Boost pressure for built engine
 

I swear I've seen quite a few threads on this but now that I'm searching for them I can't find much... so hopefully someone can help me out here.

Basically at the moment I've got an AVO stage 3 turbo running 0.6 bar (about 8 psi) on the low boost map and 0.8 bar (about 12 psi) on the higher boost map, but this is on the stock engine.

I'm considering getting the engine built with forged rods, lower compression ratio pistons (10:1), and stronger valve springs, so that we can then increase the boost without too much risk of damaging anything. But I'm wondering what sort of level it would be "safe" to increase it to. I won't be running it on E85 or race fuel, just 98 RON super unleaded petrol.

So does anyone have any experience with (or know someone who has) a boosted FA20 that has had forged internals installed, and if so what kind of boost pressure were they running and did it end badly or is it still going strong?

If I can only really get away with increasing it to 1 bar or something then I probably won't bother, as the difference between 0.6 and 0.8 isn't that noticeable, so I'd need to be getting close to doubling the pressure for it to be worth the cost of the engine build for me really.


DISCLAIMER: Yes I know there is no 100% accurate answer because everyone's version of "safe" or "reliable" is different. I'm not expecting the engine to last forever, but I drive the car a lot and do plenty of track days and drift days so I don't want it to be running so much boost that it might blow up after 3000 miles. Again I know no one can guarantee that won't happen even with relatively low boost, but I'm just asking for a general opinion of what is sensible and should be fairly safe on a built engine.

Remer lower compression decreases torque and moves the powerband higher in the rev range

but doesn't it also mean we can make the boost kick in a little earlier in the rev range too without damaging things? Or does it just mean you can have more boost pressure but it still has to be coming on at the same rev range as with the stock motor

Things to consider:
How early the turbo builds boost
The quality of parts used
How it's tuned

The killer of stock rods is low end torque, you could run more boost than you do now without changing anything if you don't add the torque at the lower end. This can be done with cam mapping and decent EBCS etc. This is why you can run decent top end power on an SC install on a stock motor quite easily.

However if you want a low end torque monster then you will need to uprate the rods. If you drop the CR of the piston then you automatically "lose" power and response. The power you can make up for by adding in boost earlier. The killer then is detonation or just too much torque for the rods or bearings to take. Spec a decent thicker oil for the bearings and hopefully Adrian should know the limits of the beefier rods. TBH with most forged stuff you are likely to hit detonation before the rods become a worry with the boost you're on about.

So as I'll be getting stronger rods, we can increase the torque at the low end and this will somewhat counter the reduction in low end torque from going to lower CR pistons right?

I saw some comments about running thicker oils but there seemed to be mixed opinions on it and no definitive answer on when it is a good idea to move to a thicker oil (or how much thicker to go).

Boost doesn't mean a whole lot. The flow/efficiency does. 20psi on an 18g vs 20psi on a 72mm will be way different. For reference, 18psi on my old avo setup peaked at ~390whp but died off horribly because it was out of efficiency. 18psi on my 6262 makes about 500whp and still climbs to redline

That's a good point and that's why I mentioned the turbo that I've got, but just to clarify it's this one: http://www.fensport.co.uk/Parts/Mode...tegory_17/4917

Apparently it has 20% better flow compared to the stage 1 & stage 2 AVO turbo and is rated for 450hp of flow. What that means in the real world in terms of when it is running past its optimal efficiency, I don't know.

You're missing the point. At 12.5-1 we saw peak torque at 4k at 13.5-1 we say a gtx30 spool at the same rate as my gtx28. At 9-1 peak torque was almost 1500 rpm later with the same size turbo.

What is the end goal? Why do you want this extra power?

Yeah I get that a lower CR means the power comes in later, but someone else just said that with stronger con rods (which I'll have) you can make the boost come in a bit sooner than you could safely do on the stock engine. So can't we use that to counter the effect of the lower CR and make it feel almost the same as it feels now, but just with more power?

I thought there was no point sticking with 12.5:1 CR if I'm rebuilding the engine because we're just going to get knock as soon as we start increasing the boost much more right? Stronger rods and pistons etc aren't going to help with that are they (unless I'm misunderstanding something). So it seems pointless doing it if I'm not going to go to a lower CR.

To improve acceleration in mid / high gears and to spin the wheels up easier in lower gears (for drifting), and to just generally feel faster and be more fun. Not sure what other possible answers there would be so I'm kind of confused why you'd ask the question.

You're still kind of missing the point. You can't magically make the boost come in sooner. The turbo needs X amount of exhaust in order to generate boost. If you lower the compression, there is no way around it, the turbo will spool later (i. e. the exhaust "energy" lost by lowering compression will be made up by higher RPMs) and thus move your power band to later RPM's. Yes, you get a big amount of margin in the knock department, but at the expense of building boost at a later RPM because of the lower compression.

Do you have E85 available in your area? If so, I'd stick with 12.5 and keep that turbo spooling as quick as it does now, but with infinite room to grow because of the beefier internals.

If no E85, I'd say lower compression at the most to 11, so get some knock margin, but not too low as to drastically move your spool RPM range too far up.

His question could be answered as in, for drag racing, for road racing more power coming out of corners, etc...

Good luck my friend. You will need it I have a feeling.

Care to explain?