[ChiragM]

Of-late, we have been seeing that a lot of the new-age turbo-petrol engines have a rather conservative rev limit as compared to some free-revving NA engines. Take the Honda City for example. It's 1.5L NA petrol revvs gloriously to 7,000 rpm, while the Figo's 3-cylinder 1.2L petrol revvs to 6,800 rpm and the EcoSport 1.5L 3-cylinder sees 6,900 rpm. Hyundai's 1.6L NA petrol was quite rev-happy and nicely went to 7,000 rpm.



In comparison, here's a list of turbo-petrols with their lower revv limits:

• Tata Altroz Turbo Petrol: 5,500 rpm
  
• Hyundai Creta DCT - 6,250 rpm
  
• Jeep Compass AT - just under 6,000 rpm
  
• Mahindra XUV300 Turbo Petrol - 5,900 rpm
  
• Renault Duster 1.3L - 6,400 rpm
  
• Skoda Rapid TSI - 6,500 rpm

What could be the reason for a relatively low rev limit for these engines? Is this to preserve the turbocharger in any way? Or is it that the makers are focusing on longevity as these downsized motors are heavily stressed anyway?

[GTO]

Just to add, not all turbo-petrols have conservative rev limits. I don't remember which, but some Germans cars (VW-Skoda, BMW) have turbo-petrols that rev almost to 7,000 rpm.

Some on the list above are way too low for a petrol. Heck, I know diesels that do 5,400 rpm!

[Dr.Naren]

Originally Posted by ChiragM

What could be the reason for a relatively low rev limit for these engines? Is this to preserve the turbocharger in any way? Or is it that the makers are focusing on longevity as these downsized motors are heavily stressed anyway?

1. Peak power is achieved earlier in turbo charged engines. If the manufacturers achieve their target, they might not simply extend the revv limiter.

2. Reliability of the turbo and engine: There are many of us who just love to redline the engine . Manufacturer has to provide warranty and they consider all types of driving conditions and overall reliability is given high importance than performance. I strongly believe that redlining a 1.5L NA engine is safer than doing the same in forced induction one.

Originally Posted by GTO

Some on the list above are way too low for a petrol. Heck, I know diesels that do 5,400 rpm!

Suzuki 1.5 DDIS was one of the best revving diesel engines I ever drove .

[AMG Power]

Current F1 engines are 1.6 turbo petrols that rev upto 15,000 rpm.

So road going turbo petrols can easily do 7000 rpm. It’s upto the manufacturer to decide rev limits depending on what they feel would increase longevity and reliability.

Tata and Mahindra engines are not to be considered for such discussions. They are an exception and belong to the Stone Age.

[Kosfactor]

A Honda City \ Verna produces peak power at between 6400-6600 RPM (NA engine), an Octavia RS245 makes peak power at 5000 RPM (Turbo), no point in revving them beyond this.

[dhanushs]

The way these turbos spool up earlier in the revv range, I think it’ll become pretty pointless after building max boost. There’s only that much a small turbo can do. And again the FE concerns with high revvs.

Overall I think for a road going vehicle, with practicality a major aspect of the design. it makes sense to achieve the torque earlier, keep it reliable and give good FE.

For a higher revving engine. IMO the main challenge would be to maintain the AFR across the revvs. Which might need twin turbos.

[Kosfactor]

Originally Posted by AMG Power

Tata and Mahindra engines are not to be considered for such discussions. They are an exception and belong to the Stone Age.

Our Indian Thar petrol Turbo produces 150 BHP at 5000 RPM (Direct injection Turbo), Nexon turbo petrol produces peak power at 5500 RPM. Considering the BMW produces its peak power at the same 5000 RPM, I thought Indian brands were doing very well considering the use case of these engines in the cars they belong.

Why do you feel they are in the stone age?

[BlackPearl]

You are not going to gain anything by higher revs in a turbo engine. That will beat the purpose of putting a turbo in the first place I believe! For sheer performance, it will be difficult to design a turbo that will cater to the entire rev range of the engine, hence they need two turbos.

