How much extra HP can I realize from adding a cam and 4bbl to slant 6.

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jeffrey356

jeffrey356

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I have a '65 dart 270 with the original 225 c.i. slant 6. I want to replace my original 1bbl Holley with a 4bbl, 650 cfm Holley and intake manifold and new exhaust headers. I also will add an aggressive cam and bore out the the cylinders .030. The stock motor makes 145 HP. It's quick, but I need more HP. I'm torn between modifying the tower of power slant 6 or changing to the 318 c.i. crate motor. The 318 makes 300 BHP, but I'm not sure I want to change everything for it. If I could boost my slant six to over 225 BHP with the upgrades that I mentioned I would stay with the bullet proof slant 6. My 225 c.i., slant 6 runs great and only has 44,000 miles on it. I hate to change it out, but I need over 200 BHP to satisfy my need for speed. If you know how to add over 55 HP to my slant 6 please eddify me. Truely, I would love to find an upgrade of 80 BHP to the 225 c.i., but I don't if I can get there. If you know how to get over 55 to 80 BHP, please let me know. I really don't want to go to the 318 c.i. because I'll have to change the transmission. Expensive!
 
You will have more looks and sound rather than power. If combined with other mods, such as milling the head about .100" and adding larger valves and headers, then your power addition will be more recognizable.
 
jeffrey356 said:
-- I have a '65 dart 270 with the original 225 c.i. slant 6. _ _ _ _ __ I really don't want to go to the 318 c.i. because I'll have to change the transmission. Expensive! --
Click to expand...
-- A mild 318 is a slick option. - See My 65 Valiant, It's a 318 dressed as a 273.
 
If staying with the 225, you will be happier starting with RRR advice, Super Six intake and 2 barrel carb. Use 2.25 exhaust with a low restriction muffler. If that is not enough for you then add a smallish cam, valves springs, and double roller timing chain. I would never bore a block that did not need it. While the head is off, replace your valve seals with Viton valve seals and find true Top Dead Center and mark it on your vibration damper. I don't guess Horsepower numbers, but this alone will give you more power and usually better mpg.
 
What I am finding with my own car, is that head porting makes a HUGE difference with these engines.
No dyno numbers to share, and I am still sorting the drive line, so I don't have any representative strip times just yet, but seat of the pants is amazing come 3500 rpm or so.
The head is one I cobbled together just as a starting point. Lots of time spent in the bowl area with just a clean up of the ports themselves and back faced stock valves.
Over the winter I plan on carving up a full race-treatment head. I'll post some pics of it as I move along.
 
you aint gonna get another 80 hp out of it unless you use a supercharger or nitrous etc . you can easily stroke the engine but that costs big bucks.

if you want a big increase in power feel, you should g to a bigger engine.
.
 
I wouldn't waste another dollar on it unless you are throwing a turbo on it. get yourself an 8 cylinder foundation.
my 2 pennies....
 
Thanks for saying that.

Way too many times I see the first ting on the "desired" list is "...bored .30 over..."

People need to check to see if it needs it. Why waste the time and money if it doesn't?

It's not some kind of badge or anything.
 
The head is the biggest weakest link.
Stock flow from what I understand is 135 cfm which is pretty bad especially since the total flow of a 6 over an 8 per cubic inch is gonna be less since you have two less ports. If you take the 2 x head flow cfm rule for 8 which takes a big cam and CR. Since /6 has two less ports have to modify the rule so 135 x 6 = 810, 810 ÷ 8 = 101cfm. So 200 hp is max available power and with a more mild cam and CR 170-180 becomes the max unless you port the heads and depending on the results you could get around 1:1 hp per cid.
Even a fully built race /6 it's hard to break 300 hp so unless you add a turbo go v8 even a 273 could hit your power goals without trying. And don't forget about torque a 225 hp 360 with 3.55 you'd need a 225 hp /6 with something like 5.56 gears to equal it up lol.
 
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whitepunkonnitro said:
What I am finding with my own car, is that head porting makes a HUGE difference with these engines.
No dyno numbers to share, and I am still sorting the drive line, so I don't have any representative strip times just yet, but seat of the pants is amazing come 3500 rpm or so.
The head is one I cobbled together just as a starting point. Lots of time spent in the bowl area with just a clean up of the ports themselves and back faced stock valves.
Over the winter I plan on carving up a full race-treatment head. I'll post some pics of it as I move along.
Click to expand...
Thanks for the info. jeffrey356
 
273 said:
The head is the biggest weakest link.
Stock flow from what I understand is 135 cfm which is pretty bad especially since the total flow of a 6 over an 8 gonna be less since you have two less ports. If you take the 2 x head flow cfm rule for 8 which takes a big cam and CR. Since /6 has two less ports have to modify the rule so 135 x 6 = 810, 810 ÷ 8 = 101cfm. So 200 hp is max available power and with a more mild cam and CR 170-180 becomes the max unless you port the heads and depending on the results you could get around 1:1 hp per cid.
Even a fully built race /6 it's hard to break 300 hp so unless you add a turbo go v8 even a 273 could hit your power goals without trying. And don't forget about torque a 225 hp 360 with 3.55 you'd need a 225 hp /6 with something like 5.56 gears to equal it up lol.
Click to expand...

Thanks for your info. I've decided to save my pennies and get a crate 318 ci.
 
A low mileage 5.2 or 5.9 magnum would be a cheap buy.
 
If you don't care about gas mileage, the absolute best thing you can do for a streeter is to gear it up.
Going from 2.76s(typical) to 3.73s will make the engine feel 3.73/2.76 =plus 35% bigger;especially in first gear. Plus 35% is 79 cubes. So the ol' 225 will feel like a 304.
Add a 2400TC and she will be a whole new animal.At this point the S/Six stuff and a free-flowing exhaust will really fill things out.
Unfortunately, 65mpg is about 3000rpm, so this is not hi-way friendly.
But I can't over-stress the change in personality,that this big a gearchange makes. And it's cheap.
And you will want at least 3.55s with a teener too. And a 2400 to 2800TC, so it's not money you will never recover.
Whereas any money you spend on the slanty,is lost when you decide it is still not powerful enough. I highly recommend at least 3.55s; for either engine.
 
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