Ben R

Why is that?

white2001s10

iTrader: (4)

I have that book, and it is VERY good.

DavidNJ

A 434 through a peanut sized carb will have a pronouced torque peak and than a sharp drop off. Shift point will be the point where the revs at 6500 or 7000 (and maybe lower) can't produce more power than at 3500 or 4000 rpm. I believe the ALMS C5R running through two tiny (37mm?) restrictors shifts at 6000rpm.

The 311 will probably produce a flatter torque curve with more linear throttle response which will be easier to control out of the turns IMHO.

ShaunSG

David, actually the air-restricted C5Rs have a very wide and strong torque curve because they are designed to run maximum power within the mass flow limits of the restrictors. It is opposite of unrestricted and usually higher power, but peakier, unrestricted engines. This does not mean an unrestricted engine cannot be built and tuned like a restrictor engine in terms of where you want the power.

Low revving high torque engines may not be aurally exciting as high RPM screaming engines, but they certainly still are effective and longer lasting engines in general.

racer7088

You will be down by more than you will ever be up and the friction rises too because you must now turn higher rpm. The reason the restrictor plate engine builders ran the smaller engines a short time was to try and get more air by the SEO plates they use. It does work as well to a degree, and you CAN get a little more mass flow though the sharp edged plates by keeping the instantaneous airspeed within bounds so the plate doesn't Vena Contracta as much either. The problems was the friction losses canceled it out from the higher rpm and the super low friction pistons and rings and bearings and low low oil pressure could always be applied to the 358s with even better results at a lower rpm and friction level.

What people that were going faster were doing was cheating in millions of ways if you know all that went on in the beginning of the plate motors and then many said that they were just running small engines and that's why they were faster! NASCAR and the engineers knew better and caught many running all kinds of air leaks both illegal and some in very gray areas and also simply forbid the smaller engines to eliminate this excuse for higher rpms with the same plate. I know one NASCAR engine builder from TAMU that was on one of those teams that flew with the plates and they were cheating and saying this themselves. They never were able to lap faster with the smaller motors legally anyway so no one cared when they put a lower limit on displacement. Most people have quite a good laugh when it comes up that I talk to.

These cars are already limited somewhat by the 4150 and cast manifolds they run so as with ALL engine they are already a restrictor plate type deal at some point but maybe only 2 inches HG instead of 15!

The cars with non-SEO restrictors just get slower when you destroke them like any other engine. You have to run MORE weight with larger engines in the rest of restrictor plate racing and that's with the same or smaller restrictors than the smaller engines get to run or the big stuff will totally dominate and run away and it still does half the time so even there displacement is an advantage for sure. I could send you a movie about restrictor engines from a 24 hour builder and you would be surprised what all the big engine people and engineers say I bet.

racer7088

It might be easier to drive in the rain maybe but probably not too fun. A gutless 310 that doesn't pick up power up high? Or are you talking unrestricted?

In unrestricted form the 434 with be TONS more fun with tons more torque and power everywhere. The 310 will not keep up. I guess it would be a good rain engine or for someone that sucked or couldn't drive maybe or if you have to run on crap tires?

DavidNJ

My experience with oval track restricted motors is a torque curve peaking in the low-mid 5k range then falling off continously. Power peak in the low 6k range. And runs out of steam a little over 7k.

With engines that have enough air, the torque curve is flatter and the power curve climbs.

Yes, the 434 would have more power. And yes, it would have a lot more peak torque. However torque at the flywheel is pretty meaningless. It is torque at the wheels that counts, and that is HP.

With a 350cfm carb, I imagine your 434 would be out of breath before 6k. Depending on cam and valvetrain rules, the 311 would probably power peak in the high 6k range, with a relatively flat torque curve to there.

In a race, the 434 would eat the 311. Behind the wheel, the 434 driver would have an interesting time coming off the turns, the 311 driver would have a more linear response.

In a little approximation, using some default engine I constructed a while ago, a 434 torque peaked at 4500, power peaked at 5500, and had a 30% rev range of 4500-6500. At the shift point torque had dropped 30%.

Change to a 3" stroke 325, the torque peak was 5500 and the power peak 6500. However, torque dropped 15% from 5200-7300. A little higher, maybe 25%, in the 5500-7700 rev range.

Yes, the big motor produced 20% more peak power, and maybe even more average hp under the power curve. However, the rpm dependent torque is not fun coming out of the corners. At least not if your goal is consistent, fast times.

Unfortunately, my print screen has checked out.

Note, my 355's curve looks a lot like the 434. And getting power down coming off the turns is always on nails. With an 'N'.

racer7088

David, I believe most of all that and thats close to what we usually see too. In fact most 350cfm carb motors don't ever acheive peak torque as high as you were saying even at 350ish inches but as they get restricted torque simply drops off at a steady rate while power stays almost the same. The bigger stuff cam keep the power up a lot lower basically is all and the friction losses at lower rpm are even lower so the bigger stuff gets more power out to the wheels all else equal. You end up with a mega huge power band with the bigger engine and can shift at a much lower rpm and still climb all over people when you have teh car set up right.

DavidNJ

You end up with a hump shaped power curve on the big engine. On the smaller engine with the same breathing it becomes an upward slope.

Yes, the big motor is faster. The small motor is easier to drive fast. Which is why Michael Schumacher and Jeff Gordon aren't afraid I'll break their records.

Old SStroker

As you said, torque at the wheels is what accelerates the vehicle. Getting the max wheel torque the car can handle thru its operating range without overpowering the tires at "torque peak" is the goal.

