high comp + boost theory
 

i was thinking if a high comp engine + boost combo would be a good idea ? :S
were im coming from
driving around town i usually try to keep the engine at low rpm to use less gas and to have it run quite as to not attract too much attention
with my small turbo (14b z6) if i punch it to make a quick lane change (with out down shifting) i build boost pretty quick and have the acceleration to change lanes or pass.
now i was planing to do a vitara build and get a bigger turbo but then i would have to down shift every time and some times you just need to react and not shift.
so what i was thinking is what if i went higher comp then stock and jut ran less boost, with a good tune for the set up would there be a huge difference or risk factor?
i was thinking this would give me better botom end power out of boost instead of boging like hell
wondering what you guys think

Re: high comp + boost theory
 

What about the same low comp pistons with added timing down low? Or is that a bad idea?

Re: high comp + boost theory
 

i've thought about this to for my old project...high compression will spool a turbo quick, & if the max boost was set real low, you can keep good off boost drivabilty & repsonse, with the added "turbo booooooost" when you need it, like an assistant

Re: high comp + boost theory
 

Look at people with b16, which there are a ton of. They seem to do well as long as there tuned right.

Re: high comp + boost theory
 

whats b16 like 10.5 to 1
what do you guys think would be the highest i should go?

Re: high comp + boost theory
 

Re: high comp + boost theory
 

10.5 isnt high compression thats what basically ever dohc vtec engine has had since 1989 so your not being that oh my god new welcome to about 15 years ago when someone first turboed a b series vtec motor




If you mean areas of mid 12's+ JG has built alot of them and all made huge torque. They also seemed to break alot at the track. Mid 10's and throwing rods half track is still kinda cool.

Re: high comp + boost theory
 

b16s are helped by their R:S. Pistons move quickly at high rpm=hard to detonate.

As for compression, the basic idea is you want to run the highest compression possible given the octane available. On E85, I wouldn't think twice on running 10's compression. On 93, I'd keep it max 10:1, but most want somewhere in the 9s. On 91 octane, I would be hard pressed to go higher than 9.5:1 compression if I was looking to make much over 300-350whp.

You won't see too many race cars running more than 9's compression with boost for the simple reason that at 700-800whp, and being limited to C16, running much higher compression, you wouldn't be able to make the 700-800whp, but maybe 650whp. You'd be octane limited. These guys want room to grow and want to keep their blocks together for a season or two at a time.

Re: high comp + boost theory
 

well id be using 94 octane and shooting for only like 300 whp with a 16g (already got one under my bed ;D)
pretty much pistons, rods and a HM port on the head
this build would be just for the hell of it to see what happens and to be a better DD

so unless you guys say its a really stupid idea im going to consider doing it

Re: high comp + boost theory
 

It is all about cylinder pressure. How you go about making it is up to you. I'm not sure how less boost and more compression would make less power. ???

Re: high comp + boost theory
 

because there is less fuel and air to explode...

high compression is over rated. it just forces your octane level up if you want to not retard it to manifold/turbo melting levels. if you can get e85, and be bothered making it work, then high compression is a goer. if not, lower = better.

moral of the story, its hard to have your cake and eat it to

meaning

if you want big power on pump gas, you need to suffer the lag.

Re: high comp + boost theory
 

is there a noticable differance in power going from 9.1 stock compression to 8.5 with vitaras out of boost???

Re: high comp + boost theory
 

someone else will have to tell you for sure, but unlikely.

take short look back in time and see that old school econobox engines ran as low as 7:1 compression to be suitable for low octane fuel with their primitive chamber/piston designs and they were still primarily limited in output by the rpm, not torque. if i wanted to run very high boost on pump gas safely with no aids, i'd consider going as low as 7:1 on a modern engine. my first choice for moderate boost would be 8:1 and light boost 10 - 15psi would be 9:1. on 10:1 you'd best keep it under 10psi on pump gas for good reliability and ease of tune. thats not to say thats the be all and end all, but its a good starting point for a noob.

Re: high comp + boost theory
 

You need to do more research on how turbos work.. the whole concept is to use the turbo to compress air, dense air = more efficient combustion.

What you're wanting to do is run the motor at stock (or higher?) compression for daily driving because you're worried 9:1 @ low rpm's wont accelerate quickly.. if you want low rpm acceleration on the fly, use a smaller turbo, such as a ball bearing T28 from a nissan. If you want top end power use something bigger. Basically, you're not running efficiently if you want high compression + low boost.

Re: high comp + boost theory
 

if you want a good explanation there is an excellent thread on evans tuning website.

cliffnotes, lower compression raises the knock threshold for your tune on pumpgas other than that who cares.

either or will work.

