Cylinder pressures juicing at low rpm. What is and what isn’t ‘safe’ and why...?

The patient – 355, 11.5:1 comp, solid bottom end, ported LT1 heads, LT4 HOT cam……

The juice – compucar wet plate, 175 shot, Jabobs NMM.

Previously we’d launched at 1,000rpm on our 2,400 stall and 3.42 gears and got a 1.59 60ft with the NMM set to come on at 2,000rpm and reach 100% at 2,500 on a 150shot and 6-deg of retard. I personally felt this was safe. Some have stated this is spraying too low due to the high cylinder pressures generated. I know the potential dangers of spraying at low rpm, but how low is too low and what factors influence this….? I have seen countless folks spraying 150 shots on heads/cam, stock bottom ends from the line (off idle) and having zero known problems. I would just like some help determining the comfort-zone as far as cylinder pressures are concerned when spraying at a particular RPM. I don’t want the pins, bearings, crank, etc reduced to matter that filters past a kitchen strainer.

So, what can I do here...? We have good rods and heavy SRP pistons. I shift at only 6,100rpm so we’re not running the thing to death. Can I start the spray at a lower rpm or should I leave it alone on the 175shot…? I do not like hitting the whole deal at once because of the shock to the drivetrain and chassis. We’ve made several upgrades here, but I do not want to come out trying to set the world on fire. I do, however, want to get this turd moving and run a good number.

Would launching off-idle (700-800rpm) and begin introducing the spray at 1,500rpm be okay or should we try launching at a higher rpm. Say we brake it to 1,500rpm and leave the nitrous coming on at 2,000 and reaching 100% flow at 2,500. I do not want to blow the tires away (11.5 ET Streets) with too much too soon. I prefer to keep the car easy to control, but am experienced enough to ‘drive’ it if I have to though I am fairly confident (no comment…there’s always an element of ‘what if’) the car will stick and go straight.

many thanks....

 

You might want to see what your convertor flashes to under a heavy hit of juice.... it's probably going to be a lot higher than the advertised "stall". If it flashes high enough, you might be out of the woods. I use exactly that approach, activating the juice at less than 2,000rpm. With a 275-shot, the convertor flashes instantly to 5,000rpm. Yet running NA, it won't flash over 3,000rpm.

In any case, you will get more responses to this on the "N2O Tech" forum than on Advanced Tech.

------------------
Fred
94 Formula A3+1: 381/TH400+OD/N2O

Advanced Tech Posting Guidelines
Detailed Mod's List
11.513@115.59 on motor; 11.162@127.67, 1.643 60' on a 125-shot. Going with a 275-shot this year

 

You should have no problems spraying the nitrous down low like that. As stated above, your converter will defineately flash higher then your stall, especially at that low of RPM. You all know the formula for finding torgue/hp numbers at a given rpm. Figure out how much extra torque you are adding to the motor, when you spray 100HP at only 1800rpm! That converter will defineately flash higher!!!! Unless you have a really high compression motor, I wouldnt worry about hte cylinder pressures, especially with only a 175 shot. If you do have alot more compression, then your cam would most defineately be much bigger, thus raising the RPM range. Dont worry, do what youve been doing. As long as you are at full throttle when you hit the button, no problems...

 

The converter flashes to about 3200-3400 on the progressive shot so I supposed the motor realistically only sees the spray at low rpm for a split second.

Thanks....

Any other opinions....

We will eventually be doing the 275 shot and for that we will be spraying at a higher RPM (we'll have a bigger stall then too).

[This message has been edited by dougg01 (edited September 05, 2002).]

 

And.....as to the content of my post.....

I was really asking an advanced technical question that I'd like answered if someone can. How low is too low and what are the factors involved...?

We have a rapidly expanding gas at low rpm that I believe could reach a point where the motor cannot effectively 'move' the volume at low rpm and destruction is the result. How would I determine this point...?

It's not that I want to juice it from idle. Heck, using a WOT switch even if I have the lower rpm ppoint set to 1,250, if I were to brake it to 1,000rpm at the line....the motor would be spinning higher than that when the WOT switch activated and the juice began to flow enough to fill the cylinders (unless using a BIG shot port injected).

Throw me a bone here. I am just trying to come up with a guyideline for how much and how soon without actually testing theory on our parts.

Thanks......

 

Note I am not a College Grad, I have never taken any classes in Fuild Dynamics, Physics, or Engineering, but I will try to use some basic ideas, to show what could/would happen.

