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Restricting the APV - why it's a good idea for the boosted Renesis
We all know that the Renesis engine was never meant to be boosted and typically think of these things as the limitations:
1/high compression
2/zero overlap
3/Limited flow through side exhaust ports
And I would like to add another : 4/ excessive intake port flow at high rpm (6port only)
Before I try to explain further it would be worth a read of this thread: https://www.rx8club.com/series-i-maj...theory-268776/
To summerise what's in that thread ..... I believe the (stock) Renesis Engine starts to choke the exhaust ports at high rpm/boost and as it gets to this point, an excess of hot exhaust gas gets carried over to the next combustion cycle . This puts the engine on a knife edge if it's running anywhere close to that point.
This effect (I believe) has cost myself and others , many engines over the years. Many have resolved the issue by running high octane gas (E85 etc) at all times but even with all that octane there still seems to be a ceiling on what can be achieved power wise.
So within these constraints, how do we get the fastest possible car SAFELY!?
The old adage that the fastest car has the most 'power under the curve' is well known.
This is why I decided restricting the APV ports was worth trying as it flattens the torque curve while reducing overall air flow. In so doing it is possible to increase boost without placing the engine in that dangerous flow situation mentioned above making the car faster - safely.
Pros:
When running a 'built' engine on high octane you can run more boost safely.
When running a stock engine on pump gas you can run more boost safely.
You can safely run higher rpms as there is less stress/torque on the engine at high rpm (think shallow apex seals and bearing loads in particular)
Very Useful where there is some boost creep from the turbo as it's possibly not dangerous if flow is restricted.
Could be very beneficial for undersized turbos. We know the stock Greddy works best with the APV blocked altogether . But what about those upgraded Greddy turbos that can run with the APV open .... making high rpm with one of these particularly dangerous.
You can have a faster car within the constraints of your fuel system.(a big deal when running ethanol through the stock fuel pump basket)
Cons
Higher boost does mean hotter intake air . This is offset somewhat by putting the compressor in a more efficient place on it's flow map (higher boost but lower flow) plus with a good intercooler it shouldn't be an issue.
Note ... I'm not trying to suggest this is some kind of holy grail for the Renesis..... just a small incremental improvement.
Almost forgot to mention : I've been kicking *** in roll racing events lately with this setup on a STOCK engine.
These charts are just general logs to show differences with APV open or restricted ... please ignore the numbers and look at shape of the curves only.
Exhaust manifold pressure :
Chart below shows EMAP with APV at full flow ... the steep gradient just after kink towards the end of the log is after APV opens.
Chart below is EMAP with APV restricted . Note how gradient levels off towards the end of the log as the restriction reduces flow.
Absolute load:
Log below of absolute load with APV fully open show how peak load is at end of log at high rpms. This is because volumetric efficiency is at it's highest after the apvs open.
Chart below shows absolute load with apv restricted : note how load is the similar for last half of log rather than peaking over last few thousand rpms.
Virtual dyno :
Chart below is VD with APVs fully open : Note how torque peaks at high rpm
Or to put it another way ................... on one of the most fragile engines out there, why make the most vulnerable rev range the place to make the most torque!?
after reviewing the data, I’m confused on how you came to that conclusion. Unless you’re referencing a specific set of conditions but missed mentioning it.
after reviewing the data, I’m confused on how you came to that conclusion. Unless you’re referencing a specific set of conditions but missed mentioning it.
I know you said to not look at the numbers, but I can’t see how you’ll arrive at the proper conclusion without doing so.
Because the numbers say that the unrestricted APV is making approx. 70 whp and 50 ft/lbs more than unrestricted. You can’t really ignore those differences wrt to the loading difference on the engine.
and the unrestricted torque output is actually flatter than the restricted roller coaster ride, just with a slight incline. Which the incline can be addressed with lowering boost as rpm rises if the means to do so is available based on equipment selection.
imo a proper test for unrestricted vs restricted would be with the boost adjusted to achieve the same output level. Because my expectation is that means lowering boost for the unrestricted apv. That in turn is going to alter how they really compare in some of the other regards.
if you disagree then ok, not interested in arguing over it. those are just a few of my observations.
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Because the numbers say that the unrestricted APV is making approx. 70 whp and 50 ft/lbs more than unrestricted.
