Forum » General Engine Building Discussion » Practical methods of finding what is restricting power

Practical methods of finding what is restricting power

General Engine Building Discussion

Talk about engine building here. New products, tricky questions or showcase your work - If it's engine building related it's welcome here.

Page 1

I'm interested in methods, devices, and techniques used in finding out what aspect of an engine is the bottleneck in power. Sure we can all throw money and parts at an engine to solve a lack of power easily, but I'm really wishing to come up with a scientific method to allow me to directly address a specific area, or areas, that are the limiting factor. This would be especially important in racing classes that either limit expenditure, or limit what parts can be modified.

In one example I have a 1994 Porsche 965 GT1. This car competed in BPR, and at Le Mans, in 1995 as a biturbo GT1 car against the likes of the Mclaren F1 GTR's, Ferrari F40LM, Jag XJ220, etc. Power was publically stated at 490bhp, but video of this car drag racing a Mclaren F1 to 200mph at Monza makes it clear that was a lie. Sadly, as is common in racing, the car was replaced by bigger better kit and forgotten. At some point it was converted to a non-turbo RSR spec and raced again. I've restored to original glory and built the best approximation I could of the engine, however no specs or photos exist of the engine. It's period correct in most areas, though I did cheat a little in using GTX28 v2 turbos and a modern ECU. For historic racing it needs to look "1995" to an experienced observer up close, so I can't go toss in some nice new bits in the engine bay such as a shiny new intake manifold. Currently, it makes 535whp at 0.95bar on my Dynapack. It completely keels over at about 5800rpm though, where I would expect to make power through 6500rpm. I somewhat suspect the throttle body to be too small as I used a throttle from a non-turbo 964 due to the race-spec factory cast manifolds being unobtanium. Would comparing plenum pressure to pre-throttle charge pressure be an effective method? What would an acceptable delta be? If that comes out good where to next? Intercooler flow and exhaust flow are certainly not issues.

That's a pretty cool project you've got going on there! I'm not a porsche specialist but the principles are the same for any engine. The fact that the engine is falling over at higher rpm normally points to something strangling the airflow in or out of the engine. I'd personally be measuring turbine back pressure and seeing what that's doing. Measuring the pressure drop across the throttle body and intercooler (which looks to be more than big enough) could also reveal a problem. Lastly the issues may be down to cam specification. Are you still running the N/A spec cams or has something more suited to a turbo application been chosen?

The cams are a custom grind based on the Porsche 993 GT2 Evo cams, which is a very similar build engine for a factory built race car. Dyno charts all bit don't exist for the GT2 Evo, but spec sheets list peak power at 7000rpm.

I'll prepare to instrument it out with some pressure sensors in various places next time I've got it ready to dyno.

If the cams are suitable for the application then the other aspect that I'd double check is the cam timing. Getting this wrong can have a major impact on your powerband.