Are there advantages to running a destroked 7M?

[Ma70.Ent]

For both turbo and N/A purposes, are there any advantages? I'm not too good with these types of engine concepts so knowing this would be nice. I don't understand the whole high revving craze and I have no idea why it could be advantageous. What's the point of losing torque to gain revs, etc? If you could answer this in-depth, that would be great. Thanks for your time

 

[Doward]

# of putts per minute = more power.

Either more cylinders, or more revs.

 

[bmoss85]

it really depends on what you are using the engine for, and where you want to make your power. destoked engines usually lose some power on the bottom end, but make up for it on top with the extra rpms its able to turn. i cant say a whole alot about the turbo application of this though, as most of the engines i have built have been chevys.

 

[Adjuster]

In general theory, if you are building a motor, you can tailor the bore v/s the stroke for the vehicle the engine is going to be run in.

A "Square" engine is one where the bore and stroke are about the same. (Or are the same.) It generally results in an engine that is quite well rounded, depending on the displacement of the engine.

The 7M is a stroker motor from the factory. It has a longer stroke than bore, and thus was/is a good engine for making tourqe, but is somewhat limited in how high the engine speed can go without causing terminal damage to something. (Rods)

Destroked, and thus lower displacement 7M's have been built that made serious power at high rpm. (HKS did it on the 80's MK3 based drag race car.. 9 second or better IIRC on 2.8L and custom everything.)

The JZ engines are also shorter stroke, and more "square" designs than the 7M. They allowed for a shorter stroke by having a larger bore to keep the displacement ability of the design capable of 2.5 or 3.0L and still have a motor that can turn quite fast with less stress on the rods.

There are some long stroke engines out there making serious power at very high speeds, but they also tend to have very high quality internal parts. (Forged, Ti and other expensive yet strong moving stuff for the crank, rods and pistons.)

Valve weight is an issue too. Most larger valves are going to hollow stems, and other tricks to keep the weight down. (Lighter materials too.)

Ok, so all things being equal here is a good rule of thumb.

Same engine design. I6 in this case, and 3.0L for discussion.

Generally, the one with the large bore, and short stroke would make more high RPM power and have less off idle tourqe.

The one with the longer stroke, and smaller bore would make more low and midrange tourqe, and possibly less power on top due to reduced engine speed.

The way around this, is to build a long stroke, big bore, and higher displacement engine with quality parts that can handle the engine speed, and yet provide the best of both worlds. (IE: My stroker 7MGTE.. ) LOL

In the V8 world, look at the LS7 engine. It uses very high tech, and expensive parts to maintain the best of both designs, but now the new LS9 Supercharged engine has gone back to the displacement and design of the "base" engine with thicker cyc. bore walls, and less stroke to help keep this motor together under the stress of force feeding it.


BTW, turbo size, or SC ratio all play into how the tourqe curve is going to look on any motor, and with variable valve control, many engines that would normally be "gutless" down low, or on top can now have very broad power bands because the valve event timing is completely controlled by a computer. (Heck, some do not need throttle bodies, they are controlling it all with valve control.. You step on the "gas" and the computer decides to now start opening the valves more to flow more air v/s a "gate" like Throttle body that allows more air into the intake manifold.. ) As you might expect, this makes throttle response very fast, and reduces some other losses, so the fuel economy and power is further improved.. (BMW calls it "Double Vanos" IIRC.)

And that is totally another subject!

 

[3p141592654]

You mean Valvetronic, not Double Vanos.

 

[Adjuster]

Ok, whatever, vanos, valvetronic, and now I see Nissan has their version in the G37.

It works, that's the key

I would expect to see some sort of variable valve timing, lift and complete control like this on new motors along with more direct injection engines.

It would not supprise me to see 12.5:1 or higher CR with variable valve control and direct fuel injection become standard on most vehicles in less than 10 years time. (To easy to improve fuel economy, power and emissions performance with these new systems.)

The really cool thing is with turbo charging and these designs, the ability to overcome lean detonation and pre-ignition is very possible. (The fuel is added only when the computer wants to burn the fuel, and depending on load, could run lean under low load, and rich enough to avoid excessive heating of stuff when making power.)

Also more diesel engines, and possibly even engines designed mainly for E85 that can run on pure gas, but will run best on the higher octane E85.

