Is building a motor necessary for FI on this motor?

[gtengr]

Originally Posted by Harey

I dont agree with that, there are more forces on the rods/pistons at low rpm as there is more time under stress. A turbo running at 10psi at 4k would have more forces acting on the rods/pistons than an N/A setup at redline.

High rpm introduces other issues such as valve float etc but not higher forces on rods/pistons.

High rpm increases the stresses due to inertia (the crank is slinging the rods and pistons back and forth in the cylinder). Not sure which one of you is correct in this case without doing le math, but the big problem with 10psi of turbo torque at 4k rpm is that the oil pressure is RPM dependent, so big forces in the rod at low rpm are more likely to compromise the oil film at the rod bearings and damage the engine. Better to have slightly higher forces at high rpm where oil pressure can provide better protection. It's all a balance though you can blow the motor from high load/low rpm or high rpm.

[gtengr]

Originally Posted by Irace86.2.0

Forces are also an issue. I don’t know what is higher but motors with an over-stroked design would disinergrate from the stresses at high rpms. Our motor is square so it can rev high. Motors with higher redlines have under-stroked designs because Force = Mass x Acceleration, so a long stroke would require significantly more acceleration to rev high versus a motor with a short stroke.

There are several under-square engines out there with higher redlines. The BMW S54 is under-square and revs to 8k rpm and was designed in the late 1990's. Generally it's done the opposite way like you noted by going over-square, but under-square isn't necessarily a problem in moderation, and in the case of the S54 it produced one of the highest NA torque per liter outputs ever available in a mass-produced car.

[86 South Africa]

Originally Posted by Xxyion

I had the same though that the immediate throttle response was what i wanted for this car and therefore i went with the Edelbrock.

Personally, i actually felt it ruined the car. There was almost too much response. Because you are hitting boost right from the get go its just push and go. The difference for me with that and NA is that with NA, theres immediate response, however theres a smoothness to it. The edelbrock response was like a humming bird, immediete and jerky. NA for me is like a snake, also immediate, but theres this gliding smooth feeling to it.

I think if i were to do it all over again i'd go with a turbo. But if i had to do a Supercharger i'd go with the Jackson Racing SC. The Jackson has the advantage of feeling like stock (just with a tad more power) under 5k RPMS, but once you pass that point its like a rocket. It's also CARB legal.

Also have you joined the Sac86 facebook group? We have a few members that have various kits (including the WORKS turbo) and i'm sure they would be more than happy to give you a ridealong.

Interesting comments;
I have had my Sprintex sps kit for about 3,000km and 2 months now and although I build torque early too I don’t have such a jerky response (it’s actually really smooth low down generally)... unless you are driving slowly and then get hard on the gas then off again - which is just bad driving I think. Haha.

I drove a centrifugal car... and while they’re great I didn’t like the fact that it doesn’t feel faster with that slow torque building, even though it is better than stock.

[Irace86.2.0]

Originally Posted by gtengr

There are several under-square engines out there with higher redlines. The BMW S54 is under-square and revs to 8k rpm and was designed in the late 1990's. Generally it's done the opposite way like you noted by going over-square, but under-square isn't necessarily a problem in moderation, and in the case of the S54 it produced one of the highest NA torque per liter outputs ever available in a mass-produced car.

I think you are understating the significance of what engineers at BMW had to do to create the S54 engine, while ignoring the fact that forces on the piston and rods go up significantly with rpms. Can it be done? Yes, but that doesn't change the fact that the forces go up significantly do to increased accelerations/forces from piston speed.

Remember, the straight six is inherently a well balanced motor. The S54 replaced the S50 from the previous M3. The S50 had a square design with a 86x86 stroke length to piston diameter (same as us), and had a redline of 7600 rpms (ours is 7400). The engine had an iron block verses the M50's aluminum block and stronger components along with improvements to the head. The S50B32 increased the stroke length more than bore (91x86.4) to achieve a 3.2L, but the redline dropped to 7400 from 7600.

With the S54, they increased the bore slightly from the S50B32 from 86.4 to 87. The S54 engine in the Z3M had a 7400 redline versus the 8000 in the M3. Some argue this is why the engine in the Z3M has a lower failure rate than the same engine in the M3, that BMW pushed the redline to 8k for marketing reasons, and that it wasn't a safe limit for the engine. Regardless, raising the redline and the stroke took M quality engineering, including significantly reinforcing the block because of the extra stresses on the motor.

[gtengr]

Originally Posted by Irace86.2.0

I think you are understating the significance of what engineers at BMW had to do to create the S54 engine, while ignoring the fact that forces on the piston and rods go up significantly with rpms.

Nah, you said "motors with an over-stroked design would disintegrate from the stresses at high rpms. Our motor is square so it can rev high." I merely corrected you by pointing out an example that shows that's not true. Explaining what lengths BMW went through to put the engine in a couple of mass production cars isn't relevant to the point I was responding to, but yes, they did have to take extra measures for strength and durability. Obviously those limitations weren't that expensive to design around because they made tons of S54's.

