Engine Building - The Right Way

[Drift2XL]

I seen people talk about how to build an engine for performance. Some have reasons and methods, most fly on a wing and a prayer.

I'm going to let you in on my rational on how I build power in Forza 2.

Having a little experience in RL, I can tell you that not any engine is going to work in every car. A real engine builder knows that and has to tailor the package for the most desirable output for the intended use.

The first thing you need to look at is the displacement/power that the car makes before adding parts.

A high-revving small displacement engine (I.E. Honda VTEC) has a small RPM band where it makes maximum effiency. A large displacement engine (big block) makes a very wide powerband that produces much more torque everywhere.

The second thing you need to look at is the intended final weight of the car.
 
Torque is the actual power being produced by the engine. Horsepower is how much is torque is being produced over time.

If your running a heavy Muscle Inc B class, ALS, or LMMS car, you need to look to find as much balanced power (HP vs. torque) as you can.
Reason : A heavy car needs more torque to pull it out of the turns A well balanced torque curve gives the car it's acceleration value.
A lighter car doesn't need as much torque to give it great acceleration. You can focus on high RPM HP building.

The third thing to look at is "What track is this car going to be raced on?"

If your going to a track that has alot of steep elevation changes, your car will need more torque than say the Test Track with its flat landscape. Having a car that pulls the long uphills at Nurburgring can take off seconds. Same thing for Suzuka West.
Slow speed corners require a better torque/acceleration value than sweepers. Maple Valley has virtually no low speed corners. Top end will give the best reduction of time. Tsukuba is nothing but hairpins accel will rule here.

The final thing to look at is "How many gears am I going to use?"

Shifting takes time. Time where no power is getting to the ground. That's time lost. You want to use a transmission that maximizes acceleration vs. top speed. Often it is the same ratios. Generally I only tune individual gears for a specific track. Wide powerbands will benefit from a five speed. Narrow powerbands will need that sixth gear to get that top end charge.
Tracks are key here too. Look at how many gears your actually using at race speed. MV, Tsukuba, Rd Atl, Suzuka East, I'm using three gears mostly. NY, Suzuka West, Nurburg, Test Track, I'm using four, five, or all six, depending on the car.

Final tip. Test the combination on the intended track. If its lacking somewhere, know what change needs to take place for further time reductions.

And a final Caveat: A built normally aspirated engine is almost always faster than a forced inductioin engine. Easier to modulate the throttle too. I normally only run to turbos and superchargers when I run out of NA parts.

[Drift2XL]

Parts lists: Which parts make torque vs. horsepower. Listed in effectiveness top to bottom

Torque builders:
Block Prep
Exhaust Upgrade
Fuel System Upgrade
Ignition Upgrade

Middle of the road:
Positive Displacement Superchargers
Lower Turbos on NA cars
Lower Turbo Upgrades on factory turbo cars.

Top End Chargers:
High Level Turbos
Head and Cam Upgrade
Intake Upgrade
Intercooler Upgrade

IMO: Centrifugal Superchargers are useless They don't really give the top end charge I have felt IRL. They just feel flat. A built engine will walk all over it. Stay away if possible.

[DirtDriver]

This is really cool stuff, Drift! Nice.

DD

[Serial]

I'm curious about Exhaust vs. Fuel upgrades.  I generally feel as though I'll get the greater benefit from the Fuel upgrades than the Exhaust...although I have no factual reasoning for it.  Is there some tangible data you've used to determine why one is better than the other ?

[Snyder005]

this brings up some interesting points.  Whenever i build a car, the selection of engine upgrades always stumps me.  Usually i just find the upgrades that will bring me to the PI desired, but i've always wondered if the upgrade selection would benefit the car more.

[Ske]

What do you think are the effective cutoff points when it comes to car weight vs build strategy? I think my definition of "lightweight" may be a bit different from the norm here, as I considered 2500 lbs heavy.

Up to now I've usually gone for balancing the figures as much as possible, looking at the power band and trying to extend the effective max power range. This usually means focusing on torque in my lighter, weaker cars and going for more revs in my heavier, more powerful cars. I usually stay away from cams altogether unless the cars have much more torque than HP.

[TheTechnobear]

thxs drift, top post !

i tended to just look to balance torque and HP, and played with engine parts till I got that at an appropriate PI,
but this info  will let me reconsider that approach, and also have a better feeling of what to change when its not feeling right.

im intrigued (think like Ske) what is an appropriate Torque to Weight ratio?
(as if i got the right meaning from what you said, Torque is more useful to offset weight (and anything that resists inertia (like a hill , so perhaps there is some rule of thumb?
N lbs of Torque per lbs of weight ... well i can hope



thxs again for sharing

[fndrbndr]

I'm reminded of Bimmer's comment a few days ago that the 'Cuda is something of an enigma; those who are fast in it are blisteringly fast, but those who aren't just can't get it to do anything.  I wonder if this is the reason.

[Placard Rat]

Great thread!

Like others here, I typically build engines with the torque and HP values as close as possible. In my case this is borne out of an ignorance of where I should focus on one rather than the other. I think I may revisit some of my project cars and apply what I've learnt to them.

