Lets Talk Power to Weight Ratio

[LowFast]

So I am to the point in my project where I need to decide if I go forced induction or stay naturally aspirated. The engine is a 2006 Ford 3v 4.6 from the mustang GT and will be running the TR3650 5 spd with a 8.8 rear and 3.73 gear.

Options are to go full on ported heads and highlift agressive cams, getting me around 400rwhp/360rwtq, or go with a positive displacement blower and hit about 475rwhp/475rwtq. Car is 65 Falcon which will weigh about 2800lbs when done. So lets say no blower = 450 engine hp = 6.2 lbs/hp and blown engine = 540hp = 5.2 lbs/hp. This will be a street driven car with some autocross/track work. It will cost about the same to go either route.

I like the idea of a high winding (7000rpm) NA engine and think it would make for a more balanced package in such a light car. I like forced induction and have owned both turbo and supercharged vehicles but they always seem to have more "stuff to go wrong" even when properly set up. I see all the cars on here with 5, 6, even 700hp or more and wonder will I be happy with more moderate power. I also wonder if these cars with 500+ hp can put it to the pavement on street tires?

What are your thoughts on Power to Weight ratio, which I think is more improtant than a big hp number. Is there sort of an ideal target for a rear wheel drive car on street tires? It seems the lighter the car the harder it will be to put down the power.

[GrabberGT]

My inexperienced .02

I agree with you about all the cars on here with 500+ hp but I think the majority of them are running 335 tires and weigh 3200+ lbs. I dont remember what tires you were going to try and squeez in but my thoughts are that even with 400rwhp your going to need just as much as the heavier more powerful cars on these forums.

I'd go the N/A route if it were me. How much extra weight is that supercharger/intercooler/plumbing going to add to the nose of your 2800lb car? Even with as little as 60, thats quite a shift in weight for such a light vehicle.

[LowFast]

I will be running a 315 tire out back, plan on something sticky. My deam engine would be to stroke the 4.6 to 5.0 built with good parts and a 8000rpm redline. 5 naturally aspirated liters screaming at 8 grand gets my pants excited.

[andrewb70]

I used to have a LS6 powered RX7 that was about 3000 pounds with me in it ( I am about 250) and it made 400rwhp. It was very fast and ultimately ran 10.95@125mph.

My GTO is 4000 pounds with me in it and it makes about 420rwhp, runs mid-12s and is easy to drive fast.

Take from these statements what you will.

Andrew

[Jim Nilsen]

It really doesn't matter when you are starting out with a fresh car and not a lot of experience on the track or auto x. If you can't control it all to it's full potential it will hurt you more than help you in my opinion. I have around 300 rwhp and just gained another 50 0r 75 hp whith my new exhaust and I can tell you that if I had started with more hp I would have been in trouble at some point in a corner somewhere that my skills were not quite ready for.

Get the car on the road/track and get the suspension dialed in and make sure the drivetrain won't break with the power you have n/a. An engine can be changed to more power easier than your total package for your suspension setup. I was a bit intimidated at 1st because I didn't have all the power I wanted compared to the boosted cars and then I found I wasn't as comfortable as I thought I would be at high speeds and the braking required. I have 2 seasons on the car now and the thought of more power is aching inside me but the need for some bigger brakes is more on my mind than more hp.

So stop bench racing yourself into more than you need and get out there and drive till you know you can out drive the hp you have.

The high winding engine is a better deal too because it makes your shift points easier to work with in the corners to come out onto the straights for better exit speed. The difference between 6,000 rpm and 6,400 rpm redline made all the difference in the way my car handled it all.

As far as hooking up? Controlling your right foot should be your only problem no matter how much power you have when you don't have weight for traction.

[79-TA]

Build what you think will be reliable so you can focus on the variable behind the steering wheel.

[camloops11]

andrewb70, Just curious, but why did you get rid of the lighter, faster Rx7 in favor of the slower, heaver GTO? Both are awesome cars though.

[Scott Parkhurst]

Learn with less power. Graduate to more.

Andrew's answer may support this...

[andrewb70]

andrewb70, Just curious, but why did you get rid of the lighter, faster Rx7 in favor of the slower, heaver GTO? Both are awesome cars though.

You're assuming that I didn't have both cars at the same time...LOL

I've had the GTO since I was 15. That car is not going anywhere. The RX7 was a side project that I started when I picked up an LS6 engine. I've actually built two second gen RX7s. The first was a street/handing car. The second was set-up purely for drag racing.

