Current Peak Cylinder Pressures on Turbo S-I engines

[TrackRat]

I have read a number of threads here regarding PCP on turbocharged engines, mostly in regard to Diesel or other experimental or developing engine tech. The minimal info. on turbo S-I PCPs however left me wanting for more info. Since virtually all major pass car companies are downsizing to turbo 4 & 6 cyl. engines for EPA mandated mpg requirements in the U.S., I thought this might be a good time to revisit this subject as the prior threads were circa 2008 and a lot of changes have occurred in pass car turbo engine development since then.

My personal interest is for a turbo 3.2L VR6 engine project where I want to be certain that the head gasket is capable of managing the PCP under maximum boost even though this would only be encountered for short duration at WOT. I've reviewed many SAE and other tech papers on theoretical / modeling techniques to estimate PCP but the best real world measured hard data I have found was on the Honda turbo 1.6L F1 racing engine circa 2000. The PCP on this engine was 16.7 MPA, i.e. ~2422 PSI @ 2.5 bar absolute(A), boost @ 12,000 RPM. I will be running a max of 2.38 bar (A) (on rare occasion), at a more modest 6,500 rpm. The PCP is likely to be some what limited by the 92 R+M/2 premium pump octane fuel used based on my past experience. Honda's 1.6L turbo F1 engine like other F1 engines of the day used 84% Toluene with higher energy content but more importantly the ability to use 37 degrees total advance at ~12,000 rpm while only measuring 101.7 RON. (The F1 rules limited the fuel octane to 102 RON). I have dyno developed numerous turbo S-I engines with <2 bar (A) boost on Indolene (simulated 92 R+M/2 premium fuel), that limited max ignition advance to ~20 deg. total @ WOT in the RPM range from ~4500-6500 rpm.

On a four cyl. turbo S-I pass car engine circa 1990 that I worked on we were seeing ~1250 PSI measured PCP in this engine with ~2 bar (A) boost. Based on that engine I'm estimating the 3.2L VR6 engine to have ~1724 PSI PCP at the increased 2.38 bar (A) boost. I know GM, Ford, VW and many more companies are currently running some significant boost in small turbo S-I pass car engines so I'm hoping that those with firsthand knowledge can offer up some current PCPs on production pass car engines along with the associated boost pressure as this obviously alters the PCP considerably. I will not be running an over-boost system so I'm primarily interested in steady state WOT PCPs to see how close my estimated 1724 PSI PCP relates to other real world measured data at similar boost pressures. I am aware that there are many variables in PCPs not the least of which is fuel type, octane, IAT, static compression ratio, total ignition timing, etc.

I appreciate any input. I'm sure those working in advanced engine development have moved on considerably from the currently available turbo S-I engines, which in themselves have come a long ways from yesteryear turbo S-I engines in performance, drivability, fuel economy and reliability. My goal is 100% reliability in this 3.2L VR6 turbo conversion engine. I'm not looking for extreme performance in this application.

 

[Lou Scannon]

Biggest variable is knock, which is difficult to quantify with anything less than a fully instrumented test cell engine. Knock can produce some really frightening peak pressures, and corresponding damage to head gaskets, pistons, etc.
I'd say, if you can keep your PCP under 2000 psi, you're probably OK. As an indicator, do a stress calculation on your connecting rods in compression, and see what they're good for.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[TrackRat]

In the past I've done real time in-cylinder pressure traces to observe if detonation was present. IME with pass car S-I engine development every point from a little above the torque peak down to around 1000 rpm is knock sensitive in a boosted engine at all throttle positions down to ~25% throttle, on premium pump fuel with static compression of ~8.5-9:1 and boost of 2-2.5 bar (A). Above the torque peak there is typically a narrow band where timing can be increased somewhat with rpm and then often capped depending on the fuel octane, chamber design, burn rate, etc. This engine will be very conservatively calibrated on ignition and AFR to insure no detonation under any conditions. The small loss in power from safe calibration far outweighs the engine damage that can occur in a matter of seconds from detonation, IME.

By design the engine PCP is being kept below 1800 PSI to eliminate head gasket leaks. I'm curious what PCPs the current pass car turbo S-I engines are seeing as the boost pressures continue to increase.

 

[gruntguru]

Hemi. I doubt that rod stress bear much relationship to PCP for two reasons:
1. Very short duration of PCP plus piston mass vs elasticity of load path below the piston mean that rod stress resulting from PCP does not approach the value obtained from a steady state calculation.
2. PCP occurs very close to TDC when inertial load on the rod is at its greatest (tension)

Link Mercedes claim puts PCP in the region of 400 bar (6000 psi)

Link Gasketless interface for boosted FSAE engine.

je suis charlie

 

[TrackRat]

Thanks for the info. I'm looking for measured current pass car PCPs at similar boost for comparison purposes. We have used gasketless turbo I-4 engines in motorsport with good success. This particular project is a street use daily driver application. The 2.38 bar (A) will only be used for an occasional run down the drag strip. Normal street boost will be closer to 2 bar (A) max. The goal is 100% reliability even on those occasional runs down the strip.

