Compression ratio for 50-60% nitro

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PaulHail

Well-Known Member
Joined
Mar 30, 2017
Messages
357
I know the older motors were set up for lower nitro. I've read through many of the older threads and have tried to cut/paste a the little gems of engine tuning and building info but never seem to get them all. I recall Andy posted a trapped compression ratio for both high nitro and pure methanol (80/20) and as I recall the cr for high nitro was 17:1 and 15:1 for 80/20.

Looks like I'll be running a pair of older engines for this season and probably next, an OS 46 vr-m and a Picco P-90. I'm planning to run them on 50-60% nitro and have a lathe so can turn down head buttons to bump up the compression ratio. I might do some port work but need to degree the engines to see where they're at.
 
These are a few numbers I have for engines I run

FW-k45 = 10.5:1

A45 = 10.2:1

Stock MAC 45 = 8.16:1

VAC 45 =11.72:1

A67 = 11.3:1

MAC 67 = 9.3:1

Sent from my iPhone using Tapatalk
 
Last edited by a moderator:
I know the older motors were set up for lower nitro. I've read through many of the older threads and have tried to cut/paste a the little gems of engine tuning and building info but never seem to get them all. I recall Andy posted a trapped compression ratio for both high nitro and pure methanol (80/20) and as I recall the cr for high nitro was 17:1 and 15:1 for 80/20.

Looks like I'll be running a pair of older engines for this season and probably next, an OS 46 vr-m and a Picco P-90. I'm planning to run them on 50-60% nitro and have a lathe so can turn down head buttons to bump up the compression ratio. I might do some port work but need to degree the engines to see where they're at.
I can give you the numbers from my EAP program for both those engines. PM me an e-mail addy.
 
Around 10:1 CR works in most engines for 50-60%. But as you can see, the range is wide on different engines. I ran a K-90 on 85%methanol/ 15%oil using 17:1 CR. It worked well. Easy on plugs and could run with all but the very fastest competitors.
 
Around 10:1 CR works in most engines for 50-60%. But as you can see, the range is wide on different engines. I ran a K-90 on 85%methanol/ 15%oil using 17:1 CR. It worked well. Easy on plugs and could run with all but the very fastest competitors.

Holy cow, is that trapped C/R? What size head button was needed for that?
default_ohmy.png
 
I find what Andy said quite interesting. With no nitro your can make a lot of power with just methanol, by using a very high CR (which is normal with methanol only engines). Question for Andy: Was there any reason you stopped, other than the "very fastest competitors"?
 
Around 10:1 CR works in most engines for 50-60%. But as you can see, the range is wide on different engines. I ran a K-90 on 85%methanol/ 15%oil using 17:1 CR. It worked well. Easy on plugs and could run with all but the very fastest competitors.

Holy cow, is that trapped C/R? What size head button was needed for that?
default_ohmy.png
Hi Terry, Yes, trapped. It's small. I will post a pic later.

I find what Andy said quite interesting. With no nitro your can make a lot of power with just methanol, by using a very high CR (which is normal with methanol only engines). Question for Andy: Was there any reason you stopped, other than the "very fastest competitors"?
I wanted to try 0% nitro, because some years ago I had a lively discussion on here with another well known boater. He contended that running a boat with no nitro would cause nothing but problems. Blown plugs, over heating, difficulty pulling away from the beach are some that I remember he mentioned. He had "been there done that". I had mentioned that I had successfully run 5% nitro in a Rossi 80 powered Mongoose back in 1990. He said that 5% nitro would make all the difference in the world.

So when my son started racing and I wanted to move him up to a bigger class, I thought that would be a great opportunity to test out straight methanol and give him a de-tuned boat at the same time. I set up a K-90 in our SGX 80 and installed the 17:1 head at a really high clearance. Maybe about .030", so it really only gave maybe 14:1 CR. It ran fine. Not fast,but not bad. Lacked a little torque. Then we started dropping the head. Each time it got better and better. More rpm and more torque. We got down to .005" and left it there. We only used one plug for all testing and racing. It was a Novarossi #8 Turbo.

We took the boat to the 2012 Fall Nats and Jason raced it in F Hydro. He finished fourth over-all, even after dropping a heat from a buoy hit. We never blew the only plug we used in that engine and the head looks like it just came off the machine. After that, Jason was ready to go faster, so we set it back up for nitro. Right after that he got a girl friend, graduated High school, got married, joined the Army and is now stationed at Ft. Carson, Colorado. They are doing well.

What is nice about the 0% is that it is so easy to burn and easy to get high rpm with a long pipe length. It does not need excessive load on the shaft to burn the fuel. It will run nearly as fast on stock 1667 as 50% nitro will run on a pitched up 1667.

