Big Issues with Cylinder Pressure (DCR)

[MadBill]

In my stock 1970 Z28, I checked the aircleaner snorkel temperature at 60 MPH on a 70° F. day. It was 125°, good for a theoretical 5% HP loss vs. ambient.

[hysteric]

Static is still static and the limiting factor. Adjusting cam timing events does change cranking pressure , but that's nowhere near combustion pressure which is what we are worried about.

Static compression ratio is useless as a predictor of performance unless we are comparing two otherwise identical engines. Never happens.

Dynamic compression ratio also has no value. Engines do not listen to compression ratios. But a DCR is very
valuable since it allows us to calculate cylinder pressures and...more important, cylinder temperature.

Cranking pressure matters little...except that it produces heat prior to ignition which then causes a very
substantial rise in combustion temperature,,,and pressure. Temperature, not pressure causes detonation.

But excessive combustion pressure for the engine design limits, tends to break hard parts. And with vibration, it will break even very strong parts.

Very interesting post David. Could you please elaborate on how you use DCR to calculate Cylinder pressure and Temp? Do you have a formula for this?

[David Redszus]

Static is still static and the limiting factor. Adjusting cam timing events does change cranking pressure , but that's nowhere near combustion pressure which is what we are worried about.

Static compression ratio is useless as a predictor of performance unless we are comparing two otherwise identical engines. Never happens.

Dynamic compression ratio also has no value. Engines do not listen to compression ratios. But a DCR is very
valuable since it allows us to calculate cylinder pressures and...more important, cylinder temperature.

Cranking pressure matters little...except that it produces heat prior to ignition which then causes a very
substantial rise in combustion temperature,,,and pressure. Temperature, not pressure causes detonation.

But excessive combustion pressure for the engine design limits, tends to break hard parts. And with vibration, it will break even very strong parts.

Very interesting post David. Could you please elaborate on how you use DCR to calculate Cylinder pressure and Temp? Do you have a formula for this?

The first step is to obtain DCR using:
bore, stroke, conrod length, IVC, chamber volume, and ratio of specific heats for the gas,
and enter the data into an excel spreadsheet. It will calculate both SCR and DCR and thermal efficiency.

Next we obtain IAT and IAP to compute and graph compression temperature and pressure
at each crank angle.

There are multiple equations involved so combining them into a spread sheet allows quick
and easy calculation.

[Sparksalot]

David, are your calculations assuming adiabatic conditions? Or are they perhaps based upon experimental data which of course would not be under adiabatic conditions?

Regarding other posts about inlet temperatures, with every engine I ever built fresh air at ambient temperature was a priority. Doing otherwise throws away free horsepower. Yes, it's difficult but challenges invite solutions.

[hysteric]

Thanks David.

I assume you mean distillation temps of various fraction?

Do you have an example you could show us if possible?

[MadmanMark]

I didn't think you need specific heat data. I thought DCR is all about calculating where the piston is when the intake valve closes. The calculation is all trig (no chemistry).

[F-BIRD'88]

In my stock 1970 Z28, I checked the aircleaner snorkel temperature at 60 MPH on a 70° F. day. It was 125°, good for a theoretical 5% HP loss vs. ambient.

I had a temp sensor in the air cleaner of my '70½ Firebird w/454 BBC also.. It was very revealing as to air inlet temps under various driving conditions. Especially during and following "hot soak" events.

[David Redszus]

I didn't think you need specific heat data. I thought DCR is all about calculating where the piston is when the intake valve closes. The calculation is all trig (no chemistry).

Yes, Mark you are correct. Specific heats are not necessary for the computation of DCR.
But what benefit is it to know the DCR? What useful information does that provide? Not better than SCR.

Unless we can use DCR to calculate compression pressure and temperature. Then we need to know the
specific heats of the gas. For a compression volume change, pure air will result in a different
(and higher) pressure and temperature reading than a fuel vapor. This where many calculator programs
go astray. We have intentionally ignored the cooling temperature effects of fuel evaporation and losses
(or gains) from metal surfaces.

I assume you mean distillation temps of various fraction?

Fuel distillation temperature curves indicate the boiling points for fuel fractions.
We can obtain the bulk boiling point using a weighted average for each fuel component.

But fuel ignition is dependent upon the auto ignition point, of each component, which is considerable
higher than its boiling point. Once ignited, each fuel component burns at a different temperature.

Compression pressures are easy to obtain. Combustion pressures are somewhat more demanding.
But compression and combustion temperatures are very much harder to obtain. And, cylinder temps
are also a factor of coolant and head temps as well as residual burned gases.

[nitro2]

We all know you can't run 87 with 13-1 even with direct injection.

Well why can't we run 87 octane with 13-1 SCR? Such a fuel would typically have a RON of 92 and a MON of 82.
Engines obey MON values, not RON nor index values. Interesting but not helpful.

Actually you can run 87 octane with a lot more compression ratio than that, been sitting on that technology for years, unfortunately electric cars are likely to be the big thing going forward.......so a technology that will get buried.

