High Comp and Turbocharged

This is sort a of a rant but i also have some questions so here i go.

I know the old rules (at least for streat cars) say to drop compression when turbocharging to support more boost. I understand the concepts behind this and assume that it is to avoid detonation and not so much to avoid engine damage from any other cause. But, what is the tradeoff from dropping the compression just to gain boost. Loss of low end torque just to be gained back when? Once full boost is reached? or maybe only very close to red line? I dont know I suppose it depends on the set up, but what is the norm. I ask these things cause i have been seeing a lot more applications where people are running more boost with stock and raised comps and are making great power numbers with great torque curves as well.

Now if i were to keep the compression stock (lets say 11.5:1 on a random DOHC Inline four) and i lengthen the stroke but use low comp pistons to keep it stock comp ratio, is that better then if i were to shorten the stroke and use high comp pistons while still keeping the stock comp ratio? Would i notice a difference on a dyno chart?

Now lets say i have a I4 with VTEC of VVTl-I or any valve timing system that has variable lift and decide to turbo it with a moderate amount of boost (say 10PSI, it doesnt matter). if i were to lower the rpm at where the cams change to high lift would i benefit from having more flow at lower rpm or would i be loosing torque in those low rpm. if i would gain power would it then benefit me to have the high lift profile always in place?

thats all for now. i hope this becomes a good discussion and i learn some things. i know there are a lot of very inteligent people on here so please share your knowledge, opinions, or guesses.

The only thing you are really combating when it comes to high CRs and high boost is heat. You combat it by retarding timing, running higher octane fuel, and anti-knock.

Typically longer stroke equates to more torque. But with more boost on the short stroke setup, there is no reason you can't make equal torque. The plus side to the shorter stroke is higher RPM AKA higher horsepower with supporting mods.
What you described, running lower CR pistons with longer stroke or running higher CR pistons with shorter stroke would depend on the setup. Esp cam timing, port size and amount of boost. But yes, peak torque would be regained when you hit full boost. Peak torque occurs at Peak Cylinder Pressure. So as long as PCP on one setup is higher at one point than any other on the opposing setup, you will make more peak torque.

Personally, I would take the short stroke+Higher CR engine just for a longer powerband/smoother powerband. Assuming I had the money to build to my preference.lol

Now when retarting timind, dumping more fuel, and reducing comp, all the losses from that are negated once you get to full boost? It just seems like that would be such a detrement to your power that would be hard to make up.

Something else. Is it eisier to size a turbo for a motor that has a lower red line since it must be sized for a smaller rpm band.

Well thats the beauty of technology. Now you can use engine management systems to pull timign under certain loads and/or dump more fuel under certian loads... You can even program when to inject anti-knock.

Turbos should be sized in flow areas. Flow changes less on a shorter stroke engine when RPM changes whereas it changes quickly on a longer stroke engine when RPM changes.

Higher compression is always better because you get a more powerful combustion, but by raising the compression, you're not getting any more air into the cylender, you're just getting more out of that air and fuel than you would with lower compression. if you lower the compression and turbocharge it to an ammount of boost that will give you the same ammount of cylender pressure, you're now putting more air and fuel into the cylender and therefore you're getting more power than with the higher compression. there are 3 ways to make more power out of an engine:1) the most common and important way, get more air and fuel into the cylender, 2) get the exhaust to exit as quickly and efficiently as possible, and 3) make the whole thing more efficient (by reducing parasitic loss in the drivetrain, valves, camshaft, etc., by making the combustion more efficient, by using lighter materials for pistons and such to make it require less force to move them, etc.). intake stuff (cold air intakes, manifolds, throttle bodies/carburators, turbo/superchargers, etc) get more air/fuel into the system. exhaust systems (removing backpressure, headers, high flow cat/no cat, cat back, etc) get the exhaust gases out more quickly. underdrive pulleys, lighter internals, balancing internals, higher compression, lighter moving parts, etc make the engine more efficient. but the most effective way is getting a bigger bang from more air/fuel in the cylender.

You can do high compression and boost all at once.

