[Mk2] [Turbo] High comp low blow anyone?

[Loser]

Hi all, sick of my engine falling out so i'm planning on doing a swap (whoever did the cambelt installed bolts that tightened on the bottom of the threads before the mount, which have since sheered off and i've only managed to get 2 of the 3 out replaced the 2 bolts i could actually get out but the xxxxxx keeps breaking them)


So onto pastures new and greater things, i've had a quick search and not managed to find anyone who's done this (on IMOC at least) and i just thought it'd be a giggle

I've been plugging numbers into effective CR calculators and worked out that a 10:1 CR @ 10 PSI is pretty much the same as an 8.5:1 at 15 PSI - does it just work like that or am i being dumb?


I'd be using a rev1/2 engine (just because it's cheaper ) with some home-brew management (will get this working before messing with the new engine though!), with whichever rev3+ cam is lumpier (can't remember off the top of my head, think it's the intake side.. anyone?), and garnishing it with a port polish and st205 CC. Oh and a 1.5m/m head skim if my maths was something like



Anyone got any thoughts/ideas/warnings??

[chron]

yep, its the rev3+ inlet cam

[Wingers]

That doesn't sound like a good idea to me.

I don't know exactly what your background so forgive me if I'm patronising you here but these engines are effectively knock-limited.
The main problem is that the engine starts knocking and we have to take corrective action to deal with this. The usual method is to chuck in a load of fuel to cool the charge down again but this costs power so is not ideal.

In the high boost, low compression engine, the charge will be heated up by the turbo pressurising it but it will then have an intercooler to reduce the charge temps again. In your low boost/high compression engine there won't be any chance to do that as the charge will be heating up inside the cylinder.
I've heard of doing exactly the opposite before now (dropping the CR and adding a turbo to an N/A engine) but never what you're talking about.

What's your theory behind trying it?

Wingers

[Loser]

The theory behind it is that i think this is more the engine that it was supposed to be in my opinion. I read a really interesting article on the celica st165 rally tuning. Toyota team europe did a high compression low boost engine and toyota team uk chose a low compression high boost engine.

Also i'm looking at the cheapest possible way of increasing power- this method would take the ct26 closer to its efficancy island (big assumption but even at 12lbs it seems on the limit of it's depth), and i noticed that crx turbo vtecs can run 8-9 lbs safely with a compression ratio of 10.2:1, and that's on a standard engine, and they can run 12-14 psi without knock- but the engine doesn't like it.

I see what you're saying about cylinder temperatures but for the above reason i think it can be done?

Obviously if i was thinking of increasing the boost as well as the cr then yeah i agree but with me reducing the boost as well surely it could work?

Out of interest, what's the first thing to go on a rev2 after the headgasket? And when in power lol

[Steve Horrocks]

iirc, wasn't James (Fonzy aka 3s service centre/TGT) building an engine like this a couple of years ago?
The one with cream coloured rocker cover seems to be jolting grey matter around!

[Loser]

plenty to be searching on there mate, cheers

[Wingers]

You'll certainly get more torque low-down/off-boost with high compression but, personally I think you'll lose out at the top end due to running a soft ignition map and/or lots of overfuelling.

Having said that, it's only a guess on my part as I haven't seen it tried. It's an interesting project though so as long as you know what you're doing when it comes to mapping it then by all means give it a go

It's always good to see people trying new things. If we all did the same thing it would be very boring.

Good luck, I'll be keen to keep an eye on how you're getting on

Wingers

[Rogue]

High compression, low blow turbo? Sounds like the specification of just about every MR2 Roadster turbo conversion!

We're just putting together the finishing touches on a 2ZZ-GE turbo conversion with a compression ratio of 11.5:1.



That said, the plan is to build up a lower compression forged long block to replace this one if (when?) it goes pop!

Patrick

[Loser]

i knew someone would be doing it, out of interest rogue what's a safe amount of boost to be running on that 2zz ge (really know nothing about that engine, pardon my ignorance )

i was thinking about it though, how come with boost you can get such mad compression ratio's? I thought the most you could get away with n/a is about 12:1 at a stretch, but if mine's 17 odd how can it be so that a can't do that on a nasp engine but can on a boosted? Or have i got to do it myself to find out lol?

Sidetracked a little there, have found an engine but can't get to it this weekend can't wait to get stuck into this.

Anyone got any advice regarding head skimming/block decking? Not sure weather i should take metal off the head or block to bump the compression ratio, if i skim the head i'll make the shape of the cylinder funny (i think, although i've never actually seen a 3sgte head) but if i deck the block i'll lose out on some water jacket. Anyone got any advice?

[bobhatton]

Hi all, sick of my engine falling out so i'm planning on doing a swap (whoever did the cambelt installed bolts that tightened on the bottom of the threads before the mount, which have since sheered off and i've only managed to get 2 of the 3 out replaced the 2 bolts i could actually get out but the xxxxxx keeps breaking them)


So onto pastures new and greater things, i've had a quick search and not managed to find anyone who's done this (on IMOC at least) and i just thought it'd be a giggle

I've been plugging numbers into effective CR calculators and worked out that a 10:1 CR @ 10 PSI is pretty much the same as an 8.5:1 at 15 PSI - does it just work like that or am i being dumb?


