CMB 27 alpha nitro

[David Ashcroft]

I got a 72" mystic cat I just started chopping and sanding ,,and I was thinking gas but I believe twin nitro will push it no problem ..it only has a 64" wetted area..OK so how about big block cmb27 with nitro .600carby and a nitro button..is the potential there,considering its much similar to the 90/101 engine,,any thoughts,,can gain perhaps 0.5hp and few thousand rpms..and can counter rotate one engine with ease ..Because of no ignition ..

 

[LohringMiller]

Does CMB offer a nitro version of that engine? If not expect a lot of R&D. We tried to rin a Quickdraw with a glow plug head. It either blew out the center of the plug or wouldn't run on a ceramic center plug. We then modified a WYK carb for nitro and ran it on ignition. The engine worked well with 15% nitro boat fuel, but destroyed the plug on 40% nitro fuel. It will take a lot of experimentation with head buttons to get a working engine. The lower exhaust temperature will need a different pipe.

Lohring Miller

 

[David Ashcroft]

I don't understand clearly ,,,,My view is a large bore CMB on gas! The base is very much a enlarged 101 nitro..

 

[David Ashcroft]

I can have a button modified and a nitro carb to suit...Or am I missing something?

 

[Danny King]

The counter rotation would be harder to deal with. But no reason it shouldnt work. But real performance will require more then one glow plug. More like 4 plugs.

Even if the engine was run on 25% nitro the power increase would be very significant.

 

[bermudadude]

just my 2¢ but the stock cmb27 carb is way bigger ,and nitro is gonna need to b equal or more to avoid lean condition,which u would b forced into "trying" to get a stable flow with hi velocity on nitro. been there- runs like crap.

u maybe right with the multiple plug thing,that MIGHT solve it

 

[LohringMiller]

Maybe multiple plugs would work, but the stock compression ratio for gasoline is too high for nitro. I promise you will blow the center out of most plugs unless you increase the head volume. If you run ignition you will disqualify the engine for both gas and nitro in NAMBA and IMPBA. Both organizations require glow ignition in the nitro classes and gasoline with ignition.

Nitro fuels have around half the exhaust gas temperature of gasoline. You will need a lot different pipe. Nitro would also need a different ignition advance. I can't imagine investigating all this without a dyno. A shop to make head buttons and tuned pipes would also be needed. A programmable ignition like the Power Spark system would help. A good start to get in the ball park is an engine simulation program. EngMod 2T is an inexpensive one.

We tested a lot of different fuels in an M&D Zenoah. Without the ability to change the above components to match the characteristics of the various alcohol and nitro combinations, the power wasn't much different. Below is a comparison. As an aside, CMB tried a nitro style AAC piston and sleeve on their 35 cc gas engine. It was a disaster and they went to a chromed sleeve with rings.

Lohring Miller

 

[Terry Keeley]

I promise you will blow the center out of most plugs unless you increase the head volume.

Just curious? What plugs did you try?

I was having that problem with my nitro motors until I switched to O'Donnell plugs with their "patented no leak internal sealing system".

 

[David Ashcroft]

Maybe multiple plugs would work, but the stock compression ratio for gasoline is too high for nitro. I promise you will blow the center out of most plugs unless you increase the head volume. If you run ignition you will disqualify the engine for both gas and nitro in NAMBA and IMPBA. Both organizations require glow ignition in the nitro classes and gasoline with ignition.

Nitro fuels have around half the exhaust gas temperature of gasoline. You will need a lot different pipe. Nitro would also need a different ignition advance. I can't imagine investigating all this without a dyno. A shop to make head buttons and tuned pipes would also be needed. A programmable ignition like the Power Spark system would help. A good start to get in the ball park is an engine simulation program. EngMod 2T is an inexpensive one.

We tested a lot of different fuels in an M&D Zenoah. Without the ability to change the above components to match the characteristics of the various alcohol and nitro combinations, the power wasn't much different. Below is a comparison. As an aside, CMB tried a nitro style AAC piston and sleeve on their 35 cc gas engine. It was a disaster and they went to a chromed sleeve with rings.

Lohring Miller

Nitro Tests.JPG

No probs Lohring ..It sounds like it would be headache to get right.

But if a custom button twin plug with a about .250cc bowl volume and squish at around .018"-.020" and a custom nitro carby may work..But I will think about it..

I remember zenoah made a 23cc nitro years ago ..but don't the specs..Thanks

 

[LohringMiller]

I promise you will blow the center out of most plugs unless you increase the head volume.

Just curious? What plugs did you try?

I was having that problem with my nitro motors until I switched to O'Donnell plugs with their "patented no leak internal sealing system".

I don't remember. Probably K&B, McCoy, or maybe Glo Bee. It was what we had lying around. I remember that the one with the ceramic center didn't blow out, but the engine wouldn't run well. All the others blew out the center well before we got close to the peak power rpm. It was a one evening project with Quickdraw to see if a nitro conversion would be easy. The only differences were a reworked WYK carb Mike simplified into a straight barrel carb and a standard head button tapped 1/4-32.

I believe the standard head volume was 1.8 cc in the 26 cc Quickdraws. We tested from 2.2 cc to 1.6 cc. We ran the squish clearance down to .002" where we started to get piston strikes. People have gone to 1.2 cc with toroidal head buttons. Zenoahs have around 2.4 cc head volume.

Lohring Miller

 

[dwb2620]

Silly question, but how come you can't use a the same type of spark plug used in the Top Fuel engines using an igniton source of course?

 

[dwilfong]

The heavy duty seal O Donnell plugs should hold up to blowing out. You will need a few of them in the head to burn all the nitro.

Just make a carb like what is on a regular nitro eng. running off the pipe pressure.

here is the carb I started for just this kind of set up. just needs the spray bar end cap made and installed.

Will bolt right up where the standard gas carb goes.

 

[David Ashcroft]

Yes that was the plan exactly,,,and a interesting carby design..

But I think now a pair of gas TKs will do the job..

Thanks

 

[LohringMiller]

We essentially did two unrelated tests with nitro containing fuels. The main difference was the head volume. We didn't bother with glow ignition on the Zenoah and had no plug issues. We didn't try really high nitro fuels with the Zenoah, but probably could have without plug issues. I don't know what fuel dragsters run, but plug replacement after a run is probably the least expensive part that gets replaced. The engine completed the dyno run fine with the fried plug. We could have run it again if the ground strap hadn't damaged the piston and liner on the way out the exhaust.

Lohring Miller

 

[Andy Brown]

CMB 27 on nitro would be great!

Glow plug issue would not be an issue if the engine is set up correctly.

I have explained to a couple of guys how to do this. Not at all difficult.

 

[David Ashcroft]

Andy can you provide some info about setting up for nitro..

Thanks