Turbo diesel trucks run 17-20.1 CR along w/ turbos....the only thing is, is that diesel engines are built absolutely bullet-proof and they have to run diesel fuel because i don't think there's gasoline w/ a high enough Octane rating to not detonate w/ a CR that high with boost. If you can find a fuel w/ octane high enough, and build a stong engine, then you can have a high cr and boost.

Well diesel engines can run high comp and a whole lot of boost cause compression is what makes diesel go boom (the good boom, not detonation). And yes you can run whatever comp and boost you want with high enough octane but thats pretty much why i was keeping to street engines.

I'm curious about at what point the cr gets too high to support high amounts of boost. i wish there were a table with two lines, one showing boost pressure and one showing cr, and wherever the lines cross or wherever they get close before detonation occurs would be a good place to set boost and cr. Come on engineers, do some math, throw some formulas around and figure this stuff out for me or us. Or at least someone steal it from a web site and post it here.

Well honestly if you wanted to know, i personally would call the local dyno tuner guy and ask him what he thinks....how high of a CR w/ boost he can tune on 110 octane.

diesels are completely different engines, on the intake stroke they don't inject fuel, they only put air into the cylender, then the engine compresses it and when it's at top dead center it injects the fuel into the cylender and because of the hot air it ignites and makes power, the reason diesels can't rev high is because diesel burns slowly and therefor can't completely burn at higher revs so it doesn't make power.

I can honestly say that on a street car you can't run an 11.5:1 compression ratio at ten PSI on pump gas without getting detonation. You're welcome to try but be ready to replace some pistons.

I just honestly don't see why people see lowering the compression as such a bad thing. As long as you can read a compressor map you can match a turbo to your engine. As long as you match the turbo to your engine you won't be out of boost long enough to worry about engine response. Just make sure give the turbo a good intake and exhaust. You will be able to run so much more boost. With lowering the compression the negatives are far outweighed by the positives. It is the other way around with high compression and advanced timing. What works for N/A will hurt a forced induction engine.

Also, diesel engine use compression to ignite the air/fuel mixture. So you can't really use them as an example here, they are a completely different animal. Also they don't inject fuel at TDC, they inject it just before TDC.

for simplicity sake i said tdc. i know it does it just before tdc to give the fuel some time to ignite and therefore allow for more power, torque and revs.

Most forced induction engines aren't very happy with less than 20* timing advance.

There is a knock limit as well for a given fuel. Its when PCP gets too high for a given fuel to remain stable until spark is applied. Basically you "compression ignite" the fuel due to its low octane.

It has been proven that lower compression ratios will support more power. You are filling the combustion chamber with air as opposed to metal. With boost, you dynamically raise the compression ratio by applying more boost.

http://www.ka-t.org/klattr1/compressiontable.jpg

The highlighted is generally the limit for 93 octane fuel.

thats a nice chart. does anyone have info or opinions on the vtec question?

A properly engineered "combustion space" both dictates and permits much higher compression (static and dynamic) with the outstanding ability to prevent detonation, and so we feel that the "shape" is everything.

The Soft Head, explained by Larry Widmer (http://www.theoldone.com/articles/The_Soft_Head_1999/)

Do I read the graph correctly, by saying that 6psi on 10.5:1 C/R is safe?

As a very vague generalization, dropping compression one full point drops the hp and torque curves about 5% across the board, but the resulting boost you can successfully push can take you back up about 70%, for a net increase of 65% power on the same fuel.

Compression is not the free ride every one purports. Sure it gives a touch more VE, throttle response, and low end torque by about the same % as the HP, but I run 8.6:1 in a 400-hp 454. Of course, torque is not my weak point with this engine, but the ability to run 86 octane on the hottest day towing a 6000 lb trailer is a very nice thing. I could pick up 20 more horses by going to 9.6, but its just not my thing. I would much rather have a long-life reliable engine that is easier to tune without catastrophic failure if you don't do it quite right.

Another thing you could do to combat the detonation is to use a piggyback computer to stage a second bank of injectors that throw Methanol in under boost. There is a good writeup on that in an EFI tuning book... can't remember which one... but its on an NSX. The owner kept the factory assembly at 10.2 and was able to run 12 psi by staging alcohol under boost.

