After a 20 year hiatus, I just purchased a 70 Cutlass. I wasn't too concerned with a #s matching engine, and I was told it was a 350. Engine runs well, doesn't smoke, and fluids were clean. So, what they hey. To my surprise after I ran the numbers I have a 403 with 350-#8 heads. I have heard that the 403/350 combination can be an adequate street performer. My understanding is that the #8's did not flow as well as the #7a's did. I have know idea what has been done to this motor, but here is my question. Is there anything that could be done to the #8 heads that would make them perform better than the 7a's. I have a complete 72 350 is why I am asking.

 

Yeah you can get port work done to them to help the flow and have them install bigger valves in it. If you want to get a little more in depth than that you can get a new cam for it. However make sure you ask someone who knows these olds engines for a recommendation. If you ask a chevy guy you will not be happy with the results, its a very different animal. You WILL have fun with this one!

 

Oh, its lots of fun, and I have already enjoyed it. The 403 will eventually be replaced by 455 short block assembly that I have had for long time. Until then, I was wondering if it is worth while to replace the 8's with the 7a's?

 

403's can run, to an extent, but because of the 'windowed' main bearing webs, have a maximium HP # and RPM they can run, before major problems begin.
Most heads can gain 30 - 40 hp with a match port, bowl blend, and a 'street - strip' 3-angle valve job, and oversize valves could give you even more.
You could find a used 455 for less than the mods would cost, and begin with more HP than the modified 403, and in less time and or hassle, too.

Just saw your above post - get a pair of those mondello heads+manifold [$1300!] a corresponding cam+kit, and enjoy 400+ reliable HP!

 

I'd say no at this point. If you were starting fresh, definitely. But re setting up the whole upper end of the motor for a small amount of gain doesn't seem justified to me.

Who knows what could have been done to the 8's. Maybe nothing, maybe 1500 worth of machine work.

 

I would have to say that Dads words of wisdom apply here. " If it ain't broke, don't fix it." I'll put the $$ in the 455. Just thought if the 7a's would be that noticeable, I might do it in the interim, but until I commit to tearing down the 403, i'll have no way to know what I have. I'll just run it for a while.

 

The 403 can be made to run, and run very well.
The 7a heads will always be better heads than the 8's because the 7a's have a 10-12 cc smaller chamber. 4A's are easily found, curious why the 8's were used. #8 heads have smaller intake valves than 4A's that will hamper performance on the 403.
The #7a head (also #4,#5,#6, and #7) will raise the compression by a point, and with larger intake valves, really wake up that old 403. A bunch.
Jim

 

Yeah what he said.

I have seen a few 10 second 403's in my days.

They aren't as weak as most say.

Earlier #4 through #7 heads will add an easy 50 horsepower over stock @8 heads.

Your dad is right, if it ain't broke, don't worry about it. Take the money you save and take your girlfriend out and buy her something, the return on your money will be much more enjoyable, atleast I always thought so, from my own personal experiences.

 

 

I have a friend that ran a 403 at the drags in his 81 Cutlass, I think he was low 11's or high 10's. The more I run mine the more I am convienced it has some work done to it, alittle loopy and revs nice, but what work has been done will remain a mystery for now. I think it can run better though seems the timming is retarded too much. Dogs just slighly from a start, but pulls strong once RPMs reach, say 1700 RPMs. Its close, but not quite there. It has a MSD ignition box, distributer, and coil; and a Holly Carb. I don't know a lot about MSD ignitions, I'll need to study up on them. She is about to rest for the winter, so i will have time to figure more out about it before the spring. I am going to run it a couple of summers, and then put the 455 in.

As for taking the girlfriend out. Well the only girlfriend I've had for 20 years is the Mrs. I think I'd be better, and get better results taking her out. The other way, would be way to costly. Besides, she understands my love of Oldsmobiles.

 

my 65 has a 76 350 w/8 heads and a m20 four speed. dont know whats been done but it pulls strong and idles real lumpy. have a set of 5 heads and wondered how much the higher compression would help. and if they breath better than the later 8's. replace the cast iron intake at the same time to save some weight.