I'd be using a rev1/2 engine (just because it's cheaper ) with some home-brew management (will get this working before messing with the new engine though!), with whichever rev3+ cam is lumpier (can't remember off the top of my head, think it's the intake side.. anyone?), and garnishing it with a port polish and st205 CC. Oh and a 1.5m/m head skim if my maths was something like



Anyone got any thoughts/ideas/warnings??

What fuel are you going to use?

Bob

[Loser]

v-power like i always do

[bobhatton]

When Toyota designed the engine to run on say 98 or so octane fuel with a boost of 0.8 bar they had a CR of 8.8. When the boost was raised to 1.0 bar the CR was dropped to 8.5.

Why do you think it will run better at a CR 10?

Bob

[Loser]

i found this article this morning which best explains what i mean http://www.motorsportsdigest.com/tech/forced2.htm

cylinder pressure wise a cr of 8.5 at 15 psi is the same as a cr of 10 at 10 psi, so i'd end up with a broader range of torque (low rpm would be improved where the turbo currently can't supply boost - below 3k or whatever), i'd be asking less of the asthmatic ct26 so the wastegate would be more open more of the time and i'd get less pumping loss (although i'm unsure if that pumping loss will be made up by the extra static compression- i guess that's why you can make mega power with ridiculously low cr's and lots of boost?), have less lag (takes more time to build up to 15 lbs than 10 with the same turbo under the same conditions?) and this is theory but i think it'll improve the fuel consumption, if you're compressing the same charge more surely it'll be a more efficient power stroke, meaning you need less throttle, therefore air therefore fuel..

I think..

[Loser]

double post

[bobhatton]

What are you going to do to stop it detonating with that high a CR and that boost?

Bob

[Wingers]

I don’t think it is the same charge though.

It’s not fair to compare turbo “compression” with internal “compression”.
If you consider an n/a engine of 2L with CR of 10:1 then you’ll end up with 2L of air in the engine, compressed to 10barA.
If, on the other hand you take a 2L turbo running 1 bar of boost and CR of 5:1 then you end up with 4L of air entering the engine (i.e. 2L at 2barA), which is then further compressed to 10bar. Obviously that has a lot more potential energy.
In reality it’s not quite that simple but the basic theory is there. Overall, the mass of air in the cylinder will be a lot higher with the high-boost application, when it’s running on-boost.

You’ll definitely gain low down torque and fuel economy but, IMO it will be at the expense of mid range & top end power. The extent of the loss will be down to a number of factors, including knock resistance which is difficult to predict.
You’ll quite probably end up having to run so little boost with a CR of 10:1 that the overall result will be virtually n/a. A meth injection kit would help rescue this to some extent but I don’t by know how much?

Whether it’s all worth doing depends on how you use the engine. If you do lots of miles at low rpm then it might be worthwhile. If you like to drive fast/accelerate hard/use high rpm then I doubt it’s value as a concept.

However, I think you might not be talking about a significant change in CR, but more a small tweak to bring the engine performance more in-line with the ct26. I guess there could be some merit in that (and I have no idea what the CR should be to do that) but to make it work you’ll need to remap the fuel and ignition which means adding the expense of a proper ECU. With that, and the cost/hassle of skimming the head/block then I’d question the validity of the project. I haven’t priced it up but given the amount of work I’d rather throw in a ct20 or even a gt28 if it were my engine and run with that.


Wingers

[bobhatton]

The CT 20 has a bigger compresser wheel than the CT26 but also a smaller turbine wheel. I feel the CT 20 comp wheel fitted to the CT26 is the way to go.

Bob

[steve b]

If you have low peak power goals but want a really nicely driving car then high compression low boost is the way to go. You are limited to overall power but off boost and general "perkyness" is much inceased with higher compression.

1800cc
9.1:1
10psi
250bhp/220lbs
GT2860RS

[Loser]

That amount of boost at that level of compression is tunable as far as i'm aware (considering what i've mentioned about vtec engines and cylinder pressures) so i shouldn't have to take any special measures against det other than a proper tune with a standalone (going to set my car up with a fully programmable honda ecu, just need to fit the dizzy, an intake air temp sensor and map sensor) which because i already have it won't cost more than 100 notes

My theory is that the power i'd lose by reducing boost pressure i'd gain by increasing the compression ratio- sure by running more boost you'll get a bigger explosion in the cylinder, but because the energy from that explosion is being used less efficently as long as the effective compression ratio stays the same i can't see any power being gained or lost (ignoring slight changes in different component's effiancy (like reducing cr and asking more of a ct26 for example)) it is my opinion that effective cr denotes the amount torque an engine can produce - if you negate pumping and parasitic losses (turbo, supercharger, cams, alternator aircon.. or whatever) (with torque comes power because i'm leaving the things that effect where power is, except the rev3 inlet cam.

Effective cr = ((boost in psi+14.7)*static cr)/14.7

Regarding cost and hassle- i'm fetching the head off to swap the headgasket for a rev3 metal one and i'm an engineer so i can get the head skimmed for free

My power goals aren't massive- i'd like to see an improvement on my last dyno (245 bhp at 12psi) but i'm pretty sure i won't see the other side of 300 brake lol. Too much money to spend to get there, but if i can make this engine more friendly for fairly cheap i'd like to do that, i do consider the standard 3sgte engine to be quite rough- alot of power but not much finesse. Purely max power maths but i should see more than a rev2 running 15psi on a ct26

edit- poor grammar

[bobhatton]

This will give you some more info.
http://www.mr2oc.com/showthread.php?t=2 ... ge=1&pp=30


Bob