My high compression + boost concept

[Sam_Q]

traditionally people have turbo'd high compression engines because they were boosing a non turbo engine and wanted to do a budget upgade. I however think theres more to it thats worth looking into. There are quite a few generalisations in my explinations below.

On most turbo engines they are made to have good strong power everywhere. To do this a mid sized turbo is usually chosen to try and have boost around as much of the rev range as possible. Also to give decent power the engine is run with a lower compression ratio to have a greater volume of air in the cylender. To deal with the high pressures heavier pistons are used. What this means is that the engine off boost is much less free reving, less responsive, uses more fuel and has a lower redline from the higher inertia.

I on the other hand don't want a huge power increase and I don't really want to sacrafice much of my fuel economy or response. I don't want to come on boost around town because I want to stay under the speed limit and I don't want my fuel economy to be affected. If I am wanting to accelerate hard my revs will be high anyway so having only mid to top end boost isnt much of a problem, yes I know first gear will be a bit slow but I can live with that. Because I will have high compression I will still have a very liveable engine off boost and it will be able to be driven around normally. Because I am not chasing heaps of power I can afford to have standard pistons and hence somewhat retain the characteristics of an NA engine.

My engine I want to do this for is as follows:

- 20v Silvertop bottom end with ARP mains + rod bolts, ground + shot-peened rods, balanced, etc...

- 20v blacktop head with 0.8mm TRD gasket, ported, matched, deshrouded valaves, etc...


I have to work out what compression it will give, I am guessing around 10.5:1


also on a more complex side is the intake trumpets, I dont think I will do this but its an interesting concept:

on an na engine they usually make them short to help with power up top
if they were to make them real long and tune them for 3000 revs on an atmo then it would push hard at 3000 but not have much up top because the harmonics are all wrong and would hinder the filling of the cylenders but if some boost was to be introduced it is a different story on the same engine that would normally have crap top end because of the resonance being all timed wrong would have air anyway because its just forced in there so I can afford to have the trumpets for low end without losing top oh good, its far from a simple thing what this might acheive is fantastic low end power off boost and good economy



I am still learning here and am open to corrections or ideas

[skiddz]

hmmmm i like your ideas man, although the only issues i can see with doing this is detonation unless on avgas or another high octane fuel. also a kinda weird laggy turbo that comes on at 3500+ rpm

everything i have read thus far makes me think you have a good concept, but it seems this is one of those things that has been done and tried again and again but to no avail, there is a bloke on rollaclub who had an sr20de+t and it apparently went alright but, he killed it cos he wound up the boost to about 9 pound or some shizzle

cheers
luke

ps, i like the thoughts on the intake trumpets and boost etc

[Cuzzo]

Meh standard 4age with 300,000kw is loving 12psi boost.

[oldcorollas]

this is the thinking with modern engines from factories...
higher compression, low boost, gives a nicer, more drivable car..
check the calcs for 3000rpm tuning.. you might find the trumpets are a little long..
also, at 3000rpm, the energy is not really there to have a huge difference using harmonics, so it may not be an issue...

good concept, might need to be just a little careful with both tuning and boost with the 10.5:1... start feeding the boost in slowly to see when it starts to detonate..

[skiddz]

edited cos im a dumbass

[myne]

Precisely right.

The key is not the compression, but the actual compression PRESSURE.

If you optimise the engine's natural breathing for the low end, but optimise the turbo for the higher end, the theory is that you'll see no higher compression pressures at the higher end than you do at the low end.

What you're doing though is effectively copying the VTEC/VVTIL type systems, except that instead of switching to a better cam at higher revs, you'll be switching to a higher pressure air source to achieve a similar cylinder filling result.

Whether it shows much improvement over a good high end cam though is another matter.
You could end up in the annoying position of having a turbo engine that performs barely better than a cammed up NA stocker.

On the other hand, if you go read some of theoldone.com's archives, you'll come to realise that it's quite possible to build a high compression turbo motor if you're prepared to do the development in the combusion chamber. Things like paying close attention to quench, swirling, cleaning up all sharp edges, and even piston shape all play a part in preventing detonation. Detonation is the real enemy, and it's not a simple matter of x pressure = detonation.
Look at the other factors, and you could be really surprised at your 10:1 4AGE running 15psi or more.