First one...


http://www.youtube.com/watch?v=_wimCgP3iY4&feature=player_embedded

Second one...


http://www.youtube.com/watch?v=nopxfs3vhW0&feature=player_embedded

Andrew

[Flash68]

Learn with less power. Graduate to more.



very wise words...

[exwestracer]

One thing about forced induction...no matter WHAT, the car shoulda' been faster "'cause its got a blower on it".

IMO, you'll impress yourself (and everyone else) more by doing it the "hard way".

How about an ITB intake? OF course, I'm slightly prejudiced to those...:twothumbs

[Jim Nilsen]

Here is another aspect to take into consideration.

I started out with a $500 circle track engine as my base and put efi on it with some really good accessories for cooling and all the nice pulley's and fuel system along with just a plastic circle track fuel cell.

I was at Road America and was out for my maiden track day to sort it all out and make sure it would work the way it was built for. My instructor didn't get it into his head that I was there to sort out the car and not there to learn how to drive like a pro. I was having problems all day saturday with my fuel pressure dropping in the corners. I didn't know why and was trying to figure it out. My instructor was on my ass for driving in the middle and slowing down in the corners as I was waiting for the pressure to come back up. All the time I thought he was paying attention to both the car problems and my driving. On sunday he told me he could get the car out for another session than I would have been allowed if I let him drive and he could show me the lines around the track. He promised me he wouldn't beat on my car and would take it easy. So I thought it would be a great idea to learn more with more time on the track. Once behind the wheel he started ripping up the track and we were right there with all the C5's and C6's holding our own. Something that my fuel supply wasn't going to tolerate, you see, I had forgot all about the pickup hose bing all the way on the right side of the tank just like they come for a circle track car. So as we were going into corner 3 and coming out hard I couldn't tell him soon enough that the red warning light was low fuel pressure and then it happened. The piston cracked and there went my engine. At 1st I blamed him but then I woke up to the fact that that is racing and it was ultimately my fault and it more than likely could have happened when I was driving.

The BIG POINT here is that I could have blown up a $5,000 engine instead of a $500 engine. Boy was I glad I hadn't had a budget that would have allowed it to be a $10,000 engine. Any engine is good enough to start with when sorting out a car for the track as long as it can get out of its own way and your arm can wave out the window to let the faster guy/not neccessarliy the fastest car pass.

I hope you can see what I am saying here because you will constanly be working on your car to go faster whether it be more power or better handling and there is no need to make your budget suffer and keep you from having the funds to get to the track this year instead of a year from now. You can go for a whole year of entry fees and travel expenses for the amout of $$$ you can spend on a boosted engine or exotic n/a engine.

I hope to see you at a track having fun and learning instead of reading your post about how you are still working on having the baddest car you think you can afford. I say this all thinking that you are looking forward to being out there with the rest of us sooner instead of later.

Did I say I really like Falcons too!

[Bryce]

I have a few small block fords between my dad and I. All built a little different for different end uses. After we built the 65 mustang we tested all the components, suspension, steering, brakes, engine accessories and plumbing using the street motor (we call it our spare motor).

[dontlifttoshift]

Build it with the blower and a big pulley to keep boost down. Build the rest of the car around the most ridiculous amount of power you would want. learn the car and add boost until it makes you pee your pants when you push the skinny pedal....

[Norm Peterson]

This will be a street driven car with some autocross/track work.
Then I'd think more in terms of a torque to weight ratio. Or overall gearing times torque divided by weight for a crude idea about whether you'll have enough grip available to put it all down with the available tires. Using some average torque value rather than peak torque, how many gears are you willing to "waste" by not ever being able to go to full WOT in?

For street and autocross, you're far more likely to be running at revs nowhere near where peak HP is developed, you've got to get through them all to even get at the big HP numbers, and downshifting to 1st isn't a particularly good alternative in the 3650 (or most other recent manual transmissions either).

Even at 4.6L, you'll be at a better displacement to weight ratio than the new Boss/Laguna Seca (~600 lb/liter vs ~700 lb/liter, and yes I know the units are "upside down"). Displacement/weight is a quick and dirty way of estimating torque/weight when you don't know what the torque is.

FI engines are more sensitive to heat, which could impose a limit on any road course running fun.

I'm afraid that focussing too narrowly on HP will lead to looking there first for improvements in lap times/run times at the expense of much more important aspects.


Norm