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Why aren't our motors rev-happy?

5K views 19 replies 14 participants last post by  NahumCC 
#1 ·
I've always wondered why Saab engines seem to be less inclined to high revving than other 4 cylinders. Even the highly modded Saabs I've seen don't even make it to 7K. What is the reason for this? What makes some motors rev to the moon and others are all mid-range power? I just want to open up some discussion. Here's a list of possible reasons I could think of:
Head design, ports, cams, springs; Crank balance, short/square stroke; balance shafts, weight of rotating assembly....others?
 
#6 ·
It also comes down to how the rotating assembly has been engineered and balanced. Saab choose to pursue driveable torque vs. high revving motors to produce power (low torque). It's all about preference of design and functionality. You could make a high revving saab but you'd need to redo the crank, rod, piston assembly as well as head work (or even a redesigned head for upper rpm flow).

Another thing to think about. When a fully tuned saab can an easy 300 torque even at the top of the rpm range it doesn't need the high revs to make the hp that the honda can make at 9k rpms with say 120 tq.

HP=(torque*RPM)/5252.
 
#7 ·
SAAB engines tend to be longer stroke than most 4 cyl. engines. Rule of thumb is the longer the stroke the less RPM they turn. Short stroke engines will usually rev very high, such as Hondas. The long stoke engines tend to make most of their power at lower RPMs and tend to like to be lugged a bit.(lower end torque) In turn most of your short stroke engines will make most of their power higher in the RPM band.(high end torque)
 
#9 ·
You take a 2.0 motor and put in solid lifters and a big turbo and cams and they will make 800bhp and rev on up to 9500rpm! They like to rev just fine, just have to get the head so it can handle it.

My motor wants to go past my 7k rev limit now and I have the 2.3 motor. I remember my 2.0 would sing to 7k and bounce against the rev limit with the gt3071. The hydraulic lifters will not support much past 7500rpm.

Check this out:

http://www.93ssr.com/engine/

http://videos.streetfire.net/video/saab-9-3-making-huge-power_14401.htm

Made max power of 850bhp way on up there.....

John
 
#10 ·
I tend to agree with John on this. I think our bottom ends should be fine, but its a matter of reworking the head and other HW to do something it wasn't originally designed to do. About the solid lifters, does anyone currently manufacture them or is it more of a custom thing?
 
#12 ·
Long stroke engine has a lot to do with it. That plus at higher revs you have to worry about valve float.
 
#13 ·
You can make any motor rev happy....take for instance the 8 valve motor we ran in our VW Drag Rabbit. In stock form, the 2.0L 8V would only make power up to 6200 rpm. After we worked the engine it spun and made power up to 9500 rpms with ease. I high reving motor is all about flow thru the head (large valves, high lift cam(s), lightened, balance, blue printed, etc).
 
#15 ·
That is true, I have seen some of the guys that make that big power from 6500-9500rpm, they don't even start going until around 6-6500rpm and then they spool-up and make the power way up top.

We start at 3500-4500rpm and run to about 6500rpm, a turbo that would make power way up top, a t76 turbonetics or a gt42rs or something like that would not even start going until 6k or more. It would be a great race car, but be pretty bad around town...

John
 
#16 ·
selection of HW is the key. for instance a B204L with HX40 is capable of providing full boost to 4000rpm and revs happily to 7800rpm. Physical engine besides head is bone stock including cams and yes it will hold just as long as you hav the mapping in order.
General statement, select a normal "garrett" and for sure you registry will be limited to 2000rpm but with other choices its possible to extend much much further up.
So the fact that if you want to remain decent midrege and extended top with big power is more than possible, but it wont happen with a Gt3071 nor gt3076;)
 
#18 · (Edited)
im not sure if anyone has mentioned this but the reason stock engines dont rev is turbo size and cams. these are kept as sensible as possible so the car almost drives like an n/a ie no lag or spool time etc. hondas with rev silly as they are designed to from the factory saabs arnt there designed for normal driving where 99% of the time your bellow 4000rpm.
if you are going to rev silly high past 7k rpm then you will need to look at lifters, valve springs and the oil pump, not to mention rod bolts, rods, pistons, cams, headwork etc. basically your completely redesigning the engine from what it was originally intended for. the 2.0 have a very oversqaure stroke and as mentioned when the above is done then can be made to rev 8000rpm plus. also remember the engine will wear out alot faster with increased revs another thing saab wouldnt want.
 
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