View Single Post
911st 911st is offline
Registered User
 
Join Date: Jan 2004
Location: Sacramento
Posts: 7,269
Dave,

I mostly agree, that sentence got left behind when I was a hurry to get out the door and did not mean to leave it in undeveloped.

However, most 930 builds here are within some type of constraints like keeping CIS over going with EFI.

If we start with a clean slate and open budget we probably want a 6 speed dialed in to how we are going to use the car. If not, a 5 speed with tailored ratios, if not better gear spacing on our 4 spd.

If not-- maybe we should at least be sensitive to the limitations of the stock transmission gearing when making other mods and trying to shoot for a fat power curve as possible so that 2700 rpm drop going from first to second dose not leave so much potential on the table.


WinRice,

I grew up dreaming about Lingenfelter. He was the master of not only increasing the VE level of a motor over anything his competitors could but also doing so over a wide power range. He could have built motors to have made even more impressive gross HP numbers if he wanted.

However, if anything his builds are a lesson in building a motor with a wide power band that would work well with existing gears than building a motor to make the highest peak HP that might need to be beat hard or use special gears to work its best.

Thus, higher average HP over higher peak HP.


The new BMW turbo 6's are a study in making as broad of a power curve as possible. They forgo big HP choosing higher compression in I think the 10/1 range and run low boost over what Porsche's approach. On top of this BMW gets to use other more modern tricks like variable cam timing, run closer to the edge with knock sensing, and other goodies like specially tuned intakes to further broaden the power band. They also probably run more boost early in the rpm range and tapper it as the rpm increases for an even fatter power band. Still, they put a lot of gears with closer ratios behind it.

What can we take from this?

We might choose to build a motor that runs say 8.5/1 compression and .7 bar boost with cams that make power early for a nice low to mid range power curve that is as fat as possible in say the 4500-6500 rpm range.

The other direction would be to build something like a 6.5/1 motor that runs 1.2 bar boost with cams timed to make its power a bit later in say the 5000-7000rpm range.

Both builds have almost the same effective compression ratio. However the high compression build is going to be noticeably more responsive making about 10% more power pre boost and coming on boost and power early in the power curve for a relatively more responsive motor.

However, the low compression / high boost alternative will end up making about 30% more gross power power over its 2000rpm power curve but will probably be a dog comparatively off boost.

Most likely the higher compression low boost motor would be a lot more fun to drive in town and with a good driver might be faster than the big HP motor in most street and Drivers Edd track situations.


Anyway, we have a major drop in average HP with our huge 2700 rpm drop going into second gear (v about 1800 drop in the other gears or ). Ignore it and we are leaving a major opportunity on the table whether we fix the gears or build a motor that is least effected by it.

I guess the same can be said for our steep first gear.

BTW, a 930 that can make about 400cwhp is making about 260chp at 50mph with the shift into 2nd. If your are racing Brian on to the freeway with his 915, he will be making about 100hp more between about 50 to 70mph than you and he is going to get a big jump on you right where it counts.
Old 02-27-2010, 10:39 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #14 (permalink)