CA18DET preffered compression ratio

[Vlade]

Hi All

I currently have some 8.5:1 CP pistons in my engine, but they sorta got a bit damaged now I need to replace them.

What is the preferred compression ratio for the CA18DET?

Reason why I ask I can get a nice deal on some 9.5:1 Forgies but, the compression ratio is way bit higher...

I know the car will need a re-tune but this ain't a problem.

[ca18datsun510]

pro's: you will ultimately make more power, just like n/a, higher compression will make more power. you will also see boost a little quicker.

con's: more heat, easier to detonate, you will have to have a good tune.

but then again there are plenty of turbo honda's running around with over 10 to 1, as well as other cars that have had turbo added.

if you have the ability to tune it, and are willing to deal with the results, good or bad, i say go for it.

[bentvalves]

what type of boost pressure do you plan to run, with what turbo?

how did you damamge the 8:5's?

what are you tuning with?

do you have 91 octane where you live or 93/94?

[good morning]

you will also see boost a little quicker.

No, you will not. Compression does not have a effect on spool. The motor makes more power off boost, so people think it spools faster.

9.5 will be fine. Compression comes down to what octane of fuel you plan on running. CAs are not the most effiecnt motors, so if you plan on running pump, I would drop it to 9 or lower.

BTW, CP pistons are softer then most pistons out there. I would go with wiseco

[ca18datsun510]

yes it does, higher compression increases velocity, and will help spool up.

i believe it is float, who has higer compression pistons and has a real world example.

also, compression has a lot more aspecs involved then just what octane you run. timing, air fuel ratios's, etc all play into that.

higher octane is better of course.

[Swedish Mike]

More N/A power -> more exhaust gases -> better spool up.

I would go 9.5:1 and tune it to perfection. Will make a nice engine!

[bentvalves]

yea i dunno bout that......nothing wrong with stock 8.5.1.

again, what do you plan to do with the engine in terms of boost pressure and turbo choice, and what type of fuel will you be using?

Greg

[boost_boy]

Hi All

I currently have some 8.5:1 CP pistons in my engine, but they sorta got a bit damaged now I need to replace them.

What is the preferred compression ratio for the CA18DET?

Reason why I ask I can get a nice deal on some 9.5:1 Forgies but, the compression ratio is way bit higher...

I know the car will need a re-tune but this ain't a problem.

First of all, sorry you damaged your $500+forged pistons/rings. Second, have you determined how you screwed them up in first place? It is important to figure out what caused damage to those pistons. I would say too much bust for an extended period (W.O.T.) could be one. I would also take a guess and say you either melted one or some on the intake side or cracked one or some of the ringlands on the intake side (only you know what really happened). So I guess the million dollar question, why in the world would you want to jack-up the compression in your engine, when you haven't figured-out what caused the destruction of your current pistons.

I don't know your financial situation, but big money or not, upping the compression ratio+wanting to spool fast+not upgrading your engine management system+not having the required knowledge or individuals available to you with the required knowledge= A recipe for failure. Go back with the stock compression ratio. Hell, you can even go back to using the stock pistons and still have a very reliable engine as long as the tuning is done correctly (as members have said earlier in this post). If people are trying to influence you to do it, tell them to help you foot the bill after the eminent catastrophic failure. Tuning on high compression engines are a bit tricky, though not impossible. And it should be left for the pros and not for the everyday tinkerer, unless you don't mind down time and you have lots of money to play with.

Dee

[iliketocrash]

not to be a d!ck but i just wanted to point out that the majority of energy imposed on the turbine wheel is thermal as opposed to to exhaust velocity. it's a common misconception for a lot of people who are new to turbo charged vehicles and that's who this is really targeted to help out.

[good morning]

yes it does, higher compression increases velocity, and will help spool up.

i believe it is float, who has higer compression pistons and has a real world example.

also, compression has a lot more aspecs involved then just what octane you run. timing, air fuel ratios's, etc all play into that.

higher octane is better of course.

I have done the closest "back to back" test you can do. Same motor, same trubo, same boost pressure, same everything besides compression (9.0 vs 10.0). The spool was the same besides the fact the higher compression motor made more power off boost. I tuned both motors.

You are correct, but this a street motor. The biggest problem he will come across is octane. Timing is a by product of the motor, BTW.

I hope that cleared things up

[float_6969]

I am running at least 9.5:1 compression forged CP pistons. It's actually a tad more since the motor is bored out to 83.5mm.

1st of all, you'd better be pretty confident in your tuning skills to do this on pump gas. I run 92 octane from BP. The timing has to be pretty severely retarded to prevent knocking.

I have done the closest "back to back" test you can do. Same motor, same trubo, same boost pressure, same everything besides compression (9.0 vs 10.0). The spool was the same besides the fact the higher compression motor made more power off boost. I tuned both motors.

You are correct, but this a street motor. The biggest problem he will come across is octane. Timing is a by product of the motor, BTW.

I hope that cleared things up

I too have done a similar comparison. I can honestly say that a CA T25 w/a slower spooling SSAC equal length manifold spools faster w/9.5:1 compression ratio than the CA T25 on the stock manifold at 8.5:1 compression ratio. The boost onset is much more linear as well with less of a "rush" and the throttle is much easier to control.