It is possible to shape the torque curve a number of ways, with valve timing and intake manifold and exhaust header tuning complimenting the head flow being the major players. If your rules have carb or restrictor plate limits as well as displacement limits, the engine designer and the driver can decide together what kind of a torque curve will work best for your car/track condition and design an engine which has that kind of torque curve. It may not have the most peak hp or peak torque, but it could be quicker. Isn't that the goal?

This is a great use for good engine and vehicle simulator programs. Once you have the current or baseline engine well simulated, you can plug the engine data and car data into an acceleration program and work with the traction coefficients to simulate what actually happens. Shaping the (rear wheel) torque curve with all the available engine and gearing parameters can go a long ways toward giving you an engine/racecar you can drive. This has been done, but not often enough at the grass roots, Saturday nite bullring level.

FWIW, F1 engines have used techniques like lagging the opening of one bank of throttle plates a certain amount behind the other bank to "shape" the torque for Michael. This is in addition to or maybe in place of engine traction control.

I have little doubt that Cup engine torque curves are tailored to specific tracks. That's legal traction control. Add the illegal TC some folks claim exists in Cup and you don't have to be Jeff or Tony to be the fastest. This in no way implies these guys are using illegal TC, BTW.

Cup restrictor tracks are unique in that there is not much acceleration once they get to speed and the engine runs in a very narrow rpm band. Not so for endurance and short track racing of couse.

DavidNJ

Unfortuately you can use the cam to trade top end for power off the turns. But you need the power off the turns to run up front (I still waiting to win when all the turns are left). Your right foot is the 'traction control' so to speak. You can't really trade off the power, since you need it on restarts and in traffic. A difference of 2' at the end of a (admittedly short) straight can be the difference between make a pass or not making it.

Old SStroker

If you have to feather it until you are half way down the straight and then are praying for more grunt the last half your engine probably isn't too well tailored to your car and the track. Unless ever other lap has a restart, which seems to happen at some tracks, you are leaving performance on the table if you aren't WOT accelerating.

Try "shaping" the torque curve with a good engine design not just sacrificing one end for the other. Look at the area under the torque curve and hp curve throughout the rev range you run. You don't have to have the highest peak power on the track, but having the highest "area under the curve" is a big help. I'm suggesting some out-of-the-box thinking on engine, especially camshaft, design. There are already enough me-too engines out there running mid-pack.

FWIW, the engine should be tailored to the driver as well as the car and the track. An aggressive front-runner can use more mid-top end than someone who is maybe less aggressive and often mired back in the pack. Most engine builders advertise/sell max power numbers because that's what most people think wins. The smarter builders build engines that fit the track/car/driver. Some of them will even admit that to you. I like those guys.

My $.02

racer7088

when you have the most power under the curve you will also have the most power and thus torque at the wheels if the car is geared right. A 427 CID C5R is geared differently than a 366 inch one and it's also faster.

When restricted apps you usually can not give up any power what so ever and stay competitive. Remember also that many of these classes are managed competition to keep brands closer so they will all race.

The rules are also often there to help bring everyone together in overall speed so people often get confused on a true heads up setup when they look at a normally santioned racing class.A NASCAR engine is NOT fast BECAUSE it's a 358 at all. It's simply a very fast 358 BECAUSE of all the money and development. If they were allowed 400 CID with no penalties both the open AND the restricted engines would ALL be 400 CID.

DavidNJ

Well, in a 25 lap race there are probably 5-7 yellow or red flags. On a yellow flag the lap is redone. So the race is usually 30-35 laps. And, although 25 15-second laps should take less than 9 minutes, the race typically runs 30-45 minutes. On a bad night over an hour. Restarts are side-by-side until halfway, then single file (if the field is really unrulely (say 7-10 yellows before lap 5, the promoter will start single file earlier).

Traffic changes lap to lap. And you never know who is behind you since neither mirrors nor radios (a recent development) are allowed. One lap you can be clear, the next bottled up, the next side-by-side. A big difference in corner speeds and how much power you can apply.

For practical purposes, the intake, exhaust, and carb are fixed. The only really tuning option is the cam or lash. If you are running a track with long straights and tight turns would want a different cam than a track with short straights and fast turns.

I'm not saying my engine is wrong for the track, I'm saying that an engine with a flat torque curve and linear throttle response is easier to drive than on with a hump in the middle. Not faster, just easier to drive.

P.S.
Everyone must have a scanner to get instructions from the starter, mostly line-up order on restarts.

Old SStroker

Yes, I was suggesting a flat torque curve.

Can you restart in a lower gear?

DavidNJ

You are only allowed to use 1 gear, with a QC, always 4th. Shifting is only done on yellow flag laps.

joecar

I've been following this thread, and have the following contribution concerning the mention of titanium vs steel springs, pins, and other components...

The tensile strength range of these alloys:
steel alloys = 50,000 to 300,000 psi.
titanium alloys = 30,000 to 180,000 psi.
aluminum alloys = 10,000 to 100,000 psi

The upper end of these ranges is somewhat expensive for steel,
but much more expensive for Ti and Al.

For any given titanium alloy, you can always easily find a stronger steel alloy.

Titanium is lighter than steel (you can accelerate Ti components quicker for a given force, or, stated differently, Ti components create a reduced loading in response to a given motion).

But, all said, steel is still the stronger.

This is why Ti springs and other components break when used in place of steel in high stress applications.

My humble $0.02, thanks.
Joe