Shark99

We have a '99 that we take to HPDE days. FM carb legal turbo with voodoo ecu that is not adjustable. Sent Question to FM but they are backed up I am sure, so thought would ask you guys: We were thinking of getting a more open exhaust system for the track. We can get a 3” system with magnaflow muffler for ~$300 or a 2.5” for $250. FM mentions issue of over boosting but if we want to future proof as might some day opt for megasquirt would the 3” be the way to go and possibly just get their restrictor plate? Also, I believe the downpipe included in our kit is 2.5” would that not ultimately be the limiting / most restrictive portion, so the 3” system is a bit of a waste of money or is more open system always better? Sorry looking for guidance. Thanks

andym

The reason that they mention that there might be overboost is that the turbo wastegate can't cope with that much flow. You can port the wastegate to alleviate that problem. Putting in a restrictor is a not ideal way to prevent the issue since you are negating the advantage of the 3 inch exhaust. A megasquirt down the road is not a bad idea. I think FM also sells a 3 inch downpipe these days too.

Shark99

Thank you for the info. What is required to port the wastegate. It appears to be a sealed unit from FM. Also, if I understand correctly, the overboost would provide more air and possibly cause a lean situation. If we monitor our AFR and it stays well below 14 under boost and we dont hear any pinging. Are we ok?

andym

The FM turbo is likely a garret gt2560r. You can watch videos online of people on youtube porting the wastegate to prevent overboost. I am not saying you will get overboost, just that it is a possibility.

The problem with overboost is that the car can run lean and detonate, but also as pressure increases due to the creep, power can effectively go up because more air. If the car is making more power and detonating the rods won't be happy.
Did you build your engine with forged internals or are you running a stock engine?

Watch a video on porting the wastegate and see if that is something you are willing to do or not.

Shark99

Yes it is a 2560 all stock internals. So if we monitor AFR and are under 14 at boost, we should be good - correct?

codrus

I wouldn't run my engine at 14:1 in boost, more like 12.5:1.

What is driving the need to replace the exhaust? Do you already have the 2.5? If so, then I would just stick with it and upgrade to the 3" when you upgrade the control system.

If the current exhaust won't work for some reason, the restrictor is a hacky solution but potentially viable. It's not any more hacky than buying the 2.5" exhaust to get the same restriction.

FM does not recommend porting the wastegate, I believe that is because of warranty concerns and because they've seen people screw it up and wreck the housing. It's up to you whether or not you can live with those risks.

--Ian

Shark99

Thanks, we are pretty much with stock exhaust, so we thought it would be good to upgrade, plus will sound a lot better :-) Re AFR will look where we are at boost to ensure we continue to be at 12.5 or lower. Watched some videos re porting does not look too hard. I am sure you are correct re FM not wanting folks to modify their stuff. Will be interesting to see what happens. Restrictor plate is only $20 so quick easy fix and albeit a hack should not be worse than whole system that is 2.5"

skidude

iTrader: (3)

3in exhaust with a 2.5in downpipe isn't quite as good as a 3in downpipe, but it would still flow better overall than a full 2.5in system.
As others have said, more flow means you could overboost, or you could spool faster. Faster spool is better as long as the ecu is tuned for it.

shuiend

iTrader: (24)

Downpipe size is mostly determined by turbo placement and what your room is. Most start 2.5" and expand out to 3" just due to spacing constraints. Quicker you can get to 3" is the better, but I don't think there is a drastic difference between the 2 sizes.