GT750 Powered Race Car (D-Sports Racer)

[sav0r]

According to the rules we can go to 850cc. Right now we are like one overbore over, I forget what that comes out to, but I believe we are under 800cc. Anyways, we started the topic of building a second top end as a result of the displacement we have left on the table. Our current one works very well, and the cylinders are plated. So we want to keep it as is. So if we were to do it, the idea is to bore to get closer to 850cc, port, maybe do reeds, and consider a head design. Once the machine work was done on the cylinders we'd probably get them plated again. We are happy with our current head work, performance across the cylinders has been great, but the head gasket situation remains precarious. As you well know. The head design would mostly be to solve the head gasket issue, not particularly there to help with performance, though I suppose that goes hand in hand.

[sav0r]

A larger impeller has been designed. This is another 15% larger than the BDK high capacity impeller. We figure we have almost nothing to lose with this. If it doesn't work we just revert to the BDK impeller and our custom housing. I will machine this impeller and a new even larger housing this fall. The new/revised housing will get some nice touches along the way.

After we picked through some data it appears we were generating the most heat at low speeds. Likely a combination of low airflow through the rads and low RPM generating less water flow through the cooling system. Hopefully a bit more flow from the pump will work out for us. Street courses are a bitch.

[pidjones]

Nice. I'm sure you have already checked proper sizing of hoses and connections as well as potential kinks developing when the hoses heat and soften. Also actual testing of both the thermostat and radiator cap (I've experienced bad new ones).

[sav0r]

The plumbing on this thing is incredibly straight forward and seems to be in good shape. We run a 6psi cap and it seems to function, and we built a new accumulator tank that appears to be working fine. We do have questions about the internal conditions of copper/brass radiators. They probably could use a good flush if not torn down and cleaned. It uses two radiators that are approximately 8" by 10". So approximately twice the radiator of a stock GT750. 'They' say, "twice the power, twice the heat", or something like that. We were okay at race pace on a proper racing circuit where average speed remained high. The street circuit however has a lot lower average speed. We are also adding some inline coolers, the finned type ones. We aren't sure they will do much good, but they look cool and offer little weight penalty so we figured it was worth a shot.

[pidjones]

That pressure should be good for 230 F if straight water (which I believe most tracks demand). Heat exchange to air is always a weakness, but I'm sure you already have a lot of air huffing through the radiators. Street races also require a lot of acceleration from slow corners, which would be heat builders. Heat up accelerating, then slow down for the corner just when the heat wants out of the block. My '77 GL1000 fans never came on except after a long 55 mph run followed by slow traffic. That heat builds up in the metal and still wants out. If weight and electric power is still in consideration, maybe bigger electric fans for the tight courses. I think you are working up a great little racer there that will alliw you to concentrate on building your skills as driver.

[sav0r]

The street track is certainly where a lot of our challenges are coming from. It's rough, it's long, it's usually hot as hell, and you slog the engine through sections at 110mph and sections at 25mph. It's quite the ask.

We don't have any fans, electric or otherwise (maybe a handful of human fans). It's a road racing car, and fans generally aren't common. We'd much rather cool the thing without the extra weight, parasitic drag from the alternator, and potentially increased weight from the larger battery requirement. On a street course it may ultimately be required though. I think we will try the larger pump first. We will probably find that the thing won't get off the thermostat when we get to proper tracks.

I've been racing for 27 years and have in the last five or so years started to unbuild my driving skills. Once upon a time I drove quite well, maybe even won some races and set some lap records. These days I am focused on other things, I suppose.

[teazer]

Heat management is quite a challenge because you need more (as in additional) cooling at low speed.  High air speed helps when going fast, but at lower speeds there's less air flow and more heat to dump.

I would try an electric water pump tied in to a thermoswitch. The stock fan switch should work or get a lighter switch.  Current draw might be high so that would potentially need a larger battery or maybe a separate battery for the pump with a relay. 

