Higher Compression Engine w/ Lower Turbo Boost

[nicholastanguma]

Would tuning things be easier if I kept a slightly higher compression ratio and employed lower turbo boost? Let's say this combination wouldn't necessarily be for attaining the biggest hp numbers possible, but rather for keeping the flattest power curve possible from idle to redline even going from Los Angeles to Denver.

For instance, according to this Effective Compression Ratio Chart (https://www.rpmoutlet.com/boost-compres ... -chart.htm), if I keep a comp ratio of 9.0:1 and run only 2 lbs of boost my effective compression ratio would be 10.2:1, and only 4 lbs of boost would yield ECR of 11.4:1.

Daring to keep 9.5:1 comp ratio and using only 2 lbs of boost would yield ECR of 10.8:1.

All of the above are below the generally standard accepted safe comp ratio of 12:1 (on 92 pump petrol).

Basically, higher piston comp ratio obviously means snappier "engine only" performance without turbo lag. But wouldn't lower boost psi also mean: 1) easier tuning, and 2) retention of power at high altitudes without having to chase after big hp numbers?

Hmmm, does this even make sense, a turbo used not so much as a big power adder but as a high altitude power retainer?

Engine is a 350cc single cylinder, air cooled, carbed, no intercooler.

[ptuomov]

If you’re interested in making similar power regardless of altitude or air density, you don’t need a higher compression ratio. Instead, you need a boost control that limits the boost such that the manifold absolute pressure is constant regardless of altitude. That is, a turbo compensated engine, like in a piston engine aircraft.

[BLSTIC]

You may find that smaller cylinders (particularly bore size) are naturally more resistant to detonation, all else being equal they require less ignition advance, and those equivalent compression ratios you've got probably don't apply if they ever even did. I've certainly run 15psi on my largely stock 8:1 EJ20 on 98RON and it didn't ping. 16:1 compression on pump gas with an engine tuned for mid-range breaks that equation. Current Mercedes M139 engine ups that to 30psi on 9:1, also on pump gas but this time with a warranty and meeting emissions.

For a flat torque curve you want detonation resistance and electronic boost control. Generally for a hot street machine you would tune most of the engine towards mid-range and raise the boost at the top end to make up for the lack of natural airflow. How that detonation resistance comes is up to you. You might run water injection, have good charge motion, use lower compression, all kinds of things.

[nicholastanguma]

You may find that smaller cylinders (particularly bore size) are naturally more resistant to detonation, all else being equal they require less ignition advance, and those equivalent compression ratios you've got probably don't apply if they ever even did.

Good to know, thanks!

[miniv8]

For horsepower, airflow is king.
The only time I'd favor higher compression for higher boost is when intake sizing is somehow severely limited.
That said, there is a line in the compression chart where engine responsiveness meets potential power output based on the fuel being used and vehicle platform.

The tuning won't be any easier with higher compression, I'd even go so far as saying it will be more sensitive for tuning, but the driveability and overall manners as a daily driver will be better.

For a 350cc single, I'd say you are in the ballpark with the numbers posted. I'd look into lowering it a tad more with chamber work and regain some of the takeoff torque with a heavier flywheel.
https://www.steahlyoffroad.com/flywheel-recommendations

[mag2555]

The 2021 Hyundai turbo motor is 13 to 1, yup you read that right!

[panic]

That chart assumes that boost and atmospheric pressure are equal contributors to cylinder pressure (14.7 ATM + 14.7 psi doubles static CR).
This is completely wrong.

[Nut124]

The 2021 Hyundai turbo motor is 13 to 1, yup you read that right!

Is it not direct injection that makes these high CR turbos possible? Detonation cannot happen if there is no fuel present during the compression phase?

The OP is going to run a carb.

[hoffman900]

The 2021 Hyundai turbo motor is 13 to 1, yup you read that right!

Is it not direct injection that makes these high CR turbos possible? Detonation cannot happen if there is no fuel present during the compression phase?

The OP is going to run a carb.

Direct injection helps, but it all comes down to knock resistance. The OEM's really do a lot of work on tumble / in cylinder mixture motion and pay close attention to the entire combustion process. It's a systematic approach, and not any one thing, but that's all engines if you're doing it right.

LoganD is a combustion engineer. Worked for AVL and now Ilmor

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.

Also a good paper: https://www.hondarandd.jp/point.php?pid=1251&lang=en
Development of New 3.5 L V6 Turbocharged Gasoline Engine for New NSX

The concept for Honda’s new NSX was to balance power performance achieving a high level of dynamic performance with a level of environmental performance meeting the demands of the times. A 3.5 L V6 turbocharged gasoline engine with a new frame was developed to realize this concept. In order to balance high power and high thermal efficiency, combustion was optimized through the use of in-cylinder direct injection and high-tumble ports, and the engine applies a twin turbocharger system featuring independent turbochargers positioned on the left and right banks and a fuel supply system combining in-cylinder direct injection and port injection. The powertrain was reduced in size and weight and given a low center of gravity in order to enhance vehicle dynamic performance. In addition, in response to the higher power of the engine, Fe spray-coated cylinder bores, a dry sump lubrication system, a three-piece water jacket, a swing arm valve train, and reduction chain drive were applied in order to help ensure cooling performance and frame reliability suited to the demands of sports driving.
As a result, the new engine achieves a maximum power of 373 kW (500 HP) and a maximum torque of 550 Nm across a wide range from 2000 rpm to 6000 rpm, in addition to complying with the European Euro6b and US ULEV125 regulations

Not much advice for the OP other than do your best to improve combustion efficiency and the knock resistance of the engine. That'll be hard with a carburetor and what I am assuming is probably a 2 valve, vintage hemi style chamber.