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The reason I said not to look at the numbers is that those logs were taken at different boost pressures. So the only comparison that is valid is the SHAPE of the curve. I don't have unlimited time (nor the inclination) to get perfect logs all done at the same boost pressure with the same setup etc
As far as the roller coaster comment ..... if you had done hundreds of logs you would know that is just what you get sometimes. In reality the straight line without the roller coaster is the anomaly not the other way around. The important observation is, VE is higher with the APVs fully open and peak torque is consistently higher at high rpms. The charts may not show this as well as you would like but that is a fact.
I don't have a dog in this fight, and I am not a turbo guy, so what I am bringing up may have been discussed on some turbo thread years ago, and I have not seen it.
A million years ago, when I was riding and tuning bikes, I spent a lot of time with cam theory and practice, and the cam manufacturers all would grind turbo cams for the big 4 cylinder engines. Suzuki and Kawasaki were the SBC of the bike industry back then.
The turbo cams all had almost zero overlap, and short duration. The NA cams were directly opposite.
So I have read how the Renesis is bad for FI because of zero overlap, but I do not see how or why.
The compression (both static and dynamic, which is what Brettus is addressing here, I think) I can see is a problem for FI.
But the biggest issue that I see, is the exhaust port design, both in terms of size and placement for FI.
Because of which it seems like a good idea to move torque peak down to an rpm where the exhaust is less overwhelmed.
Hopefully I didn't go OT here, but this seemed like a good time to post this.
Last edited by kevink0000; 03-07-2023 at 09:51 AM.
So I have read how the Renesis is bad for FI because of zero overlap, but I do not see how or why.
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I agree , saying zero overlap is bad is not 100% correct . I was alluding to the fact that ............ with some overlap, much better power curves are possible. As you would have seen on the rx7 forum.
I have a bag of questions but I'll start easy:
1) How did you block the APV ports? Blanking plate(s) like greddy did 20 years ago? Won't that come apart pretty fast, with our plastic manifold and press-fitted nuts? Why not set the APV motor to closed, then unplug the APV motor and mask any CELs that come up from this? What about carbon buildup in the now-forever-closed ports?
2) To me, it's no surprise that less airflow overall translates to less EMAP and a less pointy EMAP curve. Is it me, or is this totally expected? Ignoring boost levels and speaking strictly about mass airflows on the exact same setup; it is to be expected that never opening the APVs, letting VE drop while boost remains constant will result in less flow. This is exactly what happens on NA 4 port vs 6 port engines; one caps at 180g/s the other at 210-215g/s.
3) You mention closing APVs then raising boost. Sounds like wanting to increase airflow, but with extra steps. Why not run the least ammount of boost that still makes as much power as desired ? Cars running high boost(oem or not) drive like s*** anyway; what you seem to be looking after is indeed flat torque curve - but that's nearly impossible to do on boosted engines. Even less so, on DIY turbo swaps. Even less so, on DIY turbo swaps with pneumatic wastegates and some aftermarket boost controller with a fixed boost setpoint. What you'd need, is at least, a boost by rpm table that can get you closer to your target flat torque from X rpm to Y rpm. I've done this myself but takes ages to set up.
4) Please do one more test for me: set whatever boost level you need to achieve 350 g/s by 7500 rpm(it should be about 10 psi in the UIM, as a helper to scale the MAF correctly); then compare EMAPs at 5500-6000-6500-7000-7500rpm with APVs open and without them. I'll share my results on the same scenario once I find time for it.
I have a bag of questions but I'll start easy:
1) How did you block the APV ports?
I didn't block them , I only restricted them. Basically fine tuning to make high rpm boost easier on the engine.
To do this I used a thin plate with a hole in it where the plastic UIM meets the aluminum LIM.
Originally Posted by ciprianrx8
Why not run the least amount of boost that still makes as much power as desired ?
Agree 100% .... that would be a better way to achieve the same result. I don't have a boost by rpm EBC to achieve this however. Plus having something mechanical that can't fail do the job does add a degree of comfort.
