KieftBandit

In a recent post concerning an S4 13B Peejay mentions that the "water pump stalls at redline" . This is of interest to me as I have a 13B in a single seater that is run in the 7000-11,000 rpm range, and I can't keep it from overheating, despite two Hyabusa radiators in special nacelles, etc. The pump sheave appears to be stock, while the crank pulley may be somewhat smaller than usual. Not sure. New to Wankels.

I note that there are many engines mentioned in the forums that are being revved into five figures. If the pump does stall at c. 7000 engine rpm, what do folks do to compensate? Electric pumps? Special sheaves?

Although the engine's hillclimb runs seldom exceed three minutes, it has to put out for several minutes at a time while practicing at road courses. At this point it will clear 210 degrees in about two minutes.

For additional info on the car please see the thread "13B Semi-Blowup". Your comments on this issue will be most welcome.

rotarygod

iTrader: (1)

The water pump cavitates with the stock pullies at around 7000 rpm or so. To fix this you need to slow down the speed of the water pump in relation to rpm. You'll need a smaller main pulley to do this. The most under driven one I know of is from Mazda Racing. They are not affiliated with Mazda despite the name. There are a couple of different size under drive pulies out there depending on where you get it. Typically a street car doesn't hit redline for more than a second or 2 so cavitation isn't really an issue. If you stay at high rpm's for a long period of time you do need to address the issue. Don't worry about slowing the speed of the water pump down so it doesn't cavitate. It may be turning slower but it is still moving water which it wasn't doing during cavitation and this allows you to keep cooling. An under drive pulley will help the issue as it will allow the water pump to cavitate at a higher rpm. Someone here knows how to port th water pump for better flow but I have no idea who it is or where any threads on it are. It can be done though.

BLUE TII

iTrader: (9)

Underdrive pulley would be the easiest and first step.

In addition, Racing Beat recommends plugging the thermostat bypass and removing the t-stat for high rpm engines.

Would running a more viscous coolant like straight Evans NPG+ help raise the cavitation rpm? I know it is nice because you can run an unpressurized cooling system that will put less stress on seals. Great for Bridge ports where you have cut the outer coolant seal.

jgrewe

iTrader: (1)

We go step further and cut the stock water pump pully apart and then weld a pully from either the e-shaft feed for the air pump or A/C (I forget which) to that. You can only go larger on the water pump if you have done the smaller lower pully. Its a tricky fab job because that thin metal will grow when you weld and screw up the alignment. Next time I'll probably turn one out of aluminum.

KieftBandit

INTERESTING comments, and thanks. The plugging and thermostat removal had been done by the original builder, and is in place. Today I measured the belt OD on the pulleys and got about 4.4" at the pump (stock?) and 3" at the crank, which uses a single sheave pulley with teeth for a crankfire setup which was not in place (and may never have been). Never having owned a regular RX-7 I don't know the size of the stock sheave, so I don't know the stock sheave ratio. It looks bigger in the Haynes pictures.
I am running water with Water Wetter in it. The overheating is bad enough that I doubt that Evans would make a big enough difference; Bandit gets way too hot without even being pushed hard. I am interested in what 7000 rpm (stock) translates to in rpm for the ratio I have. 3" looks to be as small as you'd want to try at the crank; if a further slowdown is needed a bigger pump sheave would be needed, as described by jgrew.
Other evidence on this heating issue tends to finger the enormous, 3-element front wing as creating so much turbulence in front of the cooling nacelles that they aren't getting airflow for the radiators, and I may have to brute force the issue with fans. NOT elegant. But any way you cut it I think that pump is running faster than it is at 7000rpm stock, so it may also be part of the problem? Your thoughts?

BLUE TII

iTrader: (9)

In your class could your huge radiator fans pull from the bottom of the car and exhaust out the rear?

Syonyk

Ala P-51 style mini-jet assist?

(For those unfamiliar, the P-51 Mustang's cooling system had a very well designed duct setup. The incoming air was heated by the radiator, and due to it's higher temperature and volume actually produced thrust.)

-=Russ=-

KieftBandit

The car is unlimited as to its tech spec, and uses big 3-element fixed flaps ("wings") at each end to get lots of downforce, starting at a very modest speed (c. 35mph?). I believe it qualifies as an SCCA A Modified autocross (Solo II) car. It has a radiator nacelle on each side with a Hyabusa radiator in each. I plan to seal the after side of the nacelles and mount fans on top of them to suck air through the radiators. Doubt that the exhausted air will help any. I just want a cool engine, so I can concentrate on learning the beast!

This critter could not possibly be further away from a P-51. The wing drag at 125 is huge- but you're really planted at 80. I wonder what the G's are?

jgrewe

iTrader: (1)

What do you have for an oil cooler? We run two stock ones in the race car. And though I've never looked at the radiators your using, I would be suprised if they are big enough coming from the monster motorcycle. Are they multiple core? If not, can you have them modified and made 4 core. It might be worth having some custom ones made if all the ducting is in place.

And I think that Blue Tll might have been thinking about creating low pressure under the car... to clean the racing surface...yea thats why, not for MORE downforce!

Kim

Smaller diameter aluminum main pulley
http://www.wankelkim.net/rx7/fb/tempmotor1.jpg

130mm aluminum waterpulley.
http://www.wankelkim.net/rx7/fb/waterpulley.JPG

These are MazdaSpeed parts, You can find parts# in this manual.
http://www.wankelkim.net/rx7/comp.manual.pdf

-Kim

BLUE TII

iTrader: (9)

Yup, suck air out from under the car and exhaust it out the back or up and over a surface for MORE down force! Uh, I mean to keep it from overheating....

Let us know how much the underdriving helps temps when you get it worked out!

Kim

working link for above post

main pulley
http://www.wankelkim.net/rx7/fb/bpmotor.jpg

rocost

Ah the Bandit, I must say I lusted over this beast but wondered how well it was developed for constant runnig. From the pix I saw of it wit a single rad hangin off a side pod it needed some work for sure. If I remember correctly this is a PP motor with an IDA, I run a large street port with an IDA as well in close to the same RPM range in my super seven, well to 10 grand anyway.

Cooling for me has also been a bitch. First off I would say you have to chuck those busa rads. A friend runs a busa in a dwarf car and the busa rad is too small for the busa. They are single core, you need at least a double or even triple core rads. That is just the start though the real deal is getting enough and correct airflow. You can not force the air trough the rad at car speed, you need to slow it down to create a high preasure region in front of the rad. So the rad inlets need to be smaller in area and a good distance from the front of the rads to accomplish this. Next you need to exit this high preasure air from the back of the rads. You need an area of low preasure. Behind the wheels I would guess if that is possible.

As jgrewe asks, what kind of oil cooler do you have? Are you seeing high oil temps as well? I used to run two coolers, but I did not like the preasure drop so I just use one large Fluidyne one.

I hurt two motors before I got this right.

Ron

C. Ludwig

iTrader: (27)

The guys above that mentioned the rad size and oil cooling are right on. Don't underestimate the importance of oil cooling in a rotary. 1/3 of the heat rejection is taken care of through the oil cooler(3). I don't have any knowledge of the busa coolers, but if these guys say they're small at all then they're most likely too small.

My suggestion is to find an S5 waterpump. They work a little better for high RPM operation than your S4. They also come with a slightly larger OD double sheave pulley than the S4. You'll need to grind a little bit off the pump and the S4 front cover to get it to fit but it's not a big deal. Racing Beat has a description of all the pumps, pulley sizes, RPM limits, etc in their catalog. LOTS of other technical info as well. Very good reading.