The problem with avoiding detonation by "easy" methods like excessively retarding ignition, dumping extra fuel, and things like that is that they are a crutch. Its not the "right" way to do it. Dumping extra gas in will cool the combustion, but at the expense of intensely rich conditions. Clogged cats, fouled plugs, wasted fuel, and reduced power will be the side effects. Retarding ignition timing will have the effect of increased HC emissions (it richens things artificially), and only does good to a point. There is a point where enough cylinder pressure will ignite itself without a spark at all like a diesel. Retarding timing will stave off knock for a while, but as cylinder pressures rise, there is a point at which no amount of retard will help.

I say do it right. Lower CR, use 114 octane, or inject a fuel with a much higher detonation tolerance like one of the alcohols.

Do I read the graph correctly, by saying that 6psi on 10.5:1 C/R is safe?

On a correctly sized turbocharger with an efficient intercooler, 6psi with 93 octane fuel should be very safe. Could prolly even go to 8 if you are feeling frisky. But once again, using higher octane fuel or anti-knock would allow you to run much more. Say you go to 110 octane, you could run 8+17=25psi assuming your turbo and intercooler are efficient at that level. General rule of thumb is one octane point up=one more psi of boost you can run without pre-ignition/detonation.

thanks for the swift reply! That confirms what I was thinking. nice finally talk to people who know what they´re talking about.

A pitty my turbo suppliers don´t reveal their compressor/turbo efficiency maps. e.a. they cut back their Garrett GT25 compressor wheel vanes to get compressed air out temperature down.

Is there a anlogue relation between ignition retard (retarding the advance BTDC if you wish) and boost?
I´d like to map my piggy back so that my car is driveable. At least to the RR to do some proper mapping!

thanks,
david

I´m new here, but it seems I´m going to like it!

It depends a lot on the setup how much you retard timing. On KA24DEs with 9.5:1 compression ratio, we retard approx .5*-.75* per psi of boost.

No doubt the best way to tune timing is to get on a dyno and do trial and error while watching the torque curve. Start safe, advance up until torque begins to go back down.

Some guys also use MSD Boost Timing Master. Works well. I used to use one.

Thanks for the rule of thumb.
I use perfectpower´s smt6
wwW.perfectpower.com
you can tune e.a. MAP signal, 4 additional high impedance injectors, ingintion retard/advance. And all reasonably priced (about 400USD I believe) the smt7 can even tune more than that.
SplitSecond www.splitsec.com has some good stuff as well.

About my IC efficiency. In theory efficiency is pretty straightforward. In reality however I have no clue. I have a rear/mid engined car. So plumbing to the front would be deevious and producing too much head loss. I positioned the IC in the wheel arch. Unfortunately only parrallel to the wheel itself against the liner. No other options available. I hope turbulance will be enough to proof at least 60% efficiency.
I´d like to use LM-1 by Innovative motorsports and their rpm convertor which enables 5 more aux inputs, which I will be using to monitor pressure and temp diff. across the IC. What I like about innovative is that you can calibrate output for you sensor. 2V=70F nice!!!
cheers
david

well i dont know much (read as anything) about MGs but here is how we mr2 spyder owners intercool
http://hassproturbo.com/catalog/MRS-Tkit-dtls.html

i have seen this rear mount setup also with some ducting and also with fans attached directly to the IC like a fan on a radiator

Wow, nice setups they/you have there!

Low rear IC:
The IC at the low rear isn´t gonna work for me as that´s where my exhaust sits. And just above that my boot ;-)

Top mounted IC:
My car has about the same vent appeartures, but smaller.
Also, according to my workshop manual it seems air is drawn in through the side vents (cool dense air) and exits at the top of the engine compartiment (less dense warm air). Seems logical as warm air is less dense and rises up.
I believe the function with MR2 isn´t any different.
That would mean you get warm air coming into the IC from below.

Have you any measurements on engine bay temp or cooling efficiency (temp-in/temp-out?)

cheers,
david

you can check around spyderchat.com and do some searches for stuff about intercooling. an engine temps.

cheers for that !