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glen242 05-19-2008 03:31 PM

I need intellegent input concerning HP ratings.
Many people say they have a 400-500 HP SBC. Displacement from 327 to 400 CI.

Where these HP numbers come from is usually not quoted, but very rarely, an engine dyno is cited, numbers usually supplied by whoever sold the engine.

A chassis dyno reading, and I know these numbers can be massaged, is usually never supplied by the car owner, so that a comparison can be made between engine and chassis dyno numbers. Rule of thumb was, I believe, 15-18% manual and 20% and up automatic drive train loss. Does this "rule of thumb" still hold true?

My prior 350, stock 180HP, according to GM, with a cam, new heads, 9.2CR, long tube 1 5/8" headers, ST10 transmission, and Performer intake, measured 217 RWHP on a Mustang dyno. Using the 20% loss figure I came up with 271 Engine HP. Not too bad, I thought. Was my math correct?

I guess what I am asking is "how do you know what your engine specs are if it has never been on a reputable engine or chassis dyno"? Some say 1/4 mile speed will tell you, but how about driver error during the run, wheel spin, etc.? Computer sims can be manipulated, by what is input, to give different results.

I see people stating they have 450-500 HP on engines that are less built than my present 383 and wonder where the numbers came from, as they are usually never stated. Are we turning into ricers where we take each alleged HP increase from each separate aftermarket part and add them together and then add the total of them to our factory HP?

Maybe I'm just an old fart, but I find it hard to believe that 1.3 HP/CI, in a 383, is easy to get on a NA STREET CAR.

I once told someone on a Corvette Forum that there were more 400+ HP SBC cars on the internet than on the street, and I stick by this statement.

You might want to consider this post just an old man/hotrodders vent.

Comments welcome.

poncho62 05-19-2008 03:37 PM

Just like there are twice as many 69 Z28's around these days, than what were ever produced................... :rolleyes:

techinspector1 05-19-2008 03:41 PM

I've learned not to argue with these dummies. It'll only go downhill, like telling them their girlfriend is ugly.

Double_v23 05-19-2008 03:51 PM

I agree 100%, take "chevy 350 cam'd" every time he posts a different engine combination he asks the same question "do you think this will make over 400 horse?" the answer is always the

Generally speaking, people who say they have a 500 horse small block have no idea of just how it feels to be actually riding in a 500 HP car.

My BBC cost around 8 grand to build and is about as wild as you can get on pump gas, is ACTUALLY making 500+ HP at the motor. I finished it last year and can honestly say that I have never had the opportunity to open the throttle all the way. If you open it more than 3/4 it lights up the 12.5 wide mickey thompsons and you are so busy trying to get it through the first couple of gears without having it bounce off the rev chip (7,000 rpm) that you don't even notice that you are over 90 mph until it is too late.

500 Horse is a life changing experience and you know when you have got it. And if you can drive your car safely at WOT on the street then you haven't got it....

Rick WI 05-19-2008 03:52 PM

On my dyno it is VERY rare for us to have a 500 HP small block. It is rare to have a 450 HP small block as well.

All I can say is keep your wallet in your pocket if you choose to pull the trigger on an OEM new vehicle that is rated at 500 HP (Re:Vette) with your magazine claimed 500 HP build.

C-10 05-19-2008 04:05 PM

There is a formula based on the vehicles weight, trap speed, and the dreaded 'fudge number' or percentage for drive train loss all figured in to reveal the horsepower. Still a crap shoot, but an IDEA of the output.

My dyno is the width of my grin as I mash the loud pedal. :D

Even the big dyno shops correct their numbers for altitude! So what now? Even the parameters of a dyno cell change due to atmosphere. I think they have a 'fudge number' for that too.

I generally avoid conversations in bench racing when asked of my 'HP' and quarter mile times. It always leads to a joker in the crowd with a reason to ONE-UP you in some way, shape or form. Especially the rice crowd. You're doing yourself a dis-service by essentially deducing your hot rod to a NUMBER. You know, I know there is more to this hobby than that.

Even the ricers are hot rodders too. It's all the same, fundamentally. I remember the days when I started doing this stuff, only for me it was Novas and Chevelles and old pickups.

