This most likely be asked before but, here goes.
I have been playing with the idea of running NOS in my carburated bird. My compression is 10.5-10.7, I haven't quite nailed the exact number down yet. But put it this way I can not run 92 octane and not have it detonate. I usually run a mix of close to 100 octane. And it does fine. I was looking at a 75-125 hp shot. I don't want to test the rods and pistons with a bigger shot. Am I nuts or can it work?
Later
John

 

I have 10.9 compression and am thinking about spraying my car also. I use 93 octane and have no detonation. Have you checked the how much advance you have. Oh, I see that you are from Ill. What track do you run at?

 

I asked the tech line at Jegs about this when I was going to build my engine. I ended up with 11.07 CR. They said as long as you have zero spark knock, and your timing is retarded 2 degrees for every 50hp, everything should work just fine.

 

if you are detonating on 93 with 10.7 you may want to consider the timing or the actuallity that you may have much higher cr ratio than previously believed. abnormal quench can do it as well say for instance you used big dome pistons but used 76-78 cc heads and the piston does not reach the top of the deck and/or xtra thick head gaskets. one of my favorite references is http://www.speedomotive.com/Building%20Tips.htm it by no means is new info but the truth is the truth no matter who tells it. some times people read something there that hey had not taken into account before.

 

What my combo is, Flat top 4 valve releifs, They are .003 down in the hole, Stock FP permatorque gaskets,64cc Iron eagle's, Was melling cam sae dur intake 298+/-2, ex 304 deg,224/234. advance 34-36 I can't remember exactly. Temp was avg 210-240. The new cam hasen't been fired yet, to see if I still have the problem.

I usually race at Terre Haute,IN, But am planning to run Charleston, IL this year.

John

 

Compression is not a problem with nitrous, you just need to keep it out of detonation and retard your timing as recommended.

All of us know that turbos and superchargers are very dependent on compression because they introduce a lot of additional heat that can preignite the mixture. Since nitrous comes in much colder than the air with it, it is not an issue and may even lower the raise the threshold for detonation. However, since it is an oxidizer that speeds up the combustion process, timing must be taken out to make the most power (and not damage parts).

I think that the pro mod cars (pro stock with NOS) cars run compression ratios of at least 15:1 if not more with nitrous. They also pull 15 or 20 degrees timing out because of the big nitrous hits they use.

 

Your combination WILL REQUIRE RACE GAS. about 110 octane.
Do not try to bandaid the tune up with exessive ignition retard
and low octane fuel.
I'd start with a 150hp shot and 28deg total timing. Try it, check your plugs for detonation and overheating. If ok creap the timing up 2 deg to 30. If your et mph improves and plugs are ok go to 32. Be sure to provide enough fuel flow for the nitrous.
That means a seperate fuel pump and line from the tank.
Be sure you carburator is jetted right (slightly rich) before you start. On a 750 holley that would be 72primary jets and 83 secondary.
Install spark plugs about 3 heat ranges colder then stock.
Use a non-projected tip plug.
Your water temp is way too high (210-240) Fix that first.
Based on your information, assuming a .040"+ 400ci I calculate your cr at 11.58:1

 

F-bird
I have been working hard at getting the temp down, I have a different radiator, I had an advance auto repacement rad, I am trying a 89 GTA rad, It is slightly thicker and the tanks are alot larger. I have replaced the stock head gaskets, with Felpro 1044's, These have the larger holes in the water ports, The water passages are alot more open in the 1044's. I am hopeing this helps to a certain extent. I also have bought one of the plates you rivet to the back of the impeller of your water pump, Will it help I will also be using VP cool down, It is real similar to water wetter, I do know this works, I used it last year and it did lower the temps about 10 deg.

I already have been using VP C12, and I tried Torco 110??? I'm not sure of the octane rating, I think it was purple in color though.
Later
John

 

The water pump looks old. The impeller may be corroided
away inside. Wouldn't be the first time. The electric fans have got to go. They aren't capable of pulling enough air through your rad to keep things cool. I'd get the clutch fan, fan shroud and pulleys off an 86 V8 carbed car with the serpentine belt. Check the condition of the lower rad hose, could be collapseing
at high rpm.
Should scream on the NOS once ya get it sorted out.

 

I disagree with the fan thing.

Keep the electric fans. Do all of the stuff that you have been talking about and there should be no more cooling problems. I had a cooling problem at one time also. I replaced the pump, radiator, and thermostat. Still had a temp problem. I also had a clutch fan at the time. I didn't get my cooling problem fixed until I got an adjustable timing light to see what kind of advance that I had. I got a recurve kit and set my timing like it was supposed to be and it stays cool all the time now. It used to get really hot if I got on it hard. Not anymore. I had too much timing but wouldn't knock. After that I got an electric fan and that dropped the temp even more. I was thinking of also getting the impellar plate that you were talking about. Post how it works. I'm curious.

 

given your combo I would say your compression ratio is estimated as close as i could get it. Do you know what kind of lobe separation you have (104-115)? the higher usually the less overlap and with that known and without bleeding some cyl pressure during the exhaust cycle, detonation can occur. we are told and read about the wider lobe separations being better for nitrous, turbos, and blowers. some may disagree, but in dynoing I see that the truth is that in some cases a 109 or 110 (as opposed to the 112-114 usually recomended) or even 106-108 in some extreme cases can actually give us a better running "power adder car". In power adder cars obscene amounts of exhaust gas will be trying to get out of the motor and some times allowing a little out early will give us some scavengeing effect to improve the situation especially on turbo cars where exhaust = power. without some overlap, a person will have to obtain some miricle metal wire and "o-ring" his heads with it because the cylinder pressures involved with larger amounts of nitrous (as well as head and block distortion) make complete cylinder sealing a mere impossibility ( just ask a top fueler). this is a balancing act where too much is worse than none. so dont run out and buy that 104 degree "nothing but" overlap circle track cam. I know I keep addressing lobe separation as if it were overlap. but it is a good indicator of what type of and where the overlap is without having to address the subjects separately.

 

The melling cam was a 112 lobe seperation, The comp is 106. The electric fans are off the same GTA that the radiator is from, Its the dual fan setup, with both fans wired to run at the same time. The water pump is about 3 yrs old, So I was thinking about replacing it anyway, I will definantly do it now. I also have a new air dam under the car, So getting one of those won't help. Buy the way when I get NOS I most likely wont run more then a 100 HP shot, Well possably 150

Later
John