turbo question

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i have a 1990 miata and am about to put a t28 turbo in it, and i was wondering how many pounds of boost i can put on the stock motor without having to get a new fuel pump or some rx7 turbo fuel injectors. I am also worried about the well being of the motor so i only want to run a little bit for now but still feel a difference in the power. Any help would be appreciated.

posted by  240SXSR20DET

i wouldnt say no more then 10 lbs.. but google it

posted by  vlc92189

none, sell the T28 and buy a turbo kit from a tx3 laser/escort, your engine would be a 1.8ltr? yeah? then it's a BP engine, thus meaning that here in aus, ford brung out a bpT version, 1.8ltr dohc, turbo, efi, fmic blah blah blah, the works, tough car.

i wouldn't even bother trying to put such a powerful turbo on a stock engine, even the worked ones that i've seen in laser's don't really have the right gear for a T28, they'd barely even be able to handle a T25.

if you want to feel a large difference in power but not actually have such a massive amount of boost running, then i suggest that you get all the fuel gear from a laser and just try to find a td03 (16g) turbo for cheap and run it on 8psi for daily use and 12psi for racing, the engine should handle it fine.

also, if you turbo the car, don't forget that you need a turbo timer and a boost controller so that you never forget to cool your turbo down and you never **** up and run your boost to high.

posted by  HyundaGuy

Most factory N/A motors aren't very happy on much more than 5-7psi of boost on stock internals. Although I don't have first hand experience with the Miata specificially, this is the amount of boost typically run through the Honda K-Series and the Dodge 420A when a turbo system is bolted up (I'm sure there are thousands of people than run much more boost than that, but for reliability most people run 5-7psi). 12psi is what a factory turbo Eclipse (i.e. lower compression ratio) runs stock.

posted by  Bino

Its not the fact that a stock turbo car can run 12psi...stock non-turbo cars have the wrong pistons in them to run turbos at a high PSI. These cars have much too high of a compression ratio (I know for the D16A CRX engine the stock pistons are 10.5:1 while a turbo needs 9.5:1 to work at PSI's higher than 8 or so). The best way to go if you have the money is to get forged pistons with lower compression ratios and eagle rods.

posted by  Golddragoner

Yes, in fact, it IS that a factory turbo car will run 12psi reliably on lower compression ratio pistons. You clown.

posted by  Bino

I'd stay below 10 lbs until you do a rebuild

posted by  OombaIsBack

for the first time in... well... ever! i agree with oomba.

posted by  HyundaGuy

Gee I wish I could pull figures out of my hat like that.

I would have thought you would be fairly safe pushing 12 psi on standard pistons, however getting the lamda down to around 0.8 would be advisable (read you need an aftermarket ECU, sized injectors, etc). The compression ratio should be applied to your valve events and rod/stroke to get a dynamic ratio. The dynamic ratio should be about a point lower than recommended for aesthmatic engines for the fuel you are using.

Molded hypereutectic or semi forged pistons are quite suitable for a decent amount of abuse. But standard mold jobbies can also handle a lot. Whether the pins and bearings can hold up to extra stress without throwing a leg out of bed, is another matter altogether.

posted by  Wally

true, i do agree with you about 99.5% there, but depending on the turbo and also the c/r of the vehicle itself depends on the boost you can run, i mean you can't really push 12psi on a vehicle with a 13.5:1 c/r can ya? lol.

do you think you'd be able to explain why the c/r needs to be lower to handle higher boost in a detailed content? cheers wally, you're a legend.

posted by  HyundaGuy

Yeah, ya can... actually. You're only battling the octane rating of the fuel. We ran a 12:1 CR motor up to 18psi on Methanol (it was a Kawasaki Ninja). No detonation whatsoever.

posted by  Bino

I know that on my car, the stock setup handles 7PSi, and then you gotta change things. Ive even read somewhere that my motor handles up to 600 horsepower (the block) but im not sure. I would go with vlc on this one

posted by  newyorker

damn, 12:1 CR with 18lb of boost! **** me dead, that's definately an acheivement!. (that's not sarcasm).

posted by  HyundaGuy

ummm isn't that what I said? that non-turbo cars have too high of a compression ratio to run high turbo PSI? I swear that I said that...


And 18PSI on a stock engine? that I find very hard to belive unless the engine happened to be built for a turbo on it...no way could you put 18PSI on a stock engine, you'd blow out the head gaskets and seriously screw up the crank even bending the rods because of too much compression (it'd be like a diesel engine, blowing up before it hits Top Dead Center putting a hell of a lot of force on the components)

posted by  Golddragoner

I'm sorry it's outside your realm of belief. Sport bikes have very efficient combustion chambers and are dramatically overbuilt.

posted by  Bino

It also just occured to me... I never said the engine was stock, it had aftermarket pistons... it was otherwise stock.

posted by  Bino

Did someone tell you who I am? :wink2:

What you need to look at is dynamic boost. The valve events have a big bearing on this. A 20psig reading in the plenum is not what the pressure is in the pots. There is pressure drop across the valves, port runners, etc. The intake valve will not be closed for part of the compression stroke and a quiescent state of flow will occur, where there is no contribution to compression pressure. For instance you may have 20 psig at the plenum, but only 16 psig in the pots.

One of the things that contributes to det. is heat of compression. This is one reason why intercoolers are useful. But even with air to air coolers the charge will be higher than ambient. The higher mass of the charge air means it has more latent energy. When you compress it more the already higher than ambient sensible temperature will rise near adiabatically in the pots and can cause the fuel to auto ignite, thus det. Running fuel mixtures rich allows greater latent heat of vapourisation, thus a reduction the rate of temperature rise within the pots pre spark.

posted by  Wally

see there you go. Without those pistons I highly doubt the engine would work properly(consider most gas engines run 10:1 or 11:1 (can't remember exactly) and I believe turbos usually have 9:1 ratios

posted by  Golddragoner

Those figures seem a little high to me?

posted by  Wally

What? The engine had a 12:1 CR, it ran 18psi. WTF are you talking about? I never stated the engine was stock... now did I. The pistons were nothing special, little billet units with increased tolerances to deal with the increased thermal demands of forced induction. It's OK to be wrong, it happens to everybody.

posted by  Bino

Also, there isn't a "usually". Manufacturers run anywhere from 8:1 to an 11.5:1 compression ratio off the showroom floor. The City Turbo motor in my project car has a 7.6:1 CR stock and is boosted on top of that. The static C/R is all over the board, and cannot really be generalized.

posted by  Bino

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