mitsuman

iTrader: (1)

ok well i want to build a "high" compression FI setup. would be a b16a1 with forged internals. i went to this site ( http://store.yahoo.com/speedupgrade/efcominchoos.html ) and read everything. very good info i was wondering how much tuning is envolved? is it wurth to go high comp and FI? it makes sense to me higher comp, less psi. instead of low comp high psi. in the end effective comp will be equal( or close). so whats the low down on this? thnx

b18bturbo

i woudl stick with 10:1 and you should be fine. tuning ont he 10:1 is pretty easy and you can have some good spool up times.

The 10:5 higher works but its higher prone to detonation. its really finiky its picky what it likes the motor that is

DLB1994

am running 10.2:1 and it works good

Charlie Moua

iTrader: (3)

well there have been many post about this. whe search is up please use it. I'm also a fan of higer cr with mild boost (5-8psi). Tuning just takes longer and you better have a really good tuner who knows his ***** and the standalone system. BTW did i mention that you shouldn't atemp this with just the common FMU/afc hack! you'll need a reall standalone fuel management.

I was thinking about going B16 with CTR pistons (giving me 11.1cr)....still thinking about it but the pistons are prone to hot spots with turbo..so i hear.

HTH and good luck. great link BTW very useful

mitsuman

iTrader: (1)

thanks for the info. ya i think a big advantage is the quicker spools that ill get with the better low end.

anybody else?

ekb18c

10:1 - 10.5 is alright if you have some sort of fuel management. This is still considered alright compresion.

Once you goto 11:1 and up then it's starts getting tricky
to tune because you will have to work with a smaller margin of error.

accordfreak

how would I get the exact compression ratio that i want?

say you get 2mm over, 9.8:1 CR pistons..

you have to deck the head and the block to get a clean mounting surface right? so that would raise the compression ratio..

also matching the head's combustion chamber to the larger cylinder bore will lower CR.. right? who has actually matched it?

all that with a stock head gasket bore 2mm over

so how do you figure out what the final C/R output be?

evoeone

what he said , people should spend 2 g's on management before froged pistons rods and crappy management like afc hack, fmu, (compared to stand alone)

well put, lowered comp just gives you more room to **** up, higher means more precise,

colin, dont you push nice power with 10:1's?

boosted92

If you know the exact compression you want and you know all the specs of your motor, then a company like JE, Arias, Etc, will build you a custom set of pistons with the exact dish or dome that you want.

ekb18c

Yup, 426whp with 81mm @ 10:1 pistons on a clayton dyno.

Acutally my compression is really around 10.2:1.

evoeone

damm clayton dyno's. i think they suck compared to dynojets, we had a vw dyno day on one and everyone was like wtf when they got their numbers, none the less youre pushing 4 plus whp on 10:1

see guys, it can be done if you can tune right, ...... and if you work at rpm

ekb18c

Nah clayton dyno's dont suck, they just register lower numbers than a dynojet.
I think they register a little more accurate readings then dynojets. Claytons use 2 rollers as compared to one roller on dynojets. Dynojet measures how fast you can spin that roller while a clayton is a little different. On clayton's you can gun the car after first gear so you can see how many horses you are putting down at each gear (mph) as compared to just a 4th gear pull on dynojets.


With that said, I wonder what I would make on a dynojet? So maybe I didn't fail my mission of getting 500whp on 81mm pistons..

danl

Well that’s a great write-up and all and there certainly are some aspects that are true. However there are a few VERY important aspects that the author is negating. First is compressor efficiency. Boost pressure is NOT linear with respect to power output. For instance increasing the pressure ratio of the turbo charger by 20% does not increase power output by 20% as the efficiency (and thus oxygen packing) of the turbocharger is not the same. In real life this is like saying that if I was running 2 pressure ratio of boost (that’s one bar) I would make 33% more power by running 3 pressure ratio (2 bar of boost). While I would certainly make more power, it wouldn’t be quite 33% more power, perhaps 20-25% more due to the heating of the intake charge caused by my compressor wheel falling out of its efficiency island on the flow charts. Both pressure ratio and lbs/min of air affect the efficiency of the compressor.

Another aspect that favors lower compression turbo motors is the higher combustion chamber volume. A lower compression motor will have more volume with which to pack oxygen molecules in at TDC than a high compression motor with the piston nearly touching the cylinder head. By raising boost pressure you increase dynamic compression, which is in essence what the author of that article was describing. Boost pressure is how you achieve higher dynamic compression. Static compression (geometric compression with the motor not running) is only a value that has little to do with anything in the real world. Dynamic compression is more useful, but next to impossible to calculate. When tuning you merely find the highest dynamic compression you can safely run without detonation, and use fueling to help increase this threshold.

Something to ponder: If high compression turbo motors where the way to go then all the fast boys would use them. In fact, a lot of the big dogs DO run high compression turbocharged motors, but they have to use race gas to alleviate the detonation caused by the high dynamic compression ratio. Depending on what kind of power you want to make, high compression and low boost is certainly acceptable. The high compression is more thermally efficient at low boost, and almost eliminates turbo lag resulting in a very useful power band. It also results in very good response after a shift where as the low compression guy may be suffering from boost lag after a shift and loosing valuable fractions of a second in the quarter mile. It really is a balancing act, and there is no one answer because motors have a variety of tasks.

Pelican

I still like the higher CR FI machines. None of that 8.8, 9.0 stuff
Three ways to make powah; squeeze it, cram it, or rev it or a combination of the three

mitsuman

iTrader: (1)

wow very nice post. as for me. my minds made up 10.1:1 CR + FI woo hoo

danl

Carefull selection of a compressor wheel will be the key to making it work well. You have to make a HP level target, and match the compressor to that. THen you'll have to choose the exhaust side, playing a balancing act between spool and flow as a small housing will kill flow, cause reversion, and thus heating of the exhaust valves causing preignition.