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Learning how to build Lotus Seven replicas...together!
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 Post subject: Rotus7's Rotary Locost 7
PostPosted: September 11, 2005, 7:01 pm 
I have a 1982 Mazda Rx-7 (12A) that I have autocrossed for the past 2 years in CSP. It is pretty rusty and I'm reluctant to spend money to keep it looking good. Of couse if money was not a problem I'd have an Rx-8, after all I am a big rotary fan.
I see building a Locost 7 as a way to continue autocrossing with a rotary engine at minimal cost.

So at this point I have bought and finally received 3 books:
*Lotus Seven Gold Portfolio 1957 -1973
*How to build & Modify Sportscar & Kit Car SUSPENSION & BRAKES
*Build your own SPORTS CAR for as little as 250 pounds - and race it

In the Detroit Region SCCA there are 2 or 3 Lotus 7 replicas in Dmod that are very quick. They are always competing for the FTD (fastest time of the day).

So at this point I intend to build the frame this winter, compete one more year of autocross with the Mazda, and then next winter pull it apart and get the Rotus 7 ready for 2007.
Mazda Rx-7, Rotus 7, and 2007. Lots of 7's!

bruce


Last edited by Rotus7 on September 13, 2005, 11:49 am, edited 1 time in total.

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PostPosted: September 11, 2005, 11:41 pm 
Bruce,

It's my understanding that any 7 clone is automatically a D or E mod car due to the tube frame/modified tub construction. Here in the Iowa Region, we have 3 7 clones, 2 D mod(sentra ser & acura rsx-s), 1 E mod (buick smallblock v8).

I'm pretty sure that the 12A would make it a D-Mod car Under the proposed rules for 2006, you can use a 12A w/ porting restrictions (Street port I believe - no bridge porting, I believe that bumps you to E-Mod) and qualify for the 1280lb min weight, 13B w/ porting restrictions puts you @ 1380lb min weight.

For further info, I'd highly suggest joining the D/E Mod yahoo group:
http://groups.yahoo.com/group/SCCA-DMOD-EMOD/

There's a couple of 7 builders on it, including one that's built several national championship cars.

--JOsh


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 Post subject:
PostPosted: September 12, 2005, 8:44 am 
Mazdas roataries were designed to be built into the locost chassis. I'm using an NA 13b, and it fits perfectly. My only trouble is where to put all the little stuff like coils and a battery.


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PostPosted: September 12, 2005, 10:51 am 
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Joined: August 16, 2005, 10:29 am
Posts: 1076
Location: Alberta, Canada
I just sold my Rx-7. :(

nice little compact motor but my main concern is the exhaust heat and noise. And trying to get a good enough radiator under the nose cone - apparently a Griffin one will work. The exhaust noise issue could be cut down by using a turbo'd engine but they aren't nearly as reliable.


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PostPosted: September 12, 2005, 11:52 am 
My radiator is a tight fit, but it is definatly big enough to cool the engine.

The rotaries main problems are a mix of myth and distorted truth. Mazda designed some very bad exausts for the rx7's especially for 1st and 2nd gen cars, 3rd is better but most people dump the stock exaust and replace it for more power. The 1st and 2nd gen exausts at themselfs out fast, the engine itself isnt any louder than any other engine in the world. As far as the overheating problem the NA 13b, and the turbo'd version are both fine with the stock setup, the problem is the vaccum system mazda designed. They used cheap rubber which over time has proven to wear out, causing there to be turbo problems which cause over heating and subsequently engine failure. Make sure that all the vaccum tubes, and temp sensors are in good working condition and they will last forever and a day.

Back to the radiator. Here is my setup, it is a griffin 3 core drag radiator from summit. It fits perfectly in my nosecone (from CMC). It is mounted on a crazy angle, but it fits and should be able to handle more HP than my chassis ever could.

Image


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PostPosted: September 12, 2005, 11:55 am 
Here it is.

http://store.summitracing.com/


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 Post subject:
PostPosted: September 12, 2005, 12:40 pm 
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Joined: August 16, 2005, 10:29 am
Posts: 1076
Location: Alberta, Canada
My Rx-7 was pretty quiet - even after the muffler fell off. The two cats keep it quiet. I have a co-worker who races Rx-7s with no mufflers and no cats - they definitly seem noiser than a similarly unmuffled piston motor. The exhaust is definitly of higher tempature though - try putting a cheapo exhaust system on an Rx-7 - it will probably last less than a week. Lovely motor though - I thought about using mine but it was kinda high mileage and the body was much too nice to chop up (rust free, orig paint, etc, etc).

Your build is looking great!


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PostPosted: September 12, 2005, 5:10 pm 
That was my build well over a year ago.

The only reason it seems louder is because it is more constant. A normal piston engine fires each piston 1 time every 2 crank rotations, on a typical 4 cyl engineyou have 2 cylinders fire each crank roatation. On a rotary, especially the 13b, it fires 3 times per rotation, per rotor, a total of 6 times. Thats 3 times more exaust going out. It's true that the exaust has to be quite thick, have you ever really taken a look at the stock headers, they designed cooling fins into it, plus the damn thing almost weighs as much as the engine itself.

