refresh951

Very interesting write-up on CR vs Boost. It gives a very convincing argument for "lower" CR in forced induction motors as Rogue has been discussing:

http://www.modularfords.com/f17/incr...ressure-51059/

333pg333

I don't think too many people running modified motors in here are victims of huge lag. Most guys are now running decent aftermarket Mafs, Maps, Standalones etc where lag is reasonably insignificant. Sure, they're not like a modern factory small capacity, high c/r turbo motor like my g/f's Peugeot but nor do they fall off after quite low rpms. Our cars are performance cars which are driven in a spirited fashion be it on road or track. A little lag is fine by me. Feels great and is easily controllable with a modicum of anticipation.
Not to say that a high-ish c/r 16v 3L+ motor while running E85 or similar isn't appealing. But then so is a billet block / head with superior design blah blah...(Did someone say Norwood!)

refresh951

I raised CR a bit (8.3:1) for improving off-boost performance on my 3L. The 3L will have increased stroke to help down low but 10ish CR would lead to much higher peak cylinder pressures. I think a properly designed "lower" CR 3L could perform nicely off boost and could use higher boost pressure on the top end and it will hold together better for the long haul. In this way one could avoid installing a V8 motor.

TurboTommy

This is true
But, it's very difficult to increase the average cylinder pressure without increasing the peak cylinder pressure some. It's just inherent to how reciprocating engines work.
Even if one argues that an increase in BMEP can be achieved from higher cylinder pressures purely from later in the power stroke, it would be so late that some of the energy would inevitably be wasted out the opening exhaust valve.

I'm in the camp of higher CR. Let's try to get every last bit of energy out of the fuel we can; SI engines are wasteful enough as they are.
One could argue that we could help reduce detonation potential of the higher CR, with lesser exhaust back pressure on the head (bigger hotsides); but the higher CR would make up for possible increase in lag.

refresh951

Interesting that Corleone had some apparent issues with coolant "hot spots" resulting in HG issues. He went as far as to install 4 steam vent kits. Other high HP motors have reported similar coolant "hot spots". Corleone was running almost 10:1 CR. Is it possible these HG issues were not "hot spots" but rather head lift due to high peak cylinder pressures? My block is over half-filled but I saw no such coolant "hot spots". My 2.85L was running 8.1:1 CR. Any thoughts?

Paulyy

He ran a custom head, e85 and a custom cam which would have helpt. And he was revving to 7-8k because the power band moved to the right as of the high comp and cam.

LUCKY DAVE

Hard won tuning wisdom is that raising compression raises low speed torque but reduces top end power due to constricting combustion chamber turbulence, thus slowing the burn.
"Back in the day" when I raced AMA Superbike we would run different compression ratios for different tracks. Willow, Daytona, Elkart Lake, less compression (13:1 or so) to boost top end power. Laguna, Sears, Laconia, more squeeze (some guys were running 16:1 or more) to get it off the corners quicker. Of course, cams and cam timing were changed too.
Other tracks were a compromise setup. Note that we had unrestricted fuel quantity. Restricting fuel load like some classes do encourages high mechanical compression ratios, as the compression ratio is the efficiency ratio.

Dave W.

I can see a 16:1 engine having soft power up top. With compression that high it's like the entire combustion chamber becomes a quench zone.

One of the things I look for when choosing a comp ratio is the lowest ignition timing I see during a WOT pull. Typically timing is lowest at peak torque, and if peak torque occurs at lower rpm (think small turbo) then you either need to keep boost down or use a low comp ratio so that timing stays on the right side of BTDC. If you use the same comp ratio with a big turbo, then peak torque happens at higher rpm where timing is typically higher, so you can either run more boost or raise the comp ratio. So it's a juggling act between boost level, turbo size, ign timing, and comp ratio. At least that's how I see it.

Also, don't be afraid of low timing. It seems like there's a tradition in the 951 arena for having high timing. I know the APE chips that came in my car run rediculous timing during cruise. A bone stock EVO only has 4 to 6 degrees of timing at peak torque (21psi @ 4k rpm.)

LUCKY DAVE

Ya think?
And those same tuners were scratching their heads wondering why it wouldn't pull on top even with rock'em sock'em cams and way-too-big carbs ???? ()
The number of clueless tuners in semi pro racing rivals the number of restaurant owners who can't cook a tasty meal to save their lives...

Rogue_Ant

Yep - nearly always, as peak torque usually corresponds to peak volumetric efficiency.

I agree that Evos/DSMs/Hondas and other high-flowing, 16-valve, centered spark-plug, small-bore engines will be ok, and even require low timing advance. My friends ~500awhp DSM I tuned, was single-digit timing at peak torque.

However, that approach simply won't make power with the 951. They need timing, and lots of it compared to a 4G63, B16, ect. Sid's car picked up 10mph in the 1/4 just by adding timing. The large bores, off-center spark-plugs, wedge-style combustion chamber, and lower port-flow (compared to a 4-valve) all contribute to needing more spark-lead.

Lethal951

I agree Josh... but you really want to know the down side to a 800 hp Honda B16 or B18??..

Rogue_Ant

Ha, you besides having 800hp, but only 450tq?

Lethal951

LMAO!!!... That it's still a Honda POS

gencollon2

Oh yeah, coz the antique audivwporsche inline 4 is so much better than a Honda inline 4.

Obviously a modern 2.0-2.4L inline 4 with a 16 valve variable intake and exhaust head makes more power more efficiently than my low compression 2.5L 8 valve (and is somewhere around 100lb lighter from what I hear)

Black51

Ok ok, let's not get into a Honda vs Porsche flame fest people... since we all know how that'll end up.