fd3s400438

I dont know if this question has been talked about or answered in the past but I could not find anything relative so I might as well ask

Most exhaust systems of the self for the RX7 are 3inch. Take my car for example. It has a HKS cast iron manifold , 3'' downpipe with the external wastegate gases rerouted back into the exhaust , 3'' downpipe and 3'' cut back ( HKS carbon-Ti)

My car uses a small turbo compared to the american average ( I have an HKS TO4E ) but may see boost up to 1.5 bar = 22psi in 4rth gear. My power was measured at 416 fly at 1 bar .Since that I have not dynowed the RX7 but it should be close to 420-430bhp going from 15psi to 22psi in 4rth

So a friend of mine running a GT4088 with 593 fly @ 24.5 psi suggested that my exhaust may already be maxed out. The boost drops from 22psi the minute you engage 4rth down to 19 or 18 psi to redline. On his car he has a 3.5 inch = 90mm
exhaust that he fabricated himself


There are no boost leaks , brand new GREEDY boost control , all has been wired correctly. No mater what I dial in it will increase the boost but not hold it steady. It does not do boost spike , it just drops by 3-4 psi close to redline

So would you say that 450bhp or a bit less is the limit of the 3inch exhaust??? On a piston car NO but we do produce alot more exhaust gases than piston engines that is why we may spool big singles that pistons only dream about

Tell me your experiences , setups and information you may have learned or gathered over the years

Is it worth building a 3.5 inch system? Is it worth fabricating an external wastegate exhaust and re rout the gases in open air instead of throwing them back into the already small exhaust? Could it be the case that my HKS T04E rated 450bhp has been maxed out and the exhaust size does not mater that much?

Thanks alot

GodSquadMandrake

Are you using the 57 trim or the 60 trim T04E?

Basically if it's the 57 trim, it's the turbo.
If it's the 60 trim, it's something else and the exhaust could be contributing to that.





On these flow maps you want to look for 2.51 on the pressure ratio side. That is 22 PSI. We're counting atmospheric pressure plus manifold pressure here I believe. The green and red box is the flow map of the rotary engine. Their guessing from 30-40 lbs/min. It all depends on the air temperature, what kind of porting etc. So it's not an exact guess. But as you can see with the 57 trim you are going to start dropping out of the 70% efficiency range of the turbo near redline. That means your boost should start tapering off.

That's just my quick amateur analysis of your situation.

Onto the exhaust. From what I've been told you want 2.2 CFM of exhaust flow per engine horsepower. That was for piston engines and because this is a rotary it's probably 2.5 or 2.6. It doesn't mean that airflow will stop, but pressure will build in the exhaust. This increases the pressure ratio across your turbocharger making it work harder which robs power and means you get less boost. So you are telling me you have a 420-430 HP engine. I'd say you need about 1050 CFM of exhaust flow. 3" limits you to about 750 CFM. 3.5" would be about 1050. Some people will come up with slightly different numbers, maybe as high as 850 CFM for that 3" pipe. But either way you are below the requirement and you most likely have at least a little bit of pressure building up in your exhaust and you could definitely benefit from a larger exhaust system. Simply put your buddy is right.

DaleClark

iTrader: (55)

There are ways to tap into the downpipe and get a pressure reading of the exhaust. Greddy actually used to sell a big boost gauge that came with copper piping to read exhaust pressure. That's the scientific way to deduce it. Exhaust can't get out fast enough, it backs up, you build pressure in the system. Just like the old thumb on the end of the garden hose analogy.

Dale

Liborek

Boost drops because compressor is maxxed out. It has to drop - don´t forget, boost isn´t cause but consequence of flow. Since compressor can´t flow more air, but engines RPMs are rising, it simply drops manifold pressure, but flow is still same.

Certainly yes. But even better is measuring of post turbo pressure to determine if system is sufficient for given flow.

This is up to you if you can live with the noise.

Certainly yes. If you will build less restrictive exhaust system, whole combination will flow more at less boost, but it won´t change that compressor is maxxed.

fd3s400438

You have been very helpfull. I wish that somebody could tell me 100% if the T04E 57trim that I have is maxed out or my exhaust is too small.

Anyways. My next step will be a GT4088 because my HKS unit ( believe it or not ) has been running since 2003 and its time for an upgrade

So a GT4088 with double walbro pumps , 550s + 1680s and no more than 22 psi

What I do understand from the information you gave me is that even if my exhaust is not maxed out now it will definately be maxed out with the GT4088

So , what have you done. Have you bought a 4 inch system of the self or have you used a local exhaust manufacture to make your own system???

Can you get a 3.5 or 4 inch system of the self from the downpipe all the way back? If yes who does it?

Thanks alot

GodSquadMandrake

If it's a TO4E 57 trim your boost is tapering because the compressor is losing efficiency near redline, not because of your exhaust. You might be able to squeeze a few extra horsepower out by getting a large exhaust, but either way you need to upgrade that turbo to get any more power.