Howard

I have a full race motor with all the good stuff, currently running 12.5:1 CR. I'd like to raise the CR, how high can I safely go without grenading ? I'm thinking of going 13:1, can I go higher? 13.5....14........kaboom.

Thanks in advance !

Steve Weiner-Rennsport Systems

Hi Howard:

LOL,...the only limits are your pocketbook,....

Seriously though, very high CR's require frequent pistons/rings/pins replacement cycles due to logrithimically-scaled stresses on those parts as well as rod bolts and bearings.

Beyond 12.5:1, its really all about diminishing returns and only done after everything else has been exhausted; head flows, specially shaped valves, custom cams, lots of internal work,....etc.

Years ago in my IMSA days, we did a 3.2 w/over 14:1 that made over 400 HP with slide-injection and LOTS of time invested in the heads. Needless to say, it was successful, but it was a 3 hour motor.

Howard

Steve,

I have the Varioram heads with intake ports at 43.5 mm, big RS valves, slide valves at 50mm. I'm thinking, outside of going 3.8 liter I could bump the compression up, and have let's say a 30-40 hr lifespan before rebuild. I need FWHP in the 375-400 range for my class, to be competitve with the factory 996 Cup cars. I'm allowed a displacement increase and /or any other mods but not allowed to go forced induction.
Just looking for an alternative to buying a GT3 Cup Car , wife says NO. LOL

Steve Weiner-Rennsport Systems

I understand what your wife is saying,...

Honestly, a 400 HP 3.6 is not going to go near 30 hours. Thats a 24 hr motor. You can get 400 HP with REALLY good heads, all the internal mods, custom cams, and a good seat of headers/megaphones.

If you pursue this, be prepared to throw away a bunch of parts every 24 hours of operation.

Howard

So, do you think the best bang for buck is to go 3.8- 12.5 :1 CR ? Stay with 100 hp per liter and be happy? ( 380 fwhp)
That was my initial plan, but I need to tear the engine down, open up the bore to accept the new cylinders, new J & E's ( either way ). The heads were ported and flowed by EBS, and I'm running long tube headers into short ( 12") straight 2.75 diameter pipes. The rods are Carillo/Andial, so they should be OK.

Geoffrey

My experience with engines like yours indicates that in the 375hp range is acheivable. Consider the RSR Endurance trim engine which has individual throttlebody, common intake, 3.8l, 11.3:1 compression and tuned for 98 octane (our 93 octane) is rated at 360hp. You will gain the most from paying attention to the induction (50mm are probably too small for much over 375hp), intake, and exhuast. With improvements in this area, you can run a larger cam and increase the RPM range of your engine which will allow for more HP (hp=torque * RPM / 5252), however, your lower end torque suffers. In other words, an increase in hp is a result of increasing the RPM peak torque occurs at. As Steve points out, increasing RPM and/or dynamic compression increases stress in the components and requires more frequent attention.

Consider that the Cup cars are much better aerodynamically, but they weigh in the 2700lbs range. I believe if you can reduce your weight and pay attention to aerodynamic details, a lighter weight 375hp air cooled car will be competitive against a Cup car at the club racing level.