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  1. #1
    Join Date
    Sep 2010
    Location
    Nova Scotia
    Posts
    986

    How much Hp do I have? or need....

    Our cars, usually with modified engines, present a minor problem for me, and maybe you. Just how much horsepower does it have? I’ve been asked this question a few times. Not nearly so much as the “Is it real?” question. In fact most people just want to pay me compliments on the car. Yet the Hp question has been asked.

    I usually answer “I don’t really know, but it compares nicely with one of the 425 Hp versions”. I actually thought it did, but have no idea any more where I saw specs that made me think that. Anyway, it makes them happy because it meets that “it’s easy to 1 Hp per cubic inch” philosophy that is quite prevalent.

    Actually, David Vizard in his book “How to Build Horsepower, Vol. 1”, states that one should aim for anywhere from 0.8 to 1.0 Hp per cubic inch as a realistic goal for a street engine.

    My engine is actually an over-bored 410 Ford FE with a total displacement of 418 cubic inches, so I guess I should be happy with anything from 334 to 418.

    Quite a while back, someone here started a thread about how much Hp one of our cars should have. Thinking I had something close to 425 Hp and knowing I can’t use that much power I recommended about 350 Hp as a good goal to achieve. Turns out I might have been more correct than I thought, but not quite for that reason.

    Although Ford’s rating of the 428 is probably very much underrated at only 335, it has been said that only a very skilled driver could tell the difference between the 428 and 427(425 Hp the most common) by driving them. I suspect that person is correct, but I’m getting ahead of myself.

    The question of how much Hp my engine can produce has occupied my thoughts many times over the years. In fact back in the late 1990’s I bought a G-Tech meter, a small device that measures, amongst other things, an engine’s horsepower. You input the weight of your car with driver and fuel, then accelerate and the little box does the rest based on readings from its internal accelerometer.



    The new ones use GPS and can actually plot Hp and torque curves. Very nice, but possibly useless for our cars as a full throttle run might not be possible depending on what gear you're in.

    Although I played with my G-Tech on other cars, I never tried it on the Cobra as I didn’t have a cigarette lighter socket to plug it into. So I bought one and let it hang down under the dash after I fed the wires through the firewall.

    Time for the test. This was last October and we had quite the cold snap, so I waited for a warm day and finally was rewarded with one hitting a whopping 50degF.

    Now for the next problem; trying to find a long level straight stretch on public roads with little to no traffic. Not easy, but I managed to make a couple of runs.

    My list of troubles were:

    1/ My chicken quotient. Cold pavement, 24 year old hard, cold tires, unexpected arrival of traffic are real worries. So is speed.

    2/ I can’t use full throttle in second gear. In third, maybe, but the speed would be excessive.

    3/ Not focusing on what I was doing well enough and short-shifting through habit.


    Nevertheless, I managed some runs, although not at full throttle; only about 1/3 throttle in second gear.

    My maximum rear wheel horsepower cam up as 252. Hmmm…. A tad low, or so I thought.

    For that particular run, I know I only hit 4500, although I can’t remember if I “accidentally” short shifted, or traffic showed up and I let off the gas. Anyway, at 4500 rpm I would have been doing 53.5 mph in second gear and I‘m certain the Hp was still climbing.

    At the tires, engine power appears to be used in three main ways:

    1/ vehicle acceleration, which the G-Tech measures at the wheels and converts to rear wheel Hp which is the 252;

    2/ air resistance, which I can calculate for the 53.5 mph and works out to 18 Hp

    3/ Rolling resistance, basically from the deformation of the tires, which I can also calculate for 53.5 mph and works out to 6 Hp. I’m not sure if a portion of that attributed to the rear tires should be used or not, but I will as it wouldn’t be much of an error.

    Adding these up yields 276 Hp at the wheels and then allowing 15% for drive train losses, comes out at 325 Hp at the flywheel.

    Now as that's only at 4500 and part throttle, it would be interesting to predict the peak power. It's really not possible due to the throttle not being open all the way, although comparing it to a 425 Hp 428CJ using the same intake manifold as mine, an 11% increase could be expected. I got this from a dyno-chart in Jay Brown’s book “The Great Intake Comparo”. The end result is 361 Hp and that‘s still at part throttle.

