ok so i have been hearing recently that in order to use high boost on supercharged application(i am assuming that this implies turbos as well) with honda engines you need to rebuild to a lower compression ratio. but it has just come to my attention, that this is in fact false. i was reading in my Honda tuning magazine this week, and found a letter from someone who wanted to supercharge a B16A, and Hondatuning told that that it is not always necessairy to run low compression in a boosted motor. and they then go on to explain that the myth of using low compression originates from the days of old school muscle cars with horrible piston and chamber designsthat were pron to detonation at low boost levels with average compression. With Honda motors we have the luxury of some of the best designed heads in the world, and it is ok to run 10-11:1 compression ina welltuned supercharged motor. honda tuning actually supercharged a stock TypeR integra not too long ago, and the stock ITR has 11.5:1 CR. and it is infact ok to run 12 psi of boost on a 10:1-11:1 CR on a stock motor. and you would even be able to use forged pistons if you want, and retain that high CR and then increase the boost even more... I for one found this imnteresting. and hope that it answers some questions for people too

I read that too and found it very interesting, but you better have a fat wallet if you want to run 12psi on 11.5:1. I also think the low compression theory is far safer for the daily driver.

So, this is not so much a myth as merely not as true anymore? That's what seems to be the case in my mind. I really don't understand enough about CRs and turbos to really make a distinction, but from what you said, it seems you can run boost on high CRs, just not as high or as safely as on a low CR. Is that right?

yeah i saw that to but what octane gas were they using....?

ORIGINAL: Chips

So, this is not so much a myth as merely not as true anymore? That's what seems to be the case in my mind. I really don't understand enough about CRs and turbos to really make a distinction, but from what you said, it seems you can run boost on high CRs, just not as high or as safely as on a low CR. Is that right?


Partially. But these statements ultimately have more to do with how big the turbo is (i.e. how much air it's moving) than with psi. PSI looks good on paper, but think of it this way: you can blow 10 psi through a straw, but you're not moving any significant air volume. Now, a T-60 blowing 10 psi is probably going to be moving over 550 cfm depending on the trim... The tune also has a huge impact on how well a high-compression engine can handle boost.

Yea if you retard the **** out of it to stop detonation your counteracting the gains you would get from compression. Its a fine ballance and one that needs to be explored for your particular application.

Berz out.

I agree with the first statment. Im many cases you are better off funning a lower CR when running a higher boost level but with a lower CR you will be loosing Torque and we need more torque in a honda than what we have so my suggestion is to run a higher CR and a lower psi on a honda. Then your boost will pick you up at the higher RPM's when you are at full spool. take for instance a Top Fuel Dragser runs like a CR of 6.8:1 with 32PSI of boost because they have pleanty of torque because they are a big block V-8. Good luck.

thats more or less what i was trying to say in another thread about a week ago. you can run fairly high CRs with decent boost fairly reliably...but it depends on the turbo, internals, and tune you have in the car.

theres a GSR that has a couple videos on StreetFire thats running an 11.5cr on 12psi from a vortech supercharger and 93 or 96 octane. thats a really good example in favor of the argument that you can run high psi on high compression.

Ok, so in a nut shell, high CR = torque and high boost = hp? I know there's more to it than that, but that's the general idea? So if I were to (hypothetically) take my d15b7, throw on a y7 head to raise CR, and boost it, I'd be good to go so long as I get a tune and use ARP bolts? If so that may be a good direction for me.

Go for it but if your not going to use upgraded pistons and rods dont go above 5-6PSI on that setup and if you do have them I wouldnt go above 11-13PSI, anything above that on a honda you better stake the block. You will need an upgraded ignition & fuel system (pump,injectors,risenrate FPR) and i would upgrade the cams and exhaust. Any boost on a honda i recomend running a front mount IC. Good luck.

D sleeves are good for a lot more than people give them credit for.

^ yes very true

With what reliability would one expect to run 12 psi on 11.5 cr? For how long, and would that be driving it hard or taking it easy?

depends on who you get to tune it as well as the quality of parts your using.

I, on the other hand, believe that lower compression is part of building a reliable F/I motor.

A lower compression engine will run cooler, will last longer (especially the rings), and be more reliable/easier on parts that a high compression motor with boost.

I'm not saying you CAN'T run high compression, but I don't think it's the proper way to do it.

Let's just use stock internals as the meter on this scenario. How long would the stock internals hold up under that pressure? We'll say going to the track twice a month, and using it as a daily driver. I subscribe to the lower compression school of thought.

wow i didn't realize that you guys made this a sticky... cool:D

Is 12,000 miles a fair estimate? Or am I underating stock internals on a K series motor?