Thanks for the list of Builders, I've been looking for that information for a while but have drawn blanks, now I can build my engines in a more focused way and not just slap bits on until I fill the remaining PI.

Rat

[Drift2XL]

Thanks for the feed back guys. 

I'm curious about Exhaust vs. Fuel upgrades.  I generally feel as though I'll get the greater benefit from the Fuel upgrades than the Exhaust...although I have no factual reasoning for it.  Is there some tangible data you've used to determine why one is better than the other ?

The exhaust is slightly more powerful but it increases the others as a benefit (or detrement depending on case). If you have fit the exhaust first after scanning through you know what I mean. Its at least a 1 HP gain on most of the others I'm toying with a car right now. Even the intercooler upgrade gains a HP.

I haven't been logging tq/wgt, but I might have to after the response. Basically I'm looking at how much weight is the car running at and start from there. I too think like Ske and TBear. Anything over 23-2400 is heavy.

I have a displacment cut off points depending on weight. Unfortunately, the only way I know this is my memory at a love for anything four wheeled.
At lightweight status (< 2400) My usual cut off point is 3.0 to 3.5 L engine size.
In the intermediate size (2400-3000) 4.0L is usually the cut off
Over that its usually 5.0L. Except for some V8's.

The cut off is where I focus on torque vs RPM.  Within say, 0.5 to 1 liter over the cut off I'm looking to balance both, depending on PI limits. It depends on where the engine makes its power. Intake and valve trickery is great for HP/displacement ratios and shows great engineering prowess.

But there is no replacement for displacement.

Example: The OSS Solstice challenge. I built it without going to the cams. I used race everything but them. The car was around 2500 with a 2.4L four, under my rules. It helped the GM has a pretty flat powerband on that engine. Top speed was a pathetic 4.2, but accel was 4.7. It pulled the hill on the start/finish line with no problem. 
The S2000 had almost a total reversal of those numbers. 4.7 top - 4.0 accel. It was gutless. It couldn't keep up. It lacked any torque. I tried to solve the problem with gearing, going from a five to six. It kept closer until the extra and earlier shifts cost it.

Dang, I had another rule pop in my head. But I have to get off now. I'll add this if I can get back on later tonight or definitely tomorrow night after work.

[Drift2XL]

I have been tooling around in a Silvia S13 that has pushed its way to the top of my QR in B class. This was a result of the OSS Solstice. Knowing Dirt likes a mid-week addition/change, I tried out three FR Japanese cars to round out the US vs Euro battle already taking place. It knocked out the 2000GT in the QR, so you know its got the goods.

Those three cars were the S2000, S15, and the S13. The S13 turned out to be a rocket compared to the others. Almost a second faster on Suzuka West.

Anyways, I have been messing around with another hot lap deal somewhere else. Only because it is my favorite track in my favorite class. B at Maple Valley. Surprisingly, I'm not using the Muscle Inc/B class. Its about the S13. So far with three runs through the enduro, I have lowered my PB time down to a 1:32.995. Into the 32's is not shabby. This has been the source of inspiration for this thread.

However, I have come across a hole in the theory.

It is downforce.

I tried to rebuild to get better, faster top end at MV. Out came the torque build tune. One level of cams went in, as did level two of intake. And the time did not reflect the change. I have been thinking why and then writing the response, it hit me.

Aero is adding massive amounts of drag. The drag is acting like a permanent hill to pull. Once it hits around 130 the acceleration starts to drop off rapidly. The drag is overcoming the 267 HP that the car has.

So, I'm going to institute a new rule to the list.

Aero added cars, look to torque more.

It needs more to push back against the drag. In order to use the next gears worth of high RPM HP, you have to get it there. That's where a fat torque band comes in.

The only thing that would counteract this is one of those slippery top speed cars. NSX, Vettes, Charger. Where having the aero on just brings it back to the rest of the cars stock.

You've got to learn something new every day. If not, life is boring. And no one likes boring.

PS I'm going to post up the S13 build and set up here. I will hope to show a difference in multiple aero and non aero builds, as well as engine tuning with the different platform builds.

[fndrbndr]

Thanks for posting this, man.  It's quite helpful, and is already changing the way I put cars together.

[Blooze]

This is a good thread Drift.  Good job!

There are a couple numbers that I generate and watch quite a bit.  One is the Torque/Hp ratio, the other is the Torque/Weight ratio.  I tend towards cars that are in the .9 area of the first, and the higher numbers in other - I really haven't examined the ramifications of that one yet.

One handy deal I added to my build sheet was a little pre-filled out formula for comparing the Torque/Hp of various parts.  I enter the data when I am selecting engine mods and use the results of the calcs to help me make decisions when there appears to be a push between a couple parts.

$

[fndrbndr]

Yeah, this helped me make a Honda I was working on for the Sockmonkey Derby at least drivable.  I hate that dip in the torque curve, though.  The engine block helped a good bit.

[fndrbndr]

OK, hate to bring this up, but...stupid question:

If a normally aspirated engine > forced induction, why are two of the three best top-end upgrades a method of forced induction (race turbo) and something that only helps if there is a turbo (intercooler)?  It's entirely possible that I'm missing something here.  In fact, it's pretty likely, but please, do tell...