One point to think of is that with your progressive controller, you could/would/will change the start amount, and rate of change. So some of this would not apply to you.

Lets figure that typically *most* guys launch at a higher RPM (lets say 4000+). Given a 300hp system, at 4000rpms, each cylinder is taking in "X" pounds of nitrous. at 2000rpms each cylinder will have X times 2 (Xx2), and at a paltry 1000 RPMs you'd be at Xx4. Any of this making sense? Comparing the amount of nitrous entering each cylinder at 4000 RPMs, to 1000 RPMs would show the same amount as a 1200hp system at 4000RPMs.

In the midst of all this cylinder pressure has been brought up, which would obviously be sky high. Beyond that though you would've HydroLocked the engine long ago, as a Typical Small Block can only Ingest so much Nitrous/Fuel Mixture. This is why a Turbo powered vehicle can make 2000hp on 350ci, but a Nitrous car needs 700ci to ingest enough nitrous to make 2000hp. Cubic Inches and RPM will (VERY Basically) dictate how much nitrous can be ingested before Hydro-Lock.

In the end I ask why are you launching at such a low RPM? I don't know the purpose of your vehicle (daily driven street car, race car, etc). But I'd take a look at your cam specs, and converter, and I'm sure you have some ET to be found, most likely with a higher stall converter, unless as mentioned it flashes extremely high. Converter tuning is a fine art, that along with some Speed sensors in your drivetrain, several tenths can be picked up with most combinations....

All disclaimers apply. Much of this could be incorrect due to my lack of education.

If nothing comes to light for ya here, just post back in a few days and I can move this back to advanced tech for ya. I'm sure Fred and Ken will gracious enough to let some of our Nitrous Tech leak into their forum.

------------------
Kurtis Tamez
LT4-396.com
97 Z28 LT4-396
94 Firehawk LT1-396
Team NW F-Body

 

Kurtis,
the reason we're spraying low is the realtively low stall and soft gears we're running. We're running a progressive controller to keep the drivetrain from taking a hike. Am I making sense...?

Admittedly, spraying at 1,500rpm is a bit low, however on the progressive and using a 500-1,000rpm span of engagement, the full shot is not coming on too early. Flashing the converter with the progressive shot is essential on our car (at this time) to keep wheelspin in check and get the best possible 60ft.

We are moving to a bigger cam, lots more stall and bigger tires with more gear. It's money we don't have in the budget today and I am trying to keep this combination together so we can race the rest of this fall without getting stupid and ending the season early. We've not even had the car out of the garage all year.

I understand the volume-locking of the motor, but considering our modest goals....I feel that once we have the proper pieces in place, we'll be fine.

For the future, the launches will be from 2,500-3,000rpm with the nitrous (next year to be 275 shot) perhaps not being progressively added for maximum benefit getting off the line (spraying for higher flash) and that should work nicely.

Shooting a 175 shot now....I think we'll be okay. I understand your basic thought of the volumes we'd be introducing at low RPM and the engine's ability to handle that amount. There'd at least be a horrendous backfire once you reached the point of saturation. I don't think our stuff will reach that point.

Wish I had some real numbers or testing to confirm all this though....

thanks....

 

I would have to highly disagree with you on your statement that turbos can easily make 2000 hp with a 350, but you would need a 700cid monster to make that with nitrous. If you use todays "kits" and keep thinking "inside the box" then I guess you could say you are correct, but..... We have done alot of testing, and find that cam timing makes a huge difference in how much nitrous you can ingest, with a 350cid, and let me tell you, its alot more then you might think, with a relatively small cam. Now, thinking outside the box, very soon you will be seeing how a nitrous car can make a turbo car look like a punk. Not fully done with the R&D work yet, so not giving any ideas, but its all right in front of your eyes. But nobody sees it. Nitrous IS the ultimate power adder!

 

Hey I'm all for innovation. In the end as you said I guess I'm just an Inside the Box type guy, being I'm a consumer with a day job that consumes most of my resources, this being a hobby I love.

Note I did say "VERY Basically" dictate amount of nitrous etc etc etc. I do realize cam dynamics have ALOT to do with this....

I'd be very intrigued to see what you come up with when the time comes and I sincerely hope we hear about it here. I'm sure guys like Pat Musi and Gene Fulton would love some of this stuff too.

Good luck!

------------------
Kurtis Tamez
LT4-396.com
97 Z28 LT4-396
94 Firehawk LT1-396
Team NW F-Body