The reason I said not to look at the numbers is that those logs were taken at different boost pressures. So the only comparison that is valid is the SHAPE of the curve. I don't have unlimited time (nor the inclination) to get perfect logs all done at the same boost pressure with the same setup etc
As far as the roller coaster comment ..... if you had done hundreds of logs you would know that is just what you get sometimes. In reality the straight line without the roller coaster is the anomaly not the other way around. The important observation is, VE is higher with the APVs fully open and peak torque is consistently higher at high rpms. The charts may not show this as well as you would like but that is a fact.
if you say so then I can accept your word on those fluctuating differences since I only have the data that was presented, but what I’m still not clear on is the shape of the curve comment. Are you saying the shape doesn’t change relative to boost pressure and how hard the engine is being pushed? That isn’t what I’d expect the result to be.
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Are you saying the shape doesn’t change relative to boost pressure and how hard the engine is being pushed? That isn’t what I’d expect the result to be.
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I see your point.
It does depend on if the turbo is big enough to efficiently provide the air the engine can consume and if the system is good enough not to restrict flow and whether the wastegate will hold boost stable to the redline. Plus , as I've mentioned before ...the engine seems to have limits as well.
With an upgraded Greddy for example , you likely wont see higher torque at high rpm at all because it will be off it's flow map making hot air plus emap will be very high.
I agree , saying zero overlap is bad is not 100% correct . I was alluding to the fact that ............ with some overlap, much better power curves are possible. As you would have seen on the rx7 forum.
Is it overlap, or the exhaust port making that difference?
My wager would be on the latter...
But again, I am not a turbo guy, so I defer to you all.
Last edited by kevink0000; 03-07-2023 at 01:30 PM.
Is it overlap, or the exhaust port making that difference?
My wager would be on the latter...
But again, I am not a turbo guy.
It's both. But what you are saying about no overlap being a good thing only applies when the turbo isn't big enough and therefore EMAP is high. Where EMAP is kept close to IMAP you can make incredible power from overlap.. Just ask the guys over on the RX7 forum.
It's both. But what you are saying about no overlap being a good thing only applies when the turbo isn't big enough and therefore EMAP is high. Where EMAP is kept close to IMAP you can make incredible power from overlap.. Just ask the guys over on the RX7 forum.
actually there was a book from David Vizard from back in the mid-late 1980s where he went into all that turbo cam timing being different from NA timing was going down. Back then Gale Banks was making 1000 hp with V8s on pump gas. That’s an older known topic than some on here seem to be aware of. Probably around 1987 or so is when I learned about it
Just going to say this and depart the thread:
1. It’s better to make the same or even better output with less boost. There are proper ways to do it correctly. Restricting the port and thus raising boost isn’t the proper way to handle it.
2. One of the most critical, yet highly overlooked on this forum, modifications you can add to a high compression boosted engine is AI (water-methanol injection aka bug/boost juice). Most of the people mentioned to be lunching boosted Renesis engines didn’t have it. IMO it’d certainly eliminate the need to do what is being suggested here, along with just dialing the boost setting down some.
While the suggestion of this thread is only restricting the APV rather than shutting it off completely, understand that I’m stating the comments above even though ot gives credence to the intended purpose behind my old 4-port Renesis Turbo thread from 6 or so years ago. Which was all about building all teh boosts and torques as low and as early in the rpm range as possible, while limiting the top end with a lower rev limit, for making the zippiest, stoplight to stoplight Renesis possible.
1. It’s better to make the same or even better output with less boost. There are proper ways to do it correctly. Restricting the port and thus raising boost isn’t the proper way to handle it.
I would agree that just tapering off the boost towards the redline is the best way to do it .........but restricting the APV is the next best way to handle it if you don't have that option .... IMO
The point I make in this thread is real though .... Renesis engines are more fragile under boost at high rpms and some strategy should be employed in recognition of that if you want to make the most out of the engine/turbo setup.
Originally Posted by TeamRX8
While the suggestion of this thread is only restricting the APV rather than shutting it off completely, understand that I’m stating the comments above even though ot gives credence to the intended purpose behind my old 4-port Renesis Turbo thread from 6 or so years ago. Which was all about building all teh boosts and torques as low and as early in the rpm range as possible, while limiting the top end with a lower rev limit, for making the zippiest, stoplight to stoplight Renesis possible.