As was previously stated, the main reason that a turbo turns is from heat, and it's not a secret that a high compression engine has higher EGT's.

I'm not discounting your experience, I'm just stating that I've had a different one....

[good morning]

I too have done a similar comparison. I can honestly say that a CA T25 w/a slower spooling SSAC equal length manifold spools faster w/9.5:1 compression ratio than the CA T25 on the stock manifold at 8.5:1 compression ratio. The boost onset is much more linear as well with less of a "rush" and the throttle is much easier to control.

Manifolds have a big effect on the motor. I understand your experience, but you see were I am coming from.

Heat does have a great effect on turbo spool, power, etc (over all heat = energy), but ignoring the steady and unsteady gases is a bit crazy.

Take it with a grain of salt, I guess, I am done with it.

[c-rad]

I dunno about you guys, but if I want my car to spool fast, I just downshift.

[boost_boy]

I dunno about you guys, but if I want my car to spool fast, I just downshift.

Sounds like the ideal thing to do in the case of the CA18DET. Start adding cams and go fast crap and the spool-up gets slower. I say, enjoy the engine for what it's worth including it's characteristics. It likes to be revved, so stop being wussies and revv the biotch to get the boost you need .

Dee

[dash]

I recall years ago enthalpy? on fresh made a direct comprison, same motor with his 9.x:1 rebuild vs stock compression ratio sr20. Verdict; no really worth it.

Every impressive street beast I've seen, run low compression/hi boost. What particularly stands out is the higher torque output.Some prefer the 'higher efficiency' route; higher compression, lower boost, cam, ported, etc and may yield something like 400hp/300ft-lbs tq. Not for me.

[r34 gtr]

OP, you need to explain to us why you damaged some pistons that should be all but immortal in more than 90% of our cars with proper tuning. i beat on my stock bottom end like crazy (read my "holy crap i got teh det!!1" post) and it has never let go on me. maybe you should spend some money on proper engine management. it can be had for not all that much if done right.

and screw your higher compression, i enjoy my really peaky powerband. it gives the car that "nothing...nothing...holy crap its fast!" feel that none of these other turbo cars have.

[Vlade]

Damn... so many replies so quick... I found a new forum!

Ok here is some answers to your questions...

Boost I was planning to run my stock T25 for a while before upgrading to a GT2860RS later... Goal was that one day with that turbo is to reach 300Whp...

We have 93 octane fuel... There is some pumps that supply 95 but they a bit scarce

ECU i'm using a Wolf3D V4 to tune my engine...

What destroyed my engine?Well a set of broken valve guides, guess the guys who serviced my head supplied the wrong size and they seized and broke off... creating some "Solid Fuel" not good stuff

Here is a pick of one of the pistons...
Modified by Vlade at 9:17 PM 2/25/2007

[r34 gtr]

ah, well then you should have no problems tuning that thing. the wolf 3d is a pretty awesome little stand alone. if you think you can handle the 9.5's then by all means use them. actually those pistons dont look all that bad. ive seen worse in running vehicles! the arias piston in my dirt bike was banged all to hell but it never complained....until i smacked a valve so hard it cracked the piston. on second thought, never mind.

[Vlade]

Yeah I also think they don't look too bad... My tuner reckons they shouldn't be reused for reliability and possible "hot spots".

[r34 gtr]

someone might have a different opinion, but i would totally use them again. a little smoothing out so you dont accidentally scratch a bore maybe, but they dont look bad. like i said, ive seen worse. its up to you though!

[good morning]

Yeah I also think they don't look too bad... My tuner reckons they shouldn't be reused for reliability and possible "hot spots".

Your tuner is giving you good advice.

[float_6969]

Yea, those are a big no-no for reuse. Those little dings can create a couple of problems. The first being that they will collect carbon and will become hot spots that will cause auto-ignition. Secondly, I would think that the structural integrity of the piston would have been compromised and would be much more likely to fail down the road.

[r34 gtr]

sorry boys, my cheap side was screaming "youve done it before!! just re-use them" the only time i did this though the piston failed a few weeks later. at least it gave me a few weeks to scrounge up some dough!

anywho, listen to the other people i guess. and you can probably do with those 9.5:1 slugs just fine.

[float_6969]

Every time you post, my quote just makes more, and more sense...

[r34 gtr]

doesnt it?

[bentvalves]

Id prolly reuse those pistons, depending on how deep those pits are (pics dont show) you could easily clean those up with some 1000 grit, followed by scotch brite, followed by some emery paper.

[ca18datsun510]

[ca18datsun510]

Id prolly reuse those pistons, depending on how deep those pits are (pics dont show) you could easily clean those up with some 1000 grit, followed by scotch brite, followed by some emery paper.

i would agree with you if it was like a cavalier or escort or some daily driver status car. but in a project/toy/performance car, i wouldnt risk it.

[r34 gtr]

yeah, if its gonna be thrashed... i mean, i was already shut down so im just gonna say dont re use them. though if you want to send them to me for free, ill test them for an extended period of time and let you know how it goes...

[ca18datsun510]

Every time you post, my quote just makes more, and more sense...

hypocrit much float?