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Don's Dream Machine C-85 performance questions


Registered User
Leicester, UK
My C85 with 0-200/C-90 crank rods and pistons tested at 93 horse at 2475 on the Dynomometer with the timing set at 28 &30 degrees as per a regular C-85.

First question: Should I be able to run it at 2575 (like a C-85) or should I restrict the rpm to 2475 like an 0-200/C-90?

Second: Should I still use 28 and 30 degrees timing?

Thanks in anticipation for your help.

ROBL, sounds like you've got a strong engine there. the c90-8/12 can be run at 2625 for 5 min. takeoff and the 0-200 at 2750 but not sure about c85. best to just follow your original poh unless your overhauler suggests differently.

Your engine is a C85 and (assuming it is in a certified aircraft) must be set-up and operated per the type certificate.

IF your C85 is installed in a certified J3, then your limits are 2575 RPM (85HP) for Takeoff and 2525 (80HP) for all other operations, per the type certificate.

If you happen to be making more HP at the required RPMs, good for you.

I think Don has a strong recommendation to run the engine hard (don't baby it).

John Scott
Get in contact with my brother. His handle on this site is "Nathan K Hammond". He owns one of Don's Super-85's. Nate built it up using Don's mods. Its currently bolted to my dad's J-3 while Nate rebuilds his Champ. I don't know that you'll find anyone more knowledgeable about the subject other than Don himself.

From the little I've flown it its a great little engine with a lot of giddy-up.
P.S. The O-200 isn't restricted to 2475 rpm. For example, the one I fly is restricted by 337 to 2640 rpm on the J3. I use a 7142 prop and climb angle to limit it to 2640 rpm during climbout. In level flight, I limit it to 2640 rpm by retarding the throttle. Static rpm depends upon density altitude, but I usually static on the loose order of 2550 rpm.

The modified C85 is still a C85. Engine TC red line of 2575 rpm still applies. (But your airframe TC may limit you to something lower)

If you're getting 93 HP at 2475, I think you have a 'good one'. Running it at 2575 will give you a noticeable increase -- probably over 100 HP at 2575.
I built my engine with the C-85 to O-200 STC when I rebuilt my 7DC champ. I was really strong and would outperform a Champ with a stock O-200. I forget what prop I had on it but I ran my timing at 28 & 32 degrees. I kept the C-85 cam because the lobe centerlines indicated it would make more low end torque than a C-90 or O-200 cam. I wasn`t looking for speed in a Champ anyway? I wanted short field performance. It would turn 2800 WOT in cruise and about 2650 WOT on takeoff. I had mine dynamically balanced to 1/2 gram and it ran smooth as silk. I used a Marvel Scheibler carb that was approved for the C-85 and I used the O-200 jet STC. I wanted a mixture control so I put the Stromberg on the shelf and the Marvel carb was really nice. If I had another small Continental I would do the exact same thing it is a sweet setup. I would get an honest 90 to 95 mph in cruise at 2400 Skip
JimC said:
You can also put a C-90 cam in the 85 stroker to increase the valve lift.

If my memory is correct you have to machine the inside of the case to use either C-90 or O-200 lifters the mushroom heads won`t clear the crankcase.
Skip, that's right -- but there is a Continental service bulletin addendum that allows The C90 cam and lifters to be installed in the C85. I'm gonna put one in the 85 stroker that I'm building up.

The three operational 85 strokers that I'm familiar with dyno about 97 hp (one was dyno'd, the other two perform similarly). They are somewhat limited at higher rpm relative to the 90 and O-200 because of their lower valve lift. The O-200 J3 that I fly regularly outperforms them, and I have to throttle back for them to keep up with me both in climb and cruise. When running a 7142 prop, I top out at about 2950 rpm depending on density altitude, and when running a 7535 I can't apply full throttle in level flight because it substantially exceeds 3150 rpm (enough to force disassembly of the engine and stamping of the crankcase). In 55 mph climb, I turn the 7142 2640 rpm, and can turn the 7535 about 2800 to 2850 rpm -- again, depending upon density altitude. One of the nicest things about putting an O-200 on the J3 by 337 is that you aren't limited to 80 hp continuous like you are with the 85 and 90.
JimC, where did you find the Continental service bulletin addendum that allows the C90 cam and lifters to be installed in the C85?

Is this the bulletin that details the conversion of a C85 to a C90 (which also includes the crankshaft)?
I specifically remember that my STC from Aircraft Specialities specified the C-85 cam and lifters. All that was to be replaced was the crank rod and pistons. The case did have to be machined for the oil slinger on the O-200 crank. I wanted to sneak in a C-90 cam and talked to Bill Pancake about it and he told me that the case had to be ground down for the mushroom head lifters. Then I looked at the lift/duration and lobe centerlines on the 3 cams and sent the numbers to several folks I know that build Formula 1 engines as well as having the numbers run by Hollman Automotive, formerly Hollman and Moody in Charlotte. Everyone agreed that for low end torque the C-85 was the cam to use. I am pretty sure to be legal with the STC you have to use the C-85 cam.
I did forget to say that it was important for me to stay under 90 hp in the Champ . Anything over 85 hp required the repetitive spar inspection and I didn`t want to go there so it was important to me to keep the data plate that called the engine a C-85
What I built up is fairly stock.
I did polish and radius the intake spider and elbows. Smoothing them with a dremel and brake cylinder hone was easy but time consuming. When I first started, there was a noticeable lib between the intake pipes the the smaller diameter of the castings. Smoothing out the transitions certainly helped the engine breath better. One of the big tricks Don uses, is creating a venturi in the throat of the spider. He actually will use an A-65 spider, because it has more meat in the throat, and machines it to profile. I did the same with a standard C85 spider and created a slight venturi, but was more interested in getting the matting faces matched. Mass balanced the rotating parts to within .4 grams, by swapping parts around. It dyno'd at Don's right at 99hp, so I was happy. When I sent the case off, the plan was to go with a standard C85, but it came back with the machined lifter housings for C90/O-200 lifters as well as the slinger ring. So I guess they do that mod standard now.

