I've been reading a lot about compression ratios. Stock CA18DET is 8.5, NA is 9.5. 9.5 is difficult, but not impossible, to boost. You just have to be careful with the tuning. 8.5 is safer but a little sluggish at low RPM, so many modern turbo engines use 9:1 as a compromise.

You can custom order any size piston, so that's not my question. I have an NA CA18DE block. Is there any way I can get the compression down half a point without replacing the pistons? I've thought about:

1) Compare the 9.5 pistons to 8.5 pistons and machine half the the difference off of the 9.5 to get 9.0.

2) Get a head gasket that is thick enough to lower the compression half a point with 9.5 pistons.

I read somewhere about a guy in Australia milling down NA pistons, but I wonder how realistic this is. Sounds great in theory.

What do you guys think?

Thanks

I've seen guys machine their own valve pockets, but that was for 13-13.5:1 N/A applications. These pistons may not like that with boost. A larger duration intake cam will lower your dynamic compression (the only number that really matters). That would be my desired route, personally. Next time my motor goes together I'm opting for 9.0:1 and mild cams. A more aggressive cam would compliment the extra half a point. That, or just run a better fuel that will resist detonation. Maybe time for some E85?

The NA pistons can be milled down, but I wouldn't be too worried about it. AS mdb said, put some cams in it and that will help to lower the compression and bit.

How aggressive is a CA18 exhaust cam in the intake? I've got one of those I could use.

I hadn't thought about cams. They're expensive but cheaper than buying forged pistons.

How much boost do you think I could I get away with if I used water/meth injection?

They sell E85 here, but only in a few places. We do have 93 octane everywhere, though.

I was going to tear this engine down completely and put in 8.5 pistons, but I like the power I have at lower RPMs. I also put a lightened flywheel on, so maybe it's just that. I've done a few accidental burnouts when turning onto a highway from a stop sign which is always interesting...

It's not really aggressive, but it's noticeable. Water/meth + 93 octane will get you about as good as E85. Not quite as good because the E85 burns differently than gasoline, but from a detonation resistance standpoint, it's close.

What turbo are you running? I know that's a light car, so you're not going to needs gobs of power to have fun in it. I would think that even an S15 SR T28 turbo at 15 psi wouldn't be too much if you're the water+meth+93 octane. I was running 10.2:1 compression with that same turbo on 91 octane and didn't run into detonation issues until 10psi and REALLY hot days. Less compression, more fuel octane, more cam and water/meth should solve any issues I had an let you run way more boost than I was able too. And worst case scenario, you can switch to E85 and then you KNOW you won't have any issues.

Very interesting. Thanks for letting me know. I have a journal bearing T28 turbo.

So e85 and my setup would be enough? I might have to look into that more seriously. I'm going Megasquirt so I guess I could have a separate tune to get me home if E85 wasn't available.

15 PSI would be absolutely perfect. I've been dreaming about doing it but didn't have the tune. It's already pretty quick with 12 PSI and 8.5 pistons.

Thanks again.

Darian

E85 and your setup and you could tune for anything you wanted with no comprises. That's the great thing about E85. Unless you're making CRAZY power, the detonation resistance is so high that you can run optimal fuel and timing values and have no concern about engine damage.

Thanks. BTW what size cam would give me a 9.0 equivalent? I have to be honest and admit I don't know much about cams.

Hmm, that's hard to say really, but I think an exhaust cam on the intake side would get you close.

Thanks again. Is there any difference in timing between a de and det cam? I remember something about not having to advance one tooth if you're using de cams.

dhen wrote:Thanks. BTW what size cam would give me a 9.0 equivalent? I have to be honest and admit I don't know much about cams.

Time for some googleing there dhen! Look up "Dynamic Compression". Calculators galore out there.

Yeah, you'll have to know a couple detailed tidbits of info to figure up dynamic compression. Idk what'll be harder to find, our rod length, or the ABDC closing timing of cams for our motors. But I doubt an exhaust cam/DE intake cam will be aggressive enough to drop half a point. 256's would probably be minimum, but they'll do a lot more for a 9.5:1 motor than an 8.5:1 motor.

This gives me some ideas. Maybe 9.5 pistons are a good thing when combined with cams. Best of both worlds?

I'd be more worried about the sharp edges on the NA piston valve pockets than their compression rating. This isn't the 80s where "conventional wisdom" was you had to drop compression significantly when boosting. With the affordable intercooling solutions and advances in engine management and monitoring there's no reason to tolerate a sluggish engine these days.

^But we're building engines from the 80's there madsci. The ports and combustion chambers are still 80's tech, and most of us still have the 8-port head with one injector every second port. Not ideal for mixture homogenization, which is key for knock suppression.

Contrast the VQ37 from a 370z @ 11:1 CR and compare to Float's high CR (10:1?) engine. Float can't keep the thing from knocking on pump gas no matter how he tunes it iirc, but you can pump 10psi into a 370Z on 94 octane and the engine won't knock with a proper tune.

I am beyond impressed with what the VQ37 can handle as far as boost but even those types of fast-burn chambers are severely knock limited beyond 10-12psi. Beyond that you need to think seriously about lowering the CR and building the block. And you have to make sure you minimze turbine backpressure.

