Need advice...

[DJ984]

Hey guys,

A mate of mine is doing a turbo conversion on his car, at this stage he's just planning it all an gathering the last of his funds so he can do it all...

He found out recently that his compression ration is about 8 or 9:1 (can't remember the exact amount he told me... I suggested he decompress the head but that can be expensive and if there's an easier way to fix the issue, we'd like your suggestions (But we do want everything to work properly etc; no bodgy solutions)

Also, the turbo he's gonna use is a hi-flowed CT26 (T3/T4 hybrid with dual int. wastegates - of a celica GT4). At the moment the turbo's boost is set to about 10(ish).... My mate only wants to run about 5-7PSI (7 at the MOST; to be safe)...

What i suggested was a restrictive front/down pipe and the std. exhaust to limit the boost the turbo can generate... as well as that i suggested using a FMIC big enough to provide a bit of pressure drop and maybee some big intercooler piping?... I realise that if we did this to a turbo car, without enough boost you'd have turbo lag BUT when your going from his NA car to a custom turbo set-up, I really don't think he's gonna notice!?!?

The car is a Mazda SP20, about 6 months old.... How else do you guys suggest we deal with the high compression and reduce the boost?....

Any help will be much appreciated.

Cheers,

Dave.

[Roy]

If you want to run low boost, have a soft wastegate spring, and ensuring exhaust back pressure as a means to limit boost will ultimately hurt the engine more, as detonation will be harder to control if you have a restrictive exhaust impeding the flow of spent gases.

Encouraging the intercooler to have a pressure drop doesnt sound like the best approach either, as that means you then have to wind more boost of the turbo in the first place, in tuen heating the air, making the job of the intercooler harder

Id say stick with the std compression ratio, perhaps a custom head gasket to bring the ratio back to around 9.0:1, bolt on the best exhaust you can squeeze into the front of the thing, along with the most effecient front mount you can find, then fork out the money for a Microtech to ensure that fuelling is ok.

If you keep boost to around 6psi (or less if the injectors cant come to the party) courtesy of a soft wastegate spring/actuator then it should run fine. If you want to increase the effeciency of the motor, you can get power without putting too much in the way of additional stress on the engine.

Doesnt the SP20 have the same engine as the SR2 Laser, HVE in Sydney was involved in a turbo project for i think Hot 4s a few years back, i seem to recall they used a Haltec computer with a turbonetics blower, and Micks Metalcraft (may have been Performance Metalcraft) intercooler etc etc...maybe worthwhile trying to get your hands on back issues

[DJ984]

Hey Roy,

Thanks for the response...

Yeah there is a kit he can get from the states with everything and the turbo is only set to 6PSI in that one, he is looking at either a haltech or microtech computer (can't remember which)... but apparently he has to change the alternator or set it up as a piggy back... any insight?....

Yeah, I understand the restrictive exh is a bad Idea, never really thought about it till i just read what you said though... now it seems REALLY bad... definate no no....

The big intercooler idea was because trhe turbo he was looking at using was set to about 10PSI and he only wants to run about 5-7... the guys in the states have apparently been snapping rods or something with 8PSI 'cause of high compression....

I'll have another talk to him and let him know what you said.

Cheers man,

Dave.

[blind_elk]

FYI, the Nissan / Winfield GT-Rs ran 9.1:1 C/R and 1.8bar boost. Granted, they would have run forgies and all the other good bits. But it suggests to me that your mate should still be able to get away with 10psi (approx 0.7bar), even at 9:1 C/R .

[Roy]

You could always take a hat out of the CAMS NAZI Book and do what they did to the Sierras and GTRs, install a pop of valve on the inlet manifold. I think you will find that FJ20s had them std. Essentially looks and acts in a similar manner as a BOV with a spring that is rated to a certain pressure.

It is mounted after the throttle body, and the minute the thing sees a pressure higher then its spring is rated it will crack open and bleed air saving your engine from seeign higher boost levels then intended. Only prob is that you could be running the turbo harder then it needs to as you are bleeding off boost pressure which it has had to trade thermodynamic effeciency to produce.... so not ideal but if its the case of 8psi v 6psi, may not be such an issue and could be an easy fix

As for compression ratios around 9.0:1 , i dont see that as a big deal, though if the enfien was never designed for forced induction, then the rods coulc become as issue much earlier in the equation, so as long as only running say 6-8psi, sue same amounts of revs to preserve the rods form failing

[MonstaS4]

Roy, these overboost valves are too harsh to be controlling boost. VL turbos also had them standard. When i wound up the boost after fitting a cooler, when boost reached 12psi it felt like the spark or fuel had been cut. Air is bled off so quick that if you kept the boot into it, the car would surge then slow very violently.

[Roy]

Yeh perhaps, it would be very dependant on the surface area of the piston that is bleeing the boost, big area = quick change so your right you would have to keep your foot into it. But small surface area piston with a bit of stroke, we are only talking about 8 v 6psi....could be wrong though

[Merli]

He found out recently that his compression ration is about 8 or 9:1 (can't remember the exact amount he told me... I suggested he decompress the head but that can be expensive and if there's an easier way to fix the issue, we'd like your suggestions (But we do want everything to work properly etc; no bodgy solutions)

You could always install a thicker headgasket and decompress the engine, but if he's only going to be running 7psi max, then I wouldn't bother.

What i suggested was a restrictive front/down pipe and the std. exhaust to limit the boost the turbo can generate... as well as that i suggested using a FMIC big enough to provide a bit of pressure drop and maybee some big intercooler piping?

Already covered by Roy, restrictive exhaust is a bad idea... Have it as free flowing as you can get it.

As for the idea of a restrictive intercooler, what purpose does that serve? So your turbo will spool upto blowing 7psi of pressure, but the intercooler creates a 4psi drop, so you're only getting 4psi at the intake plenum... What does that do? Well your boost controller is asking for 7psi at the intake plenum, so now your turbo has to blow 11psi to make 7psi at the intake plenum. Restrictive intercooler doesn't sound like too great an idea now does it