RONIN FC

I see your comparisson, but are our rotaries closer to the old pushrod V8 in terms of flow compared to the renesis?



I think the *main* purpose behind the no overlap is emissions and stopping unburned fuel from entering the exhaust.



So its pretty unanimous from the responces that a marginal overlap is beneficial to making power

mazdaspeed7

I never said our engines flow poorly. Actually, its quite the opposite. Show me a piston engine than can hit 130% VE. I do agree than less emissions is a good reason for no overlap, BUT, there has to be more to it. If overlap is always required to make the most power, why arent indy car engines running overlap? I consider than one of the highest levels of tuning/building. Tons of money is spent to extract every last bit of power.



High overlap engines are peaky, and more sensative to exhaust and intake. It will make more peak power, but its a 2 edged sword. You also have to deal with the reversion at low rpm hurting flow, and bsfc. Its always a compromise, each person has to decide what suits them best. There is no single, perfect answer here.

RONIN FC

I think your missing my point, i know rotaries probably flow more than most piston engines. But how do our engines compare to the renesis? thats why i likened our engines to old pushrod V8's compared to the renesis.



And here is some food for thought, most (if not all) of the near 1000 hp supras, 600 hp mitsubishi 4G63's all run overlap. Lets face it, were not pulling trailers with our Rx's. Were driving 2700 lb rockets do we really need the low end power?

White_FC

I think you hit the nail on the head with the BSFC comment mazdaspeed7, i'd say that'd be a huge consideration for a Indy or any other high level form of motorsport like that.

mazdaspeed7

Ill elaborate a little more. If zero overlap reduces the bsfc(a good thing) and makes for a broader, more usable powerband, without sacrificing much peak power, then whats so bad about that? Contrary to what many of you believe, you dont own a race car. You own a road car. Peak power is not the only thing that matters. I drive my car A LOT, and peak power is far from the top of my list. Drivability is by far number one, followed closely by ovarall efficiency. Im not taking purely about gas mileage, or I would drive a civic cx. Im talking overall efficiency of the motor. An efficient motor will simply drive better than an inefficient one, as well as get better gas mileage, and somewhat redice stresses on the motor.



Overlap can, and often does make more power. BUT, it is not without downsides of its own. With the experience Ive had, I would much rather have a motor with minimal overlap. One example was my last 2 engines. Both have the race ported exhaust, but on this last motor I moved the port down some. The port opens and closes sooner, and therefore has less overlap that the first one. The intake porting was nearly identical, and EVERYTHING else onthe car is exactly the same. This last motor with less overlap pulls more vacuum at idle, has a noticable increase in the low end and midrange, and the power doesnt fall off as quickly. Basically, it made my torque curve broader, and gave me more area under the curve, with no noticable downsides. I cant wait to get it on the dyno.

j9fd3s

yah it depends on the combo. i think on a street motor with mufflers the less overlap the better. we're getting ready to go to 4 port motors, for basically that reason. it closes the intake earlier.



the real reason though is cause we know it will do 200rwhp, without the experimant



mike

mazdaspeed7

I was referring to the exhaust port openign and closing earler, reducing overlap. Since youre using 4 port irons to reduce the duration of the intake port by closing it earlier, I think youre just shooting yourself in the foot. The late closing is what gives the high redline, and high VE at elevated rpm's. With a standalone and goot intake/exhaust, 200 rwhp shouldnt be a problem on a 4 port. But, I think it will prove to be a somewhat elusive goal with a stock intake manifold/ecu, unless you have some other tricks up your sleeve.

mazdaspeed7

And before someone labels me as anti 4 port, and assumes my statements implied that 4 ports are the work of the devil, and are useless for power, I was referring to one specific reason that I feel should NOT be a reason to choose a 4 port. There are plenty of advantages of both 4 port and 6 port motors. Its all about what suits the individuals goals.

RONIN FC

I agree with you but, i still think that *some* overlap is better than none on a performance street engine.



The way i look at it is why try to go against the characteristics of the engine, i would rather emphasize the strong points! Now im not sayin cut the engine open. but just enough to to force the last of the exhaust gasses out. Im not worried about power below 2500 rpm. But then again im not building a daily driver just a "streetable" machine.



Mazdaspeed7:



Im not doubting your knowledge. Infact i learned from your input! I just want to see where you stand and what your beliefs are. What im willing to deal with you may not like, and vice versa.



BTW I haven't seen anyone in here that thinks they have a race car only people that are willing to compramise low end power for more usable power.

mazdaspeed7

I do respect your goals, and every one elses. I have absolutely no problem with people who sacrifice some drivability for a little more power. But I just like to make it clear than I am NOT one of those people. Ive been at the bottom as far as drivability goes. At one point my car would stall every time you let off the gas, you could NOT drive it smooth through parking lots, it bucked like crazy at low rpm's, low throttle in low gears, etc. After than, it progressively got better. I got to drive a mostly stock FC today. It only had a catback and intake and pulleys. My car drove so much better, in all aspects. Thats saying a lot for my car. But its still nowhere near where I want it to be. To me, the ultimate goal is perfect drivability, with enough power. I want my car to drive at least as well as something you would find on a new car lot. Nothing less will suffice.



The race car thing was a little too general. I should have been clearer. My point was that pretty much everyone here has a modified street car. If you have a fully track-preppef car that gets trailered to events, and never sees the street, this obviously applies. But to everyone else, with modified street cars, drivability matters. No matter who you are, or how little you drive the car, drivability matters. Its just a matter of how much you require to consider the car livable. My standards are VERY high ion this concern, but I know not everyone is me. Again, there is no perfect answer to anything aotomotive. It is all a compromise. EVERYTHING that makes up these cars is a compromise. Which side of that compromise you want to work towards(or th emiddle, if thats you), is completely up to each individual. I do me best to present the facts as best I can, and let each person decide what suits them.