View Full Version : what year 12a is best?

10-23-2001, 01:31 PM
ok pony up guys, what year for best rotors,housings,fly/counterweights,intakes

Dave Damouth
10-23-2001, 02:35 PM
Common practice is 84-85 motor (lightest rotors), 79-80 carb &intake (intake is slightly better, carb must stay with intake per the rules) and some would suggest the 82 flywheel and counterweight because its the lightest. I would argue that the 84-85 flywheel and counterweight have a lower moment of inertia and work just fine, although the big clutch from the 84-85 weighs a little more. Most suggest the 79-82 oil cooler setup, although the 83-85 water to oil intercoolers can be made to work.

10-23-2001, 03:01 PM
Dave Lemon has everything on his website www.mazdatrix.com (http://www.mazdatrix.com)

Dave D's right though

10-23-2001, 03:18 PM
Dave, if 81-82 flywheel which you can use with 79 rotors is lighter wouldnt that be the bomb?

10-23-2001, 03:33 PM
The trick is... finding a 79 Motor with Rotors worth using. Dave L. has more parts than anyone around here, he would have trouble I bet..

10-23-2001, 03:48 PM
have em.

Dave Damouth
10-23-2001, 06:56 PM
All rotors from mid 70's to 82 weigh the same. Approx 1/2 lb more than the 83-85 rotors. Look for the engines that had a water to oil intercooler sitting under the oil filter. They have the lightest rotating assembly. If you want to use the 82 flywheel, you have to use the smaller clutch and change to the 82 counterbalance. Should really rebalance everything also, but apparently works ok for most people if you don't. I think that the 83-85 flywheel is probably ok, as it probably weighs a few lbs more but takes less energy to spin it because the weight is not located as far out from the center. I may be the only person who thinks this way, as common folklore suggests you should use the lighter flywheel.

The flywheel and clutch do not have to belong to the original year motor, but don't try putting light rotors in an older motor, sorta illegal and all...

10-23-2001, 07:28 PM
which of these has less rotating weight

(1) 79 rotors with 81 counterweight/flywheel.

(2) 84 rotors with 84 counterweight/flywheel.

10-23-2001, 08:22 PM
In most of the Pro7 engines we have built we are using the heavier 79-82 rotors. We like the shape, and consistancy, of the pocket in them better than the 83-85's (*NOT fully tested yet). The power we get seems to be ok.

The 79-80 side housings seem to work also - they have a slightly smaller port cross section (in the front and rears) than the 81-85's. Our view (*NOT fully tested yet) is that it helps bottom end torque.

We prefer the lighter 81-81 flywheel and front counterweight, and the lighter/smaller clutch.

But remember!! the important power gains are in correct jetting and timing -- which have been different on EVERY engine we have built.

(*) By that I mean we found a combo that works, and have NOT been willing to buy the parts, assemble an engine, break it in (for MANY hours), and do full dyno runs to find out if 83-85 rotors, and 81-85 side housings make LESS power. I am not at all sure they do.

10-24-2001, 01:04 AM
thank you Dave and Dave, but you didnt answer the question though.
which of these has less rotating weight

(1) 79 rotors with 81 counterweight/flywheel.
(2) 84 rotors with 84 counterweight/flywheel.

Dave Damouth
10-25-2001, 12:01 PM
#1 has the lower "rotating Weight". I'm not sure it has the lowest moment of inertia though......

Dave Damouth
10-25-2001, 12:27 PM
Oh, BTW. Everything else being equal, I would do what Lemon says. He's built more engines than I've seen, and my stuff blows up anyway. My point is that weight and polar moment of inertia are 2 very different things.

Dave- Interested in finding out how the new rotors and housings work if someone else is buying the parts?

10-25-2001, 12:42 PM
you will find out because thats what im going with.
84 rotors/81 fly/counter/79 housings
heres why.

quote from mazda comp

"The combination of 1981-82 flywheel and 1983-85 rotors is recommended because they are the lightest stock components available.
The complete rotational component group (eccentric shaft, rotors, flywheel, counterweights must be rebalanced when changing any of the components".

