Modern Engines & Static CR vs. Boost Levels?

[NewbVetteGuy]

All,

I'm really interested in understanding how to think about the traditional relationships between static CR and maximum power / boost levels in the context of a modern engine with dynamic timing of both camshafts, ignition timing, boost curves, efficient intercooling, and more efficient turbos.


-I've been looking at the BMW B58 and S58 engines lately and it's been quite the learning curve going from SBCs and LS engines to these modern engineering marvels, but I'm still curious just how much the traditional relationship between static CR and Boost levels can be "stretched" by all this technology and what role/ impact static CR still has today.

The B58 has an 11:1 static compression ratio and people are pushing what to me is a shocking amount of boost into these things. I'm just curious at a hypothetical level, what performance benefit/trade-off there would be to dropping the CR with a dished/reverse dome piston in one of these things - let's say you could drop the static CR down to 9.5:1 with all else being equal, how much more boost would that theoretically allow you to run and what would be the difference in power if you ran that additional boost? (Just rough "order-of-magnitude"-type numbers discussion.)



I keep hearing about the benefits of these high static CR boosted engines, but when push comes to shoove and BMW wanted to increase output levels, in the S58's they chose to drop the static CR and increase boost levels via 2 turbos instead of a single twin-scroll, so it seems the old "rule-of-thumb" regarding static CR and boost still exists, it's just been "stretched" quite a bit... Trying to understand the trade-off in a modern context and roughly how to calculate the benefits of dropping the static CR...



Adam

[mag2555]

The fundamental rules and pluses In regards to power of having a motor with high static compressions don't change even with 6 camshafts and a 10 71 blower feeding 4 turbo's!
The higher your boost pressure the more heat gets made and more you need to cool it before it enters the Intake tract to make full use of it.

In regards to BMW don't forget that they need to sell product and a 2 Turbo car is sure to sell better then a single Turbo car!

[hoffman900]

Between refinements in the cooling systems and A LOT of focus on tumble and subsequently better combustion, the engines today can get away with a lot more compression before being knock limited.

The naturally aspirated engines are no different, with compression ratios of 13.1 + commonplace on bore sizes 3.5-4” range on pump gas.

I would be cautious to drop compression and not give up the tumble / squish characteristics of what the factory has figured out, as you may actually lower the detonation threshold. No doubt you could do it, but the “how” would have to be thought out.

That’s really it in a nutshell though. It’s a function of cylinder pressure and the fuel type, and how much can you get away with without causing detonation. The OEMs are going about it by pulling heat out of the chamber and piston (multiple oil jets per piston), as well as a huge focus on tumble (4 valve engines) - which is effected by the port angle / included angle / bore size / and piston top shape. Also modern (OEM and high end aftermarket) ECU’s are helping a lot as well.

[peejay]

Well said!

In addition, notice those BMW engines have electric water pumps. They can run the coolant through the head full-bore all the time instead of compromising flow everywhere but a small island with a fixed water pump to crank speed ratio.

[hoffman900]

The old school way (think WWII bombers) of combating high cylinder pressure and heat, was more octane. I’ve read that fully loaded bombers were using leaded fuel in excess of 150 octane. Yikes!

No different than 60s muscle cars.. the knowledge and computational / engineering ability wasn’t there to know how to deal with higher cylinder pressures other than a lot high octane leaded fuel. 50 years later.. engineers have figured out how to improve combustion efficiency (and cycle to cycle repeatability) that raises the detonation threshold, and thus high cylinder pressures can work with a given octane.

It’s still a function of compression vs. boost pressure, but again, a lot of thought needs to be put into lowering compression without disrupting the combustion efficiency of a modern engine design.

Check out the cooling system on the new CBR1000 (214bhp/L in full emissions garb): https://www.cycleworld.com/2021-cbr1000 ... cati-v4-r/

[ClassAct]

The old school way (think WWII bombers) of combating high cylinder pressure and heat, was more octane. I’ve read that fully loaded bombers were using leaded fuel in excess of 150 octane. Yikes!

No different than 60s muscle cars.. the knowledge and computational / engineering ability wasn’t there to know how to deal with higher cylinder pressures other than a lot high octane leaded fuel. 50 years later.. engineers have figured out how to improve combustion efficiency (and cycle to cycle repeatability) that raises the detonation threshold, and thus high cylinder pressures can work with a given octane.

