LM1 Clive

Has anyone built a 4.125" bore 3.5" stroke turbo motor here ? I would love to get a turbo motor to rev to say 7800 with peak power at say 7400..

I have a very light car at around 2200lbs so a revy motor tends to work best and has been proven in alot of time trials but that's with a NA motor....

Could i build a turbo motor and do the same ??

HAZ-Matt

iTrader: (8)

Yes it is possible...

There are those out there that don't think there is a lot of point in trying to rev sky high with a turbo motor though.

S10xGN

iTrader: (11)

Looking at your sig, it appears you've had a few cars but have you ever actually had a turbo car? They make tons of torque, you won't need a lot of revs with that setup. My S-10 weighs 3750 with me in it and runs 12.0's @ 117 with a 235" Buick six cylinder and 24# boost...

bww3588

iTrader: (49)

He has a light car, no need for tons of torque. He wants an engine that makes power up top, and a short stroke with a matching turbo that doesn't come in until higher in the revs would be great.

bww3588

iTrader: (49)

To the op, I'd look for a custom crank around 3.25 stroke with a 4.1 bore and a custom cam. Nice set of trick flows and a single plane victor intake. That thing will scream up top with a matching turbo.

343 cubes BTW.

HAZ-Matt

iTrader: (8)

It doesn't make a lot of sense from a reliability standpoint.

Not to mention that making the engine oversquare is really the least important thing as far as getting it to rev to 8000RPM. That's going to take some thought into the valve train.

As far as the torque goes, the torque accelerates the car and power is a linear relationship to it. It doesn't matter if it is light or not. The point is that it is easier and usually safer on a turbo car to get more power by increasing boost, not by spinning the hell out of it.

bww3588

iTrader: (49)

Is he building a street car or purely track car...maybe we should find this out. Nothing wrong with making the turbo come in later and push all the power up top if its a track car.

ls1 1990 VN

The amount of times i've heard this and understand it.....it gets me though that Rotarys are as fast as they are & can be easy 10sec street cars yet couldn't pull a turd from my butt.

So then how does the torque theory work with them?

3pedals

iTrader: (3)

its the most hp you can lay down THROUGHOUT the whole run that gets you the quickest et's

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That can be done easily, concentrate on valvetrain and turbo setup. Building the engine with a shorter stroke is not going to help achieve this.

HAZ-Matt

iTrader: (8)

I think we are talking apples to oranges, but it is still the same physics.

A rotary is soft on torque but easily spins very high as the parts don't really change directions like in a reciprocating engine. I don't really know how reliable a 10s daily rotary is. I remember seeing a 3 rotor conversion a while back that was in 12's maybe... that was NA. The apex seals don't really like boost though. Maybe that has changed since I stopped paying attention to them years ago

Horsepower = Torque * RPM / 5252

Average power / weight ratio is a decent estimation of how quickly you can accelerate something. In a rotary car, you need to rev it to make up for the torque because of the relatively small displacement (even if the power density is somewhat better than a standard engine). With more RPM you can take advantage of gearing to make up for engine torque and get similar torque at the wheels. A 2.4L F1 engine makes moderate torque (~200lbft), but spins to 18,000RPM and puts out over 700HP. In reality this sort of logic should be applied to any naturally aspirated engine that you want to build for pure performance. Keep reving to get more power out of the torque you have to work with (which by and large is dependent on displacement).

A turbo motor is a different animal because it's torque is not as closely related to displacement as you can dial up the boost to effectively increase displacement (or rather more precisely mass airflow through the engine). In this case you aren't nearly as dependent on RPM and gearing to reach your power goals. Sure you can have your cake and eat it too (boost + RPM) but that probably is going to be more expensive.

So yes you could build the motor in the original post. If you want to see 8000RPM you can with the right valvetrain and turbo. It's just that you don't have to necessarily spin it that high to get power to make the car fast. And without doing the math to see what the piston speed actually is, you probably didn't have to worry about destroking it.