    So what about software programs to predict the peak Horsepower? An expert on the FE engine site has his own software and came up with only 347 Hp. When I originally called Crane Cams about a choice of camshafts I wanted as much performance as I could get while retaining a decent idle. They plugged some numbers in their computer and came up with an incredibly optimistic 480 Hp. On line calculators have yielded anywhere from 361 to 408 Hp with many in the 370 Hp range.

    OK, so with my prediction at 361 Hp with only part throttle, a peak ought to at least be 370 to 380.

    So why not a lot more? According to the experts on the FE Engine forum, my Offenhauser dual-carb 360 single plane manifold is a dog, even though that Cobra Jet 428 did hit 416 Hp with the same manifold. My cylinder heads don’t flow particularly well either. So if the airflow is restricted mostly by the intake and cylinder heads, going to full throttle might not achieve all that much even with those twin 600cfm Holleys wide open. Apparently my cam choice could be more aggressive too.

    To summarize the result, if I am putting out 375 Hp, that works out to 0.9 Hp per cubic inch, right in the middle of David Vizard’s recommended range with lots of room either way in case I’m off a bit. Also, as you can see, if I can’t use all the Hp my engine can put out, then 350 Hp sounds like enough to get that original Cobra experience.

    One other thing I wish to emphasize; my car is dangerous, especially in low gears due to the massive torque at the wheels, and of course in higher gears due to the speed. The tires don’t help, BUT although I intend to replace them this year, it won’t be with those atrociously expensive and short lived Avons. I only drive on the street, not the track and it’s a rare occurrence when I push the car really hard due to the fear factor and worry about crashing something I built and worked on for so many years.
    John

    “A prudent person profits from personal experience, a wise one from the experience of others.”

  2. #2
    Join Date
    May 2016
    Location
    Hockley ,Essex
    Age
    58
    Posts
    441
    My take on this subject is that generally most of us
    will go with a quest for more power.
    Then in reality most of these machines will give plenty
    and will certainly be enough to get us into trouble !
    My cob only has a 240 cubic inch RV8 engine and this has
    certainly given me plenty of fun.
    I only use this on the road but would probably go for a few
    more cubes for other uses like track etc .
    I am a professional driver for the road like most of us but don't know
    enough about real racing to be able to use these cars to their full
    potential.
    Stay safe,
    Chris.

  3. #3
    Join Date
    Nov 2006
    Location
    Shropshire/Wales Border
    Age
    50
    Posts
    6,079
    Hi John
    There is the Cobras fatal flaw to try to calculate also which is a serious lack of rear grip. Off the line it seriously limits useable power. Even once rolling the grip limits power usage to a decreasing degree as you gather speed.
    I tried running a 400hp/400ftlb Dax with decent and newish Yokohama tyres at the drag strip. Even with those tyres launching on what is essentially a rubber surface I’d estimate 50% throttle was about as much as you could use off the line. The cars 60ft times were poor, normally around 2 seconds. I doubt much more than 200ft/lbs was useable at that point.
    Once up past 30-40 mph then progressively applied full throttle was useable, although care letting out the clutch after upshifts was still necessary. From 50 onwards the limit was then completely torque, although how much spare grip was available I don’t know. Clearly by this point you’re well exceeding legal speeds.
    I’d guess around 400hp/ftlbs could be considered a sweet spot? Much more doesn’t really become usable until you’re past legal speeds, but the 30-70mph acceleration is maxed out?
    Re difference between the 427 and 428 I’d guess any noticeable difference may be around throttle response. The 427 is a lot shorter stroke and should rev more happily but as you say I suspect it would take a very skilled driver to notice!
    Just random thoughts.
    Lloyd B
    Current: Crendon #54 in build - 427 Side Oiler/Cobrajet Heads/Dual 600cfm Holleys/4 speed toploader/Vintage cast knock on wheels
    Dedion Dax/Clarkson 383 Chevy with roller 4/7 swap cam, AFR195 heads - SOLD

  4. #4
    Join Date
    Jan 2002
    Location
    Marlow
    Age
    70
    Posts
    1,863
    Both Lloyd and I have dragged our cobras, from what I can remember he was a bit faster than my 12.25 sec pb. the biggest difference I have found over the years is tyres and first 60 feet. Now as to how much power is required for a road car, well if you try and drive like your on the 1/4 mile at Santapod, you need locking up! I have 400 bhp and very importantly 440 ft/ibs of torque, miles more than required to stay legal on the road. For my track days it's about enough , my slow cornering is balance by huge grunt on the straights. So I think may be 500 bhp for drag racing, 350 for the road and with good tyres and around 400 bhp for the track with track tyres.
    Just my thoughts.
    Cheers Clive