[CamKing]

Would tuning things be easier if I kept a slightly higher compression ratio and employed lower turbo boost?

Yes, we've been doing that in racing, for decades.
Easier to tune.
More power off boost.
No noticeable turbo lag.

In IndyCar, we figured this out in the late 70's and early 80's. The rules makers kept lowering the boost, so we kept increasing compression, ind increasing RPM. Right now, they're only running about 20psi.

The only down side we've seen, is increased heat in the cylinders.

[mt-engines]

So you want to build an na engine at sea level and a boosted engine at altitude?

Build an NA engine with say 10.5:1 compression.
Buy an electric boost controller, a big intercooler and a couple Baro sensors.


If you ran co2 wastegates, you could program a controller to activate at a pre set bar figure. No need for springs. Just air pressure will control the boost level

Low compression engines suck at high altitude. Higher compression makes up some of the loss

[ptuomov]

Beyond controlling the manifold pressure to absolute (not gauge) pressure, here’s another altitude compensation idea: a tiny bit of nitrous oxide.

When turbine has spooled and wastegate is partially open, the turbo compensation delivers the desired torque regardless of the altitude. However, at high altitudes the turbo will take higher engine rpms to spool than at low altitudes.

The way I believe this can be cured is with a tiny bit of nitrous controlled with a rpm and manifold absolute pressure map. If the nitrous switch is on and the throttle is at WOT position, then the engine will get a small nitrous shot as long as the actual absolute manifold pressure is BELOW the target absolute manifold pressure. If the target absolute manifold pressure is set to slightly below the steady-state load-holding WOT absolute manifold pressure at that rpm at sea level, it’ll only use nitrous to eliminate lag at sea level. This setup will automatically use more nitrous at high altitudes to spool the turbo better.

[rustbucket79]

What’s your thoughts on cylinder and head temperature during sustained load in the L.A. Heat?

Being air cooled, and unable to control engine temperature, coupled with non inter cooled and positive intake pressure would be of great concern to me.

[miniv8]

Can we discuss the effect a draw through carburation has on charge cooling v's a blow through on a single cylinder application like the OP states?
Will a drawthrough turbo effectly tune the intake as being longer, hence more torque in the lower rpm's once the turbo is up to speed?
Will the turbo chop up all resonance even if it's a single cylinder?
Will the draw through effectly cool the intake charge more than a blow through, allowing either more boost or compression?

[nicholastanguma]

The 2021 Hyundai turbo motor is 13 to 1, yup you read that right!

Is it not direct injection that makes these high CR turbos possible? Detonation cannot happen if there is no fuel present during the compression phase?

The OP is going to run a carb.

Direct injection helps, but it all comes down to knock resistance. The OEM's really do a lot of work on tumble / in cylinder mixture motion and pay close attention to the entire combustion process. It's a systematic approach, and not any one thing, but that's all engines if you're doing it right.

LoganD is a combustion engineer. Worked for AVL and now Ilmor

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.

Also a good paper: https://www.hondarandd.jp/point.php?pid=1251&lang=en
Development of New 3.5 L V6 Turbocharged Gasoline Engine for New NSX

The concept for Honda’s new NSX was to balance power performance achieving a high level of dynamic performance with a level of environmental performance meeting the demands of the times. A 3.5 L V6 turbocharged gasoline engine with a new frame was developed to realize this concept. In order to balance high power and high thermal efficiency, combustion was optimized through the use of in-cylinder direct injection and high-tumble ports, and the engine applies a twin turbocharger system featuring independent turbochargers positioned on the left and right banks and a fuel supply system combining in-cylinder direct injection and port injection. The powertrain was reduced in size and weight and given a low center of gravity in order to enhance vehicle dynamic performance. In addition, in response to the higher power of the engine, Fe spray-coated cylinder bores, a dry sump lubrication system, a three-piece water jacket, a swing arm valve train, and reduction chain drive were applied in order to help ensure cooling performance and frame reliability suited to the demands of sports driving.
As a result, the new engine achieves a maximum power of 373 kW (500 HP) and a maximum torque of 550 Nm across a wide range from 2000 rpm to 6000 rpm, in addition to complying with the European Euro6b and US ULEV125 regulations

Not much advice for the OP other than do your best to improve combustion efficiency and the knock resistance of the engine. That'll be hard with a carburetor and what I am assuming is probably a 2 valve, vintage hemi style chamber.

Super informative, thanks.