We're running a new McCauley 1b90CM7146 which is a lot of prop, but the motor soaks it up nicely. On the Cub (25" Goodyears, no VG's) it will get airborne around 200'-250' but can cruise at 95-100mph. Understand, this is a Champ prop and I didn't want it re-pitched. We tried several other props; CM7445 and a Sensenich 7242, but neither pulled as hard or cruised as fast. I like it that way personally because it will pace with any other aircraft in the class. We flew to Ohio with 3 other Champs (65hp, 75hp, and 85hp.) The Cub was faster, and climbed harder, then any of them.

With the CM7146:
static: 2250-2300rpm
WOT: 2550rpm
Cruise @ 2150 = 80mph 4gph
Cruise @ 2300 = 90mph 4.5-5gph
Cruise @ 2500 = 100mph 5.5-6gph

I've got another C85 were building up soon for a 'project' and it will be for max power. I've got both LyCon, moly coated 10/1 O-200 pistons, and a set of C85 rods/pistons; when we figure out which makes better power I'll let you know. I'll probably change the cam timing by a couple teeth. I'd use the 35* C-90 cam, except it only shows an advantage with an open exhaust; and we will probably be running a system similar to a J-3's. Plus I've got a C85 cam and lifters already. It would be nice to custom profile a cam, but thats above my pay grade. We will dynamically balance the crank this time though. And maybe bump the timing to 30/30. I'll clean the intake system again, but also have the jugs flow balanced. Then maybe get a new CM7642. I don't want the shortest takeoff possible; thats not practical for us. I would rather see the power be put to use in both climb and cruise.

I actually have a copy of the service bulletin - I will dig it out tomorrow.

You have a choice - use the STC, or use the service bulletin. If you use the STC, you have a C-85. If you go for the bulletin instead, you are converting to a C-90, and must re-stamp the data plate. This is an "either-or" choice, you cannot combine the two.

For a J3, be careful if you go to 90 - the dash 12 is not on the type cert, but the C85-12 is. Does this help?
Thanks for all the responses. Mine has the stock C85 cam, but we did have to machine the case for the slinger. The case doesn't have the through-bolts in the center crank casting, so I guess we'll keep it to stock C85 rpm limits.

I would be interested in seeing the C90 cam Service Bulletin. I think it may be M47-16 (2/26/60), which modifies the C85 series engine to the C90 series. If this is the case, I do have copies of if in .jpg format if anyone is interested (I don't know how to attach images here).


Edit: let's try this:

..... I've got both LyCon, moly coated 10/1 O-200 pistons, and a set of C85 rods/pistons; when we figure out which makes better power I'll let you know.

Nathan, compared to the stock 7.0:1 O-200 pistons -- for the same timing, the C85 pistons with the O-200 crank will give you 8.686:1 compression for a 7.02% increase in power and torque throughout the rpm range. The 10:1 Lycon pistons will give you an 11.29% increase in power and torque throughout the rpm range.

Me, I'd go the 10:1 pistons. They should run just fine on premium autogas.
P.S. A McCauley 7440 seems to have more static thrust than most anything else, up to about 2750 rpm. Above that, a 7535 has more thrust. From memory, the crossover point is at about 505 pounds static thrust (too lazy to look it up).

With the 7142, in level flight the O-200 J3 that I fly tops out at 118 mph indicated at 2950 rpm, with the 7535, about 110 mph at a much higher rpm. Based on gps runs, this particular airspeed indicator is fairly accurate in this speed range. In practice, I limit the engine to 2640 rpm or less either by retarding the throttle, or by climb angle. The 7142 gives me 2640 rpm at 55 mph indicated. With the 7142, average fuel burn is 4.29 gph at about 80-81 mph indicated and 2350 rpm with two on board at 3500' DA (including one climb from 310 MSL to 3500 DA)
......The case doesn't have the through-bolts in the center crank casting, so I guess we'll keep it to stock C85 rpm limits.

The two through-bolts can be added. One without issues -- the second requires o-rings for sealing, since (if I remember correctly) it penetrates an oil passage.
engine mod

i'm running a 0-200 with 85 hp pistons its supposed to put out 118 hp shound work the same in your modifyed c-85
i have homebuilt pa-11 or sporttrainer 11 i cruse 92 on flouts
....its supposed to put out 118 hp shound work the same in your modifyed c-85

An O-200 with C85 pistons will increase the compression ratio from 7.0:1 to about 8.68:1, and will put out 107 hp at rated rpm of 2750.
JimC said:
The two through-bolts can be added. One without issues -- the second requires o-rings for sealing, since (if I remember correctly) it penetrates an oil passage.

You are correct, Jim. Unfortunately, it invalidates all sorts of TCDS's and/or STC's (as I understand), my memory is slightly lacking on exactly how. Anyway, it's too late now for my engine.

Rob, if I remember correctly, there are either STC's or previously approved 337's to add the throughbolts.

One of my friends recently overhauled and converted his C85 to a stroker, flew it about 30 hours, then disassembled it to do further work on the internals, reassembled it, and is trucking on. It's quite a nice engine., and it only takes a few hours to disassemble or reassemble a small Continental.
engine mod

thanks jimc for letting me know the hp of my engine before i built my engine i talked to a guy that had one built at a shop and was told it dynoed at 118 thats why i sain it was supposed to put out 118
lycon pistons sound interesting where can i get info on them thank you