i saw 2 dynos tests on one of the SCCA Speed Touring Car Sentra SE'R Vspec's. one was done when the engine was brand new and it made 210 at the wheels. then at the end of the season (or after so many races) it made 185 to the wheels. what exactly causes such a decrease in power as the engine is ran, and how do they fix it?

 

hmmmmmmmmmm you think your engine would still have as much HP at the start of its life as it would at the end of 100,000 miles of hard driving???


not to mention RR is can do a number on a motor, missshifts, downshift at really high RMP's.


think about this road race is normally what 20-40 minutes long, right, well they spent 99% of that time in the top 1000 rpm. high RPM = more stress more stress= parts breaking down.

 

Well, I sort of agree but that is a lot of power reduction after just one season. 210 down to 185 is pretty significant.

One season and that much loss?... I think it's a combination of both, how it's built and how hard it's ran. I've seen stock honda motors with over 100k miles on them put up close to stock hp numbers on a dyno.

I think I need to research the details of this significan loss in power more closely.

As for fixing it, most if it has to do with freshening the head (cleaning valves and ports, port matching, new cam and whatnot). When it comes to leakdown, most racers here will live with 3% but will look at rebuilding the motor with new bearings and rings after 5%+.

 

I would think it is mostly valve and cylinder head related and to a much lesser extent the rings. I know Formula Vee guys who are lucky to get mid-50 hp number are regularly freshening the heads with valve jobs on an extremely regular basis, some every few weekends. They also set the rings up with as little drag as possible so that makes them quicker to wear and lose compression. Look at the small bore Midget and Bugeye racers who are fastest with a light trail of smoke coming out. At the high end at high prices, those things are a volitile as a hand grenade with the pin pulled. That is what we with the less stressed, reliable Japanese engines racing against them are hoping for.

The fresh tight valve job with as little drag from valve guides and from rings while balancing no compression loss is the key.

 

I might also question the accuracy of the dyno numbers. Unless they were obtained from the same exact dyno under similar conditions, they may not be very reliable.

 

Why would the factory Nissan team not rebuild the motors over the course of the season if they were experiencing such significant power losses?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Sean O’Gorman &raquo;</TD></TR><TR><TD CLASS="quote">Why would the factory Nissan team not rebuild the motors over the course of the season if they were experiencing such significant power losses?</TD></TR></TABLE>

im not really sure. most of the teams only rebuild once a season. there are a few that rebuild 3 times a season but the majority only once.


but im guessing the majority dont experience that much of a hp loss.

 

As the bearings wear, it should actually free up some HP due to the reduction in drag. I don't know how significant it is, but in theory some of the lost HP from compression / guide wear should be picked up in wider clearances over time.

 

Also need to take into consideration how much a motor needs to be built up to be competitive in Touring. The more built the motor, the more it will wear.

The Mazda teams were re-building motors much more often then the Hondas and BMWs because their engine needed almost 100% boost in power from stock. High comp, big cams, etc.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Sport_Injected &raquo;</TD></TR><TR><TD CLASS="quote">i saw 2 dynos tests on one of the SCCA Speed Touring Car Sentra SE'R Vspec's. one was done when the engine was brand new and it made 210 at the wheels. then at the end of the season (or after so many races) it made 185 to the wheels. what exactly causes such a decrease in power as the engine is ran, and how do they fix it?</TD></TR></TABLE>


Did they mention if there were Any "restrictions" on the motor?

SPEED Touring and GT are very well known for "evening" the field with weight rewards and power restrictions. Did they mention what if anything was different between the motors tuning when new and at the point at which is was again dynoed?

 

210whp seems very low for a World Challenge car. I am guessing this is not a front running car like the RTR Nissan SE'R.

Anyway, I lost about 20 whp when I bent all 8 of my exhaust valves on an over-rev. It still ran fine in VTEC but it did not idle very well. Surprisingly the loss of power didn't effect my lap times all that much and I went on and raced three weekends with them bent.

 

Same place, same conditions, same correction factor? If not that will obviously account for some of it.