Re: high comp + boost theory
 

This is only true to a point. Compression ratio, like boost, just increases your VE. Turbos are neat because you can have a shitty engine, just increase the boost, and you will make more power because you are just going to shove my air into the engine. NA on the other hand it is very hard to increase air flow so it's all about taking whatever air you can get, and combusting in a manner which yields the most power. Combining both ideas will yield the highest efficiency. Realistically if I want to make 300whp and run 93/94 octane, I know 9:1 compression I have plenty of margin for error(given my turbo setup doesn't suck). So I increase my compression to 10:1, I still have room for error but not a ton. Now I'm at 11:1 and I'm entering the "if you ---- up, I'm going to turn your pistons into scrap metal" zone. At this region you are making the most power most efficiently but are running a low margin of error. It's all about what your priorities are. Racing super competitively, where winning counts for something, you want to run close to a 1.0 safety factor. Granted if something goes wrong, you're fucked. But if things go right, you have a chance to win. Running with a large safety factor, sure you may finish every race, but your chances of winning are decreasing.

With a D-series engine, I would run 9.5:1-10.0:1 compression, and a T3/t04e 50 trim .48/.60. If you were talking about a B-series engine, I would suggest 10-10.5:1 compression with a similar turbo.

Re: high comp + boost theory
 

i have a 11:1 Stock prelude type s motor in my prelude, t3/ t4turbo on 6lbs. runs awesome. its been boosted about 3 months now.
i spent a lot of time tuning it so it doesnt knock. but its all the the tuning. i have plenty of power in the lower rpms and turbo spools a lot faster on this car compare to my other car lower compression (9:1) same turbo. high compression low boost is all in tuning the ignition timing.

Re: high comp + boost theory
 

Here's a thread comparing 10:1 compression to 9:1 compression, on a boosted gsr.
http://forums.evans-tuning.com/viewt...hp?f=15&t=1612

The graph is no longer up, but they talk about it.

It's a 15whp difference, running ~9psi on both setups.

Re: high comp + boost theory
 

Not a honda but i am currently supercharging a my toyota 4age with 10:3 c/r . basically i had two options high boost + 8:0 gze pistons or low boost + 10:3 stock pistons. The latter seemed more attractive and unavailability of the forged pistons helped influence me too. 8psi + 10:3 c/r + good intercooling + good fuel and timing management = fun ass car to drive.

Re: high comp + boost theory
 

idk if it was mentioned, but make sure u do a turbo ring gap, and not a N/A gap.

Re: high comp + boost theory
 

you will make much more power with low compression and lots of boost than with high comp and low boost.
in a way boost is like adding displacment to an engine without expanding the bores or lengthening the stroke.
you force more air and fuel in a smaller place(turbo honda) than sucking n/a air in a big place(v8).
thats where we detonate......
pistons with a 10.1:1(STATIC) COMP RATIO are running like 20.1 comp ratio under 18lbs boost(numbers are guesses but you get my drift)
pistons with a 8.0:1 (STATIC) cr are only running like 14.1 cr under 20lbs of boost.
find a happy medium that gives you a good cr under load and a turbo that moves enough cfm of air at your desired boost pressure.

thats pretty much the secret to top end horses

Re: high comp + boost theory
 

i knew this must have been done thats why i asked
i know how a turbo works and all that
since this is just a minor build (pistons rods 300hp) in the mean time and to learn off of before i do my full build in the future i thought about this

no i see that there isnt that much gain on the lower end
ill still do more research towards this before i make up my mind
but it was just a thought

Re: high comp + boost theory
 

Overall, high comp+boost > low comp+equal amount of boost. But that is given enough fuel to avoid knock on the high comp motor...

Re: high comp + boost theory
 

May be its me but i think the OP is looking for better spool time and off boost response as opposed to max power.

Re: high comp + boost theory
 

that was obvious....

Re: high comp + boost theory
 

He won't get it by just raising static compression. Squeezing the air/fuel mix more doesn't magically give the charge greater volume. You need to increase the volume of spent gases moving through the turbine to make it spool faster. The only effect of increasing compression is off-boost performance and having to use higher octane fuels.

Re: high comp + boost theory
 

I always thought squeezing the air/fuel more does create more heat and turbos spool from not just the spent gases but also from the heat expansion from those gases (why its hard to get performance out of rear mounted turbos without heatwrap on everything) But you may be right, I cant seem to find any reliable source that talks about static CR vs spool times

Re: high comp + boost theory
 

Ok well I may have slightly underestimated how much the spool time will be affected. But the slightly quicker spool will be negligible, and NOT worth the extra risk needed to prevent detonation. And whatever power is gained will probably be lost because you've got to retard the timing so much to keep detonation in check, and you'll also be dumping buckets of fuel into the cylinders to keep things cool.