HAZ-Matt

iTrader: (8)

Sure that's a not a bad idea. But if he was concerned about too much torque he could still soften it at the bottom just with a boost controller. Vary the boost to come in whenever you want, or for whatever gear.

bww3588

iTrader: (49)

The thing I like is to keep the stroke shorter on a high rpm engine is to keep piston speed down. There is a fine line between what is too fast and too slow...and what's just right. With a turbo car I would imagine a higher piston speed is not needed to pull air in quicker at a higher rpm because your forcing air in rather than pulling it in.

I would assume you could get away with a more stock like piston speed with a turbo and make the most power.

SSCamaro99_3

iTrader: (1)

Let's say you can build two motors both turbocharged:

One makes great power from 3000 - 6500 rpm.

One makes great power from 4000-7500 rpm.

One is going to be cheaper, and more reliable. Gearing can take care of any type of usage (street, drag, roadcourse) that the car will see. From everything I have read on here. Gaining rpm is the most expensive and detail oriented part of building these motors.

3pedals

iTrader: (3)

yes, but it also rewards you with copious amounts of hp

LM1 Clive

Automatic cars can make the most of a Higher Torque figure lower down in the rev range, but a light 6 Speed manual car is better suited to a harder reving engine. That's why you see all the Porsche and Ferrari motors rev higher and faster....

I've had 400bhp in 2 cars about 2200 lbs in weight and the car that beat me ever time was a 377ci motor that reved to over 7500 and beat me by 2 seconds.....

Yes lots of torque is great but torque tends to break tire's loose, and even though I have 61% of my weight over the rears i'd sooner have higher rpms...

But no not had a turbo motor so need more info....

Ps thanks for the help so far lads...

LM1 Clive

Mainly street but have a Pro driver and am going to try and beat the Laguna Seca Track record for a road tires car if we can.... and set some fast times at the strip also 0-200-0 etc

Makes for good PR on selling my car.

HAZ-Matt

iTrader: (8)

Porsches and Ferraris rev higher because they have less displacement, not because they have manual transmissions. Ferrari doesn't offer any boosted cars either, so they have to make due with the displacement they have and wind them out.

Consider the rev limits in the Porsche lineup though:

911 GT3 RS - 8500RPM
Carrera GTS - 7500RPM
911 GT2 RS - 6750RPM
911 Turbo S - 6750RPM
911 Turbo - 6500RPM

The NA ones rev higher than the turbo's. The transmission choices are more or less the same.

As far as manual versus automatic, if anything a stalled auto is less dependent on having a decent power curve since you can sit in a narrower RPM range. With a manual you end up having to sweep a rev range that depends on your gear spacing... certainly the more revs you have to play with the more you can get away with gearing advantage as you can hit a higher speed with a lower gear. But turbo motors can make whatever torque you want, and they also don't mind load so you would probably end up being better able to utilize a little less gear and wider ratios than you would if you were gearing a car with an NA motor.

I don't really know what would be ideal. You would have more torque with the turbo if you cared to utilize it, so you could potentially change the gear ratios and make them a little higher and maybe get away with more gear spacing. Perhaps less shifts. :shrug:

SSCamaro99_3

iTrader: (1)

That was kind of what I was getting at. Why not build an engine that makes the same average power in a given range, and gear the car appropriately. Adding rpm for the sake of making the car softer is expensive and detrimental longevity (unless you get extremely high quality parts). You can run a softer rear gear to help with traction. Oldsmobile had a roadracing program in the late 90's that utilized big torque BBC's and Hewland 5 speed sequential transmission. They raced against high revving small displacement cars, and were very competitive.

LM1 Clive

Just to though in some specs Enzos are 6 liters so not small capacity
they have 650bhp at 7800 and are over square by 22% 92mm x 75.2mm

That's more HP than a LS9.......

Yes I don't know what the turbo motor's going to be like at gaining revs but that's kind of why I'm asking for advice...

Ps here's a video of a similar car 377ci SBC reving to 7800rpm and hitting 100 MPH in 5.6 seconds, and the suspension is designed for a road racing not drag...
I need to beat this that's my main objective.....

http://www.youtube.com/watch?v=OuvBLWciOXs