High Comp and Turbocharged

[Reed]

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.

[nissanfanatic]

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

[Reed]

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.

[nissanfanatic]

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.

[beef_bourito]

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.

[Schister66]

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.

[Reed]

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.

[Schister66]

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.

[beef_bourito]

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.

[Polygon]

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.

[beef_bourito]

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.

[nissanfanatic]

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.



The highlighted is generally the limit for 93 octane fuel.

[Reed]

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

[454Casull]

Originally Posted by Larry Widmer

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

[jupilerman]

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