[ashwinprakas]

Why bring Turbo into this discussion?

Lets go back to basics.

Lighter Pistons/Shorter Strokes = Higher Revving

Heavier Pistons/Longer Strokes = Lower Revving


A 250cc single cylinder motorcycle(CBR250R) revs to 12,000RPM, a 4 cylinder motorcycle(ZX25R) of the same displacement revs to 20,000RPM.

Both are short strokes but 4 small pistons are individually lighter than 1 large piston. This is purely physics at play.

Simple. Case Closed. Kapish!

Regards,
A.P.

[v1p3r]

Originally Posted by ChiragM

What could be the reason for a relatively low rev limit for these engines? Is this to preserve the turbocharger in any way? Or is it that the makers are focusing on longevity as these downsized motors are heavily stressed anyway?

A few reasons:

1. It's not needed: Very often, turbochargers are added to engines to make them more relevant. For example, the beloved 1.8T from VAG (which came in the old Octavia RS, Audi A4, Beetle, etc.) began life in the 80s as an NA Audi engine. To pass modern emissions and make the same engine architecture more driveable, a turbocharger was added. This lowers max torque points (way more important for driving than bhp) and makes the car more efficient. As a result, there is no need to have higher revs. The 1.8T's max torque rpm went from ~3500 to ~1750, lowering gear shift points and increasing efficiency.
2. Turbos choke: Point 1 also means that a turbocharger is optimised for a rev range. This is normally the mid range, where most people drive. Because of how air flows, a turbo that works well in the mid range will choke the engine at higher RPMs. This choking will also overcome the inherent ability of an ICE to produce more power with higher revs. So the engine will be quite useless past a certain rev limit.
3. Detonation: Higher revs increase the chance of detonation. Add to that the extra heat generated by compressing the charge, and the lack of absolute reliability on both fuel quality and driver skills, and a high rev limit is suddenly a bad idea.

Most of this is also true of smaller vs bigger engines. Small Japanese NA engines - bikes, Honda car engines - have to rev high to produce power that a larger American NA V8 will produce at lower RPMs. This is purely a function of how power is made - digging into the basic power equation of P = W.t will show that.

Originally Posted by GTO

Just to add, not all turbo-petrols have conservative rev limits. I don't remember which, but some Germans cars (VW-Skoda, BMW) have turbo-petrols that rev almost to 7,000 rpm.

Some on the list above are way too low for a petrol. Heck, I know diesels that do 5,400 rpm!

With the petrols, it's a case of what you're willing to spend. Better intake optimisation, better camshaft optimisation, more turbochargers, now with electric actuation, and you'll find the proverbial sky is the limit. When Ferrari launched the F154 (first in the California T), it was greeted with trepidation. The 488 GTB soon changed that and people wondered why they'd dissed turbos. Except for collectors, you'd be hard-pressed to find a buyer saying the 458 was a better engined car. The diesels, however, are at their limit. Diesel simply burns too slowly and doesn't have a uniform flame front, so 5500 rpm is as high as you'll possibly go.

Originally Posted by Dr.Naren

Suzuki 1.5 DDIS was one of the best revving diesel engines I ever drove .

What vehicle was this engine in?

Originally Posted by dhanushs

For a higher revving engine. IMO the main challenge would be to maintain the AFR across the revvs. Which might need twin turbos.

Your post is almost entirely correct. However, turbos and AFR are not related. Maintaining the correct AFRs across the rev range is a function of injectors, ECU and a suite of sensors. Adding a turbo will not do much. Of course, a bad AFR will literally melt your engine.

Originally Posted by BlackPearl

For sheer performance, it will be difficult to design a turbo that will cater to the entire rev range of the engine, hence they need two turbos.

Well, you could spend a large amount of money on R&D and charge a lot for it. That's what the Europeans do!

Originally Posted by ashwinprakas

Lets go back to basics.