  5. #5
    Join Date
    Sep 2010
    Location
    Nova Scotia
    Posts
    986
    Quote Originally Posted by Chris1 View Post
    I am a professional driver for the road like most of us but don't know
    enough about real racing to be able to use these cars to their full
    potential.
    Yup! And the real danger is when we think we are better than we are....

    Lloyd & Clive: It's quite gratifying to have both of you coming to basically the same conclusion as myself, but yet from having different experiences. I do wonder about those guys who insist on a 500+ Hp engine for the street. Low rpm performance might make it a bit of a dog and it's not likely possible to achieve the peak Hp anyway. I suppose it's a lot about "bragging rights" which I think is silly.... yet still, I kind of liked telling those that asked that my engine was probably about 425 Hp. Guess I can't do that anymore.
    John

    “A prudent person profits from personal experience, a wise one from the experience of others.”

  6. #6
    Join Date
    Aug 2004
    Location
    Christchurch, NZ
    Age
    60
    Posts
    2,509
    Do like Rolls Royce used to and quote it as 'adequate'.
    Cheers

    Myles D-W

  7. #7
    Join Date
    Jul 2012
    Location
    NW England
    Posts
    740
    Quote Originally Posted by Eggbert View Post
    .. yet still, I kind of liked telling those that asked that my engine was probably about 425 Hp. Guess I can't do that anymore.
    Why not? It’s not like they’d know any different

  8. #8
    Join Date
    Nov 2006
    Location
    Shropshire/Wales Border
    Age
    50
    Posts
    6,079
    Best I ever managed was a 12.34 so you got me there Clive . I remember someone running an LS powered car that was claimed around 500hp and was into the high 11s? It would have been interesting to see his 60ft times. I bet they were similar to ours.

    Certainly I’d concur with Clive that the best times were invariably a result of getting the power/grip balance right off the line. From 50mph up you’re just a passenger. That would have been where the LS car eased ahead.

    I’m switching from a small block to a big block car, but have not driven it on the road yet. I’m guessing 50-100 more ft/lbs. It will be interesting to see what the difference is on the road. I may have to take it to Santa Pod to find out although the new cars on a very different type setup.

    400hp feels like a great target for a super fast road car but bragging rights are great fun (although I find enquirers are usually more astonished by the cubic capacity, after all 400hp is common in cars and even 4x4s these days)
    Lloyd B
    Current: Crendon #54 in build - 427 Side Oiler/Cobrajet Heads/Dual 600cfm Holleys/4 speed toploader/Vintage cast knock on wheels
    Dedion Dax/Clarkson 383 Chevy with roller 4/7 swap cam, AFR195 heads - SOLD

  9. #9
    Join Date
    May 2004
    Location
    Finchampstead, Berkshire, UK.
    Age
    55
    Posts
    3,836
    I’m in the “too much is too much” camp, pre supercharger I was about 400Hp which allowed me to really push it “occasionally” on the road, with the blower I’m at about 550Hp and now if I log a day of hooning around the trace for the throttle never gets above 80%.

    It’s all a bit moot though, our toys are not dangerous or overpowered per se, it’s all a question of how sensible is the person at controlling the right foot go faster pedal!

    Take it easy and enjoy it appropriately

    Regards

    Jim
    GD Mk3 Jag based
    Supercharged LS1
    GD J1M

  10. #10
    Join Date
    Oct 2017
    Location
    Oxford
    Posts
    1,168
    I previously had a Westfield that I slowly developed over a six year period into a real flying machine.

    When I sold it it was 276bhp at the wheels or approximately 335bhp at the flywheel and weighed in at about 600kg. That equated to 550bhp per tonne.

    It was a brilliant track car, but was simply too fast for normal use. Whilst I could drive it sensibly I could not enjoy wringing it’s neck and driving it hard as it was savagely fast on normal roads.

    So for my use that sort of power was too much!
    Classic Replica Viper IVA passed 25th March 2021

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