Adding a little power to the 403?

[rebelgtp]

Alright guys I have my 403 that I recently acquired and I know these engines are not great for huge power numbers but surely we can do better than the 180 or stock horsepower by opening things up a bit. I also know about the inherent weakness of the windowed mains that limits big number builds.

So how would a fella on a fairly limited budget (I would love to take the car to the GRM Challenge) go about freeing up some of the horsepower in this engine? I would love to get even just 100 more horse power which I figure should be reasonable and safe for the motor and keep it a good fun street machine. I know I can open the exhaust a good bit with a set of headers and a nice free flowing exhaust. From what I understand swapping out the intake manifold for one of the Edlebrock units you most are gaining a weight savings (is that correct?). I also have heard of swapping out the heads for a set of earlier 350 heads or even modifying 455 heads to bolt up to the 403 engine.

So in what ways can I massage this engine to release a little bit more power without breaking the bank. I figure on buying some headers for it and hopefully can find someone selling a used aluminum intake. Where else should I look to try and squeeze a little more power out for this engine? I would be happy with hp number sitting between 250 and 300 for this car.

 

[G-Body_Vet]

IMO, your best bet is to find some 7 or 7A heads from a 71 or 72 350 and modify them for use on the 403. A nice compression bump and a cam will really wake that engine up. The 403's only had something like a 8.2 to 1 CR.

 

[rebelgtp]

What has to be modified on those heads to bolt up to the 403 and about how much do you figure a set will cost? I was actually thinking the stock compression was even lower at just 8:1.

 

[G-Body_Vet]

What has to be modified on those heads to bolt up to the 403 and about how much do you figure a set will cost? I was actually thinking the stock compression was even lower at just 8:1.

From what I recall, I think the only thing that needs to be changed if the head bolt holes. I think they need to be reamed out for 1/2" bolts. Price wise, I have no idea what they go for. Maybe one of the more seasoned Olds people will chime in.

 

[DRIVEN]

Some good info here:
http://www.442.com/oldsfaq/ofe403.htm





I'm so jealous right now.

 

[willitell]

What has to be modified on those heads to bolt up to the 403 and about how much do you figure a set will cost? I was actually thinking the stock compression was even lower at just 8:1.

From what I recall, I think the only thing that needs to be changed if the head bolt holes. I think they need to be reamed out for 1/2" bolts. Price wise, I have no idea what they go for. Maybe one of the more seasoned Olds people will chime in.

Reaming them out is all you need to do, Should run about $100 or so. The heads you want are 5, 6, 7, or 7A (69-72). Best bet is to look for a deal, find heads that have already been prepped by someone. I got my pair of 6s for around $350, and they had been cleaned, checked, milled, set up for stud mounted rocker arms, and a 3 angle valve job with fresh valves. I had my machinist check them out, ream the holes to 1/2" (later 350s also use a 1/2" head bolt) and set-up a new set of springs. Total for my heads: $600 not a bad deal for what i got.
The earlier heads will improve your compression ratio. It should be good for at least one whole point (8.2 -> 9.2).

A dual pattern cam (Like a Lunati Voodoo) had been recommended to me, and i went with one of their milder profiles. I've had good experiences with Edelbrock's performer cam in other applications, and strongly considered it for my 350. There are also reproduction W-30 and W-31 cams out there

You're on the right path with your ideas. An aftermarket intake should help you with some gains. Edelbrock and Holley Dual Planes are out there used for a very reasonable price. As far as Headers, I'm going to use the cheapo Flowtech ones for now. I figure for $150 you really can't go too wrong.

Finally to wrap it all together, a decent torque converter. Just pick a reputable company and have their techs recommend one for your application.


Before you get that far into it, I'd recommend doing a compression test, to make sure that your foundation is solid.


There's plenty of Info out there on it. And a good number of Oldsmobile forums too.

 

[rebelgtp]

I'm so jealous right now.

How come?


willitell
thanks for the info. Considering my hopes to get the engine in by the end of summer unless I find the heads ASAP I think heads and cam will need to wait until next year. However it gives me a good idea of what to look for. I should go out to the local engine builder and rummage through his shelves and see if I can find some heads.

 

[DRIVEN]

How come?

I've been looking for a 403 for mine. Seems like the stuff I've come across has been straight junk, suspect, or really over priced. You know that yours ran (drove to the yard) and the price seemed fair.

On the head swap, I remember reading somewhere that the '72 350 heads are the best to use due to hardened exhaust seats. You may want to verify that before buying a set.

 

[rebelgtp]

Tell you what DRIVEN you keep an eye out on some parts for me down there and I will keep my eyes out for another 403 out here.

Yeah from what I have now read the 7a seems to be the top choice.

 

[FE3X CLONE]

The head bolt holes will need to be enlarged to 17/32". If you ream them out to exactly a 1/2" it'll be tough to get the bolts/studs in. You may also need to enlarge the dowel pin holes as well.