Inliners International Forums General Discussion Engines what is then gain

hi again, i have a bone stock chevy 250. but i will be putting clifford headers, offy intake and a holley 390 carb on it, what will the horsepower gain be?
thankx.
kev

Not too much.
The stock cam will hold you back from getting more HP.

Maybe 30 HP ish? Just a guess.

MBHD

what would you recommend for a street able car, as far as doing head work( bigger valvs,springs,shaving ect) and a cam?

All those things combined really complement each other very well. The best bang for the buck would be to change to any performance grind cam that is available for your intended purpose. Like Hank said, the stock cam holds you back from any of the other add-ons to be of much benefit, even doing headwork and leaving the stock cam in the engine will be disappointing.

looking at some came that say "might not have enough vacuum for power breaks" is this o for the street?

Yes there is a point when your vacuum could not be high enough.

There are ways around this problem though.

A 12 volt vacuum pump,that came on GM cars like Caddys & other GM vehicles.These work great!

Also, if you have a power stearing pump you can run a hydra boost brakes,which is what I will be using,so I can hold the brakes while I boost launch


MBHD

Several ways to deal with this.
1. do you have power brakes?
2. almost all disc are power, but they don't have to be. A stick car can be converted with a different master to give good pedal pressure.
3. the amount of cam that kills a vacuum boost is a variable, with compression a major factor. The brakes still work (on stored vacuum), but only once with boost (after that, they work but the pedal is very hard), and need a brief interval of engine vacuum to boost again.
An extra vacuum can in the hose allows several more stops.
An electric pump cures most of the problem (but it will cycle continuously in heavy traffic). More details, read my article:
http://victorylibrary.com/mopar/pump-tech.htm

Gearset,
Here are some computed numbers from my DeuceCoupe Gonkulator.
It matched up fairly well with Tom Lowe's recent dyno testing so here is what it computes for:
Stock 250
Torq 224 at 2300
Powr 152 at 4000 (rated 155hp)
Add Offy 4v, 390 Holley, headers
Torq 227 at 2800
Powr 173 at 4600 (gain of 21)
Add Clifford 264 cam (206-206 duration)
Torq 241 at 3400
Powr 201 at 4800 (gain of 49 total)
Or Clifford 270 cam (214-214 duration)
Torq 241 at 3500
Powr 207 at 4900 (gain of 55 total)

Then of course there's the head....
But first, what vehicle is it in, trans, gear, exhaust, usage, and weather (doesn't it get cold up there???) Big cams are liveable in the summer but a nuisance in the winter.

Cool, thanks for all the numbers!!!!! The car is a 69 chevelle 2 door with a stock 250 and stock power glide trans. The 250 and glide were stock with this car.
As far as gears go you know better than I do as far as whats in it. I will have offy intake, holley 390,and clifford headers.

Mufflers and pipe size I have no idea yet.the car isn't going to be winter driven. As far as temps go, 50 to 85 degrees F is normal for summers hear. keep in mind that the car will be a weekend driver and i still don't know what a "streetable" car is.
one more thing, is that at the rear wheels?

thankx again. kev

Kev,
OK I guessed you have a 3.08 gear now from the info I have.
Here is a recipe and if you want to have the most fun, take the car to the strip both now (it will never be that slow again and you have a comparison) and when you're done - even at each stage if possible. You can see below that adding a cam can make the car SLOWER with a setup like yours.
By the way, all the power numbers are SAE gross, that is, 60F dry air at 29.92 barometer and sea level into the carb, no accessories or air cleaner or exhaust. Wide open. Net power (the way they are rated today) is much lower, and rear wheel is lower yet.
Stock guess (152hp)
2.96 60ft
12.63 at 57.2 1/8 mile
19.64 at 69.4 1/4 mile
14.0 0-60mph

With Offy 4v, Holley 390, headers as you plan (173hp):
2.92
12.28 at 61.1
18.86 at 73.6
11.9
My rule is that a full second change in 1/4 mile is a totally different car, so even with what you plan, it will feel that way.

Now add the Clifford 264 cam (201hp)
3.01
12.42 at 62.7
18.80 at 75.9
11.7
Note the 60ft and 1/8 mile got SLOWER. Bad move. With that heavy Chevelle, a 2-spd powerglide and 3.08 gear, you can't give up ANY low end. That's ok, next step is fix the trans (to a 200-4r lets say) and rear (to a 3.73 posi now that you have overdrive):
2.52
10.80 at 65.0
16.95 at 79.3
9.2 0-60mph
Now you're in business.

For comparison, Road&Track tested a 71 Chevelle 350 back in the day and got, with 2 people aboard,
16.90 at 82
So, you're in range. But it looks to me like once you put a cam in there, you will want to look at trans and gears next.
good luck

neat stuff. Thank you for all the numbers, it really puts it in to perspective now.

kev.

Sounds like a great project.

Two recommendations :
1st - lose the Powerglide

2nd - lumps & valve work (you already have most everything else except a cam - and of course a cam)