The problem with a larger impeller is that it pumps even harder (bigger power loss) at high revs and you have sufficient cooling capacity at top speed already.

What about air flow across the radiators?  Is there a way to improve low speed air flow without adding drag at higher speeds? Time to build a wind tunnel.......

[teazer]

According to the rules we can go to 850cc. Right now we are like one overbore over, I forget what that comes out to, but I believe we are under 800cc. Anyways, we started the topic of building a second top end as a result of the displacement we have left on the table. Our current one works very well, and the cylinders are plated. So we want to keep it as is. So if we were to do it, the idea is to bore to get closer to 850cc, port, maybe do reeds, and consider a head design. Once the machine work was done on the cylinders we'd probably get them plated again. We are happy with our current head work, performance across the cylinders has been great, but the head gasket situation remains precarious. As you well know. The head design would mostly be to solve the head gasket issue, not particularly there to help with performance, though I suppose that goes hand in hand.

I didn't realize the bores were plated.  That sounds like a good idea. Stock liners are a little thin and not centrally placed, so going larger may require new liners and if so, might as well go with aluminum (plated) liners.

Head gasket leaks are an issue with those motors and I believe one cause is the uneven stud spacing.  And using inserts tends to exascerbate that. Brett DeS bolted his inserts to the top of the block with a ring of smaller bolts on his big bore motors to stop the leaks.

[sav0r]

The alternator is axle driven, so it only charges over like 45mph. It doesn't appear to be in an issue as the data shows the voltage remains steady throughout the course, but it could be an issue if we run fans or an electric water pump. I proposed using some RC batteries and a fan to my father and he wasn't super into it lol. I figured I could just turn them on in the tight sections, turn them off in the fast sections. It would probably help. At this point I think we will take the hit with the larger water pump, it probably won't cost us much, but it should serve to move more water even in the lower RPM ranges.


On last tear down it seemed that since the tube nuts are in a bind on account of the rather inconsistent studs, that we aren't likely getting accurate torque numbers when torquing the head. So we made some new tube nuts that don't fit quite as tight, we might have to pin the head as a result.

My father turned over a cylinder and a few heads. I haven't gotten started yet, but I plan to work/explore a new head design as well as potentially converting a cylinder to reeds.

Yep, our good cylinder is plated. We were just hoping for better longevity out of the cylinder since it's fairly one-off.

[pidjones]

The LiPo idea might work well with am RC motor, controller, and ducted fan like they use on electric jet models. They have by design very strict weight limits.

[sav0r]

We have an additional aluminum radiator we might add as well. I dont think any active cooling is in our future. lol

The Dave man does as the Dave man sees fit.

[IR8D8R]

My 125 shifter kart had its radiator in the back behind the driver. Slightly offset for weight balance. It would overheat under yellow flag laps and generally ran a little hot. I opened a hole in the nose and mounted a tankless aluminum heater core and ran hoses front to back. It stayed at 130 degrees always after that. No fans. I think a lot had to do with the extra coolant capacity from 12 feet of 5/8" hose. I wasn't that weight-conscious. It was 15 lbs over the minimum for the class which was with driver. I could have lost 15 lbs...

IR8D8R

[m in sc]

to jump in late in the game here... regarding cooling, try to use aluminum coolant lines wherever possible, will shed heat along a pipe where normal rubber lines insulate heat in. every little bit helps. even adding fins to the lines along the length helps.  .02

[sav0r]

Asides from some connections it's all aluminum lines. The lines are actually made out of a collection of aluminum tubes sourced from folding chairs lol. Nice thin wall and the preformed bends made for short work of the plumbing.

We are going to flush the radiators. There is a good chance they aren't flowing properly. We fought with this on my friends Elva, no matter what we did with that car (MGA motor) it would overheat. Finally we got fed up and did a radiator flush and that solved the issue.

[sav0r]

Snapped a photo of the inline coolers. Just to reiterate, the car cools fine when average speeds are high, but on the street course the tight sections tend to build a lot of heat.