I know that it is very common for n/a 7's to turn up to 10k. I understand that turning this fast doesn't help unless you port to allow for decent VE's up there.

My question is:

1) Why aren't high boost turbo engines (with extended ports) doing the same?

2) What are some of the problems encountered when doing this? Seal float? If so when?

3) Heat generation issues?

4) Stat gear/ e-shaft flex?

5) What rpm are some of you high boost (15-20psi) guys running as far as rpm? Whats mods have you done to support this?


Thanks,
Justin

 

I am building a high revving 13b with a GT40 turbo. I am semi peripheral porting (small peripheral ports), stock primaries, and street ported secondaries, using a custom intake manifold.

I have RX8 stationary gears with 3 window bearings, race clearenced rotors, 3mm ceramic apex seals, deep groove rotor bearings, Cermet coated side and rotor housings, ceramic coated rotors, high pressure oil regulator, underdrive pulley, dynamic balanced rotating assembly. The engine is NOT put together yet.

It will rev up to 10,500 RPM; however, I dont expect to make peak power that high. I will probably be shifting at 9-9,500 RPM. I don't see why I shouldnt run 15-20 lbs of boost; the engine will easily withstand it, but I am looking for more than peak power. I want the best combination of boost response, broad powerband (where torque is most flat), peak power output, and reliability.

Anything over 8,000 for a stock S4 and 8,500 for an S5 and FD will result in apex seal chatter due to excessive seal sliding velocities in relation to the apex seal weight, cavitation of water in the water pump due to excessive rotational velocity of the pump, and possibly physical contact of the rotor to the side housing due to eccentric shaft flex. Also, revving higher requires deeper bearings for more oil "cushion" and higher oil pressure to counteract the higher loads on the eccentric shaft.

There are some other things that you must do to be efficient and safe at high rpms, but there's a short list...

 

The last guy pretty much hit the nail on the head.

Most people who build engines don't like running steel apex seals much pass 8,500rpms. (You can, but its not as safe as other seals designs) If you where to run ceramic seals, perform the correct oiling mods, clearancing etc, a boosted motor could take more rpms. While carbon apex seals don't like boost, then can take alot of rpms. The engine that is pictured in my avatar saw 11k rpms alot. I've had very large street port motors that made peak power 8,600rpms+, and never had a problem.

When Rob Golden builds my next motor, I'm shooting for a 8k rpm peak power point.

CJ

 

Great info guys!

shm21284 that engine is going to rock!

So it looks like the real problem is apex seal weight vs. strength. Steel is too heavy for >8500 and carbon is too weak for boost. I guess ceramic is the only way to go on high revving boosted engine.


Thanks,
Justin

 

Yep. Ceramic apex seals rock. But there is more to just apex seals being the limiting factor of high rpm/high boost engines. I do have a write up on building a 12a, most of the things apply to a 13b, just at higher HP levels than a 12a. Here's the link:

https://www.rx7club.com/1st-gen-archive-71/engine-how-build-high-output-12a-turbo-without-blowing-up-your-motor-380353/

 

I'll touch a little more on some of the stuff I know.

According to Rob Golden, you need atleast 10psi for every 1000 rpms you're going to be spinning the motor. This is acheived by certain oil pump mods, oil flow mods internaly, and externaly (Including oil coolers.) customized oil regulators, bearing clearances, etc. I'm not going to say exactly what I know, because I don't want to step on Robs toes. But I will say, that on 3 of my peripheral ported engines, it was'nt uncommon to see 100+psi of oil pressure.

One peice ceramic apex seals don't like "hot starts". If you have the money, I highly suggest the 2 peice ones.

Port timing- You need *alot* of port timing to have a street ported engine (or a realy big freakin' turbo.) want to make peak power, over 8k rpms. On my engine that made peak power at 8,400rpms (Not 8,600 like i typed above.) it had port timing like that of a peripheral engine. It had a cool "lope" to the idle.

To be honest, I think there are very few engine builders out there that can keep a engine together for long periods with such internal loads. If I'm not wrong I think Rob also told me, that load goes up exponentionaly (I can't spell.) with RPMs. Anyways, I don't pretend to know everything, and most my knowlege comes from N/A stuff. Perhaps it would be better to e-mail/PM people like chrispeed, Rob@pineapple, Xcessive, etc. They could tell you alot more than me. CJ

 

If you really want to get crazy with the rpm's, go with the Guru racing 2 piece e-shaft. The two piece design allows for a center bearing and higher safer rpm's because the center bearing virtually eliminates the e-chaft flex. You can also lighten and balance the rotating assembly. This will also cut down on some of the increased stress at those stratospheric rpm's.