10-25-2001, 12:56 PM
Correct me if I'm wrong, but somewhere in the GCR there is a section specifying the definition of an updating/backdating system and its relevance to engine balancing. As I remember, it says that the flywheel, eccentric shaft, rotors, and counterbalance are considered one system (or the piston equivalent). That means you can't legally mix and match different years of rotating engine parts.

10-25-2001, 01:46 PM
oh, didnt i mention this was for the E/P 7 im putting together?
no actually my ITA/7 has to have a claimer motor that a group of us have agreed to, but I plan on doing what Dave L is doing this year and run some pro7(which the above is legal for) and E/P stuff as well this year, have four cars might as well.

[This message has been edited by 7'sRracing (edited October 25, 2001).]

Dave Damouth
10-25-2001, 02:12 PM
Here we go again! http://Forums.ImprovedTouring.com/it/wink.gif

Engine is one assembly, clutch and flywheel are another assembly. Engine may be "balanced and blueprinted", and any "harmonic balancer" is permitted. Given that most of us are cheap, its easier to leave it however it is, and probably doesn't much matter for a stock motor. Recommended motor for ITA has traditionally been late model motor with 81-82 flywheel and counterbalance. I would guess that that came from people reading the rules and reading published weights, then assuming they had the best combo. Depends on where the weight is located relative to the center whether that conclusion is correct. Probably couldn't measure the difference if you tried, so I always stayed with the original flywheel despite it being 3 lbs heavier. Dave may have a valid argument for the rotor depression, plus the older rotors could go in the front or rear, whereas the new rotors are front only, rear only. I doubt if it makes a lot of differences, although it does seem like the different rotors like different timing.

10-25-2001, 02:29 PM
what are those FWD guys doin in our space anywho, get a rope

Dave Damouth
10-25-2001, 03:05 PM
Whoa boy! He's one of us. You know, the other Rotary Rich. (long story) Calm down and keep an eye out for that miller wabbit guy. Then use the rope...

10-25-2001, 03:49 PM
Having never been in a position ($$) to just assemble and dyno ANY engines to "see what they do", we tried to learn as much as possible, as soon as possible, early in the Pro7 program. (Had never dreamed we'd need to try and get power WITHOUT a header, have to keep the shutter valve butterfly, and no carbon seals).

Rotating weight versus polar moment of inertia ARE two very different things - NEITHER one of which shows up on a normal (old and cheap) engine dyno !! (We use a 30+ pound flywheel on the dyno).

I assumed the lighter 83-85 rotors + lighter 81-82 front counterweight and flywheel would be the hot ticket (it's what we have told customers for years). Funny thing is, it still MIGHT be better ON THE TRACK! However, ON MY DYNO, the first engines we did, which had 83-85 rotors, wanted a LOT more ignition timing, and still were a few HP short of the later ones we built with 79-82 rotors.

Once we arrived at a "best" combo (on dyno), that is what we needed to stay with for customer engines (and my own).

(BIG) However, having now finished a year of these cars/engines (Pro7), and knowing what is in each of the engines we were involved with (built and/or dynoed), I can GUARANTEE that driver, tires, chassis, jetting, etc is a LOT MORE important than rotor / flywheel combo. --

10-25-2001, 04:27 PM
thanks Dave and Dave, we haveta watch them wabbit varmits sneakin in here and such, ya know their a few parts shy where it counts of a real racecar.
besides have you ever seen a rabbit run on its front legs?

[This message has been edited by 7'sRracing (edited October 25, 2001).]

10-25-2001, 04:46 PM
Dave do you equate more ignition timing into more heat therefor less hp? and could this be offset with a lower running temp?

Dave Damouth
10-25-2001, 05:22 PM
Which Dave, the smart one or me? I'd say slower burn due to combustion chamber shape. Slower burn is not necessarily bad, but if the real Dave says a few less hp, that is bad. Unless the manifold is affecting exhaust port flow differently depending on the combustion pocket. I'm gonna stop now, this is making my head hurt...

10-25-2001, 05:46 PM
Never looked into any way to "measure" it. My guess was more required timing = less efficient combustion, which we then attributed to the shape and irregularities of the pockets on the 83-85 rotors.

If you look at a bunch of these 12A rotors, the 83-85's are normally a pretty bad blend of the pocket casting edge into the machined face of the rotor, where the '79 (actually '76)- '82's have a much more consistant pocket shape and blend into the face.

