Ive built a couple of turbo hondas before but both on stock blocks.
I currently have a B18c that i am going to build but im wondering what factors do people consider when choosing a compression ratio ?

The build will go something like this.
B18c overbored to 81.5 (currently has a damaged bore)
Eagle rods
Forged pistons (brand depends on what compression ratio is chose)
B16a head.

With the b16 head and 0.5 overbore im looking at about 9.9:1 what should i be looking to drop this down to ? Im wanting somewhere between 300-350hp. I know there will be plenty of people that say you dont need to do the internals for this kind of power but im doing it for longevity.

 

No need to drop it down at all. Even with your built internals, you can stay at that compression, and is actually encouraged

 

Totally agree. Turbo Hondas under 9.0:1 become a dog when not in boost and spool a bit slower and more boost is needed to make more power (depends on setup though). I like to keep compression for boosting 9:1-10:1 but I tend to stay more around 9.6:1-10:1.

 

ok thanks for the info.

Out of interest at what point would you guys say that a drop in compression is required ?

 

Prob when only using pump gas,non forged internals, and don't have a good tuner

 

standard pump gas here is 95ron and super is 97-99ron which is available pretty much everywhere now. Im considering tuning it myself so maybe worth dropping the compression a little to the mid 9s for a small margin of safety

 

^^^ In addition to over 11.5:1-12.0:1 regardless of fuel, unless you're an extremely great tuner

 

you ever try tuning this even on elf 110 or anything? i've never witnessed a turbo car over 11;1 but just for discussions sake was curious if it becomes more difficult to tune even if you ran crazy gas like that ELF 110 leaded

 

84 mm b18c5 11:5:1...pro 1's. golden eagle i/m. top mount. 62/65 journal bearing...2200cc injectors dual bosch 044s..hondata s300...800 on 27 lbs.

 

I will be running 11:1 in my race motor. Jeff Evans @ Evans Tuning uses 11:1 off the sheld cp's and with his head it is bumped upto 12:1. I'd stay around 10:1.

 

I am rebuilding with 11.75 this time on E85. I also think 10:1 is very reasonable.

 

High compression is done all the time on race gas but these are all out drag cars, for the street its best to stay in the 9's or low 10's under 9 allows for more power out of pump gas build because reduces the ability to knock.

 

same impression i always had as well. if you're on the street and on pump gas you're pressure limited with the 11:1 ish numbers

 

Negative, the car runs more efficient at 10:0 compared to 9. If it bothers you that much, simply add another half point of fuel to 11.0 and call it a day.

...Or don't bother trying to make 500whp with shitty gas on the street and expecting it to hook anyway, because it doesn't.

 

So what do you mean by negative? i wasnt saying anyting bad about running 10:1 compression on pump gas i was refering to 11:1 and up. mid 9's is fine too and yes I know 10:1 is more effecient and still capable of running pump gas and making great power.

I didnt imply that anything bothered me about the fuel on 10:1 compression. I personally am at 9.8:1 on 93 octane with over 400whp and its great. I tuned my car and it runs like a top and makes great power in and out of boost. 500whp wont hook untill 4th gear and depending on tires and chassis it still may not hook on the street. It may hook in third if your running big BFG's like 235/60s which is a 26" tall tire but not my cup of tea, i prefer my 400 and change that hooks perfectly in 3-4 gear and boost by gear does it job in 1-2

 

550whp hooking in 3rd on a 205 non drag radial..630ish in 4th



Im at 10.5:1 but run on E85 all the time.. Makes great power per lb thought about putting it on pump gas just out of curiosity but im not trying to make a number..

When choosing a CR, might not be a bad thing to ask your tuner what he is comfortable with

 

you must be running slicks or have a really really long custom gear on 3rd to hook on 3rd at 550whp. unless this was at the track that was prepped very well

 

Nah, he is just rocking some Nitto NeoGens in a very well sorted chassis.

 

i have to call bs on 205 neogen hooking on WOT 3rd gear at 550whp... unless this thing makes no tq

 

Its not that its exactly hard to tune or you need a great tuner. You'll just be limited to less boost unless the octane level is increased. All the tuner is doing is adding ignition timing, watching the power output and listening for knock. If the engine hits the det wall then he needs to back down the boost and call it a day or increase the octane rating and carry on tuning.

 

I went 11.57@137 on the neo gens... i set the car up pretty good on these tires... I do have a GSR tranny with an LS final drive.. Only gives 3mph more in 3rd...

It only made 417tq when it made the 659whp... So yeah id say tq was mid 300's?

 

For the compression game, it's about maintaining a certain degree of margin of safety.

Sleeved block? Yes, you can reap all the benefits of compression and I'd go above 9.5:1 CR for a pump gas setup even for 500 WHP. The lower boost required to make power easily offsets the drawbacks of higher compression.

I wouldn't go much higher than 9.5:1 on stock sleeves, if your plan is to make power at the "upper limits" of stock sleeves on 91 or 93 oct.

It's always about power and octane.

 

Tony what do you recommend for the 94 oct up here in ontario, on stock sleves and lower boost levels <20

 

Higher compression will allow for more efficient power levels at lower boost levels.

There's a lot of misinformation in this thread due to "hear-say" Listen to the people who actually tune or have hands on experience with the situation, not internet trolls.

I personally have tuned quite a few setups that are running stock sleeved setups running 18-20psi of boost @ 8:8 or 9:1 CR , the car has no *** until 6k. I'll leave it at that.

 

nothing wrong with 10:1 compression and 300-400whp on good pump gas