It’s still a function of compression vs. boost pressure, but again, a lot of thought needs to be put into lowering compression without disrupting the combustion efficiency of a modern engine design.

Check out the cooling system on the new CBR1000 (214bhp/L in full emissions garb): https://www.cycleworld.com/2021-cbr1000 ... cati-v4-r/

So you need a subscription to Cycle World to read the article? The page opens but I can't find the article.

[hoffman900]

The old school way (think WWII bombers) of combating high cylinder pressure and heat, was more octane. I’ve read that fully loaded bombers were using leaded fuel in excess of 150 octane. Yikes!

No different than 60s muscle cars.. the knowledge and computational / engineering ability wasn’t there to know how to deal with higher cylinder pressures other than a lot high octane leaded fuel. 50 years later.. engineers have figured out how to improve combustion efficiency (and cycle to cycle repeatability) that raises the detonation threshold, and thus high cylinder pressures can work with a given octane.

It’s still a function of compression vs. boost pressure, but again, a lot of thought needs to be put into lowering compression without disrupting the combustion efficiency of a modern engine design.

Check out the cooling system on the new CBR1000 (214bhp/L in full emissions garb): https://www.cycleworld.com/2021-cbr1000 ... cati-v4-r/

So you need a subscription to Cycle World to read the article? The page opens but I can't find the article.

Pops up fine for me and I don’t have a subscription

[Truckedup]

The old school way (think WWII bombers) of combating high cylinder pressure and heat, was more octane. I’ve read that fully loaded bombers were using leaded fuel in excess of 150 octane. Yikes!

No different than 60s muscle cars.. the knowledge and computational / engineering ability wasn’t there to know how to deal with higher cylinder pressures other than a lot high octane leaded fuel. 50 years later.. engineers have figured out how to improve combustion efficiency (and cycle to cycle repeatability) that raises the detonation threshold, and thus high cylinder pressures can work with a given octane.

It’s still a function of compression vs. boost pressure, but again, a lot of thought needs to be put into lowering compression without disrupting the combustion efficiency of a modern engine design.

Check out the cooling system on the new CBR1000 (214bhp/L in full emissions garb): https://www.cycleworld.com/2021-cbr1000 ... cati-v4-r/

I have quite a few books on WW2 engines and aircraft....
All US 4 engine bombers used dual stage supercharging, a turbo that maintained seal level air pressure to maybe 25,000 feet, and a mechanical blower to provide boost for engine power. All of them had intercoolers to reduce intake heat and fuel was 100/130 octane...Normally bombers only used full power for take off, about 50 inches of boost max, about 10 psi....The fighter aircraft mostly used dual stage supercharging at much higher levels..War emergency power, allowed for maybe 5-10 minutes ,could see over 80 inches in liquid cooled engines and a bit less for air cooled radials...Sometimes 150 octane was used but lead flouling spark plugs was an issue...Some used water/alcohol to limit detonation, others used various types of intercoolers..Limited use "sprint" engines used 100 inches of boost , intercoolers, water injection and exotic fuel blends like Tripane to control detonation...

[ptuomov]

BMW s58 and n58 engines may be instructive, does anyone have good piston photos? I think the lower boost engine runs 11:1 and the higher boost engine runs 9.5:1 compression ratio.

[lefty o]

with small modern engines, it seems to me that most of them that run what was once considered hi compression for a naturally aspirated are now running 10.5-11:1 with a turbo to top it off, and what i see as the common denominator is direct injection. seems that being able to time exactly when a shot of fuel is injected into the combustion chamber allows them to avoid much of the pre-ignition.

[ptuomov]

BMW s58 and n58 engines may be instructive, does anyone have good piston photos? I think the lower boost engine runs 11:1 and the higher boost engine runs 9.5:1 compression ratio.

Errata: 9.3:1 in s85 and the other engine is b58 not n58. As the original poster noted. Anyone have good piston photos?

[hoffman900]

with small modern engines, it seems to me that most of them that run what was once considered hi compression for a naturally aspirated are now running 10.5-11:1 with a turbo to top it off, and what i see as the common denominator is direct injection. seems that being able to time exactly when a shot of fuel is injected into the combustion chamber allows them to avoid much of the pre-ignition.

