What really limits rev capability on these cars?

[serafins]

I've read every thread out there on this. So please don't tell me to search. My question is what limits the rev capability besides oiling on these cars? Specifically gen 2 but since I suspect the answer involves main bearings, and they are the same for all gens, it's sort of universal.

Oiling is an obvious problem but relatively easily fixed. Beyond this, there is still a lot of doom and gloom about higher RPM. I have read a lot of talk that the engine has an inherent design limitation that will not allow it to rev without eating bearings. There is some suggestion through use of a smaller diameter main bearings in the GTSR race engines that the oil wedge was breaking down at high rpm due to too tall/skinny of a bearing. This was the same issue the old Ford FE engines had when Ford started running them in endurance racing. However, the Viper uses 340 sized main bearings from what I understand, and the 340 was a 7k rpm engine from the factory. So why is it different for the viper?

Additionally, I am just as inclined to believe that they switched to a smaller diameter bearing to gain a few horsepower, since this is routine motorsports practice. Does anyone know if this was specifically a mod aimed at improving endurance?

I am mostly curious about this just from a technical standpoint. However, I fully intend to rev the engine I am building to 7k rpm so I guess I will find out firsthand if it's up to it. It's a forged (99) gen 2 engine. My only mods to the bottom end are an external wet sump from exotic engines and the included ATI crank dampener. Plus new H rated bearings. I see no technical reason it will not be able to hit 7k without an issue.

FYI before someone comments "it won't make power up there", this is the top end: race used GTSR heads, T&D shaft mount roller rockers, custom cam, gen 3 intake port matched to the heads (at least to start - but will keep the gen 2 in case its a dog on torque), big volume racetronix fuel pump like Dean posted in the fuel pump thread, bigger injectors, single blade throttle body, etc.

 

[Goggles Pizano]

Let's just stick with the engine since that is what this thread is about.

What's your planned maintenance schedule for it?

 

[GTS Dean]

Why don't you surf the Gen 3 forums for Pokeyl's threads?

 

[MoparMap]

RPM is usually limited by the valvetrain more than the bottom end. Oiling is a good point, but controlling float and dealing with pushrods at high rpm is usually where I've heard of problems with any engine, not just the Viper. You have to put such heavy valve springs on it to get them to close that you stress the whole valvetrain pretty hard and I'm not sure how big of pushrods you can fit in the stock head clearance holes.

 

[GTS Dean]

Here's a quick summary thread from Gen 3 Forum. https://www.viperclub.org/vca/threads/rpm-limit.679138/ There's a lot more backstory on this build.

For the Gen 1 and 2 models, the mechanical rev limit can be raised substantially, along with the fuel cutoff. However, the fundamental issue is the intake manifold being a huge bottleneck for high rpm usage. This is why you see Gen 3 & 4 heads and intakes. For road course duty, you'll need a more reliable bottom end, with all that entails. The FIA kept trying to strangle the GTS-R with smaller restrictors and more weight. I think the answer was more torque, modest rev limits and total reliability.

 

[serafins]

I'm really more interested in the engineering explanation behind why people thing the bottom end won't hold up at 7k rpm on the V10, when the 340 used similar design with 60s machining and metallurgy and its fine at 7k rpm. Maybe it's a function of v10 harmonics, idk. That's why I'm asking.


Some more details on my engine though:
the top end has all the rev capability it could need. Gen 3 intake ported to match the GTSR heads. Heads are running dual valve springs with titanium retainers. The heads are pretty tricked out from their time at CDI. They have the beryllium copper valve seats, bronze magnesium valve guides, etc. Milled for correct geometry with longer valves and the T&D rocker setup. The oil return galleys have been polished.

Valve Spring info:

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Compression will sit right around 11:1.

Without giving too much away on the cam, duration at .50 is 239 intake/244 exaust. LSA 114. Modern smooth ramp rate design. .624 lift intake, .615 exhaust.

Pushrods will be 3/8". More than enough room in the GTSR heads for them.

All that to say the valve train will be pretty bulletproof.


In regard to the bottom end, the main piece is the exotic engines external wet sump. Then you have the 10qt pan, trapdoor baffling, revised drainback tray, etc.

Will get new H series bearings but otherwise the crank/pistons/rods/main caps are stock forged 99 units.

 

[serafins]

Why don't you surf the Gen 3 forums for Pokeyl's threads?

Thanks Dean. I have seen his stuff. I don't think he's done much to the bottom end at all. I really think I'll be okay. Springing for the external oil pump like you recommended on messenger I think will make it live. I've talked to at least 2 guys running a very similar setup on a forged gen 2 shifting at 7k rpm for years with no issues.

That's why I'm really not overly concerned about putting it together and seeing what happens. I am confident it will live. But my confidence is derived from other's trial and error. The general consensus on this forum is that the engine will never live at 7k rpm. I don't believe that's true but as you know, I analyze arguments for a living so I like to consider all sides.

So if there is anyone who has some engineering reasoning/theory behind why the bottom end of the viper motor will not live at 7k rpm once oiling is fixed, I'd love to hear it. Not looking to argue with you, just would like to know what it is so I can research it further and at least add it to my list of concerns/possible failure points.

 

[Old School]

Without knowing anything else, I thought the 9L became popular because of limited RPM capability, so just add displacement instead.

 

[serafins]

Without knowing anything else, I thought the 9L became popular because of limited RPM capability, so just add displacement instead.

Even with the added stroke they rev those out to 6500 rpm. And that’s with an internal stock style oil pump I believe, though its already on the Gen 4/5 revised oiling design and prefix installs better OPGs.

The main issue with the factory Gen 2 oil pump is the pickup is too small for the volume the oil pump wants to **** at high rpm. And the pickup tube is a rectangular shape. And/or the rotor is too small and spinning too fast (debatable). Bottom line is it causes cavitation that not only destroys the gears over time but sends air to the bearings.

this stuff is well documented on the forums by reputable builders. When oiling is mentioned as a solution, such as in the thread where Dan Lesser announced his external oil pump system, there’s another group of people that come out of the woodwork and allude to an inherent design flaw that oiling won’t fix. They never name it though from what I’ve seen.

If it’s just the cross drilled crank, or that a couple rods don’t receive oil for 270 degrees of their rotation, I’m aware of that and not concerned. Again, these are aspects shared with the 340, which used heavier rods and pistons and lived just fine for 100k miles of abuse on 60s conventional oil.

If no one knows or has an answer that’s totally fine. I just want to make sure there’s not something right in front of my face that I’m missing. Like, for example, the main bolts or rod bolts are only good to 6500rpm. Or the main caps are hollow. Etc.

 

[99RT10GTS]

The facotry did make the GTS-R blocks with a 2.5 Main bearing size vs. 3" to get a little more RPM out of the race cars. Of course they also got better heads. The intake manifold and heads are a huge restriction. The Gen 3 manifold or a custom manifold will help to get the RPM limit higher than stock

 

[Dan Cragin]

I had quite a few track motors back in the day that could run 7 to 8 k. It’s just the right combination of parts.

For the street I always found a broad torque curve with big power in the stock rpm range with great driveability was most desirable.

I must admit, a short stroke, bad ass, solid cam, dry sump, G2 to G5 top end converted high rpm streetcar could be fun. Maybe something I would consider for my 97 now that I have time on my hands.