would you go with 10:7/1 or 8:5/1 compression if building a 383 stroker for a s/c that you plan to run about 7-8psi?

 

I'd go 10:0/1 if it were me. Maybe 10:3-10:5/1 if it will be intercooled and tuned by a top tuner. 10:7 seems too high, and 8:5 is way too low. 8:5 is for a turbo motor running 25+ psi.

 

8:5 being to low is what i thought as well, but i talked with a tuner (whom shall remain nameless so i can keep this post up) and he reccomended 8:5 - 1 on 7psi....

please more feedback....anyone got a 383 / s/c setup??

 

When choosing CR on a 7-8 psi motor, consider the following:

1.) Forged piston motors admit more oil into the chambers in cold start and more deposits form over time. Deposits destabilize combustion. So a 10.7:1 CR when the motor is new running 7-8 psi on pump gas and reasonable timing advance may seem safe and on the edge but down the road when the motor gunks up a bit, it won't unless you really back off timing and/or pour in lots of fuel (pulling too much timing or pouring lots of fuel has their own drawbacks over time).

2.) Variable fuel quality that you will run into on long road trips.

3.) Extreme heat and dryness and special circumstances like idling for long periods in the summer in traffic and then suddenly gunning it

4.) On blow-through blowers lots of high rpm, low throttle load driving causes a lot of hot charge air bypass, ready to be admitted into the engine the moment you crack into it.

Combine all above and you need some margin of safety. CR has to be in the mid to upper 9.x:1. Now, 8.5:1 will eat into throttle response, part throttle driving response, fuel economy (lower brake specific numbers), etc. 7-8 psi with intercooling is too conservative for contemporary late 90s to present motors. However it does allow room for you to grow. I'd say 15 psi safely on pump 93 with lots of timing. 20 psi is near the limit if mileage on the motor is low and seal is excellent everywhere with some lesser timing.

 

I don't think stroker are conducive to FI. An all bore is probably better since you wan to run low boost anyway.
If you want to run 6-7lbs boost the base C5 10.1CR should be OK, lots of people on this forum are doing that.
If you want more power then you might want to consider a forged stock cube motor w/ 9.7ish CR. You can run up to 12lbs boost safely w/ a net yield of 600+rwhp.
I have such a motor for sale, barely broken in, check the part for sale and/or FI section. Moving up to a 422
It will be waaaaay cheaper than anything you build on your own & w/o all the associated hassle.

 

well i am definatly going with an all stroke no bore bottom end. so you guys think about 9:7 / 1 and about 8 or 9 psi will work best?

 

I agree with above that an All-Bore would be better, but back to your compression question...9:7:1 is a good compression for 7-8 and even allows a little room for more boost. You will have the torque from being stroked.


[Modified by C5-VERT, 8:59 PM 10/1/2003]

 

The greater the bore, the greater the opportunity for detonation to occur, particularly at high load low rpm driving as combustion has a long path to travel and if combustion isn't completed fast enough then there are other flame initiation sites that could form. This is why large bore motors sometimes use dual spark plug designs. Block rigidity also decreases which has ramifications on ring seal

More stroke has its own drawbacks like the potential to compromise the ring packing, increased structural loads for given rpm, more bearing wear, and potential for more side loads if the rod geometry isn't lengthened.

If I had a centrifugal mechanical S/C, then I'd lean to all bore increase if you HAD TO choose between all stroke or all bore. This is because the high load/low rpm detonation isn't an issue since such blowers don't make too much boost down low and any combustion instability can be taken care of through calibration.

But on positive displacement mechanically supercharged or turbocharged motors, I'd go all stroke (again if you were forced to choose one or the other) since you can indeed generate nearly full boost at very low rpms (1000-2000 rpms) and therefore you want to minimize bore to keep detonation tendencies to a minimum. On the pos. displacement blown and especially turbocharging, given the already generous 5.7L displacement, you can probably spend the $$$ elsewhere such as going up one stage in cylinder head flow porting and get the highest hp since your torque curve is still going to be thick everywhere.

However on LSx motors, F.I. has been proven to work darn well on big bore AND stroke motors. So the debate is not so critical.

 

thanks....thats exactly what i wanted to know....

since the price is the same, maybe i will go to an all bore motor.

 

I highly suggest getting a few books to read up on.

Corky Bell's Maximum Boost is a great book. It's written mostly for turbo'd cars, but it makes alot of great references to the super chargers.

He also has a book bout about Superchargers... Design, Test and Build. Great book for those that are wanting info on designing the best motor suitable for blowers.

Both of these books will help give you the best combination detail one would want.

Here are a few links for you.
http://www.bellengineering.net/maximumboost.html

Good luck! :cheers:

 

Can you state where you are getting your facts for this post from? Sounds like :bs to me. What do you mean "combustion has a long path to travel?" and what does a larger bore have to do with other flame initiation sites? Also what does low rpm/high loads mean? Also some of your statements on cylinder wall loading and ring seal make sense theoretically, but I don't think anyone has proven this to be an issue with any LSX engines yet.

Maybe you know a lot more than me but a lot of what you said makes zero sense to me.

 

The most fundamental fact that all reputable engine designers and even enthusiasts know (refer to any internal combustion engine textbook) is that larger bore engines (all other factors kept constant) are more prone to detonation under low rpm and high throttle load (such as when you're at 2000 rpm with foot to the floor). This is because combustion flame propagation occurs at a certain pace (maybe 10-40 meters/ second for controlled combustion). The larger the bore, then for given propagation speed, the longer it takes for the flame to spread. The longer it takes to spread, the greater the chance the !!!radiative heat emanating from the main combustion event!!! causes areas not yet reacted to begin reacting. And hence this is why spark advance (just go check your Autotap or scanner) is kicked down a lot when you mash it from 1500 rpm in 5th gear.

Now, at higher engine rpms, things just happen too fast for unreacted areas to gain the opportunity to prematurely ignite before the main combustion event reaches those areas. And hence on N/A cars, you see a general trend toward increasing spark advance with increasing rpms with above being a big PART of the reason (on top of the basic fact that flame propagation does not scale strongly with engine rpms).

Of course things get complicated with centrifugally s/c'd mechanically blown setups because you're superimposing the phenomena of increasing charge air temperatures with increasing engine rpms due to a slightly exponential boost curve. so in such cases timing curve is less advanced down low, more advanced in the mid-range, and then once again less advanced up top.

At any rate, the discussion about bore and combustion stability is a very very very fundamental discussion and any internal combustion engineering professor (check with U. Wisconsin - Madison, leaders in combustion modeling or U- Mich - Ann Arbor, good at everything automotive) and this discussion will be basically mirrored.

Feel free to IM me and I can point you to various specific sources (books, people, professional instructional videos). All in all, the trend for the past century is to increase bore/stroke ratio (or better put the bore's contribution to the total displaced volume) but practical discussions aside, the main limiting factor is combustion stability. Faster and "smarter" control systems are surely going to up the bore/stroke ratio but it's mostly incremental from here.

So the idea behind moderating bore, twin plug designs, and fast burn combustion chambers is to deny unreacted sites from reacting ahead of schedule. The faster you get combustion over with, the less likely radiation (from the main combustion event) will heat up other unreacted areas enough to light prematurely.

As I said earlier, for tuners have found numerous bore/stroke combos to work with various LSx F.I. setups.


[Modified by STAGED, 10:55 PM 10/6/2003]