custom turbo project... general questions

[curtis73]

Hey all. I'm pretty well versed with the basics of forced induction, and very well versed with N/A engines. So I have some basic questions to start me down the right path.

A little background. I have a Caddy 500 in a 66 Bonneville. It is currently set up to tow and I have a cam that actually peaks torque LOWER than stock. It peaks at about 550 ft-lbs at 1700 RPMs. I call it the gas-powered diesel. Long story short, the car is no longer used for towing and will be used for occasional driving. The combo that was well matched to tow (low end torque, plus 3.73 gears) is getting the heave-ho for mid range power and 3.23s

I have two options with the Caddy. I can either do it N/A with head porting and a cam that is suited to 1600-6000, or add a couple small turbos. The N/A version will make in the neighborhood of 450 hp and peak torque at about 2300 RPMs.

The turbo option is very expensive but attractive. My question is this. When building for a turbo, do you build the engine for the target RPM range, then tune the turbo to expand power in that range? Or do you tune the turbo to extend the RPM range and therefore power? In my example, my Caddy stops making good power above 4500. Is it an efficient choice to use the turbo to extend that to 6000, or do I need to build the engine with a little more flow first. Here is a dyno chart of my current combo (based on desktop dyno and input from other caddy owners). One more way of asking the question; Does adding a turbo shift the curves up the vertical axis, or out the horizontal axis?

[curtis73]

By the way, EFI will be involved if that was a question

[CBFryman]

well it depends on what you are planning to do with the car. with a turb's wastegate you can paint a torque curve like a belt driven s/c or N/A engine could only dream of. you could work with say twin T3's and work on your current torque curve to bring it up. or you can go with a step up, t3/t4 or 2 t4's to make good boost in the upper RPM's to provide a torue boost in the lower while only working with a few PSI but helping flow and then extend that torque curve, flat as a board i may add, into the 5500-6000 or more range.

since you are looking for a drive around fun car i would say look for increasing your current torque curve, but if you think you may do hish speed multi power band racing give your car a little more RPM range to work with. fit the car to your driving style in the type of driving you want it to perform best in. dont go for a middle of the line 'ok' at everything but doesnt accell in any particular thing car.

[curtis73]

Do you know where I can find specs on the T3/T4 units? I want to plug them into Desktop Dyno and play around with how they affect output. DD isn't very good with forced induction, but I can at least see what the trend is for different combos. Specifically, I'm looking for flow, surge flow, efficiencies, and anything else I can find.

I agree with what you're saying about increasing the current curve, but if you look at the dyno chart above, adding torque isn't a real issue. I want to extend the powerband to the right. Maybe not add torque, but at least extend a flat band out as far as say, 3500 or 4000. That will bring HP up to the 500 area and that would make me happy.

The current combo is 8.7:1, but that's with slightly milled heads. I could go back to stock heads and get about 8.4, but I only want to run about 6-7 intercooled psi or less. I have no problem running 91 octane. Its all expensive here in L.A. so what's a few more bucks?

So, you were talking about painting the curve with the wastegate. Can I choose a smaller turbo to get boost sooner, then 'gate off the pressure to keep 7 psi all the way to 5500? Is that how its done, or does that introduce too much exhaust backpressure from the small housing required to do that?

[caddycarlo]

he is not talking about a specific turbo but a type of turbo .........you might look at the maps on this site

http://not2fast.wryday.com/turbo/maps/

[CBFryman]

well he does have the right idea. but smaller turbos dont flow like larger turbos. so you cant get too small of a turbo and expect it to keep up with the flow of such a large engine at higher RPM's with out it over spooling.
look at some turbomaps and flow charts. choose one that will create w/e PSI you want at w/e RPM yo uwant and then afterthat turbo timers and electronic boost controllers will take car of keeping your torque and boost curves as flat as possible up untill redline.

after talking another look at your dyno chart. you may look at 2 T3/T4 hybrids. look here for a little tutorial on how to read compressor maps.

now assumeing 100% engine displacement efficency and 70% turbo efficency your engine will be using in excess of 60lbs/min of air if runing 10psi of boost. now yo usay you only want to run 7psi maxx. on an 8.7:1 with some ignition retardation you could easily run 10psi on pump gas, maybe even 15psi. and when you dont feel like buying premeiom you can buy plus and when you dont feel like buying plus buy regular and just turn your boost down... its as simple as a twist of a knob on manualy controlled boost controller or a push of a button on electronically controlled boost controller.

question that came to my mine... is this engine carburated? if so look for some fule injection heads or drop the turbo idea. tuning a carburated turbo engine is very hard and a pain in the ass. but it can be done. its just my reccomendation not to. you will see a torque cruve flatten out just by transfering to fule injection.

we need Saab in here. he is a turbo guru. or Hypsi87. if they dont find this thread soon try PMing one of them.

[curtis73]

Great advice, great references... exactly what I needed. Thank you. I also bought a couple books on turbos and I'll read up on them in the next couple days. I already have "maximum boost" and its a good intro, but lacks some "meat" that I crave.

I would absolutely love to run big boost on this big caddy, but unfortunately it has an entirely cast bottom end. The crank is nodular and I'm sure it would take it, but the stock rods are weak, as are the cast pistons. Forged pistons are available for a crazy price, but the hypereutectics that I would want are not available. Caddy guys have reported problems above 8 psi or 550-600 hp on a stock lower end, so I don't want to kill my reliability in the process.

Keep the opinions and tech coming, and thanks!