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No ..... while I have no doubt a very successful build could be done as you suggest ........... it would have a narrower power band and need to run at several psi more boost than a six port (restricted apvs or not) to match it. Which goes against the very objections you made in point 1. in the same post!
And at the risk of going OT again, a quick look at cam specs low/no overlap is still a thing, at least in the turbo bike world, perhaps because those heads flow so well vs displacement. Maybe not valid for American V8 power, or rotary though.
1300cc/450hp street Hayabusas, zx-14 etc. Some 700-800 hp race only.
When the first VTEC was announced, (with 2 cam profiles, intake only) we all thought it was brilliant, since all cam timing was such a compromise. Now, even the cheapest econobox engine uses variators on int/ex timing.
The type of stepped, low bandwith intake valve variation the Renesis uses with solenoids and vacuum motors was becoming common in Japanese auto engines in the mid 80s.
So, the tech in the Renesis to optimize timing events is 40+ years behind piston at the current time, which is good to take note of, and try to mimic, since all IC engines have some inherent, concrete similarities.
Or if you can't outright mimic the advances in metal, then look there for the compromises made by the manufacturer for areas of improvement that can actually be attained.
Imagine true variable intake and exhaust port timing, optimized for rpm/load.
By which I mean the exhaust is the primary impediment to more power, either NA or FI, for the reasons of necessary compromise.
Unfortunately there is very little that can be done by way of port timing to improve that situation. My small PP exhaust experiment did prove (to me anyway) that there were gains to be had high in rpms (8000 plus).
4) Please do one more test for me: set whatever boost level you need to achieve 350 g/s by 7500 rpm(it should be about 10 psi in the UIM, as a helper to scale the MAF correctly); then compare EMAPs at 5500-6000-6500-7000-7500rpm with APVs open and without them. I'll share my results on the same scenario once I find time for it.
I will do this at some point. I also want to compare peak flow at 10psi APVs fully open, and see what boost I need to run to get the same flow when APVs are restricted.
Came across this on FB which is basically the exact opposite to what I'm suggesting (lowish rpms and high boost). IE maxing out rpms and running low boost.
Here we have 407whp @ 9200rpm and 12psi boost. Spoolup is obviously poor and torque is low but peak hp is great and it would still be very quick car if kept in the 6500-9000 rev range. Basically on a stock Renesis with rx7 apex and studs and 60mm s300 BW turbo.
Will be interested to see if anyone follows suit and how well the engines hold up.
At what power range would you recommend installing the APV restrictor and are you able to post any pictures of the restrictor you have tried? Is there an alternative like a way to reduce how far the APV valve opens?
At what power range would you recommend installing the APV restrictor and are you able to post any pictures of the restrictor you have tried? Is there an alternative like a way to reduce how far the APV valve opens?
Depends .... on some Upgraded Greddy setups ...I'd definitely fit one to keep the turbo inside it's efficient flow chart . For a good system with a decent turbo .... anything over 10psi.
You mention zero overlap as a reason that it doesn't make as much power as it could in FI configuration, but I also remember you mentioning there's no point boosting a bridgey renny due to the overlap. Is this because the overlap is at the wrong part of the stroke?
Originally Posted by Brettus
I would agree that just tapering off the boost towards the redline is the best way to do it .........but restricting the APV is the next best way to handle it if you don't have that option .... IMO
So to confirm:
Best from the engine's perspective: yes APV, reduce boost towards redline (poor compressor efficiency, possible overspeed issues)
Best from turbo's perspective: no APV, increase boost and leave boost flat (better efficiency due to higher PR, but higher intake pressures)
You mention zero overlap as a reason that it doesn't make as much power as it could in FI configuration, but I also remember you mentioning there's no point boosting a bridgey renny due to the overlap. Is this because the overlap is at the wrong part of the stroke?
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Yes , beneficial overlap requires overlap from both intake and exhaust either side of TDC.
Originally Posted by ZenVirZan
So to confirm:
Best from the engine's perspective: yes APV, reduce boost towards redline (poor compressor efficiency, possible overspeed issues)
Best from turbo's perspective: no APV, increase boost and leave boost flat (better efficiency due to higher PR, but higher intake pressures)
kind of , although no APV is only a good idea when the turbo is too small to supply air efficiently at high rpm.
03-06-2023, 05:21 PM