Speaking of Chevelles, do you think GM rated the LS6 454 accurately? Hmmm. I remember a 5000 pound sled that would MOVE.

richard stewart 3rd 05-19-2008 04:24 PM

I can't imagine where some of these figures come from, A few days ago I did a computer Sim for a new guy here, the engine had poor flowing heads,(that were supposed to have bin fully ported?) low CR, & the wrong cam, He thought his engine should have had over 400hp, it didn't, (was in the 300 area), so I jacked the CR to over 11:1 & the RPMs to over 6,000 just so he'd stop complaining about the low hp#s(real hp#s) posted previously, I would bet that even though his engine couldn't produce the HP he thought it should, he's telling people that his engine has over 400HP & it was computer simulated. It doesn't make sense to me, running off at the mouth, telling people you have way more then what you really have, what do you say when you loose the race? I left half of my HP at home tonight
Take care,

sam-missle 05-19-2008 04:59 PM

i joined a few internet forums less than a year ago, and it amazes me at how many engine experts and builders the internet has created. i think most of these inflated figures are from computer programs, like EA, etc. that were fed the wrong info.


PatM 05-19-2008 05:56 PM

I have both EA and DD. I use them just for fun. I actually DID use EA to aid in the design and component selection of my engine (not yet fired). But whenever anyone asks about the HP, I am always careful to qualify "my dyno simulator says . . . ". EA is a little optimistic, especially with hydraulic roller cams. I recall reading a magazine test comparing different cams, to see how a solid flat, hydraulic flat, solid roller and hydraulic roller (with similar timing) would compare. I charted those (and many other test results) from magazines to try to calibrate my simulatar. Not perfect, but better than being totally deluded. EA is optimistic pretty much across-the-board, but particularly with hydraulic roller cams. I don't recall exactly, but I think it was somewhere around 5%.


techinspector1 05-19-2008 06:07 PM

The worst thing anyone can say to me is "ported and polished" AAAARRRRGGGGHHHH :spank:

Well, that and "how will this cam sound?" DOUBLE AAAARRRRGGGGHHHH :spank: :pain:

sam-missle 05-19-2008 06:19 PM


Originally Posted by techinspector1
The worst thing anyone can say to me is "ported and polished" AAAARRRRGGGGHHHH :spank:

Well, that and "how will this cam sound?" DOUBLE AAAARRRRGGGGHHHH :spank: :pain:

yep, lot of head porters out there too, but without the bench.


Guy Hiltz 05-19-2008 07:52 PM

I can give you a real life comparison.A friend of mine did a dyno sim program on my big block for me,with all the right parts & specs.His program came up with 637 HP.On a real dyno,with no "fudging",it showed a best of 554 HP.This is a huge difference.

coldknock 05-19-2008 10:27 PM


Originally Posted by sam-missle
yep, lot of head porters out there too, but without the bench.


That is a gross understatement in my neck o' the woods.

I have a huge pile of junk heads, that were once useful, because of these hacks. I've also had these a-holes buy heads from me and have their names carved into the accessory pads, or put their valve cover stickers on an engine that I built. :spank:

Ehhh, this kinda junk is a real sore spot with me.


carsavvycook 05-19-2008 11:16 PM

I understand what you say! I cringe every time I here the kid in the shop next to mine says "all of my motors have 650HP they cost me 5K". I know better having tuned them for his engine builder who states the rated HP off of a desk top dyno program. His builder tells me they should be 420HP realistically because of the early camel hump heads the kid wants to run. They are lucky if they have 380HP in chassis with my built in dyno our Lord gave us after tuning.

I know how much you have to spend to get real horse power having spent 22K total for 560HP out of a Boss 302 Ford (twin turbo-ed in the early 70's, 65K total to get 981HP out of a 481CID BBC(dual stage nitrous injection) in the late 80's. Both were engine dyno tested. This Corvette (in my pictures)was 405.5HP in chassis 4 years ago, and it is much faster now(he now calls it his Screaming Yellow Zonker). It is going back for a re-dyno later this summer
after suspension replacement changes to the 35 year old parts.

curtis73 05-20-2008 12:35 AM

I think with common engines, its just the proliferation of experience. If I post a question that goes something like, "how much hp will I get with X heads, this compression, this cam, these exhaust manifolds, etc..." chances are there are four guys on this board who have done a nearly identical assembly and had it dynoed.

Its also just good old fashioned guessing. You can get really close just knowing about how much cam and compression it takes to get a 350 to make X hp.

The other phrase I HAAATTTE is "balanced and blueprinted." Bulls#!t. You slapped new bearings and pistons in an untouched 30-over shortblock. That does NOT mean B&B.

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