HP Per ci

[headcase]

Hi

I was wondering about fuel consumption and other factors after looking at different engine ci - power possibilities.

Eg:

2 identical cars, both at 650hp.

Car 1: 428ci (stroked 351 small block) = 1.5 hp per ci.
Car 2: 360ci ---------------------------- = 1.8 hp per ci.

Which of the 2 will be a more efficient engine in terms of street cruising and which would be more efficient under wide open throttle (racing)?

Which would have better acceleration (flatter torque curve)?

[skatinjay27]

You cant know with just the displacement and power numbers... Need more info ie: compression,cam profile...

[MattO]

There's too many factors to make a judgement based on that. There's just so much that goes into it. Yes, hp/ci can be a measurement of efficiency, but again, alot goes into that. I remember a bunch of Honda S2000 fanboys saying that, at the time, the motors in those cars made more hp/ci than any other n/a motor produced at the time. Problem: You didn't really make any power until the VERY top end of the RPM range. Down low, the cars were pigs by comparison. Same deal with the rotary cars I've driven (Mazda RX7/8).

[headcase]

So there is many ways to get a 351 at 650 hp with different cams etc and still produce same power even though the bits were all different?


Hmmm.

[Steve Chryssos]

The 351 (C or W) is not known as the world's most efficient engine. As such, 1.8 to 2 HP per cube usually requires a lot of race-only design elements including high compression, and aggressive cam and revs.
You might be happier with goals in the 1.2 to 1.3 HP/cube range with a carb and 1.4 with EFI.

DISCLAIMER: I'm talkin' real horsepower, not internet horsepower.

[DTM Racing]

Originally Posted by streetfytr68

DISCLAIMER: I'm talkin' real horsepower, not internet horsepower.

Wait you mean I can't take my engine builder's word for it? But he SAID my 225cid Slant Six should be good for 650HP with this 3/4 cam!

I love taking cars to the Dyno. Complete heartbreaker for some folks. A customer of mine swore up and down that there was no way my Road RUnner only made 400rwhp, since my car was way faster than his "600hp" 383 Stroker 350. We spent the day tuning his car on the dyno...256rwhp.

[hp2]

Horsepower is like that hot looking, busty gal in a bikini that we can't help but look at when she walks by. Torque is the fair looking girl with a nice smile and a good sense of humor that we take home to meet the parents.

Torque is where its happening. More torque at lower levels will make a car more enjoyable on the street. Torque production is also a better measure of efficiency than horsepower, IMO.

It also is fair to say that more cubic inches will always produce more torque or horsepower than less cubic inches. It also will produce that power at lower rpm levels with less effort. However, it also will require more fuel to feed those cubic inches. You can improve the efficiency of the larger displacement by squeezing the charge tighter or introducing better atomization of the charge so it burns the fuel more completely (thus needing less fuel for the same power). This is where the engine build gets tricky because there are multiple factors involved that can help or hurt you and it is never as simple as saying I want X compression producing Y power. It becomes a balancing act of compromises. Also keep in mind that 600 horses from 500 inches requires a whole different level of components than 600 horses from 360 cubic inches because of the rpm levels involved. That can become a huge factor in the cost to achieve the repsective power levels.

Additionally, when looking at the weight you're carrying and how it relates to the handling, with your mustang, you will have a greater range of spring options that can be used to offset the additional weight you would carry with a big block over the small block. You may wear out tires faster, but, that may actually equalize the overall wear of the fronts as they carry more weight to match the burnouts you'll produce with the rears as a result of massive torque under your foot.

One issue of concern with a mustang is that they do not fit big blocks very well. Sure, some were put there by the factory, but their sheer size means service can become more of an issue than an equivilent small block. Ford engine bays, and mustangs in particular, are unfortunatly small. In this regard, your 351 small block stroked to 427 would be an easy answer.

If you don't track the car regularly, I wouldn't get too hung up on racer based perfomance. Race cars are very tempermental, require lots of upkeep, and regular maintanance and need to be inspected regularly (remember the bikini girl above). A street car with lots of torque is more fun to drive within a moderate rpm range and it won't require as much meticulous maintanence procedures as often.

Enjoy what you have for a while before jumping into big future plans. You may be surprised by what your mustang has to offer you. Once you get to the point that you are bored with its power levels, then see where you want to go with it.