Lighter Pistons/Shorter Strokes = Higher Revving

Heavier Pistons/Longer Strokes = Lower Revving


A 250cc single cylinder motorcycle(CBR250R) revs to 12,000RPM, a 4 cylinder motorcycle(ZX25R) of the same displacement revs to 20,000RPM.

Both are short strokes but 4 small pistons are individually lighter than 1 large piston. This is purely physics at play.

There are a couple of cardinal errors in your assumptions.

1. The weight of 4 connecting rods and a 4 cyl crankshaft are higher in terms of reciprocating mass than a single cylinder's will be, given they are similar in size and design philosophy.
2. You have not given the weight of the piston - just assumed that 4 smaller pistons are lighter. The Kawasaki has 4 x 50 mm pistons. The Honda has a single 76 mm piston. It is very unlikely that your Kawasaki has a lighter cumulative piston weight.

If you look at the dyno graphs of both bikes, which is the right way to ascertain power, you will notice that the Kawasaki and Honda both make the same power at 10k rpm - which is about 25 rwhp. The Kawa is built to make nearly double that but it needs to rev to over 15k for that. Revs are a great way to make more power - this is a basic fact of physics that Honda has applied really well, if you notice how their B and K series engines worked.

I must disclaim that I've spent a significant amount of my working life testing and optimising all kinds of engines, from CVs to cars to motorsport ones.

[Nav-i-gator]

Why would we want to rev higher if there is no gain in power? I have a city and I dont feel revving it to the limits....there is no need, simple! I mean, it is good to rev the nuts off if you are test driving a car and want to test its limits, but once you own a car, you drive it "normally". You dont drive like a rally car anymore, right?

[landcruiser123]

No point bringing F1 cars & superbikes into this discussion about mass-market cars.

• F1 cars have multi-million dollar budgets for design & manufacturing a handful of cars
• Superbike engines have shorter strokes as they require lesser torque

I think it is due the lower rev-limit is due to one main reason: The Turbochargers. Metallurgy & Manufacturing also plays a part too.

With a higher operating rev-band, we need more complex technology like VGT or electric supercharger assisted Turbo (VAG) to keep the engine responsive at all RPMs.

Also, the lesser exhaust back-pressure of the gasoline fuel compared to diesel makes running smaller turbos a cheaper option - both engineering & manufacturing cost wise.

[Dr.Naren]

Originally Posted by v1p3r

What vehicle was this engine in?

I had driven Ciaz 1.5 DDIS

[Jeroen]

What is with the fascination of high revving?

We have countless threads suggesting trost Indian drivers never ever exceeds 1500RPM?

We are concerned about lugging not revving I thought?

Jeroen

[ashwinprakas]

Originally Posted by v1p3r

The weight of 4 connecting rods and a 4 cyl crankshaft are higher in terms of reciprocating mass than a single cylinder's will be, given they are similar in size and design philosophy.

Yes, but at the same time being a 4 cylinder engine it is has better primary balance presuming the ZX25R fires at 180 degrees.

Then comes the question of secondary imbalance, even here the ZX250R would have an advantage due to its stroke being just 31.8mm.

Compared to that you have the CBR250R with a 55mm stroke trying to make do with its counter balancer, a common point of failure on such designs when sustaining higher RPM's.

You have not given the weight of the piston - just assumed that 4 smaller pistons are lighter. The Kawasaki has 4 x 50 mm pistons. The Honda has a single 76 mm piston. It is very unlikely that your Kawasaki has a lighter cumulative piston weight.[/list]

Yes, I had not gone into the data sheet at the time of posting, as these two were the first to pop into my head when thinking of a comparison and you're right the cumulative weight of 4 pistons would most likely be greater than the single piston of the CBR250R's.

BUT having said that, rather than the cumulative weight of all 4 pistons, the individual weight and stroke length matters more as it determines the tensile load faced by the connecting rod and even without going into the specifics and merely estimating by displacement and number of cylinder I can be sure that the CBR250R's connecting rod faces more tensile load than the ZX25R's at the same RPM.

Do correct me if I've made any errors.

Regards,
A.P.