A good offseason question, pretty much spurred on by the 914 powered Sports Racer in this thread- https://grassrootsmotorsports.com/forum/200x-classifieds/porsche-914-in-legrand-dsr-1200-obo/69846/page1/
And the observation that the wheelbase of the car is the same as a Fiat 850. Add in a very small space for an engine- where the VW transaxle would be a huge space benefit, a very strong potential engine for that theoretical DM car would be a rotary.
But there are restrictions- from the SCCA rule book:
" On 12A engines: no peripheral-porting or J-porting is allowed. Bridge-porting that does not cut into the water O-ring is permitted. On 13B engines, 4- and 6-port: Maximum porting permitted is street-porting. No bridge-porting, J-Porting, or peripheral-porting."
The base weight of a DM car is different between the two cars, too- the 12a has a base weight of 1280lb, the 13b of 1380lb. Both with driver, and given the chassis, one would also need to add 40lb for the tube frame- so the lowest weight would be 1320 for the 12a and 1420 for the 13b. Assuming that the weight targets can be met, which engine would be better?
Given the porting restrictions, how much power can be made on each engine?
Thoughts?
Are you allowed to use aftermarket fuel injection?
are they assuming the 12a with be na while the 13b will be turbo?
Brett_Murphy said:
Are you allowed to use aftermarket fuel injection?
Yes, engine controls are free.
MrChaos said:
are they assuming the 12a with be na while the 13b will be turbo?
No- both are NA. As far as I can tell at least. The last line in the weight rules is:
" Forced induction w/ displacements per 18.0.B, up to 2000 cc w/
inlet restrictor ..................................................................................1380"
And that's a 30mm restrictor for boosted engines. Not sure if that's just piston engines or all 4 stroke engines....
For this argument, w/o clarification, make both engines N/A.
Not being a rotary person, are there many different 13B designs? A 4 and a 6 port version?
alfadriver said:" On 12A engines: no peripheral-porting or J-porting is allowed. Bridge-porting that does not cut into the water O-ring is permitted. On 13B engines, 4- and 6-port: Maximum porting permitted is street-porting. No bridge-porting, J-Porting, or peripheral-porting."
The base weight of a DM car is different between the two cars, too- the 12a has a base weight of 1280lb, the 13b of 1380lb. Both with driver, and given the chassis, one would also need to add 40lb for the tube frame- so the lowest weight would be 1320 for the 12a and 1420 for the 13b. Assuming that the weight targets can be met, which engine would be better?
Given the porting restrictions, how much power can be made on each engine?
Thoughts?
A 12A with a bridge port that meets those rules will be a roughly 240-260hp engine depending on noise restrictions and how much you want to spend on the exhaust system.
A 13B (preferably using a Turbo II or FD engine as a starting point) will be a roughly 200-220hp engine, maybe a bit more if you really have your ducks in a row. It will not have the midrange torque of a bridge ported 12A. It will, however, be able to be muffled more effectively without hurting power.
The other issue: 12A parts are rare as hens' teeth. Mazda stopped producing them when they started making the RX-8, and supplies have really dried up. So for practical purposes, your best bet is a 13B.
My personal experience is that an intake manifold from an '86-88 RX-7, port matched to suit a Turbo II engine, with nonturbo internals, is an incredibly flexible and powerful engine. The 6 port engines' port timing closes far too late to have good midrange torque, and the Turbo II intake manifold is horribly pinched in order for turbocharger clearance.
alfadriver said:Not being a rotary person, are there many different 13B designs? A 4 and a 6 port version?
There are three different "major" flavors, with several different subspecies. Without writing a book:
There's the '74-85 engine, which is a long 12A. '84-85 were six-port engines, '74-78 were four port using the same port dimensions as the 12A engines. Finding parts for these is like finding parts for a 12A with the exception that rotor housings are somehow still available, and you can use later 13B rotating parts in them.
Then there's the '86-02 engine, which underwent many revisions through the years - two flavors of six port, at least three different flavors of 4 port (turbo). The 4 port engines all have larger intake ports at a minimum than the 12A-based engines. The twin turbo Cosmo engine has enormous port runners and is the go-to for a max effort naturally aspirated build - people have made roughly 300hp with one of these in a form that meets the rules you posted, with a lot of R&D and attention to detail. But you can't find them anymore.
Then there's the RX-8 engine, which has three different subspecies - series one ('08-down) had four port and six port variants, series 2 had a revised engine and all engines were six port. The four port engines are merely six port engines with the auxiliary ports unmachined, they are not performance engines in the least. That said, there is a racing class for old IT7 cars where people throw in eBay 4 port engines because 12As are impossible to find, and I guess in the limited trim for racing they make about 200 horsepower.
And now that I've hopefully answered your questions, or at least helped you find more questions to ask, I'm going to offer an opinion, because of course I am.
Rotaries are tough competition engines but they are heavy and bulky. The engine itself weighs about what a VW watercooled four cylinder does. Figure another fifty to one hundred pounds or so for a free flowing exhaust system that will pass noise regs. This is not an exaggeration. And you'll have to find a place to package all that in a mid engined vehicle, it's hard enough with a front engined car that has a 95" wheelbase. You will also need to package a beefy radiator AND oil cooler, due to the oil's doing duty as part of the cooling system, and the engine's horrible thermal efficiency in general.
If I were to build something under a 2 liter limit, that already had a VW flat four in it, I'd look into Subaru engines. Or, heck, a VW watercooled four cylinder, which is about the same length as a 12A. The 88mm bore center makes them SHORT engines.
In reply to Knurled. :
Given my theoretical build has to include the Challenge, cost and availability matters, so 13B it would be. Noise isn't specifically laid out in the rule book other than it's up to the local group based on their locations. So having one that is easier to do is a good thing.