 

[Lou Scannon]

Check out this thread on the Ford 2.3 Turbo Forum.
These engines can handle a lot of boost but the known weak links of the stock configuration are the headgasket and connecting rods (in compression). The weak links only get exposed under extremely high boost and/or poor tune conditions, i.e. high peak pressures.
Naturally, there are several robust, aftermarket solutions for both weak links.
If you doubt the assertions about bent connecting rods, go ahead and sign up, and challenge them.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[TrackRat]

Back in the day of the 2.3L Turbo I-4 Ford engine, we tested a dual sparkplug cyl. head intended for marine use. The second plug actually created rumble and unacceptable operation likely due to the unusual chamber shape.

This VR6 engine is actually quite robust as the engine is reported to have been originally intended to be a Diesel engine. The OEM forged steel rods and crank as well as the beefy block are far stronger than a typical hi-perf S-I engine. With the stock rods these engines are known to run completely reliable with 8.5:1 and 2 bar (A) pressure. That being said, this engine will use Crower's strongest forged steel I-Beam style con rods, with Mahle forged 9:1 turbo pistons, pins, rings. Special stainless valves and bronze guides are replacing the OEM components to address the increase thermal load. This engine is being upgraded for the loads it is expected to experience.

So far I have not found any reliable source that indicates that the head gasket leaks on the VR6 engines even at 2.6 bar (A) boost, which I find amazing based on other engine development work that I have conducted in the past. The OEM gasket is a multi-laminate stainless steel (MLS), design that is reported to be totally reliable.

I'm looking for real world measured PCPs on similar boost pass car S-I turbo engines to see how they compare to my past experience with in-cylinder pressure traces and projected PCP for this engine.

 

[Lou Scannon]

FWIW, when I tore down my 2.3T after blowing the headgasket, I found that all my rods were "squished", i.e. shorter than the lower spec limit by 0.002 to 0.025". Subsequently, I calculated that a PCP of around 2500psi was sufficient to reach the yield strength of typical conrod forging matl(based on a somewhat careful estimation of the I-beam cross-sectional area).

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[gruntguru]

Link
"Maximum combustion pressure has been increased from 200 to 210 bar (3046 psi)" (Latest BMW 750D.) Diesel I know, but getting up there for a production car.

je suis charlie

 

[TrackRat]

The Ford 2.3L rods are reported to be just std. material and design so they are probably less than ideal for use with high boost.

Pass car Diesel engines are starting to approach PCPs typical of HD Diesel designs for increased efficiency and power. As the PCP increases attention needs to be given to everything else for structural integrity, engine vibration, etc. It's really cool to see the high torque and fun drivability of the newer HP Diesel engines and still obtain excellent fuel economy.

 

[gruntguru]

Hemi - it would be an interesting exercise to calculate the piston acceleration (at say 15 deg ATDC) at the lowest rpm you suspect PCP was yielding your rods. Then re-calculate the PCP required (after accounting for the tensile effect of accelerating the piston)

je suis charlie

 

[Lou Scannon]

gg, agreed!

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[PackardV8]

Given the variables of iron vs aluminum blocks, number/diameter/material of head bolts/studs per cylinder, head gasket construction, bore diameter, RPM, type and sophistication of EMC, is there really a PCP number valid for more than the one test engine?

And just for S&Gs, imagine the BRM V16 turbocharged with today's EMC capabilities.

jack vines

 

[TrackRat]

Jack - The answer is yes. By knowing what the existing application can withstand for PCP, the goal was not to see what design/construction is viable for other applications, but instead to see what PCPs were being experienced in current boosted pass car engines of similar boost pressure.

 

[durablack2]

Most GTDI engines on the market today operate around 100 bar peak cylinder pressure. You find a few engines such as the new AMG 2.0L that will get upwards of 120-130 bar. Diesels are higher but I don't have much experience with them so i cant say what they get up to PCP wise.

 

[TrackRat]

Thanks for the info. durablack2. I suspect many boosted port injected pass car gas engines are in the 100-120 bar PCP range primarily due to the octane limitations of pump gasoline. With DI of course you can run a tad bit more static compression or boost due to the cooling effect of DI over port injection.

 

[gruntguru]

. . and with rapid combustion courtesy of Mahle TJI or similar it is possible to use much higher PCP without detonation.

je suis charlie

 

[TrackRat]

Yes, when rapid combustion tech is cost effective for pass car use, the PCP can increase some depending on all the other variables in the mechanical composition of a given engine design.

 

[140Airpower]

Hi TrackRat, can you document this? "...the {greater?} cooling effect of DI over port injection...".

 

[TrackRat]

140Airpower -

Any number of auto companies can confirm in their testing that the DI gas vaporization process in the cyl. produced such a significant drop in induced charge chamber temp that they were able to increase the static compression ratio by as much as a full point, i.e. going from 11:1 to 12:1 in numerous engines with no other changes than going from PFI to GDI. There are likely SAE papers on this if you're interested.