[NewbVetteGuy]

On that vette the dual electric fans are probabily a big mistake... Blocked airflow at speed... The factory GM system is better. big mech clutch fan proper fan shroud.
Proper fan pulley speed. big rad with proper ducting to n front including the critical chin spoiler..

I see trouble. correct ducting of cool air to the carb is also critical chin on these cars.

Thanks for the heads up and will monitor. Cold air intake is tough on these cars, especially with the standard hood (not LT1/BB hood) which, btw, is why the shorter EPS manifold vs RPM AirGap. And, will need to get airflow through the three core rad, the Vintage Air condenser in front of that, and an auto trans fluid cooler blocking part of that as well. Will be running an 170 degree thermostat, a cooler plug, ensuring the chin spoiler is in tact, and also sealing the aluminum shroud as needed to ensure no issues with rad airflow. Yes, this could be a challenge on warm summer days.

The dual snorkel CAI's from the later C3 L82 are a good solution for cold air on C3s, unless you're making big power, but they usually get sold pretty fast and aren't super cheap.

These will REALLY cost you, but they work well even for big power and look fantastic. Designed by Paul Dana from the Corvette Forums; he ended up with 510 RWHP with that CAI in his AFR headed 427 SBC.

https://www.ebay.com/itm/Custom-made-CA ... xyTjNSiUjf

I didn't catch what year C3, but Paragon and Ecklers sell a modern day copy of the rubber extended front air dams that came with the later C3's as a major component of the "HD Cooling" option for around $70. https://www.youtube.com/watch?v=rtdP5fIiZDY&t=3s The pace car spoilers also greatly increase the air being forced through the radiator; from the factory when you selected the PC front spoiler the HD cooling option and rubber extension got "deleted". (GM decided one or the other was sufficient.)



Adam

[hysteric]

Thanks David.

Unless we can use DCR to calculate compression pressure and temperature. Then we need to know the
specific heats of the gas. For a compression volume change, pure air will result in a different
(and higher) pressure and temperature reading than a fuel vapor. This where many calculator programs
go astray. We have intentionally ignored the cooling temperature effects of fuel evaporation and losses
(or gains) from metal surfaces.

Some of that will depend on the fueling aspect in relation to the quality of the mixture and how homogenous it is in the chamber. I think the "fuel vapor" is the moving target that may be very hard to qualify.

[hysteric]

We all know you can't run 87 with 13-1 even with direct injection.

Well why can't we run 87 octane with 13-1 SCR? Such a fuel would typically have a RON of 92 and a MON of 82.
Engines obey MON values, not RON nor index values. Interesting but not helpful.

Actually you can run 87 octane with a lot more compression ratio than that, been sitting on that technology for years, unfortunately electric cars are likely to be the big thing going forward.......so a technology that will get buried.

Can you share what the technology is?

[mt-engines]

Well why can't we run 87 octane with 13-1 SCR? Such a fuel would typically have a RON of 92 and a MON of 82.
Engines obey MON values, not RON nor index values. Interesting but not helpful.

Actually you can run 87 octane with a lot more compression ratio than that, been sitting on that technology for years, unfortunately electric cars are likely to be the big thing going forward.......so a technology that will get buried.

Can you share what the technology is?

Injector timing, valve timing, ignition timing, combustion shape torque management etc.

Its amazes me how little people are willing to understand just to be stuck in the stone age. Thats why threads arent like they used to be. You get the same 5 people talking about 1980s technology when it comes to anything, changing peoples builds around that outdated information, and not wanting to listen to people that have real world experience.

I've run 12.5:1 with 25lbs of boost on non exotic fuels, but if i were to post the build sheet, and say im buiding a new engine i bet i would get about 3-5 people telling me to run a 115 lobe sep cam, 7.5:1 compression, tunnel ram with a 144 blower and 3 2bbl carbs.etc

look at an L86 engine in a Cadillac Escalade.. 11.5:1 compression, 100,000 mile warranty towing capacity of 8,000+lbs
Heck, I think it was 10 years ago when Mazda announced their Skyactive engines that run on regular pump fuel and is 14:1 compression.

But according to the speed talk know it alls, its not possible.

Lots of the knowledge here is still lurking around.. I speak to a few of them from time to time. And they just don't want to respond to help people because of how the posts end up going, or the fact that every other post is some imaginary build with a budget of only stuff they have sitting around.

[mt-engines]

On that vette the dual electric fans are probabily a big mistake... Blocked airflow at speed... The factory GM system is better. big mech clutch fan proper fan shroud.
Proper fan pulley speed. big rad with proper ducting to n front including the critical chin spoiler..

I see trouble. correct ducting of cool air to the carb is also critical chin on these cars.

I ran dual fans on my 70 vette, and never had any problems. Engine was a 358" Sb2.2. Even when it was the Dart headed L46, there wasn't a problem. keep the oil cool, the tune in line and the cooling system doesn't have to overwork.

[David Redszus]

Just to prove a point, Honda once built an engine with a compression ratio of 25 that ran on 25 octane fuel.
Not hard if you know how.