One more question- does E85 bring anything to the table? Not sure if knock is an issue, but in theory it should help keep things cool, and it brings a little more O to the combustion chamber- if the engine can deal with it.
In reply to Knurled. :
The VW I4 is that short? Never knew that.
Not having a chassis in front of me, the other option that is not known is a transverse engine. Like will a Honda K20 powertain interfere with the suspension? Or will a smaller B series fit (and gain 100lb leeway)?
Without seeing the package space, it's really hard to tell.
I don't know much about the subaru flat 4's or how much they can make.
Interesting that the engine is so heavy- again, not being knowledgeable, my impression is that they are small and light.
In reply to alfadriver :
Detonation is a non issue. You can crank up the ignition timing until the engine loses power and you can run it so lean that you melt the spark plugs, but it won't detonate. When Mazda was running GTU, they used to cut race fuel with kerosene to make an 80 octane blend.
All that said, the largest issue with E85 is gumming up the combustion chambers/seal slots. You WILL be wanting to premix 2 stroke oil in the fuel, and the alcohol friendly 2 stroke oils, even the "zero gum" options, leave a whole lot of crud behind. I'm curious how running E85 could help cooling as I tend to run rich for cooling purposes (around .8 lambda, best power is around .95) but the gum issue concerns me. I premix with gasoline on the order of 50:1 with whatever oil I can find, and find no real carbon deposits in the engine after many tens of thousands of miles.
alfadriver said:
In reply to Knurled. :
The VW I4 is that short? Never knew that.
Interesting that the engine is so heavy- again, not being knowledgeable, my impression is that they are small and light.
The VW four was intended to be run in a longitudinal front wheel drive application. In a Rabbit or Golf, the bellhousing face is almost centered in the engine bay!
Rotaries are not particularly small due to their width. They're almost as wide as boxers once you stick manifolding on them. And they are DENSE engines - there is very little airspace inside them, it's all water jackets and iron. That said the rotational mass is very low, which is what keeps me interested in them. The rotors weigh 10lb each give or take (many revisions over the years...) and orbit around a roughly 60mm radius and rotate at 1/3rd engine speed, the eccentric shaft weighs about 20-25lb and has a very small polar inertia. The whole of the rotating assembly including counterweights and flywheel weghs less than most piston engines' crankshafts alone weigh.
In reply to Knurled. :
Thanks for the input- made me a smarter person today.
It would be fun to run into a BM car like that other thread, and find someone who will let me take a mold of their 850. That would make a very cool DM car, IMHO. Easier than trying to make my own X1/9, or Europa (even with the free ones that continue to show up every once in a while). Not sure if any of the national BM cars use car engines, but the entire BM field was faster than the DM winner- so it seems like a good starting point for a competitive DM car.
Knurled. said:
The other issue: 12A parts are rare as hens' teeth. Mazda stopped producing them when they started making the RX-8, and supplies have really dried up. So for practical purposes, your best bet is a 13B.
unless you want to spend $$$$$$ for the billet engines that pac performance makes
In reply to MrChaos :
I was unaware that PAC was making 12A parts.
Their 13B/20B parts are about what good used 12A bits cost, so I guess they are a viable alternative, as long as your class rules don't dictate OEM only.
13b are cheap and stock computer with street port is 200hp if everything is perfect. An aftermarket ecu could see as much as 220 hp.
Well, I guess I just reiterated knurleds info.
Everybody dismisses 12A's because they are "expensive" yet there's no market for them now that IT7/Spec7 isn't really a thing anymore. Good runners can be found cheap from what I can see.
In reply to Javelin :
From my vantage point, IT7/Spec 7 isn't a thing anymore because they used up all the 12A parts.
Side housings can be reground and re-Nitrided fairly inexpensively, as these things go, but the 12A rotor housings were notorious for losing their chrome as a failure mode. Chrome loss would start to occur after 100-150k or so and the housings would usually be done by 250-300k. The larger problem is that the 3mm seals mean the rotors are also a wear item, the heavy seal beats the slot in the rotor and it opens up. When it opens up, the seal no longer has good gas pressure behind it and it starts bouncing. This bouncing also kills the chrome in the rotor housings. High RPM makes this worse - 12As do not like high RPM anyway,
The '86-up engines have their faults but the chrome doesn't flake, and the 2mm apex seals can handle high RPM a lot better. And the .7mm side seals (down from 1mm in the older engines) seem to be a lot more side-housing friendly as well. And the rotor gears are held in with 12 pins instead of 9, and I can dig up some SAE papers that show how this has a remarkable effect on tooth loading on the stationary gear, so you don't need to run hardened stat gears with the modern rotors if you take them over 8000...
The hidden answer- which doesn't seem to be disallowed anywhere- would be 13B GSL-SE housings on an otherwise completely Renesis engine. You need the GSL-SE housings because of the O-ring grooves, which are on the housings of the -SE and Renesis, but every other engine (save the 12a) has them on the plates. This is a combination a friend of mine and I have been discussing for some time, and he is amassing the parts to do it.
This should net you around 250hp, without any special porting. It WILL require some custom work including a completely custom header- you'll end up with 5 exhaust ports! If you want to KISS, then a crank trigger ignition and a 13B to Renesis intake adapter with a carb will make it run.
In reply to TheRX7Project :
Technically the majority of rotaries have the coolant grooves in the rotor housings. Mazda just did something goofy for a decade and a half.
The last of the RX-8 engines has a lot in common with a '74 13B, to both good and bad. The tension bolt locations in particular should have been moved for the FD, but they didn't.