It's going to be fun, no matter what

 

[madseacow]

weird, I was just looking this up last night, and came here to ask about 5m crank swaps. I'm not done reading aaron's thread, so forgive my ignorance. I don't know much about the subject, but I enjoyed reading this Wiki page.
If anyone feels like commenting, I'm thinking I could sell my current 7m crank/bearings and get closer to square with this. I havn't come across the stock 5m stroke, stock 7m is 91m, I believe I can get 84mm max out of the stock block's bore correct? It'll be damn close to square. I'm interested in square for the throttle response benifits, and reliability at sustained high rpms (nothing crazy, 7-7.5). Is that a bad idea?

If this is answered in "high revving 7m" or "Bigaaron's" threads, I'm sorry... I'll get to it.

 

[annoyingrob]

If you picture your piston movnig up and down, it's obviously experiencing acceleration forces as it's being pushed up and down.

Now picture two motors, one with 10mm of stroke, one with 20mm of stroke, both spinning at 1000rpm. Both pistons are moving up and down 1000 times per minute, however the piston with 20mm of stroke must move much faster, as it has twice the distance to cover in the same amount of time as the motor with 10mm of stroke.

What's happening is the more stroke the motor has, the faster the piston is moving, and the more force it being exerted on it. If you were to look at a 7M revving to 6000rpm, the pistons would be subjected to the same amount of force as say a 2JZ would experience at 7 or 8k (That's not a mathematical calculation, just a wild guess used so you get the picture).

By de-stroking the motor, you're decreasing the stress you're placing on the pistons and rods, effectively allowing them to spin faster. By decreasing the stroke, say 10%, the motor can spin I believe about 19% faster (just doing some math in my head, I may be way off)

For de-stroking a 7M, I wouldn't bother with a 5M crank. Too much custom work to be done to it to fit properly in the block. What I would to is take teh 7M crank to a machine shop, and have them re-grind the rod journals offset to decrease stroke. Then it's just a matter of getting some custom rods with the right length and cam diameter, as well as right bearing size.

 

[madseacow]

Cool, thanks for the input. It seemed like I could just drop in a new crank with new bearings and be ready to go. But I guess not, if there's no easy/painless (Relatively speaking, without gobs of machine work) method then it's not really worth it for my build. Thanks again.

 

[Keros]

What's happening is the more stroke the motor has, the faster the piston is moving, and the more force it being exerted on it. If you were to look at a 7M revving to 6000rpm, the pistons would be subjected to the same amount of force as say a 2JZ would experience at 7 or 8k (That's not a mathematical calculation, just a wild guess used so you get the picture).

By de-stroking the motor, you're decreasing the stress you're placing on the pistons and rods, effectively allowing them to spin faster. By decreasing the stroke, say 10%, the motor can spin I believe about 19% faster (just doing some math in my head, I may be way off)

Just to add a comment here, that on a 4 stroke engine, it's the exhaust stroke (where the exhaust valves are open and the piston moves from bottom to top), that is most likely to cause catastrophic failure at high RPM's. When the exhaust valve is open, the pistons are pushing exhaust out, and there is therefore, no compression to slow the piston down as it approaches the peak of its cycle, this causes the rod (and only the rod) to slow the piston down and prevent it from rocketing through the hood of the car... however, this puts the rod under immense tensile stress, and like all metals, tensile stress is where it is weakest.

On two of the strokes the piston is moving down (compressing the rod) and the other stroke the rod is aided by compression in slowing the piston.

 

[Adjuster]

Thus the reason I have Pauter forged billet rods on my stroker...

Exhaust stroke stresses at stock rpm seems to be no problem..

Even on a few over-rev moments into the 7150rpm range, it has not broken anything up to this point...

No need to rev this engine higher either, it's very powerful down in the 3.5k to 6k range, right in the sweet spot of this combination.

 

[Doward]

Also why I'm running Eagle Forged rods here. Hell, it's the big reason to go aftermarket forged rods - it handles the loads much better

 

[turbodriz]

I see in the begining of the thread that the conversation was about rev. I just wanted to say that it has been proven time and time again that the 7m can be reved at it's standard stroke. There are many guys here on the site who's 7m make power in the 7.5 8.0 grand range.

 

[supradjza80]

For what it is worth the 2jz is perfectly square at 86 x 86 (bore x stroke). I believe the 1jz is 86 x 77. From what i have seen I think square motors have the most desirable characteristics, the best of both worlds design.