Partly. But none of the sportbike engines have DI. It’s all about mixture motion in the chamber as well as pulling heat out of the piston and chamber as well. Obviously EFI does help A LOT. Controlling cycle to cycle variation in combustion is another big component.


The bike engines, at least, have relatively large bores as well (equitable to a SBC and the like, but with shorter strokes (and much higher rpms), so the time for combustion is shorter).

[hoffman900]

Read these. Logan is a combustion engineer for an outfit that does a lot of work with OEMs, F1, etc.

Modern turbocharged engines run a huge amount of tumble. Not only does this help with mixture uniformity, it actively cools edges in the cylinder and prevents hot spots. It's like....having a fan on the inside of the cylinder before combustion starts.

This is true even for port injected engines, probably the best production example is the Ricardo designed McLaren V8. Those heads rate very high on the tumble index. The reason you don't see DI on superbikes is that they need all the valve area they can get and it's not possible to package central DI with their valve sizes and cooling jackets. As was stated earlier, superbike engines have quite a bit of tumble, although not as much as say....the new Civic Type R engine or A45 AMG engine.

Why is it beneficial to transfer heat from the cylinder walls, piston, and combustion chamber roof into the charge on a pump gas, knock limited engine? This is not a rhetorical question.

Hot spots act like a spark plug. Controlling hot spots is more important than an increase in pre-combustion air temp. On modern high-BMEP production engines there's also the issue of material longevity, this is an issue particularly on the exhaust side.

This is the same reason they use oil squirters on pistons.

"Why is it beneficial to transfer heat from the cylinder walls, piston, and combustion chamber roof into the charge on a pump gas, knock limited engine? This is not a rhetorical question."

Hot spots act like a spark plug. Controlling hot spots is more important than an increase in pre-combustion air temp. On modern high-BMEP production engines there's also the issue of material longevity, this is an issue particularly on the exhaust side.

This is the same reason they use oil squirters on pistons.

Ok, so if the problem is low speed preignition, I can see how more tumble is better. If the problem is spark knock, then more tumble is not necessarily always better.

If so, for downsized passenger car engines, or engines running very high octane fuel, more tumble the better. For a "tuner engine" running on pump gas and making power at higher rpms, there may be a point where more tumble is no longer better.

This used to be the old thinking, and it was based on the fact that as RPM rises charge motion naturally gets more chaotic so you don't need to design a port specifically for tumble at high RPM. Now we know you want to control that charge motion and design it to do things deliberately, this involves designing the entire intake system correctly.

This is how you've got Ferrari and McLaren turning 8500+ RPM with turbocharged engines making 200hp/L on 91 octane.

Ok, so if the problem is low speed preignition, I can see how more tumble is better. If the problem is spark knock, then more tumble is not necessarily always better. If so, for downsized passenger car engines, or engines running very high octane fuel, more tumble the better. For a "tuner engine" running on pump gas and making power at higher rpms, there may be a point where more tumble is no longer better.

This used to be the old thinking, and it was based on the fact that as RPM rises charge motion naturally gets more chaotic so you don't need to design a port specifically for tumble at high RPM. Now we know you want to control that charge motion and design it to do things deliberately, this involves designing the entire intake system correctly. This is how you've got Ferrari and McLaren turning 8500+ RPM with turbocharged engines making 200hp/L on 91 octane.

The new quote display for this forum sucks hairy scrotum.

With port-injected engines, the "old thinking" certainly looks it has been the dominant thinking. Maybe the solutions have changed. My impression was that in the port-injected engine era, the idea was to get a lot of tumble at low rpms or low loads such that the burn would be fast enough in those conditions even with a lot of EGR. In the port-injected engine era, the high rpms and high loads would take care of themselves. At high boosts and high rpms, sometimes charge motion was considered excessive and an intent was to slow it down.

That McLaren is a low compression, port injection engine "relic" so it's of particular interest to me. What's inside there? It has long intake runners and I am guessing very much a tumble port feeding a conventional four-valve head. But that's just a guess. Another guess is that the variable valve timing and using them right with turbos is largely responsible for the wide power band. With those log exhaust manifolds, the narrow bore spacing is going to allow it to rev higher, especially if the VVT takes out the valve overlap, I am guessing.

