79 403ci SBO powerband
#1
79 403ci SBO powerband
Ive got a 403ci SBO in my 79 Trans Am
As Ive detailed befor in other posts, I went thru it and added a mild Edelbrock camshaft, intake, hooker headers and pypes dual exhaust.
I am very pleased with how it runs after adding some timing to the distributor.
My TH350 is set to shift at 5k rpm when at WOT.
I do notice that the engine will rpm up quickly to 4250 or 4500 or so.
After that, it seems like it loses steam a little bit. It will get to 5k but just not as smoothly as I would like.
I had installed new edelbrock valvesprings when I installed the camshaft.
I am thinking that the stock (I believe 4A) heads on the 403ci are giving my engine a case of asthma.
Am I thinking correctly? would it be worth pulling the heads and porting them a bit? Maybe a head swap? Or just lower the WOT shift point to 4250 or so?
Any thoughts/recommendations appreciated.
kg
As Ive detailed befor in other posts, I went thru it and added a mild Edelbrock camshaft, intake, hooker headers and pypes dual exhaust.
I am very pleased with how it runs after adding some timing to the distributor.
My TH350 is set to shift at 5k rpm when at WOT.
I do notice that the engine will rpm up quickly to 4250 or 4500 or so.
After that, it seems like it loses steam a little bit. It will get to 5k but just not as smoothly as I would like.
I had installed new edelbrock valvesprings when I installed the camshaft.
I am thinking that the stock (I believe 4A) heads on the 403ci are giving my engine a case of asthma.
Am I thinking correctly? would it be worth pulling the heads and porting them a bit? Maybe a head swap? Or just lower the WOT shift point to 4250 or so?
Any thoughts/recommendations appreciated.
kg
#2
The heads are definitely a choke point. But could also just be where you cam has run out if its power band. Or running out of fuel if a stock fuel pump is still in place. A head swap could wake the engine up across the entire range.
#5
I went with 3.08 ratio rear gears with my TH350 and 255/60-15 tires.
Carb was performance tuned by Everyday Performance LLC
Everything runs and drives great. No complaints.
I would just like the rpms to keep climbing to 5k at the same rate as before 4250-4500
Not sure I want to pull the heads though.
CAMSHAFT: http://www.summitracing.com/parts/edl-3712
Carb was performance tuned by Everyday Performance LLC
Everything runs and drives great. No complaints.
I would just like the rpms to keep climbing to 5k at the same rate as before 4250-4500
Not sure I want to pull the heads though.
CAMSHAFT: http://www.summitracing.com/parts/edl-3712
#6
That's a very, very mild cam. You might want something more along the lines of 218 to 222 would work. Like:
http://m.summitracing.com/parts/cca-42-229-4
http://m.summitracing.com/parts/crn-804541
http://m.summitracing.com/parts/cca-42-210-4
http://m.summitracing.com/parts/hrs-510991-08
http://m.summitracing.com/parts/cca-42-229-4
http://m.summitracing.com/parts/crn-804541
http://m.summitracing.com/parts/cca-42-210-4
http://m.summitracing.com/parts/hrs-510991-08
#8
That big of a cam for a 7.8 to 1 403? Maybe the Performer cam has been holding me back all this time. The 4A heads are awful on the exhaust side and have huge 84+cc chambers. #8 heads with mild milling and bowl work would be better.
#9
They are not really that big.
The seat to seat on the comp 268H is about 274 to 276 degrees, which is shorter than his current cam.
Crane rates theirs at .004, so it's a 272. Which is eight degrees shorter than his current cam.
The Comp 275 DEH and the Howards are both about 281 to 283 degrees, which is only a degree or three longer than his current cam. Not sure about the Erson but it should be in this range as well.
They are all more aggressive cams that can improve the airflow through the engine. With 403 c.i. to feed at 5000 rpm, you need more time/area without much more seat to seat duration than it already has. Especially since the OP likes the way it drives as it is. Well, in my opinion anyway.
The seat to seat on the comp 268H is about 274 to 276 degrees, which is shorter than his current cam.
Crane rates theirs at .004, so it's a 272. Which is eight degrees shorter than his current cam.
The Comp 275 DEH and the Howards are both about 281 to 283 degrees, which is only a degree or three longer than his current cam. Not sure about the Erson but it should be in this range as well.
They are all more aggressive cams that can improve the airflow through the engine. With 403 c.i. to feed at 5000 rpm, you need more time/area without much more seat to seat duration than it already has. Especially since the OP likes the way it drives as it is. Well, in my opinion anyway.
#11
I was playing with the idea of backing the distributor off 4 degrees to its original setting of 18 BTDC initial. Just to see the effect.
Wondering if I am getting some detonation at those hi rpms...I dont hear it but?
Wondering if I am getting some detonation at those hi rpms...I dont hear it but?
#12
backed initial timing down 4 degrees to 18BTDC, which takes that 4 off the total at WOT.
The power still noses over in the low 4250 range. Dont get me wrong it pulls fine for my use, it just seems to peak there.
I think I will put a little more weight in the governor and drop shift points a little.
Will advise.
The power still noses over in the low 4250 range. Dont get me wrong it pulls fine for my use, it just seems to peak there.
I think I will put a little more weight in the governor and drop shift points a little.
Will advise.
#14
Not true. The peak will be determined by the whole combination with the best tune.
I've done many dyno pulls that had too much or too little timing that once optimized, changed the peak number and rpm.
#15
Changing that cam timing would probably help. The Performer can doesn't build much cylinder pressure. That would explain why your car likes similar timing to my low compression 350 and 403. I am only running 12 degrees vacuum advance on my current 9.5 to 1 350. So only 50 degrees max cruising. Mark, so lazier timing curves produce more power?
#16
I heavied up the governor weights a little and got some good results.
It took some monkeying around and a couple tries but Ive got 1-2 WOT shift at 4500, chirps the tires real strong
2-3 WOT shift is at about 4750-4850 now, the last 300 rpm or so feels a bit wrung out, but it shifts strong.
I adjusted the Pontiac ratchet shifter and it works like a dream now.
I also noticed that I was about a quart low on DexMerc...yikes! That slow leak from wherever added up to some fluid loss.
I am going to screw in the adjustable modulator a little bit to tighten things up and get a crisper part throttle feel
This car gets up and goes! Feels much better with the shift point improvement. 4500 is the right spot. I MIGHT add more weight to get the 2-3 at 4500. That would drop the 1-2 a little lower than I want, but I could compensate by Manual shifting.
Moving in the right direction!
It took some monkeying around and a couple tries but Ive got 1-2 WOT shift at 4500, chirps the tires real strong
2-3 WOT shift is at about 4750-4850 now, the last 300 rpm or so feels a bit wrung out, but it shifts strong.
I adjusted the Pontiac ratchet shifter and it works like a dream now.
I also noticed that I was about a quart low on DexMerc...yikes! That slow leak from wherever added up to some fluid loss.
I am going to screw in the adjustable modulator a little bit to tighten things up and get a crisper part throttle feel
This car gets up and goes! Feels much better with the shift point improvement. 4500 is the right spot. I MIGHT add more weight to get the 2-3 at 4500. That would drop the 1-2 a little lower than I want, but I could compensate by Manual shifting.
Moving in the right direction!
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