A formula one car for example has a really large bore and an extremely short stroke, (Ill see if i can find the exact numbers when Im not at work). So a formula one V-8 (2.4l) makes about 300 ft-lbs of torque maximum but since it revs to 19000 rpm it makes 750+ hp.

More interesting info is the equation for hp and torque and how they are related.

Hp=TQ*RPM/5252, so basically at 5,252 rpm horsepower is always higher than torque. This is also why making your torque curve go into higher rpms produces more power than the same torque curve at low rpms. And for what its worth, more HP is always better than more torque in terms of vehicle acceleration.

 

[annoyingrob]

^ 1JZ is 86x71.5mm

Random fact, RB26 is 86x72mm

 

[figgie]

A formula one car for example has a really large bore and an extremely short stroke, (Ill see if i can find the exact numbers when Im not at work). So a formula one V-8 (2.4l) makes about 300 ft-lbs of torque maximum but since it revs to 19000 rpm it makes 750+ hp.

more HP is always better than more torque in terms of vehicle acceleration.

F1 motor specifically.

the Bore is double the size of the stroke or put another way. the stroke is excatly half the bore size.

As for your last statement.

WRONG. HP = work but without Torque, nothing will happen (Hp= Work,TQ=Force, RPM = Time). Torque is a force that without TIME (very very important) is does not produce any work (put a 1 lbs weight on your breaker bar that measures 1 ft, means 1 ft/lbs. Problem is the weight is hanging there and not moving. A Force but no work is done). HP and Torque are related. You can have torque but no work done. But you CAN NOT have HP without the Force * time (simple work equation).

 

[Poodles]

Hp=TQ*RPM/5252, so basically at 5,252 rpm horsepower is always higher than torque.

Nope, it's the crossover point and they're equal.

Take two exact cars (assuming they have the ability and aren't already on the edge of max flow) and spin one up higher than the other. The one that revs higher makes more HP. These cars would be equal until they had to shift and the higher reving car would win as it would be higher in the powerband where there is more torque/horespower.

On a side note, the WRC cars run restricter plates and are tuned for the most power at 5252 RPM. They run out of air and getting more under the curve is the way to more power in their application as their max flow is limited.

 

[figgie]

Nope, it's the crossover point and they're equal.

Take two exact cars (assuming they have the ability and aren't already on the edge of max flow) and spin one up higher than the other. The one that revs higher makes more HP. These cars would be equal until they had to shift and the higher reving car would win as it would be higher in the powerband where there is more torque/horespower.

Just an FYI

More revolutions per minute equals more work done in the same amount of time (Second, minute, hour). That is of course taking into consideration that the head can flow the the amount of air for the given RPM.

On a side note, the WRC cars run restricter plates and are tuned for the most power at 5252 RPM. They run out of air and getting more under the curve is the way to more power in their application as their max flow is limited.

To be more exact. The restrictor plate is exactly a 34mm opening for Group A cars, 32mm for Group N cars. The Restrictor plate must be within 5mm from the turbos Impeller and is attached via a tamper proof mechanism by race officials. Once installed, it can not be removed or bypassed. That is why WRC Cars have ALS on thier systems so they are always in positive intake pressure. IF by chance they fall of boost, the motor is sucking wind literally to try and produce power.

[thumb]http://foo.is/~baldur/wrcevo1.jpg[/thumb]

Restrictor on the right hand side attached to turbo inlet.

 

[annoyingrob]

^^ Are they feeding air into the exhaust manifold too?

 

[figgie]

^^ Are they feeding air into the exhaust manifold too?

Glad you "caught" that

Actually that is the new version of the anti-lag system based on EGR. It works both ways for them

The NEW ALS system that particluar car uses (EVO) is actaully pretty intresting. The "valve" you see on the exhaust manifold serves as the basis to bypass air into the exhaust system while the ECU adds fuel. But instead of the old style where you are dealing with pumping losses to get the air through the engine, this way, unrestricted air is injected directy into the exhaust stream while the unlit fuel comes into the exhaust, at the right time the valve closes, the mixutre lights and you have an ALS that reduces the EGT, is less destructive to both turbo and exhuast parts but still able to maintain race level boost when off throttle with less fuel consumed.

It is actually pretty f'ing brilliant. Also notice no BOV All that work the turbo did is used to keep the turbo spinning off throttle. So no venting to atmophere for them anymore and if you think about it, why would you when you can keep all that nice pressurized air in the system and help in keeping the turbo spinning.