On a seperate note, Mazda rates the engine as a 1.3l, this is quite untrue, try and find a copy of Sport Compact Car, I want to say around Jan-May of 04, in the editorial tachnobable it explains how based on conventional engine design the 13b is really 2.6 liters, but mazda only counts one compresion area from each cylinder at a time so it gets cut down to 1.3


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PostPosted: September 13, 2005, 11:59 am 
CR_Turboguy wrote:
Bruce,

It's my understanding that any 7 clone is automatically a D or E mod car due to the tube frame/modified tub construction.

I'm pretty sure that the 12A would make it a D-Mod car Under the proposed rules for 2006, you can use a 12A w/ porting restrictions (Street port I believe - no bridge porting, I believe that bumps you to E-Mod) and qualify for the 1280lb min weight, 13B w/ porting restrictions puts you @ 1380lb min weight.

--JOsh


Thanks for the correction on DM or EM. I have seen a Corrola and a Honda S2000 powered cars here in the Central div.

I thought for next year the porting restriction was no P-Port and therefore Bridgeport would be allowed for a 12A. Of course CSP doesn't allow any porting so this is another reason to like DMod.
thanks for the link.

Bruce


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PostPosted: September 13, 2005, 12:15 pm 
Dave wrote:
My Rx-7 was pretty quiet - even after the muffler fell off. The two cats keep it quiet. I have a co-worker who races Rx-7s with no mufflers and no cats - they definitly seem noiser than a similarly unmuffled piston motor. The exhaust is definitly of higher tempature though - try putting a cheapo exhaust system on an Rx-7 - it will probably last less than a week. Lovely motor though - I thought about using mine but it was kinda high mileage and the body was much too nice to chop up (rust free, orig paint, etc, etc).


I like to build my own exhaust sytems including the muffler.
I'm thinking an extra muffler across the back behind the rear axle might be needed to keep noise down. Although a long resonator (packed with high temp insulation) along the side may be enough. I can already smell the charred paint on the body!
I'm also concerned about the hot exhaust shield and a passenger getting a serious burn off the pipe when they get in and out.

Bruce


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PostPosted: September 13, 2005, 12:29 pm 
One problem I'm facing with this car (1982 Rx-7) being used as a donor is the unusual wheel bolt pattern - 4x110. Very few manufacturers used this pattern and therefore very few wheel options are available. I want to have 13 inch wheels for autocross and 17 inch for the street. That way the ride height is lowered without making changes to the suspension.

Therefore I'm considering using 4X114.3 (used on 84 GSLSE Rx-7's and the later FC Rx-7) or 4x100 (used on the Miata). For either pattern there are infinately more choices. But to do either of these means using wheel adapters or not using my current rear axle/brakes and front suspension/brakes (if I was to cut the strut down for an upright). Not real happy about using adapters. Both choices also offer the opportunity to hace an independant rear suspension.

Bruce


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PostPosted: September 13, 2005, 12:37 pm 
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Joined: August 16, 2005, 10:29 am
Posts: 1076
Location: Alberta, Canada
I had these Rikken wheels on my Rx-7. Very light, good width, and looked good (better than picture shows). But the bolt pattern is very limiting especially for 17" (only Mazda used it btw and only for a few models). The 84-85 GSL-SE has a live rear axle with rear discs and LSD with the more common 4x4.5" bolt pattern - its a touch wider than the 12A axle. For the front spindles you could use Saab 900, Toyota van, TR6 ones. Saab has some nice Minilite type alloy wheels too.

Image


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PostPosted: September 13, 2005, 3:59 pm 
My spindles and rear axle are from a 1997 blazer that had a tree dropped on it, parts were dirt cheap because the car was uninsured at the time of its demise.


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PostPosted: September 13, 2005, 9:43 pm 
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Joined: September 13, 2005, 9:04 am
Posts: 126
Dave wrote:
(only Mazda used it btw and only for a few models)


Infact the 85 12A RX-7 was the last car ever to have it ;) It was also used in the RX-2, 3, etc during the 70s.


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PostPosted: September 14, 2005, 6:07 pm 
derf wrote:
On a seperate note, Mazda rates the engine as a 1.3l, this is quite untrue, try and find a copy of Sport Compact Car, I want to say around Jan-May of 04, in the editorial tachnobable it explains how based on conventional engine design the 13b is really 2.6 liters, but mazda only counts one compresion area from each cylinder at a time so it gets cut down to 1.3


I read that article, and the reasoning behind it is that since the conventional piston engine (4 stroke) fires once per every 2 revolutions, it's displacement would be half based on the rotary system. Or vice versa. The 13B displacing 1308cc on one revolution, since it fires every revolution, would make it akin to a 2616cc motor. (Because it fires 6 times per revolution, this would make the power curve similar to a 12 cylinder 2.6L conventional 4-stroke piston engine.)


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