A little long but I hope this helps you

There are a few manufacurers making what they call "light boost" motors.. They run normal compression ratios and small turbos with
light boost for a little more top end.

Kappa22 and Larsenracing as well as others gave you some good advice..
You had better have a well built motor and high end controllers to control things such as ignition timing retard etc. if you plan on doing this...

The more compression you have the more cylinder pressure you create. Aspirtating the engine creates huge cylinder pressure..

As we know. engines are air pumps. Simplfied, as there are hundreds of variables... Let's say one cylinder normally contains 25 cubic inches of air. With a good turbo or S/charger @ about 15 PSI that cylinder will now have about 50 cubic inches of air.. So your engine acts like it is twice it's size..
Now comes the downside.. The compression in that cylinder is now probably 15-1 or more.. So you need controllers to properly handle that much compression.
You need to greatly reduce the total ignition timing and add enough fuel to light this off..

With lower compression pistons, you can still pump in that same amount of air and gain that cubic inch increase without as much compression increase as you would with high compression pistons..
That is the reason most turbo motors run lower compression ratios.
I hope this helps you..

its been true for me to get more power out of 5psi with a good tune then 12 psi with a poor tune.just my .02

d-series rods are like twigs they are no bigger than my finger.and they snap like twigs.trust me,i now have a b18a as aresult of them.just keep the boost low (6-8lbs.)and have a reliable boost controller.and dont street tune, dyno it,its fragile

ORIGINAL: StinkBOMB

Is 12,000 miles a fair estimate? Or am I underating stock internals on a K series motor?


K's are built like tanks, that's why they're so popular, new technology and better design.

if i'm correct, you are definitely underestimating the k's internal's. I'm not sure what trim, but i've seen a mid-500hp boosted setup on stock internals at over 2 bar, and it could handle that, but it's not recommended for a dd, or reliability with that setup.

i know im gonna get some laughs for this one but its for a friend and he's not trying to do much right now....

1.5L SOHC
stock internals
upgraded fuel injectors
boost controller
no intercooler (for now)

how much boost will be safe on this engine? 6-8 psi?

thanks guys.....

the head doesn't fail on boosted honda engines, the bottom ends do -- especially piston rods, piston heads and block walls/sleeves...

ORIGINAL: stubAdub

i know im gonna get some laughs for this one but its for a friend and he's not trying to do much right now....

1.5L SOHC
stock internals
upgraded fuel injectors
boost controller
no intercooler (for now)

how much boost will be safe on this engine? 6-8 psi?

thanks guys.....


No laughs from here my freind. I <3 1.5L's.

Read the FAQ sticky of this section. You'll find "psi" or "lbs" doesn't mean squat.

Try a DSM T25, get an intercooler, and use a Z6 head/IM//ECU/etc. Check out a HF/CX/STD manfiold on homemadeturbo.com also. Keep fuel (premium) around 12:1and run conservative iginition timing. It should be good for ~200hp.

I got a 100% complete turbo kit that'll slap right on your motor. ;)

ok just to make sure i understand you all. if your want to take a stock n/a motor and boost it and still have it as a dd, for relability and safety, it is good to lower the cr. so after you have a complete turbo kit, plans for a good tune, what would be the easiest way to lower your cr and how much lower would it have to go. i don't know alot on this subject just trying to fully understand every aspect of boosting before i even turn the first nut to do it.

dished pistons

now before i ask this no one please give me the you don't even need to begin boosting your car speech cause i'm not. i'm just learning all i can so that when the time does come to boost it i am well imformed. so now i'll bring on the newbieness. what exactly are dished pistons and how will they low the cr?

they're concave therefore they increase the volume of the combustion chamber at TDC therefore they reduce compression kinda thing.

ORIGINAL: pWnEdU

I, on the other hand, believe that lower compression is part of building a reliable F/I motor.

A lower compression engine will run cooler, will last longer (especially the rings), and be more reliable/easier on parts that a high compression motor with boost.

I'm not saying you CAN'T run high compression, but I don't think it's the proper way to do it.


I couldn't agree with you more. If your engine is going to do everything you just pointed out...run cooler, last longer and be more reliable, why take a chance on high compression? Unless you're made out of money of course and I KNOW that I'm definitely not. Low compression is the way I'll go. :D I'd like my engine to make it past the stop sign at the end of my street without going POOF! haha