Would it take out plugs? Yes, I am sure it would. But I think the mixture would have to be set extremely lean to the point where the engine was way off on power, so it would be obvious when the mixture is too lean. Not the case with high nitro. It can take out plugs even sounding rich and making great power.

On the other hand! If we would have run the K-90 with the stock head which gives very low CR, I think about 6.5:1, there would have probably been all the problems that the other boater mentioned.
 
Around 10:1 CR works in most engines for 50-60%. But as you can see, the range is wide on different engines. I ran a K-90 on 85%methanol/ 15%oil using 17:1 CR. It worked well. Easy on plugs and could run with all but the very fastest competitors.
Holy cow, is that trapped C/R? What size head button was needed for that?
default_ohmy.png
Hi Terry, Yes, trapped. It's small. I will post a pic later.
I find what Andy said quite interesting. With no nitro your can make a lot of power with just methanol, by using a very high CR (which is normal with methanol only engines). Question for Andy: Was there any reason you stopped, other than the "very fastest competitors"?
I wanted to try 0% nitro, because some years ago I had a lively discussion on here with another well known boater. He contended that running a boat with no nitro would cause nothing but problems. Blown plugs, over heating, difficulty pulling away from the beach are some that I remember he mentioned. He had "been there done that". I had mentioned that I had successfully run 5% nitro in a Rossi 80 powered Mongoose back in 1990. He said that 5% nitro would make all the difference in the world. So when my son started racing and I wanted to move him up to a bigger class, I thought that would be a great opportunity to test out straight methanol and give him a de-tuned boat at the same time. I set up a K-90 in our SGX 80 and installed the 17:1 head at a really high clearance. Maybe about .030", so it really only gave maybe 14:1 CR. It ran fine. Not fast,but not bad. Lacked a little torque. Then we started dropping the head. Each time it got better and better. More rpm and more torque. We got down to .005" and left it there. We only used one plug for all testing and racing. It was a Novarossi #8 Turbo.

We took the boat to the 2012 Fall Nats and Jason raced it in F Hydro. He finished fourth over-all, even after dropping a heat from a buoy hit. We never blew the only plug we used in that engine and the head looks like it just came off the machine. After that, Jason was ready to go faster, so we set it back up for nitro. Right after that he got a girl friend, graduated High school, got married, joined the Army and is now stationed at Ft. Carson, Colorado. They are doing well.

What is nice about the 0% is that it is so easy to burn and easy to get high rpm with a long pipe length. It does not need excessive load on the shaft to burn the fuel. It will run nearly as fast on stock 1667 as 50% nitro will run on a pitched up 1667.

Would it take out plugs? Yes, I am sure it would. But I think the mixture would have to be set extremely lean to the point where the engine was way off on power, so it would be obvious when the mixture is too lean. Not the case with high nitro. It can take out plugs even sounding rich and making great power.

On the other hand! If we would have run the K-90 with the stock head which gives very low CR, I think about 6.5:1, there would have probably been all the problems that the other boater mentioned.
That was a Awesome weekend I remember racing that boat heads up neck and neck back and forth in this one heat and I just inched ahead of it and I was running 60% great race!
 
Around 10:1 CR works in most engines for 50-60%. But as you can see, the range is wide on different engines. I ran a K-90 on 85%methanol/ 15%oil using 17:1 CR. It worked well. Easy on plugs and could run with all but the very fastest competitors.

Holy cow, is that trapped C/R? What size head button was needed for that?
default_ohmy.png
Pictured - Stock head bottom, 17:1 head top.

Correction - The plug used was an O'Donnell 99 Turbo, a cold plug. Not the Nova #8 that I mentioned previously,but it is also a cold plug.

K-90 heads.jpg
 
Thanks for everyone's feedback.

If my math is right, it looks like cutting 0.010" off the head for a 1.078" bore will drop the head volume by 0.15 cc. Now to set the squish and measure the head volume to see if I need to shave the head a bit to bump up compression.
 
Thanks for everyone's feedback.

If my math is right, it looks like cutting 0.010" off the head for a 1.078" bore will drop the head volume by 0.15 cc. Now to set the squish and measure the head volume to see if I need to shave the head a bit to bump up compression.
Whatever combustion chamber volume you use, Run the head close. No more than .014" on any engine. .008" is good on .45 engines.

Thanks Andy.

"If you always do what you always did..."
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Last edited by a moderator:
We once tested .002" squish clearance on 26 cc Quickdraws. That was the minimum possible. You could start to see head strikes with that clearance at 19,000 rpm. We ran the standard Quickdraw head button with 1.8 cc volume, but others have tested turbo combustion chambers with 1.2 cc volume and no detonation on gasoline.

Lohring Miller
 
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