what will be an optimum CR on a CA18DET

[ByReaL]

Hi

i serched a lot but i canot find the answer to the folowing question

i want to buy forget conrods and pistons metal HG and a GT2871R turbo A/R 0.64 (the one with -15in the part number code)

anyway my problem is that i do not know for what Compresion ratio to aim

8.5:1 is the stock CR, i also know that the lower theCR is the higher is the boost you can run.

i'm not afther 3 bar of boost on 5:1 CR or some wired figures, i'm just afther the maximum HP (hi CR + Hi boost = hi power , or foked engine ) i can get out of my engine in this setup (piston/rods/turbo...) with a nice power curve and withoult sacrificeing the reliability....


anyway the qestion:
for what CR will you aim haveing this setup ?

PS: the car use 98 RON fuel

[Fake Ben Taylor]

im not sure what the exact figure is but i would have thought a 1.9mm HG would be best.

also low compression and high boost means high top end power but low bottom end power.

high compression plus boost and strengthened internals means a slightly lower top end figure but a much better low down figure and i believe probably more area under the graph

[ByReaL]

the car is build/used be me 95% for the plesure of driveing an SX and 5% for drag racing

1.9mm HG is irelevant somehow, because the CR is afected also by the pistons i'll buy, by how much i'll take from the head.... and other things (the HG is just one of them)

the ideea is should i go on the GT2871R to something close to 8.0:1CR or try to acive 9.0:1CR or just stay to 8.5:1CR

or what other CR will be best for this setup ? (without blowing the engine)

PS: i want to take the best out of the GT2871R

[Fake Ben Taylor]

i personally would be aiming for 8 - 8.5 :1, as this is already imo quite low yet will maintain some low down power.

just my thoughts. paul_s13 and/or sideways danny will probably be able to help more

[SM]

This is no "optimum" just best comprimise for use.

If you are building a race or full-fledges track car, probably about 7:1
If you do lots of road driving probably as high as you can get away with, maybe even standard 8.5.

[Pete C]

If you do lots of road driving probably as high as you can get away with, maybe even standard 8.5.

I ran standard compression ratio on my CA and still managed 292bhp & 276lb/ft - I'm sure you could get more than that with careful mapping.

[bren]

If the car is real used in anger just once then it has to be built towards that goal unless youre going to run higher octance fuel, drop the boost etc.

No point running high comp ratio and mapping for the odd blast on the road and then doing the one and only track day of the year, caining the car properly and detonating it to death as the comp ratios too high for the useage.

[SM]

Mapping will allow for standard compression ratio @ 400bhp if done well, ok so it migth be quite retarded, and low AFR, but it doesnt mean it has to blow up.

[Sideways Danny]

the key IMO is to run a high as compression as you can without incurring high EGTs. I built my engine to be relatively high comp compared to some similar power target builds but I have removed the squish area. I believe that the squish on a CA will actually encourage detonation because it isn't well spec'd as standard. my engine runs 8.17:1

[Jez]

I agree with Danny - the shape of the comb chamber will affect the chances of detonation.

To get the ideal CR for max power you need to be able to have the ignition timing so that it is just on the safe side of det but with peak cylinder pressure around 15 deg ATDC.

[bren]

Mapping will allow for standard compression ratio @ 400bhp if done well, ok so it migth be quite retarded, and low AFR, but it doesnt mean it has to blow up.

Retarded timing - high egts.

[SM]

Retarded timing - high egts.

more fuel, lower EGT's

As I said everything is a comprimise, I'd happily comprime lower power, retarded timing and low AFR for more economy and off-boost performance on my daily drive and run high compression. But on the trackcar I'd want low compression, less fuel for cooling, more power, lower EGT's and would be bothered about the off boost and economy downsides.

you just have to pick your priorities

[Sideways Danny]

more fuel, lower EGT's

but if you're building an engine to spec rather than re-mapping a standard one you can compromise a lot less, using fuel as a coolant can still lead to power loss overall so building the engine right in the first place is surely the better way

[SM]

Yep, your right Danny

I did say it was a comprimse, and you would lose power, saying that everything is a comprimise you just have to pick whats most important to you, for your useage needs.

[Fake Ben Taylor]

am i right in thinking you could use a water/methanol injection to do the cooling and as such run a lower afr?

obviously youd have to watch the water level and knock down to low boost if it ran out.

[ByReaL]

No point running high comp ratio and mapping for the odd blast on the road and then doing the one and only track day of the year, caining the car properly and detonating it to death as the comp ratios too high for the useage.

true, but this is the beauty of the setup i'm tring to achive, a nice to drive road car that can also perform verry well on the track, i'm not woried at the moment because of the maping, for teh moment i'm tring to gather all the needed hardware, to suport as hi power as posible, and doing all reserch and beein at the point were i'm afther pistons and rods, i was hited by the qestion

what CR should my engine have?


afther i'll chose a CR i'll do the math to achive that on a 1.6mm head gasket , so i can have a little room to adjust ti going to 1.2mm HG if necesary or to 1.9mm HG.


detonating to death i strongly beliefe that will not be the case because it will be properly (i hope) maped on a dyno, and will have for shure at leat 3 set ups

street
race no nitro
race with nitro

so i can enjoi my car on the road and also to be able to race it



as someone said it is all about the compromise and about the achived power curve.


PS: if i'll not reach a conclusion that X.Y:1 CR is the best for my setup, i think i'll stay with the stock value of 8.5:1, but till then i wayt for your opinions

[SM]

Yep, WI will allow more advance on any engine, no matter what the CR. And is another way to tilt the comprimse in your favour, only downside are when it doesnt work (Jez seems to have got on top of his now) and the wallet damage and availability of DI water if you dont carry enough.

EDIT - I'd probably go lowish if you wanting to run NOS - just more room for error in setup.

[Sideways Danny]

am i right in thinking you could use a water/methanol injection to do the cooling and as such run a lower afr?

obviously youd have to watch the water level and knock down to low boost if it ran out.

in theory, it's effective if you know how to set it up. It's ballache though and Jez had his repeatedly stop working this year at time attack. I'd be very hesitant, there are better ways (IMO ) depends of your power aim really



to answer the original question in simple terms, I wouldn't run less than 8:1 on a road car (upto 350bhp) personally. anything over 350 becomes a bit of an impractical toy IMO so it becomes less important

[Fake Ben Taylor]

in theory, it's effective if you know how to set it up. It's ballache though and Jez had his repeatedly stop working this year at time attack. I'd be very hesitant, there are better ways (IMO ) depends of your power aim really



to answer the original question in simple terms, I wouldn't run less than 8:1 on a road car (upto 350bhp) personally. anything over 350 becomes a bit of an impractical toy IMO so it becomes less important

i know the better ways albeit only because you enlightened me

i was just curious, as for a road/occasional track car it could be an effective way (if you could get it to work) of maintaining a driveable car but not killing it on track the once or so a year its on track.

[Chris B]

I'd estimate somewhere between 8:1 and 8.2:1

With that turbo it's probably best to aim closer to 8:1, it is going to have 1.6bar (and maybe more ) being fed into it

It could be a good idea to get adjustable longer-duration cams in there too so you can move the power band up the rev range a bit.

Definitely remove the squish for what you say you will use it for