JacRyann

The way I see it, is like shoving clothes UP a laundry chute. Sure you have a lot of force when you shove a shirt back up. But then you relax to pick up the next item and the column comes back down. Then you shove something else up and it comes back down.

The momentary high-pressure available when the exhaust is open and piston pushing up can shove out the current exhaust. But later in the crank circle when the piston is near TDC and there's no more pressure, exhaust BP being > boost will move things backwards. It's like having built-in EGR. In an optimized NA motor, the exhaust-flow would still be moving outwards and pulling in the intake-charge.

333pg333

I'm behind most of you guys commenting here in terms of tech knowledge. However, continuing to think out loud...we clearly saw a large change in terms of response rate with the new Xover and smaller housing. Yet we didn't see a significant change in the total hp or shape of curve. We are monitoring knock and have knock block plus monitor at the track. The only thing we don't have is EGT probes. Might have to get around to these as well.

Raceboy

And it is not combustion pressure that breaks the engines, it's the detonation pressure where flame speed increases to ~100 times of that of a normal combustion (flame speed ~15 m/s in normal combustion, 1000+ m/s during detonation, meaning 100 times higher pressure).

While you can hear detonation, you cannot hear preignition and this is as destructive but cannot be detected, it just burns a hole in the piston quietly These two are often mixed up, but they are different things, though one can cause another.

Voith

Here is some good info.. http://www.modularfords.com/threads/...boost-pressure

gruhsy

Thanks for clarifying. That sounds familiar. I believe the books talked about the two different aspects. I will have to go back and reread to refresh my brain

Alan 91 C2

Another good discussion for peak cylinder pressure PCP. Basically 600-2,000 PSI

Our engines making 400 RWHP are probably in the ~1,200 PSI. To make 600 RWHP probably in the ~1,800 PSI.

http://www.sdsefi.com/meltdown.htm

Voith

Great article, thanks.

refresh951

I completely agree wrt CR. I would not recommend over 8:1 CR with the 8V head on a big power turbo motor.

If I was going to go high boost (25+ psi), I would build a stroker motor to help bottom end and go something like 7:1 CR. The logic in the Mustang forum posts is sound (see here). You can make more power with lower CR and more boost and keep the peak cylinder pressure lower. The bore size on our motors makes the argument even more compelling (especially with the 8V head).

Corleone's motor convinced me that 9:1 CR was going to lead to trouble. As awesome as that motor was he had issues with his head and I am convinced it was because of CR. The only big power turbo motors (8V) that survived any length of time always had lower CR (8:1). My motor is now in its third year with no head issues (8:1 CR).

Voith

I will also go with 8:1 4v.

Here is a link to 2012 Mahle book titled Pistons and Engine Testing, lots of very interesting stuff covered. (for instance ferroprint is tin covered for first run-in, thats why our pistons always look to have worn skirts but are still covered with steel plating, its only tin that's missing)

https://www.fsb.unizg.hr/miv/MSUI/Ko...ubner-2012.pdf

(please do not post it elsewhere as this is supposed to be university internal material)

Geneqco

Sean's (JET951) 16v 3.0 l is also 8:1 CR IIRC and he also runs almost exclusively on e85 and makes very good power.

I think a high power expectation from an 8v 2.5 l running 9:1 CR on e85, whilst possible with the correct components, makes the whole issue of back pressure paramount.

Whilst most of the discussion thus far has been biased towards HP which is obviously important, in my mind, the more important issue has been that just raised by Raceboy, Voith & Shawn... how long can an engine be expected to last under such conditions? The tell tale signs (head lift & associated issues) are already emerging after the very first dyno session with that configuration.

Voith

I wonder what the result would be if the same engine would run 7.0:1 CR high boost.

TurboTommy

Keep in mind , you can go with higher CR if you utilize a more aggressive camshaft. You get less dynamic compression (the real in cylinder compression) with a later intake valve closing and more overlap.

Geneqco

Yes, but high back pressure would tend to work against you on that.

Raceboy

Lol for a treet car I would not go lower than 8.5 with 2 valves and lower than 9.5 with 4 valves per cylinder.
SDS article was written some 10+ years ago or something? Please find me a factory turbocharged 4valve per cylinder car older than 5 years and with CR less than 9? I would guess that there are none or some with ancient drivetrain.

There are lot's of fears that are mostly based on other's failures and self inexperience. If one will tune 9.5 CR car like he would tune 7 CR car, it will obviously blow up. Tuning the combo to drive well is the single most crucial thing for longevity.
If you build 951 with 8v head and 9:1 or more CR and use factory Motronic, it WILL cause issues because the tune in it was not meant for this.

Thom

The "dynamic compression ratio" is a combination of static CR, cam profile, boost and ignition timing.

Corleone ran an unusually sharp ignition profile with a reasonable boost level to make high power from a responsive engine.
We can lower the ignition and increase boost to get a beefier torque curve at low and mid engine speeds and we may get the same peak power (since power = torque x rpm), but the engine will feel much lazier at higher engine speeds.

It's up to the end user to choose what they want, the two simplified extremes that I can see being :

1) Low CR + small overlap cam + "some" back pressure + boost + little spark advance to get a modern turbo diesel feel super efficient at low/medium loads and low/medium engine speeds

2) High CR + high overlap + as little back pressure as possible + contained boost level + high spark advance to get a super responsive engine at full loads between peak torque rpm and peak power rpm.

OEM will tend towards first choice for environmental reasons and user-friendliness, while racers will tend towards the second choice (but then this is probably changing since diesel is being used more extensively in motorsport).
Keeping an engine streetable but fun enough is finding the good balance between the two, at least as far as I have noticed.