Re: high comp + boost theory
 

Uhh, come again? You're confusing some things here, the only reason why a ring gap would change is if you're using cast or forged pistons.


For the OP, since I take it you want to try rockin' that 16g you could do one of 2 things...
1. You could swap the 7cm^2 housing from the 16g back over to the 14b's 6cm^2 housing if you like the spool response of the 14b, or...
2. Lean spool. Yes, I said it...lean spool. All newer Evo's run this on the stock setup, basicly it runs a bit leaner (and therefore hotter) in the first few thousand rpm's to get the turbo spooling faster. I wouldn't recommend it on a high c/r build, but if you do a vitara setup you shouldn't have a problem. At a throttle point over 50% my afr's will stay near the 14:1 mark into boost and around 13.5:1 up until near 4-5psi. After that the turbo is spooled up enough to drop to the standard (in my case) 11.3 afr until I hit my 23psi max. With a 16g6 and a 10.5cm^2 hotside on my 2 liter I start to spool by 2300 rpm and hit max boost by 3300rpm. If I disable lean spool I don't start hitting boost until around 3200 and peak isn't until 4000 or a hair later.

Re: high comp + boost theory
 

i considered the 14b/16g Frankenstein but quickly turned away
my buddy had it oh his zc and said that it lost top end power compared to the normal 16g

but the running lean... ill have to do some research
so i guess the boost makes up for the restarted timing to prevent knock

Re: high comp + boost theory
 

Read what I typed. Everyone else is half correct. I'm about 80-90% correct. Although I will agree with slo_crx mostly about the turbo spooling. at 0 inHG, I wouldn't be at 14's AFR and feel safe(would you feel safe if your NA engine ran that lean under that load??) but I do agree running a bit leaner there and retarding timing will increase spool time. 5-10ft/lbs in the midrange on a 300whp engine is a significant increase. Higher compression makes up for the shitty VE of a D-series engine.

Re: high comp + boost theory
 

ya i most likely will go low comp maybe even try for faster spool with the afr's
but like i said i wanted to see what you guys thought and go from there

Re: high comp + boost theory
 

alone, that statement is untrue as a blanket statement.

this kinda gets to the point though

for a given two comp ratios and a fixed octane, if you run through and plot power vs boost, there will be a crossover point where the retard you need to avoid knock moves pcp too late in the stroke to be useful. at low boost high comp gives more power, and at high boost, low comp gives more power (if high comp can survive at all). somewhere there is a crossover point.

in the gsr example above, the 9:1 should have been running 15psi safely and easily whereas a tight tune would be required to make 15psi on 10:1. along those lines anyway. at 15psi, on the same pump gas, you would probably have found that the 9:1 engine would make more power than the 10:1 on pump gas and with a tune that wont ping no matter how hard it is flogged. ie, a safe good tune.

the spool difference is going to be fairly marginal, whereas the power increase from the higher boost allowed by the lower comp will be significant. that is what it comes down to.

Re: high comp + boost theory
 

well said fred

Re: high comp + boost theory
 

Wow, I think some of you have some serious misconceptions about how forced induction actually works. :3

When the charge-air density is increased (Via boost) The Effective Brake Mean Working Compression is increased. Example:

A 6:1 static comp in a 100cc cylinder in an atmo engine is still 6:1

A 6:1 static comp with a charge-air density of 1.25 (3.675psig) will have an brake effective C/R of 7.5:1 under boost

A 10:1 static comp in a 100cc cylinder in an atmo engine is still 10:1

A 10:1 static comp with a charge-air density of 1.25 (3.675psig) Will have a brake effective C/R of 12.5:1 under boost

A 12.5:1 static comp atmo 100cc cylinder is (once again) 12.5:1

A 12.5:1 static comp with a charge-air density of 1.25 (3.675psig) will have a BME C/r of 15.6:1

Cliff notes: The higher the static compression the higher the effective working C/R during the power stroke

Note: Temperature is a function of charge-air density, and obviously the adiabatic process comes into play here, however the system is squishing the air pre-induction charge then further compacting it in the cylinder. The harder that a charge is compressed, the high the average cylinder pressure will be, whether the charge is less-than-atmo , atmo, or positive atmo(boost). The result will be increasingly greater average cylinder pressure the higher the boost though.

The higher the static C/R is though, the less margin of safety the engine will be able to enjoy. E.G. High boost+high C/R+hot day = :X engine.

My bike's static compression is 11.8:1 and I've run up to 12 lbs on a 98o day in Tucson, AZ. so...I think you'll be alright with a slight bump and moderate boost levels.