[BLACK KNIGHT]

I understand that there's a difference between RWHP and HP at the flywheel, the HP at the flywheel is more because you lose some via the drivetrain. But when Pontiac or Chevy says that the LT1 is rated at 275 hp, what exactly does that mean? And what exactly is Brake horse power (BHP)?

Sorry for the noobish questions!

 

[Chris 96 WS6]

The factory numbers are flywheel HP. They are SAE Net HP ratings.

Gross HP is the engine run on a dyno stand with no accessories. When you see motors tested in Car Craft they are Gross HP rated. Also older cars before 1972 were all gross rated.

SAE Net HP is a standardized procedure that is also on a dyno stand but you have to have all the belts, accessories, exhaust, etc. all hooked up. All factory HP numbers are SAE net. So when GM says 275 hp for the LT1 that's what the engine puts out to the flywheel in the car with all the junk hooked up.

Rear wheel HP is tested on a chassis dyno and measures the output at the wheels minus hp loss to the differential, transmission, etc.

 

[BLACK KNIGHT]

Very cool info, thanks! I'm hoping to find a dyno in MD this spring maybe, I want to find out what the rear wheel horsepower is.

 

[Firehawk 526]

Excellent explanation! I know it varies, but isn't it generally between a 12-15% drivetrain loss?

389 motorhp vs 334 rwhlhp = 14.2% loss (In my situation anyhow).

 

[Chris 96 WS6]

15% loss for manual trans cars and 18% with automatics and the stock tq converter. Aftermarket high stalls will increase that number.

 

[BLACK KNIGHT]

15% loss for manual trans cars and 18% with automatics and the stock tq converter. Aftermarket high stalls will increase that number.

I take it you mean aftermarket high stall will decrease the 18%?? If it increased it then you'd get more horsepower loss...what would be the point of an aftermarket converter? Oooh, and would gear ratio have an effect as well? Like I have 3.23 gears but I understood that some had 2.73?

 

[AJ52486]

Torque converters with a higher stall rating achieve this by increasing slippage. More slippage = more drivetrain loss. People buy them so that their engine is closer to its peak power when they launch. If you had a cam that helped power in the 3,000 to 6,000 range for example, you would want to launch the car close to that rpm, so you have more power to work with. For your question on gears, maybe this will help.

 

[Chris 96 WS6]

No, aftermarket converters INCREASE the percentage. More stall speed = less efficiency. THey vary by brand but generally the more stall speed the more HP you'll lose. The advantage of the high stall is in torque multiplication.

Gear ratio does not affect the measurement of HP due to ratio alone, as HP is tied to engine RPM. a deeper gear (higher number) will be slightly less efficient due to more frictional loss, but its usually not noticeable.

Stronger aftermarket rear ends eat up hp too. A 12-bolt is 2% less efficient than the factory rear, and a Ford 9" is about 3% less efficient than a 12 bolt, partly due to mass and partly due to design.

 

[revolutionary]

Chris - the man with the super duper tech explanations!

 

[BLACK KNIGHT]

So a high stall converter will slightly decrease rear wheel horsepower but makes up for it because the 60 ft time is so much improved?

 

[Chris 96 WS6]

So a high stall converter will slightly decrease rear wheel horsepower but makes up for it because the 60 ft time is so much improved?

Yes.

Really big stalls can eat up 30+ hp, so I'm not sure I'd call it "slightly decrease"

 

[BLACK KNIGHT]

Wow, up to 30 hp? How would it help your 1/4 mile time then? Does it only help at launch or does it somehow help when shifting into 2nd and 3rd?

 

[Chris 96 WS6]

2 ways it helps.

1. 60ft times. .1 drop in 60ft time is commonly said to gain you .2 in ET. So you can afford to give up HP down the track because you get such an advantage out of the hole.

2. The concept of shift extension. The extra slippage means you use a narrower (and higher) rpm range through each gear, which means AVG hp to the wheels for the period of time you're in each gear is greater. So even though you're giving up HP through the converter, you're basically shifting your racing RPM range up, which essentially recoupes the HP loss.

You can't worry about peak HP numbers too much. Its all about the average.

 

[BLACK KNIGHT]

Oh I see, very cool info!