help you find the right gear/tire combo...shifting into 5th isn't the end of the world, but you won't be gaining much MPH with the stock trans ratios before you reach the end of the straight.

For the street, I'd think traction is going to matter most - I'm guessing you'll need a drag radial to have any chance of hooking with 3.73s.

Bottom line: you're going to have to make a compromise of some sort. Shifting before you reach the peak in your power band is going to cost you drag racing, so I'd want a higher rev limit with a cam. For 20-30 minute sessions at a high performance driving event, the lower rev limit isn't nearly as big of a deal performance-wise unless you are trying to set records. Your engine will most certainly last longer that way under those conditions.

[Aevus]

For the drag strip, you can go to a 28" tire on an 18" rim (M&H or MT) versus the stock 27" tire on a 19" rim if you want to keep the 3.73s, but that only matters if you are banging the rev limiter before the traps.

I'm running Kuhmo ACR's 355-30-19 and according to Tirerack it's 27.5'' (727 revs per mile)

...and according to xse site calculator, with 3.73 gears and 27.5'' tire I should reach 133.8 mph trap speed @ 6,100rpm.
While 6,500 rev limit would get me to 142.5 mph.

Then, according to https://robrobinette.com/et.htm
With 3650 lbs weight and 710 bhp, I should reach 137mph trap speed.

All in theory, of course.

But let's say my tires are worn down to 27.3'' and I really have 3650lbs/710bhp, then I would need 6,300rpm rev limit to get 137.2 mph.

[Steve M]

Don't forget that your trap speed is the average speed over the last 60' of the quarter mile; it is not your instantaneous speed, so your trap will always be a bit lower than how fast you were actually going right at the end.

I'd say low to mid-130s would be more reasonable to expect at your average track in average weather. Gen 4 guys with full bolt-ons (~600 RWHP) in great weather are usually in the 133-135 range. Gen 5s are about the same, although I have seen many much lower than that.

[Aevus]

Don't forget that your trap speed is the average speed over the last 60' of the quarter mile; it is not your instantaneous speed, so your trap will always be a bit lower than how fast you were actually going right at the end.

I'd say low to mid-130s would be more reasonable to expect at your average track in average weather. Gen 4 guys with full bolt-ons (~600 RWHP) in great weather are usually in the 133-135 range. Gen 5s are about the same, although I have seen many much lower than that.

Just saw your 130.06mph video, Steve :-)
Honestly, I'd be more than happy to reach the 130mph mark, but I doubt that would even be possible given the tires and clutch I have. I also doubt I'd be able to shift that fast with the gen3 transmission... We'll see!

For what is worth, here is the list of things I have or will have:

- Camshaft 710r Roe racing kit (with Valvetrain/lifters/etc...)
- BBK 67mm throttle body
- K&N air intake
- UD crank pulley
- 93oct tune
- Comp Coupe oil pan
- M&M headers + hiflo cats + Vibrant race muffler
- Wavetrac diff
- 3.73 gears
- Centerforce dual friction clutch
- Kuhmo 355 rear
- Yokohama A052 295 front
- ACR solid sway bar F&R
- IPS Sway bar bushings
- Engine & transmission poly mounts
- BC racing coilovers
- 12/14kg per mm spring rate Swift springs
- Corner balance 48/52
- Camber F -2.1 :: R -0.8 :: Caster 7.3 :: Toe-in Total :: F 0,38deg :: R 0,44deg

[Aevus]

Gen 4 guys with full bolt-ons (~600 RWHP) in great weather are usually in the 133-135 range.

With a full engine rebuild and the parts I'll have, with a dyno tune, I've been told that 700bhp would be realistic. So I guess 620-625whp. My car is slightly lighter than a gen4, so I guess an excellent drag strip driver (not me), it would be possible to be in the 135-137mph range then?

So if the actual finish line speed is around 140mph, then the cutoff should be 6,450rpm.

[Steve M]

With a full engine rebuild and the parts I'll have, with a dyno tune, I've been told that 700bhp would be realistic. So I guess 620-625whp. My car is slightly lighter than a gen4, so I guess an excellent drag strip driver (not me), it would be possible to be in the 135-137mph range then?

So if the actual finish line speed is around 140mph, then the cutoff should be 6,450rpm.

That's a healthy set of mods...if you hit that 620-625 RWHP mark, 135-137 would certainly be in reach with good driving.

I'm trying to squeeze everything I can out of the stock exhaust manifolds, so I'm about at the limit of what I can do without either adding more power or stripping some weight out, neither of which I intend to do. It drives so nicely on the street as-is, and I don't want to ruin that.

Either way, keep us updated on how it all goes. Should be rewarding!