Noob question on engine compression.
 

Hi everyone,
I have a lot of time to think here in Iraq while waiting to get shot at, so while I researched to find an answer to this I could not. I have read a lot of stuff on this forum about N/A's and TII's since I was thinking about a turbo on my N/A. Although I have decided not to do it, I was wondering, since an N/A has a higher compression but that changes when the car gets more miles on it due to worn out rotor housings, seals and gaskets.....and when doing a rebuild without changing the rotor housings the compression of the engine usually does not go back up to factory spec., right? So if the compression is kept close to that of a TII, can you put a turbo on the N/A and run a higher boost, or normal TII boost.....or am I just completely missing the point. Thank you.
By the way here are pictures of my cars: 1986 N/A rx7, 1983 Datsun 280zx.

Sure you can. Just make sure you have enough gas upgrade , so you wont lean out the N/A engine..

Thanks, so can you make the N/A match the TII's comression and run the same boost without worrying about the reliability issues I have read about?

oh , one more question. Would wiring the 5th and 6th ports open or closed help or hinder the whole thing, wouldn't the open ports provide more air and therefore more backpressure for the turbo?

Are you thinking that you can turbo an old n/a engine because it has lower compression? The lower compression in an old n/a motor is because other parts are wearing out. The rotor's compression hasn't changed. I don't think that's what you wanna do. Turbos have rotors that are lower compression.

Now you CAN turbo it if you want with the right amount of fuel, but if that motor is already on it's last leg and is losing compression...turboing it is just going to speed up the process.
I believe all turbo motors are 4port.

^^^what he said. Adding the stresses of forced induction to an already weakened set of Apex seals is a great way relieve yourself of having those pesky Apex seals.

good line of thought however.

plenty of people are running forced induction on higher compression N/A rotors, like everbody is saying, it all comes down to tuning, this is also true with the lower compression rotors. I wouldn't have any second guesses about going to forced induction on a 6port high compression engine.

Do a little bit of searching, there are a few on the forums that are already doing what you want to do, and there are great write-ups on it.

Thanks. I really appreciate the help. So since you say that all turbo motors are 4 ports, would it better to wire the 5th and 6th ports on an N/A open, shut, or just leave them alone. Again, thank you for helping me learn some valuable info. I am gonna search the archives for more writeups on turboing the S4 and hopefully get the thing done when I get back home.

That depends on how you plan on using the car... It will be less drivable with the auxiliary ports wired open, and will have a pretty rough idle...

I'd wire them shut

Hows the weather over there in hell?

Anywho, the first thing you want to do is not look at boost (psi) as your 'power' or 'goal', high PSI doesn't always mean a lot of power.

With a higher compression engine, you'll run LESS psi, but make MORE power than a 4port counterpart, so don't look at it like making the same boost as a TII, because you really don't need to do that to make equal power. But as has been said before, with proper tuning, and proper care, they can run a turbo just fine (Just remember, less boost = more power, more power = more fuel, and intake temps become a LOT more important, so you can't just crank the turbo up to 15psi like you see others do, it's simply not that efficient and it creates too much heat for the engine to handle, (not exactly, but just as an example))

Good luck, and watch out for those pesky explodin pedestrians!

The weather is actually getting sort of nice here, I think it's down to 110 degrees now, but there seem to be a lot more of the latter. I don't really want to run much more than 5psi, so I am not too worried, plus I found someone in Sav. who might be able to help me with installing all the stuff and getting the parts required together. Thanks for the advice.

you are confusing compression PRESSURE with compression RATIO.

compression pressure is what you test when you do a compression test, and it is a measure of the amount of air that leaks past the seals when cranking.

compression ratio is an integral part of the engine design, and is the ratio of the volume of the chamber at its largest (BDC on a piston engine) to the volume at its smallest (TDC on a piston engine)

when talking about boosting an engine, you are talking about the ratio. a VERY oversimplified way to look at is in terms of total compression. if you look at it this way, you can imagine that there is a certain point where the compression ratio becomes too high for the strength of the combustion chambers, and the ignition resistance of the fuel.

total compression is fairly simple... on a NA engine it is just the base compression ratio. with forced induction, it is base compression + boost pressure/2


this is the reason why turbo cars have lower compression ratios, because when starting off lower, it is possible to add more boost, which means more power, without reaching the max safe compression.

in real life its a little more complicated than that, but that gives you a basic idea

pat

Actually it's the measure of the amount of air that doesn't leak past the apex seals... :p:

So what do you think a safe boost level would be for an S4 N/A would be without modifying the engine to compliment that boost.

5-7 psi

cool, thanks...thats plenty for me, I just want enough power to try to keep up with the pesky turbo'd Miata at autox and to be able to drift cause I salod my 240 and ended up trying it in my 280....oh boy was that a bad idea....thought the old bitch was gonna fall apart. That thing is all stock and although it's in great condition it's a 2+2 and the power to weight is retarted, plus those cars have really slow initial pickup, so I don't think I'll be attempting that again.=)

if you get a standalone ECU, you can pretty much run as much boost as you want. i run 14 psi in my TII, with s4 NA rotors (and therefore compression ratio)

on the stock ecu, 8 psi is pushing it.

another point is that if the engine is weak enough to have poor compression, then it wont last too long with boost.