sentry65

iTrader: (11)

just curious,

been thinking about building up my engine and running a huge shot of nitrous with 8.5 compression pistons with 101 octane race gas.

But for normal day to day NA driving, i'm guessing you could safely run 87 octane gas? That'd probably give you slightly more power NA than 91 octane right? - assuming you have the lower compression ratio going on right?

axxizzer

lower compression ratio means lower power actually. For FI you want low compression, for NA you want HIGH and a built/tuned motor.

What youre asking is if crap gasoline will make more power on lower compression?

NO

sentry65

iTrader: (11)

yeah i know you want low compression for FI hence why i said I was considering doing a large 250-300 nitrous shot on a low compression engine

but when not shooting nitrous how would the car drive with lower octane fuel?

for example I thought it was possible to lose a slight little power by putting in higher octane fuel in a low octane car/truck.

g35irish

It's important to make a distinction between static compression ratio and dynamic compression ratio. The static compression is what you measure on an engine stand with a burette and graduated cylinder. It's a simple mathematical comparison between the volume above the piston at the bottom and top of its travel. Dynamic compression is much more difficult to measure because it depends on how efficiently the cylinders are filled when the engine is running. Factors such as camshaft timing (specifically the intake valve closing point) and the tuning of the induction system have a major impact on dynamic compression. Do you plan to replace the cams during your rebuild?

Dynamic compression ratio is what really counts in a high performance engine because it determines the actual cylinder pressure. Think of two engines that are identical except for their camshafts. The engine with the shorter camshaft duration will typically have higher dynamic compression at low rpm because the intake valve closes earlier on the compression stroke. The engine with the longer duration camshaft will have less dynamic compression because its intake valve closes later after the piston has traveled farther up the cylinder. On the other hand, if the long-duration cam does a better job of filling the cylinder at high rpm than the short-duration cam, more air and fuel will be trapped in the cylinder and the resulting dynamic compression ratio will be higher. So, if you plan to reduce the static compression ratio you will want to consider shorter duration cams to maintain the static compression ratio. However, keep the trade-offs in mind.

So what the rewards of higher compression? Raising the compression ratio generally increases power output across the engine's entire operating range. Compression increases the efficiency of an engine by extracting more energy from the fuel that is burned. Low-compression motors are usually not as consistent or as sharp on throttle response as high-compression motors.

Higher compression also tends to make the cylinder more "active" by increasing the turbulence in the combustion chamber. This turbulence in the chamber can actually forestall detonation by creating a finely atomized, homogeneous fuel/air mixture that burns quickly. It is a serious mistake to cut compression ratio by installing a thicker head gasket or running more piston-to-head clearance because this destroys the "squish" between the head and piston that is essential to producing turbulence.

All in all you have to establish your perfomance goals and be aware of the trade-offs that will occur. If you want an all out "bottle baby" (and I have built these cars) you will give up power when you are NA to allow for the additional power (i.e. Nitrous) when you are spraying the motor. If you want the best of both worlds (if there is such a thing) I would simply upgrade the internals to handle the heavy dose of nitrous and leave the dynamic compression ratio alone.

xephiron

I don't know why everyone wants to lower compression to run N2O. That is a misconception... Nitrous isn't FI, it is basically a huge increase in efficiency which allows you to burn alot more fuel and burn it all completely. That is why running N2O calls for larger injectors, increased fuel pump volume... or with wet NO2 it adds an additional fuel injector, and with ported N2O it adds an injector per cylinder. I am trying to use a very simple analysis to explain this in lamens terms....

But basically, increase your internals' strength. Don't lower compression. N2O and high compression are old high school buddies. As a matter of fact, if you are gonna run a N2O beast with basically no other power adders... then when you do decide to get forged pistons go ahead with 11.0:1 or 11.5:1 compression ratio pistons, depending on what octance gas you have available or are willing to use. my .02

sentry65

iTrader: (11)

g35irish,

thanks for the great long post.

I currently have the JWT cams installed on my car. I want would love to increase my compression for more power/throttle response, but would also like to add a 75 shot of nitrous (or more if it would be safe). There's only 91 octane available at the pump, but there's also race 101 octane gas available if needed.

would it be ok to get a 11:1 compression ratio pistons with my JWT cams (which are 260 DEG DUR. / .428"(10.9MM) LIFT) and run nitrous?

thanks,

It sounds like a better route for me to go than FI because I'd like to safely track the car (AZ gets hot in the summer) and go to the drag strip. This setup seems like it'd be the best setup for me.

g35irish

What you are proposing should work well. You should be able to bump us the static compression given the increased overlap of the JWT cams and still run 91 octane gasoline. It would be interesting to compare the cranking compression of your engine to that of an engine with stock cam shafts. This would give you a better indication how much room you have to work with.

Where the nitrous is concerned it is always a better idea to use higher octane fuel. If you are planning to only use a 75 horsepower dose of nitrous I think 91 octane would still be OK, although with the higher ambient temperatures there the 101 would be good insurance.

Why not run a larger dose of nitrous? If you are planning to place the pistons, you should also upgrade the connecting rods, wrist pins, etc. If you do that you could safely add 150+ dose of nitrous.

sentry65

iTrader: (11)

this is sounding good

what about a stroked motor - like the 4.3 AEBS stroker kit? would there be any negative effects of detonation or anything by using a kit like that?

Also, what about timing? With the high compression and nitrous, would you have to pull some timing to make everything happy?

g35irish

You are asking good questions..questions that would take pages and pages to answer completely. However, let me simply answer your questions without going into all of the back ground information and other possible combinations you could consider.

Yes, the stroker kit would work great and I can't think of any negative effects (except to your wallet) from putting together that type of combination. One thing to keep in mind though is that with the larger swept volume associated with the larger engine, it becomes easier to build dynamic compression. You need to keep this in mind and discuss it with your engine builder/cam supplier.

Timing: Yes, you will have to pull out iginition advance as you add larger doses of nitrous to prevent detonation (plus the larger engine will probably want slightly less advance to begin with...). The supplier of your nitrous system can give you a good "rule of thumb", usually somethng like 2 degrees/50 horsepower of nitrous, but you will have to do some testing and tuning to get it dialed in.

sentry65

iTrader: (11)

thanks for the great info

i'm kinda curious about doing a direct port install for nitrous while the engine is totally taken apart. How much might that cost to do? With direct porting, would risk of backfiring pretty much be eliminated?

g35irish

I am also going with a direct port set-up on my G-35. I should be receiving my Crawford Cast Plenum within the next couple weeks and I plan to do all of the plumbing before installing it. I have been using NOS for over 20 years and direct injection is definitely the way to go.