I was recently talking to another board member when the subject of different HP came up between NA and blower application LT1's. For some reason we came to the conclusion that NA power seems to be more effeicent than blower power......For example I will use two well known board members which I personally know. This comparison is based on weight, mph, and rwhp....

Now Jason Short's 93 formula has trapped at 126 mph with a race weight of 3550lbs and a very close estimate of 470 rwhp. Rich Krause's 95 Z28 has trapped a best of 125 mph with 610 rwhp at a 4000 lb race weight.....

So if Rich came down to Jason's race weight of 3550 he would be trapping ~129 mph. How does this make sense if Rich is making ~140 rwhp more.

Can anyone explain this? I would consider both good drivers so that really couldnt factor in much. I just dont get it.

Without getting flamed can I go out on a limb and say NA power is.....better? Based on the numbers it looks that way.

EDIT: Also thanks to Jason and Rich for letting me use them as examples.

 

Gearing and torque. It depends on where each car makes it's peak HP and where they are when they cross the finish line. The high HP car may not be at red line at the finish line. If it had gearing better suited then the mph could be higher.

It's hard to make a comparison just knowing HP and weight. Low HP, heavy cars can run quick if the chassis and driveline are modified to best use what little power is available.

As a better comparision I've seen 12.0 cars usually running in the 106-110 mph range. When I ran a 383 SBC, I ran 12.0 at 117 mph using 3.27 gears and a 26" tall tire. I had the mph showing I had a lot more HP than the other cars but my gearing was all wrong. I shifted at 6800 rpm and crossed the finish line around 5500. If I was able to use 3.90 gears, the car would have ran mid 11's with the same MPH.

Both cars would need to be identically setup except for those 2 engines to use them as comparisons.

Changing the race weight changes a lot of other factors. You don't need deep gears to get the car moving off the line. The engine can rev up quicker so deeper gears are not needed going down the track shifting through the gears.

 

O.k. I'll take a stab at this.

What you need to look at to check this out is the average power over the certain RPM area in which the engines run.

If I remeber right Jason has a 396 LT1 and Rich has a 355 cube blown LT1. Since Rich has a centrifical blower he will produce more power up high and most likely less max TQ. A big cube engine can get you alot of average TQ which is what wins races.

One thing you really need to look at is the race weight, that 400+ lbs is a big reason they are so close. Another thing is that to go faster you need much more HP, there is not a linear relationship to HP and 1/4 speed it is much more logarithmic. That's why a 125 mph car is 500hp and a 200mph car is 1300 and weights much less.

I am a fan of a big cube NA engine, just like I would rather use a N2O system over a centrifical blower in a rules class because I can get much more average TQ in the RPM band I am using.

Bret

 

Good points Stephen Im sure Jason and Rich will chime in with their gearing, trap RPM, tire size, etc. to clear a little of that up.

Bret I also never thought about the logarithmic relationship between the two...

Hmmm

 

True to a point, cars with centrifugal blowers trap a bit slower than the rwhp suggests compared to NA. I think it's because of the "peaky" nature of the boost (an hp) curve for a centrifugal.

In terms of analyzing what's going on, here's my attempt to reason it out. My car is very difficult to launch without the nitrous turned on. It tends to either bog badly or spin madly. I really don't know if I ever had one completely satisfactory non-nitrous run. The configuration that I ran the car in most often made 713rwhp with the nitrous. The best run in that configuration was 135mph. For a 4,100lb car, it should take 780rwhp to run that speed. Using the same formula (no pun) for 126 and 3550 nets 549. So the difference is 79hp for Jason's combo and 67hp for mine: basically ther same.

I think that convoluted reasoning implies that you are on to something. The two cars, when my nitrous configuration is compared to Jason's NA, perform as expected. When I run with the SC, my car traps ~4mph "slower" than it should. And I think this is due to primarily one thing: the launch difficulties with my car without the nitrous. I think that if everything were optimzed though (gearing, tires, etc.) there wouldn't be much of a difference.

This whole exposition wasn't too clear when I read it over. What I am trying to get at is that at least in my experience, the torque curve of a centrifugal setup, combined with a heavy car, makes a difficult combo to optimze at the strip. To be honest, though it was what I was hoping for, I was sorta surprised when the nitrous turned out to make the car so much easier to drive to its' potential.

BTW: FWIW the configuration I usually ran without the nitrous was the ~590rwhp "street" setup, so the difference isn't quite so dramatic. Maybe 3mph and not 4-5mph.

Rich Krause

 

If we're talking about a centrifugal style blower, it's pretty easy to explain.

The airflow of a centrifugal blower increases with the square of engine speed; that is, it moves 4x the amount of air at 6K as it does at 3K. But, the engine "consumes" air in proportion to engine speed (2x the amount of air at 6K as it does at 3K, assuming a constant VE, which obviously isn't the case). The next result is that boost pressure climbs in proportion to engine speed (which is a well-known effect).

Well, the result of this is that a centrifugal blower will not add xx% of power to the powerband of the motor itself; there will be a larger power increase up top than there is down low. Because of this, the peak power numbers on such an engine will be larger than a NA motor for the same effective area under the curve.

A motor with a positive-displacement blower (like a Roots) should theoretically produce constant boost across the rev range (although it seems like that's not always the case, due to VE variations and blower inefficiencies), as the blower moves air in direct proportion to engine speed. Turbocharged motors are the real wildcard, as the compressor speed is not directly related to the engine speed. Depending on turbo sizing, you can get either a super-peaky and very narrow powerband, or a dead-flat horsepower curve (which is probably how Dale Basemann can get an Impala SS to run 134 MPH trap speeds with "only" 675 RWHP ).

The bottom line is that peak numbers can't be accurately compared between two motors of different types, as the various power adders will affect the horsepower curves (and therefore the area under the curve) in different ways.

 

What tranny is Jason running? A loose automatic is going to come up with appreciably less RWHP than an M6, but the performance isn't going to differ that much. You need to compare flywheel HP.

And nitrous throws a complete monkey wrench into the calcs, because nitrous is the opposite of a centrifugal blower..... large amount of torque added across the full rpm range, really fattening up the "area under the curve".

Rich:

You might want to try my HP calculator. It shows 135mph in a 4,100# car is 730rwHP for an M6 (12.2% loss)....... .

 

I've had the same results and agree that it's due to the 'peaky' power curve of the blower cars.

With 416rwhp 373rwtq and 3,600lb raceweight, my car ran 120.5mph

With 634rwhp 580rwtq and 3,800lb raceweight, my car ran 128.4mph


Another thing I noticed is that when my car was naturally aspirated, it would gain about 1mph if the tires spun, and would lose 1mph if it hooked. It would also lose mph going from a radial to an ET street.

With the blower, it would trap 125-126 with radials and a 2.0-2.2 60ft.

With ET streets it ran 128+ with a 1.7 60ft. (and that was on low rpm bogged launch)

Would have been interesting if I got a really good launch out of it.

I'll never know now though because I'm switching to turbos