Problem is, there's a lot of work to be done to achieve it. Most average engine builders dont know or really care about these things. They simply build to old set formulas that are outdated. Engine builders nolonger advance engine technology like they used to. It's mostly up to the manufacturers to advance.
Higher than 10:1 compression is now common in most new engines. Most of them can run ~10psi with no other mods. Tell that to someone in the 80's and they'd look at you like you're mad. Honda S2000's run 11.4:1 if I recall correctly, and there are a few out there with low pressure turbos (6-10psi) and no internal mods.

It's all in the little details that make up the head, the ports, the pistons.
That's why the new engines can do it where engines of the past cannot.

[Sam_Q]

skiddz: a 20v head is very resistant to detonation, right now with my silvertop its fine with 95 octane E10, I have been told with the right tuning 12:1 is possible with 98 RON fuel and some good tuning. I will be using 98RON E10 from united.

cuzzo: I assume 300,000km not kw? a typo or are you joking around with me?

oldcorollas: yeah I will look into the trumpets more, I wasnt on planning on tuning them for 3000 revs but something a bit longer than usual, I might experiment a bit and see what happens. I will be very carefull applying boost, i want to get a 4psi wastegate actuator and raise it slowly from there.

myne: interesting points, all of which sound right on. I am thinking though that by using a 20v head that is designed for 11:1 compression standard and then running it at 10:1 I am already down the right path. I will be taking any sharp spots off though. I plan on building this myself with the assitance of a friend. So I am hoping that Toyota already did every trick in the book to make their head as resistant as possible and I am making use of it. I do hope I do end up with an engine that has a power curve like an exagerated VTEC engine, but time will tell.

Also I never mentioned to have a turbo for high end and only for high speed use I chose something big, maybe too much I just took a chance. I bought a TD-05 off an evo for a cheap price, I think this will be interesting.

[skiddz]

skiddz: a 20v head is very resistant to detonation, right now with my silvertop its fine with 95 octane E10, I have been told with the right tuning 12:1 is possible with 98 RON fuel and some good tuning. I will be using 98RON E10 from united.

mmmm this would be the great lack of "squish" area, but im still not convinced, thats probably cos im not very learned on this topic, but i shall sit and listen without interupting again

[Sam_Q]

nah relax, you had some good points, I dont pretent to know that much myself, hence why I post the topic on here. Thanks for your input either way.

I so wish I knew more about toyota and their lack of squish area, it goes against every combustion chamber princible I have ever heard.

[Turdinator]

I don't know heaps about turbos but by matching compressor and turbine housing i thought you could adjust where the boost would start to kick in. ie small compressor so that it keeps boost low. Large exhaust housing will boost later and probably be less of a restriction down lower in the revs.

I haven't read this article because i don't have a subscription anymore. But it used to be an informative site and could be quite relevant.
Click me

[Sam_Q]

yeah sounds like a good idea, I will see if its possible for me to mix and match the turbo I have lined up.

[ViPeR_NiPPleX]

high(ish) compression plus a turbo with a 'too' big a turbine wheel (but normal compressor wheel)? is that what you are thinking Sam? Low down it acts all n/a and after about 4krpm the turbo starts to spool?

[skiddz]

i think he [turd man] meant a smaller turbine and larger exhaust housing, which to makes sense in that a majority of teh flow will try and go around the wheel, and only a little will make it spin, with more flow..... yeah you know how a turbo works...

a big turbine and small compressor..... sort of makes sense but im not quite sure, so it spins a shiteload but the compressor isnt working as hard so to speak and therefore comes on boost later?

[ViPeR_NiPPleX]

wouldn't a smaller turbine in a larger exhaust housing spool up quicker? or has the 2am crazies gotten to my brain already?

[Sam_Q]

yeah it would, but starting with an oversized turbo initually offsets that, or something

damn this heat I need more sleep also