Flow rate of 2L 6 cyl vs 2L 4cyl

[rbsileighty]

I keep hearing that the SR should spool the same turbo faster than a RB20DET. This may be true, but I'm led to believe differently...

Given the argumument that the SR has bigger valves and bore than the RB, which they say makes it flow more, the RB still has 8 more valves than the SR and the same displacement. With more valves, the overall surface area of the valves would greater in the RB, considering the size diffence is not huge. I will be getting the measurements of the RB20's valves soon (friend has an extra RB head) and will do some calculations on the flow.

How would the bore size make a difference? I would assume it may effect rate, but I am drawing a blank. I'd imagine it would would be negated by the fact that one revolution is still 2L of displacement in both motors.

So given these two motors, which do you think would flow more, and which motors rate of flow would be higher at the same RPM (given the valves are not insanely different in size)?

I always hear this argument, and typically see SR's running more agressive A/R's than the RB20's do in a turbo. Let me know what you think!

[Q45tech]

"negated by the fact that one revolution is still 2L of displacement in both motors." ???????????

All engines take 2 rpms to fire all cylinders, it's 1.0 liters per rpm.

A I4 would have 2 big slugs of exhaust, the I6 has 3 smaller but closer together puffs.

In theory the more lighter [smaller bore] pistons the faster it revs for the same displacement.

[rbsileighty]

Yes, I do know that it takes 2 to make one complete... cycle. We all have brain farts, and that was one of mine (please just replace a one with a 2 in that statment). I remember changing that yesterday for some reason when I was at school (wrote this in a lab when taking a break from studying). I have a really big test today... been studying for it day in day out for the past 3 days... wish me luck!

Hopefully this does not undermine what you thought I know... I know it's a 4 stroke motor and takes 2 full revs to make it's full 2L. I am very familiar with the Otto cycle, but my fingers were going faster than my brain yesterday, who knows what was going through my head. Enough excuses, back to the point...

That is what I was thinking... It would be cool to have the two motors w/ the same turbo on it to see the actual. OK, have to run... study break is over.

[rbsileighty]

Ok... I really don't have time, but here is something I was thinking.

It almost still comes down to the one dimensional flow per cyl of the motor. Since ((Sectional Area*Velocity)/specific volume) = mass per unit time... and this has to be factored in with the flow efficiancy of the cyl, one still may have an advantage over the other. This is why I want those valve sizes. Solving that eq for exit velocity should give us an idea of which motor flows faster/more. With the theory of 3 pulses vs two big pulses... that would mean that on one revolution (got it right this time!) both motors are flowing 1000cc's... and it would come down to exit velocity.

Actually, writing that out might have helped me solve it with no calculation. The smaller valves should have the higher exit velocity, and they are both flowing the same amount of volume. Therefore, the 6 should be pushing the same volume of air as the 4 but at a higher rate. This is probably what you were saying/implying Q45tech... I just needed to sort my way through it for a concrete answer. Thanks for the help!

It would still be nice to know the actual flow rate. I will still try and do the calcs when I get the valve sizes.

[Q45tech]

You are forgetting the 3 dimensional aspects of flow.Many times small valves have more lift to compensate. What counts is the exhaust valve curtaian area and duration vs. lift curve.

Also the exhaust gas temperature is where the power is. How closely the turbine is to the exhaust ports.

[rbsileighty]

That is a good point. I had not thought of the higher lift. So what do you think? Do you still think the 6 flows more after stating those conditions? If so why?

The velocity would still be higher due to the nozzle affect of the opening's smaller diameter wouldn't it... and volume would be at least close to, if not at least equal with the higher lift.

[Q45tech]

It is not the velocity [which would denote excessive back pressure] but the total heat that turns the turbo charger.........assuming the turbo is correctly matched.Kind of a chicken/egg problem.

An I6 is a much stronger basic design than an I4, in a heavy car where 100-150 pounds is insignificant, it would be my choice.

[s13sr20chris]

just a thought...if mb says 4 valves is bad for converter light off(i know they are moving back to it) then might 4 valves siphon off less heat in a sr20 than in an rb20? that could affect spool without affecting flow much. it would seem that a square motor like the sr20 would not have as much flow as a big bore motor like the rb20(i think thats right). so many variables. thanks for putting up with that rambling mess.

[rbsileighty]

The bore on the RB is 78mm and the bore for the SR is 86mm. Thanks for the thought... the good thing about ideas is, right or wrong, they still make you think.

[s13sr20chris]

really? i was under the impression that the rb20 was oversquare. well thanks for the info and sorry for the misinfo.

whats your opinion on the total valve area? if it is larger with the rb(all else equal), i would assume higher top end with longer spool up.

[rbsileighty]

Yeah, the stroke is only 67.9. Don't worry about it... at least you were thinking about it. Who knows, your comment might spark someone elses thinking about something we haven't even considered.

[s13sr20chris]

dang, you nice bro. i have seen some folks get megaflamed for just thinking and not knowing. by the way, i edited my post to add some stuff.

[s13sr20chris]

ok, what is the stroke. i also cant wait to find out the valve specs so we can do some more speculating.

[psychic_mechanic]

Maybe I'm over simplifying this, but couldn't you just compare the volumetric efficency of both engines?

Personally I'd go with the stronger 7 main bearing engine if given a choice.

-Vinny

[Q45tech]

The peak torque is the point of highest VE...........but on a turbo the extra pressure sustains [partially overcomes the rising resistance].

Since acceleration is a derivative of time [Calculus] one would just measure the turbine rpm..........rate of change to derive the gas volume and temperature measure inlet /outlet temps..

http://home.arcor-online.de/ni....html

[s13sr20chris]

what he said.

[s13EastTN]

so...find a graph of an sr20det with say...a t3/t4 turbo that shows the turbo's rpms through the full power band and then find a graph of an rb20det with the same turbo that also shows the turbo's rpms through the full power band and compare and that should give us a good idea of which engine pushes more exhaust?

[s13sr20chris]

this dyno 2000 program i have should do that except i am having trouble trying to get the program to understand my turbo specs. the program will show rpm dependant boost, torque, hp, ve, etc.

[bigtom]

6 is more than 4...it must be better:). the I6 is more balanced in nature and generaly produces more torque. also I6 will rev much higher and in the lb-ft/sec game thats good. if it revs to 10,000 RPM is 3700 too late for full boost??? just a different perspective.

[Chally]

Very interesting topic. Had the same arguments with friends over here.In my thinking, because the 6 has more power strokes per rev, you would think it could turn the turbo more evenly & therefore quicker. But, saying that, my z18t would be at full boost at 2,200rpm, & stay there until redline.The turbo RB30, found in Holden Commodores, would also be at full boost at approx the same revs.The GT-R R33V-spec, I did up, also had full boost around the 2,200rpm & it has TWO turbo's & only 2.6l.

It all seems to come down to a lot of trial & error, as we've found out the turbo recommended for a certain unit worked worse than one that was supposedly too large. Go figure...