Temperature wise, my dyno keeps the water at a solid 190 degrees, and I usually keep the oil under 210. (Outside water supply, fans, spray bars etc.) (actually, the water spray onto oil coolers is misting stuff right out of Home Depot). All race engines that leave here are 1700+ exhaust temperature.

Btw, I promised myself that the next Pro7 engine we dyno, I would make a movie of it and post it on my www.mazdatrixmovies.com (http://www.mazdatrixmovies.com) site. It is SO cool when you can SEE the exhaust gases INSIDE the cast iron manifold!!

10-25-2001, 05:54 PM
thanks Dave, as I said I have all possible combos so I guess ill go with 79 rotors and housings with 81 fly and counter and leave the 84's for the E/P motor and lighten them down to 2 pounds, hehe

10-26-2001, 12:49 PM
Could you give any advice/tips on a simple procedure to optimize timing using a chassis dyno? Also what about tuning jets? Please consider that some of us don't own our own dyno and can only buy "runs" at a chassis dyno ($$$). You've written several times that optimum timing and jet tuning are very specific to each engine and now I'm worried that I've been missing some extra HP.

10-26-2001, 02:14 PM
What class, engine, mods, exhaust do you have?

10-27-2001, 12:08 PM
ITA/7 with legally blueprinted/balanced 12A, Racing Beat header with dual pipes back to axle, then one 2 1/2" over axle into muffler. One MSD with direct fire coils.

10-30-2001, 05:56 PM
"Tuning" --
I have not done full dyno testing on an ITA engine (nobody ever wanted to pay for dyno time) -- so I can only "point" you in the direction we saw with the Pro7 engines (NO header, just the stock cast iron manifold).

We ALWAYS run with an EGT gauge !! Anything above about 1740F and you are either too lean OR too rich !! (Yes, you do not know until you change the jets). If I see it heading toward 1800, I stop and change something.

--- This is ALL Pro7 stuff here -----

Timing wise - I start with 25 lead and 13 trail. Then you MUST do carb jetting before timing.

Try 125 primary and 155 secondary, then lean the secondary out till you no longer gain power, then go back up one step.
Then try the primary down one (120) and see if the gain on the top end is worth the loss of power on the bottom.
Etc Etc Etc Etc Etc -----

These can be VERY lean settings, and it is not hard to kill the engine while you are trying to "find the ballpark!!"

AFTER the jetting is optimized,THEN you play with timing.
Try about 16-18 degrees trail.
Put trail back at 13, and try about 28 lead. If it likes that, then try decreasing the split.
Etc Etc Etc Etc ----

On a chassis dyno, watch out you do not set the floor of the car on fire (NOT joking!!). Watch the paint, any undercoating, etc, anywhere near where the exhaust runs.

Dave ---

11-06-2001, 03:45 PM
dyno time
how much is dyno time Dave?

11-06-2001, 07:18 PM
Engine dyno is $500/day.

Sounds expensive, but more changes can be done in one day than 4-10 race weekends.

Every one I do, I think about doubling the price when I look at how long it really takes me.

Has to be scheduled WAY ahead - MANY restrictions:
Can't run it during working hours, after about 9:30, Thursday nights (farmers market behind me), many weekend afternoons and evenings if they are having a concert in the STUPID concert area they built behind me!!, anytime the condos 2 blocks away call the cops about "that loud chain saw place again", etc etc.

Rotary ONLY, doubt any OEM fuel injection would be cost effective - WAY too many sensors, boxes, and wires to pull out of the car. It would be most of a full day just to wire/hook everything up.

Break-in is NOT included !!! I break-in ALL race motors AT LEAST 6 hours with variable load. Rough "break-in" cost on a fresh engine is maybe $300 ??
Costs are human time only - extras are gas (a LOT), jets, plugs, filters, gaskets, etc.

I normally do not charge any break-in time on engines we build. A Pro7 engine (and probably ITA) would normally be done in max of one day and it's easiest to get it set up and ready to run on a Friday night, then dyno on Saturday.

I supply the ear plugs, and I encourage the customer to come and help (but I DO make you work). I find it very good for the customer to see what happens with various changes, and know why the changes are made.