Lowering a rotaries compression ratio?

[StavFC]

As title (not with a broken engine, lol), is this ever done, and if not, why not?
 
You rarely see rotaries running more than about 1bar without either race gas or water injection and lots or prayers, as obviously det is the big killer.
 
Standard RX7 turbo comp is 9:1? For a hot running turbo engine susceptable to det thats fairly high, but is there a proper reason why people dont go a little lower?
Should massively reduce the chance of det without needing higher octane fuel (ie non pump gas), meaning more boost can be safely run.
 
In my experiences of owning and driving countless cars which have had the compression lowered, its of little concequence, the low down tradeoff you barely notice as it doesnt affect how soon a car reaches boost (its only a small % you lose, lose a lot less than lairy overlap loses low down, and nothing is that powerful down low anyhow), and once one boost its a huge advantage, as you run a lot more boost.
 
I mean, even using an RX example, the EGI FCs dont feel mentally more powerful low down do they, despite being .7 of a point higher compression.
 
One things for sure, an engine thats got 500bhp from higher boost smaller turbo sure as hell is a crapload torquier and nicer to drive than a 500bhp car running 1bar.
 
Any particular reason? (ie not enough meat on the rotors etc)

[BlitzBoy]

Have a read of this on differences between the FC and FD engines, it has info when 13BT run with a 8.5:1 compression rotors
http://se3p.com/index.php?title=RE:FC3S_13BT_vs_FD3S_13B-REW

Main reason is cost, as Mazda dont offer lower compression rotors then stock it would take another 3rd party manufacture. Given stock Rotors are £400 each a pop, a cost to a third party company in R&D would be even more.

You can run up to 1.2 BAR safely on pump gas if you have good tuning and a decent size turbo with a big enough hotside not to cause a build up of heat and restriction.

IMO just run a decent WI system to go past 1.2 BAR and they dont cost the earth, good bang for buck

[StavFC]

Interesting link, thanks
 
I have aquamist to go on my car, its a safe bet regardless, but from owning cars running 2bar on pump fuel, even running 1.2bar feels a bit odd, lol.
 
I presume stock rotors cannot be machined to have a larger dish for slightly lower comp then? SOME pistons have enough meat on them to be modified enough to drop the comp at least half a point and still safe at 2bar+ and 150bhp a piston, but then again most arent. Ive no idea how much meat is on a rotary to know whats safe there.

[Kash_]

The thing is on the Rotary engine you don\'t need to push high boost to get good power

[BlitzBoy]

Interesting link, thanks
 
I have aquamist to go on my car, its a safe bet regardless, but from owning cars running 2bar on pump fuel, even running 1.2bar feels a bit odd, lol.
 
.

yes but the amount of power produced by a rotary at low boost it would take a piston engine a lot more boost to produce the same power.
Look at my power 555bhp thats produced only using 18 psi on a T51, it would take more boost say on a Supra to produce the same power.

I assume you can machine the bowl of a rotor to lower the CR but then you run into balancing issues and of course you have to make sure the bowl is done correctly shaped for the combustion and how the flame front is developed etc etc (beyond me and my knowledge to explain lol) therefore it comes down to cost and R&D.

A piston to do is easy you just machine a certain thou off the top and just have to make sure its flat lol

[rx jamie]

and rotaries with big ports such as a bridge dont really suffer from as much lag as piston engines.

[StavFC]

The thing is on the Rotary engine you don\'t need to push high boost to get good power

No they dont but its not about peak power as such.
 

Look at my power 555bhp thats produced only using 18 psi on a T51, it would take more boost say on a Supra to produce the same power.

Maybe so, but thats down to head/cams as much as anything, ie they could get it at 18psi, but they not stupid, theyd rather run less cam/head and more boost to get far more torque and drivability.
Also they have the oppertunity to push the whole 750odd out of it without too much worries rather than stuck at 555! I mean, the spoolup you have now, plus an extra 200 hose and at least that much extra in torque too, it wont happen, but it would be nice! :mmmm
 

and rotaries with big ports such as a bridge dont really suffer from as much lag as piston engines.

They dont for their size, but regardless of when things spool up or what power they are, low boost means low torque.
Mine at mo for example, although its on boost from very low down, and has great power at high rpm, it drives like a powerful but small n/a engine, not much in the way of low/mid torque due to lack of boost.
But an engine running 200bhp per litre almost always has the same torque as power if running 2bar boost.
 
Regardless of all that, it certainly doesnt sound like something thats easily done on a rotary, and thats all I was interested in, cheers :Thumbsup!

[mx1311]

the only people who seem to be lowering compression ratio\'s are the big drag boys in p.r, the u.s and aus

most of them have their own machine shops, quite a few threads about it on rx7club

getting tips from said dragracers on how to do it etc dont bother, you wont even see their motors on show in the pits (covered with blankets and towells)

rotor motors are strange in regards to power figures, take blitzboy for example, 555fwhp and running low 10sec qtrs and terminal speeds that 800+hp supras struggle to do (even with his granny shifting )

was having this convo with kevin webb from aus, he was saying the physical limit of pump fuel in a rotary (largeport large turbo)he has found without water is 1.3bar


nose around on rx7 club.com in the racing section, it passes the time for me at work

colum

[Prof]

For torque you just need to port big - boost is the other way to get there - so a combination of both means running much less boost for large torque figures.  I think you\'ll find Blitzboys engine also makes a lot of torque due to the porting he\'s had done :driving
 
So if you\'ve got mild street ports and stock induction then you will not be getting the torque you could get.  Induction mods - removing the inlet restrictions, throttle body porting, inlet manifold, and then engine porting.
 
Same goes for the exhaust

[BlitzBoy]

Im getting just under 400 lbs of torque by the way

[mx1311]

still dont help the way you change gear

col

[StavFC]

Mines got modified inlet and exhaust manifolds, not like I think they have much of an effect on torque. Power yes..
 
But regardless, nothing, barring nitrous (or increasing compression hugely), you do to an engine comes remotely close to boost pressure for giving torque, its not how engines work. Prof, have you got any info on other high hp RXs barring Blitzboy thats not running 1.5bar+ thats got anywhere near as much torque as power?
Most I see seem to have torque in the low 300s even when power is 500+?
 
His at just under 400 with 550bhp isnt bad. But when 2litre turbos running more boost and pump fuel can have the same power but still 100lbft more, you see where im coming from.
 
But you cant have it all, just wondered if you can, not the end of the world!

[BlitzBoy]

still dont help the way you change gear

col

lol at least I dont go through gearboxs like they were packs of clunge protectors

[Rzla]

lol at least I dont go through gearboxs like they were packs of clunge protectors

And it begins

[Rzla]

Mines got modified inlet and exhaust manifolds, not like I think they have much of an effect on torque. Power yes..
 
But regardless, nothing, barring nitrous (or increasing compression hugely), you do to an engine comes remotely close to boost pressure for giving torque, its not how engines work. Prof, have you got any info on other high hp RXs barring Blitzboy thats not running 1.5bar+ thats got anywhere near as much torque as power?
Most I see seem to have torque in the low 300s even when power is 500+?
 
His at just under 400 with 550bhp isnt bad. But when 2litre turbos running more boost and pump fuel can have the same power but still 100lbft more, you see where im coming from.
 
But you cant have it all, just wondered if you can, not the end of the world!

All this and the rotary will still be faster!
100lbfts still won\'t help the piston beat the rotor!