[Irace86.2.0]

Originally Posted by gtengr

Nah, you said "motors with an over-stroked design would disintegrate from the stresses at high rpms. Our motor is square so it can rev high." I merely corrected you by pointing out an example that shows that's not true. Explaining what lengths BMW went through to put the engine in a couple of mass production cars isn't relevant to the point I was responding to, but yes, they did have to take extra measures for strength and durability. Obviously those limitations weren't that expensive to design around because they made tons of S54's.

Correct. There are examples of built motors with small stroke to bore offsets that can handle the forces.

[Xxyion]

Originally Posted by 86 South Africa

Interesting comments;
I have had my Sprintex sps kit for about 3,000km and 2 months now and although I build torque early too I don’t have such a jerky response (it’s actually really smooth low down generally)... unless you are driving slowly and then get hard on the gas then off again - which is just bad driving I think. Haha.

I drove a centrifugal car... and while they’re great I didn’t like the fact that it doesn’t feel faster with that slow torque building, even though it is better than stock.

Honestly its really hard to explain to people why i disliked the kit even though i'd still recommend it to everyone. It was just a feeling i had. Unfortunetly thats just hard to explain.

[Harey]

Originally Posted by gtengr

High rpm increases the stresses due to inertia (the crank is slinging the rods and pistons back and forth in the cylinder). Not sure which one of you is correct in this case without doing le math, but the big problem with 10psi of turbo torque at 4k rpm is that the oil pressure is RPM dependent, so big forces in the rod at low rpm are more likely to compromise the oil film at the rod bearings and damage the engine. Better to have slightly higher forces at high rpm where oil pressure can provide better protection. It's all a balance though you can blow the motor from high load/low rpm or high rpm.

I just know in the WRX FA20DIT, the rods can't handle more than 350wtq under 4000rpm. But after that people are running over 400wtq without a problem.

[Grady]

Stress on the rods pistons ect has nothing to do with square or not. It is the physical stroke that is the limit. An 86mm stroke at 8k your speed change of the rods pistons will be the same no mater how large the bore is. Blow the math out of proportion. You could never get a 200mm stroke engine to 8k no mater how big around you made the bore. What a larger bore than stroke gives you is a larger area to install valves. That makes it easier to get fuel in and exhaust out.

[86 South Africa]

Originally Posted by Xxyion

Honestly its really hard to explain to people why i disliked the kit even though i'd still recommend it to everyone. It was just a feeling i had. Unfortunetly thats just hard to explain.

I actually do kind of get it... quite hard to describe the changes a SC make beyond the extra power. It definitely changes throttle response and feel + a bit of extra weight up front for sure.
I guess I adapted to it a little differently to you. I’m loving the extra shove.

[Xxyion]

Originally Posted by 86 South Africa

I actually do kind of get it... quite hard to describe the changes a SC make beyond the extra power. It definitely changes throttle response and feel + a bit of extra weight up front for sure.

I guess I adapted to it a little differently to you. I’m loving the extra shove.

Yeah the weight was a huge thing I noticed. Especially the Edelbrock as the kit itself added about 80lbs on top of the engine. I could feel it every time I put my foot on the gas. I’ve decided I’m more of a turbo guy as that’s what I’ve always had.


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[Irace86.2.0]

Originally Posted by Grady

Stress on the rods pistons ect has nothing to do with square or not. It is the physical stroke that is the limit. An 86mm stroke at 8k your speed change of the rods pistons will be the same no mater how large the bore is. Blow the math out of proportion. You could never get a 200mm stroke engine to 8k no mater how big around you made the bore. What a larger bore than stroke gives you is a larger area to install valves. That makes it easier to get fuel in and exhaust out.

This seems reasonable in a practical sense, but not in absolutes, right? Like an F1 engine proportionally enlarged to the size of a bus would still rev high, right?

[Grady]

Originally Posted by Irace86.2.0

This seems reasonable in a practical sense, but not in absolutes, right? Like an F1 engine proportionally enlarged to the size of a bus would still rev high, right?

No the piston has to travel from TDC to BDC back to TDC in 1 RPM.

AT 86mm stroke, piston speed is X (No time for exact math right now)

At 172mm stroke same rpm speed would be X*2 because it has to travel twice as far in the same time.

And so on and so on, at some point piston speed becomes the limiting factor.

[Irace86.2.0]

Originally Posted by Grady

No the piston has to travel from TDC to BDC back to TDC in 1 RPM.

AT 86mm stroke, piston speed is X (No time for exact math right now)

At 172mm stroke same rpm speed would be X*2 because it has to travel twice as far in the same time.

And so on and so on, at some point piston speed becomes the limiting factor.

Piston speed (velocity) isn’t the problem. It is piston acceleration because force = mass x acceleration, so greater acceleration means greater forces.

What I am saying is that if a motor could rev to a given rpms then a proportionally larger engine could as well. And in that case, the larger motor would have a longer stroke, even if it was proportional, so there isn’t a fixed maximum stroke length, and it is more about undersquare and oversquare.

You would have to show that mass increases disproportionately because of surface area to mass or some other metric.