Well, I guess it depends on what you consider "old". You have to be careful because an engine released in 2005 had the combustion system designed at least 5 years earlier, so there's a time lag. Basically any engine with the combustion system designed in the last 15 years is going to have very high tumble due to shallow valve angles. So that would basically be any production engine after 2010. In the late 90's and very early 2000's high end CFD was still so expensive that it was being used sparingly and time-dependent CFD was virtually impossible with automotive development budgets. It was also pretty impractical from a time perspective, what used to take 6 months to run now takes 2 weeks and can be done on computers that are 1/100th the cost you would have paid 20 years ago.

I guess what I'm saying is that the drastic increase in speed and drastic reduction in cost for high end CFD has caused us to change a lot of our previous perceptions about engine design. The new Mercedes M139 is a perfect example, 20 years ago if someone were going to design a 2.0 turbo 4-cyl to make over 400 hp on pump fuel they wouldn't make it heavily undersquare (83x92) with almost no valve angle. That engine revs to 7400 RPM and makes 370 lb-ft from 121 ci.

It makes a lot of sense if you think about it. If you're knock limited and not airflow limited, which most is the case for most heavily turbocharged engines, it makes sense to design the entire engine around reducing knock instead of just trying to make it flow more. This is where the aftermarket is behind, a Coyote or LS head doesn't need more flow to make 1000 hp on pump fuel, it needs more knock resistance.

This thinking spilled over into naturally aspirated engine design. They now give the engine just enough airflow to make the RPM/power target, and then they design the rest of the engine around maximizing efficiency and cylinder pressure. That's why the new GT3 engine has 13.5:1 compression and pretty shallow valve angles for an engine that revs to 9000 RPM. They spent a great deal of time making the large bore engine very knock resistant, and that's a hard thing to do.

So does anyone have a recipe how to cost effectively modify an oversized, small cam, early 1990’s dump port into a more modern tumble port? Or is that “go straight to the foundry with passing go” sort of proposition?

This is going to be completely counter-intuitive for you, but the answer is to lower the roof. You want the angle of the port generally to be more out of line with the angle of the valve. You also want to get rid of any "turn" into the valve, you actually want the port exit to have an angle relative to the valve. This will absolutely reduce your bench flow readings, the key is to minimize that loss while still getting the tumble.

This is the exact opposite of what race heads do, they always raise the port and try to bring the port more in line with the valve angle. This is very, very bad for tumble. Part of the reason this is happening is that CFM sells heads, in the aftermarket it's always more more more bigger bigger bigger. Unfortunately combustion efficiency isn't an easy sell.

and...


This is all just a refinement of what Keith Duckworth figured out with the Cosworth DFV.

[Kevin Johnson]

with small modern engines, it seems to me that most of them that run what was once considered hi compression for a naturally aspirated are now running 10.5-11:1 with a turbo to top it off, and what i see as the common denominator is direct injection. seems that being able to time exactly when a shot of fuel is injected into the combustion chamber allows them to avoid much of the pre-ignition.

Partly. But none of the sportbike engines have DI. It’s all about mixture motion in the chamber as well as pulling heat out of the piston and chamber as well. Obviously EFI does help A LOT. Controlling cycle to cycle variation in combustion is another big component.


The bike engines, at least, have relatively large bores as well (equitable to a SBC and the like, but with shorter strokes (and much higher rpms), so the time for combustion is shorter).

Two articles from Cycle World separated by four years time:

https://www.cycleworld.com/2015/07/21/w ... m-phasers/

https://www.cycleworld.com/honda-develo ... rica-twin/

[gunt]

2 tings

if you were to run these compression with a fixed can and crap fuel you would have a problem on traditional injection the vvt ability gives the effect of a much larger cam and bleeds of dynamic compression , kinda the same as say the honda guys running std engine making big figures on boost , due to the fact the honda high comp n/a cam is far to big for the boost application and helps bleed dynamic comp,

if you look at the direct injection , between all the technology they have the ability when crusin to run 34:1 lean in the Mitsubishi GDI 54:1 so to run a little boost is realy no biggie