I guess what I'm saying is there's still the compromise between boost and CR, the modern heads just let you get away with more than most of us are reasonably capable of eking out of the CA head.

mdb4879 wrote:Yeah, you'll have to know a couple detailed tidbits of info to figure up dynamic compression. Idk what'll be harder to find, our rod length, or the ABDC closing timing of cams for our motors. But I doubt an exhaust cam/DE intake cam will be aggressive enough to drop half a point. 256's would probably be minimum, but they'll do a lot more for a 9.5:1 motor than an 8.5:1 motor.

All of the necessary specs can be found in the FSM. Most at the end of the Engine Mechanical section.

Buddy is right. 10.2:1 compression on a CA18 with pump gas and anything but low boost simply isn't possible. Even that's not really true as I still had detonation issues with 91 octane and hot days, so it still wasn't a viable option. I'm sure there were things I could have done to help the situation some, but you're never gonna run 10:1 compression and 15psi on 91 octane on a CA and not have to pull back the timing from optimal, even with a big intercooler and water/meth injection. Knowing what I know now, maybe I could could pull off 10psi on pump gas and a lot of detonation suppression tactics and still maintain optimal ignition timing, but I wouldn't bet any money on it either.

Here's where cams come into play again. People throw boost on top of 10:1 Integra GSR motors and even 11:1 RSX Type-S motors, but they have massive cams compared to us so the effective cranking pressure is close to the same. Granted, the internal engine design does help suppress knock, but for the most part the basic principle is the same. If they had tiny cams like we do they wouldn't have much better luck.

I saw a guy post some interesting info about the correlation between CR and ignition timing. It was meant for all motor, but it still applies to boosted applications. Say the threshold for knock is 800psi of cylinder pressure. You can either run 10:1 static compression with optimal timing, or you can run 13:1, but with far from optimal timing as long as you haven't reached the knock threshold. The cylinder pressure is the same and you'll make the same torque.

The same would apply to a boosted motor. You can run high compression and low boost, or low compression and high boost. Ultimately the cylinder pressure will be the same and the same power will be made. It doesn't change until you change fuel type and the knock threshold is raised, allowing you to reach MBT with the higher compression. But on the lower compression motor you had already reached MBT, so the fuel change doesn't really net any more power.

Of course it's not really this simple. There are many other factors that would change the knock threshold between the different setups and how the power was delivered. But for simplicity's sake, the basic principle applies. On Float's 10.2:1 engine he probably made gobs of power (relative) on 7psi. He could've made the same power running 9:1 on 12psi (random numbers), but the tuning would be more optimal with the 9:1 motor, and even though the same power was made it would probably be a safer motor. He went the other way and changed fuel type, allowing him to reach MBT with his current setup.

Come to think of it, some of this may be wrong in the boost application because the amount of air flowing into the motor is different. With all motor the amount of air is the same, only the timing has to change. So it really gets a lot more complicated than I may be making it out to be. But y'all get idea.

Check this out! We've neglected talking about the cooling system!

We should be talking about cooling mods and more effective ways to get the heat out. I think it was Darin Morgan who wrote, "If you can't get rid of the heat, the heat will get you!"

But check this out, Nissan went to great lengths to increase coolant volume and flow around the combustion chamber. Even reducing the amount of metal between the chamber and water jacket!:


Now compare to the CA18. Way more metal, way less coolant:



What kind of things can we do to help get the heat out??




Also check out the quench pads, they've rounded the edges like themadscientist was talking about re the valve pockets:

The FJ20 ran 9:1 compression with an intercooler the size of a phone book, a distributor and an antique Ljetronic engine management system and the same sort of squish pad pentroof chamber. I don't buy into this idea that the CA18 can't live at that compression.

To make sure my opinions about the issue are clear, for todays technology, unless you're running big boost (over 20psi) there's no 4 valve engine made that needs to run 8.5:1 compression. It's overkill anymore. 9:1 is totally workable and will require VERY little work to prevent detonation. 9.5:1 will probably require you to be more careful, and may start to limit the max boost you can run, but for a low to moderate boost levels, isn't much of an issue. 10:1+ (on a CA18 anyway) can be problematic. As I've said before, on 91 octane, with a moderately sized A2A intercooler, I had issues with detonation at high rpm's on really hot (100°F+) days and as such, had to run timing that was less than optimal. I'm not sure that a larger intercooler or water injection or cams or even cam timing, wouldn't have fixed the issue. My point being you can't just throw some 10:1 compression pistons in there, fill it with 91 octane and a chip tune, and be good to go. It won't work. Even on 9.5:1 compression engines (my experiences with turbo KA's), the distributor had to be backed off a bit to keep the engine alive, even at low boost levels. But there's a lot that can be done to optimize an engine for higher compression ratios and boost.

Just be aware that there is a trade off between compression ratio increases and the total amount of boost you can make. And it's not a linear trade off. If I were to try and build another fun, mid-boost street engine that ran on pump gas, I'd run 9.5:1 compression, stock divided exhaust manifold, some Tomei PonCams, a GOOD water injection system, and an S15 SR T28. That setup should make damn close to 300whp with supporting mods (fuel, intercooler, CAI, exhaust, good tune), and would be a blast on the street.

float_6969 wrote:If I were to try and build another fun, mid-boost street engine that ran on pump gas, I'd run 9.5:1 compression, stock divided exhaust manifold, some Tomei PonCams, a GOOD water injection system, and an S15 SR T28. That setup should make damn close to 300whp with supporting mods (fuel, intercooler, CAI, exhaust, good tune), and would be a blast on the street.

my build on 9.0:1 minus the water injection and the good tune

LOL! Still a good setup and will be lots of fun!