5A heads on my 307
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5.0 and 5.7 Discussion, 5A heads on my 307 in Cadillac Engine Technical Discussion; Hey guys, I'm looking at getting some 5A heads and intake for my 88 brougham. I already have a cam ...
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    pimpin88's Avatar
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    5A heads on my 307

    Hey guys, I'm looking at getting some 5A heads and intake for my 88 brougham. I already have a cam from the vin 9.

    I just want to know if it's worth me putting these parts on there. How much power would I really get out of doing that.

    And I know the best way to get more power is to swap in a 350 or bigger, but for right now, I'm gonna stay with the 307.

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    Johnny Bravo is offline Cadillac Owners Fanatic
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    Re: 5A heads on my 307

    Quote Originally Posted by pimpin88 View Post
    Hey guys, I'm looking at getting some 5A heads and intake for my 88 brougham. I already have a cam from the vin 9.

    I just want to know if it's worth me putting these parts on there. How much power would I really get out of doing that.
    And I know the best way to get more power is to swap in a 350 or bigger, but for right now, I'm gonna stay with the 307.
    5A heads along with a vin 9 cam is worth an honest 40 hp at least.
    However, it's a major job.

    Also, you'll need the matching intake, exhaust manifolds from a 5A 307, mabey new pushrods as well.

    The '88 307 is roller cam motor so I hope you have a vin 9 roller cam.

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    Re: 5A heads on my 307

    Yep, I have a vin 9 roller cam. It's been in the car for about a year now. Huge power difference. I was just looking a getting some more out of the 307 without swapping it out.

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    Johnny Bravo is offline Cadillac Owners Fanatic
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    Re: 5A heads on my 307

    5A heads alone should be worth another 20 hp at least, especially in the upper rpm range.
    Again, it's major job and I wouldn't swap heads without getting them rebuillt: Three angle valve job, valve guides, springs and resurfacing. That could run between $500-600.

    However, it possible to pick up a few more horses in the process with some valve bowl porting and exhaust AIR bump removal. Resurfacing will bump up compression a bit too. Whether it's worth it or not is a decision you'll have to make.

    Another excellent performance upgrade, that is a heck of a lot easier and cheaper than changing out cylinder heads is swapping in a complete 3.08 or 3.42 ratio rear.

    Many, if not all, early to mid '80's RWD Devilles with the 4.1 engine had 7.5 inch 3.42 rears.
    And, it's not too difficult to find later FWB, Buick Roadmasters or Chevy Caprices with 3.08 or 3.42 ratio, heavy duty 8.5 inch rears in the junkyards.
    Although, you'll need the driveshaft from the donor car as well for that one.

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    Re: 5A heads on my 307

    I have a rear end from my last roadmaster sitting in my shed and it has 3.08's that I was looking at putting in there. It has a nice thick rear swaybar too. I asked the guys over on the impala forum if the whole axle would fit in and they havent responded yet.

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    Johnny Bravo is offline Cadillac Owners Fanatic
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    Re: 5A heads on my 307

    Quote Originally Posted by pimpin88 View Post
    I have a rear end from my last roadmaster sitting in my shed and it has 3.08's that I was looking at putting in there. It has a nice thick rear swaybar too. I asked the guys over on the impala forum if the whole axle would fit in and they havent responded yet.
    Well, the rear is a direct interchange, but there may be an issue with the lenght of the driveshaft. You'll also need to bleed the rear brakes afterwards of course, but thats a good opportunity to flush out the old brake fluid.

    Also, check the RPO codes decal on the underside of the trunk lid to make sure you don't already have a 3.08 ratio.

    You may want to post on Oldspower.com, there is a more activity over there and this is an Olds powered vehicle afterall.

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    N0DIH's Avatar
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    Re: 5A heads on my 307

    Basically there is a few shafts.

    8.5" axle, THM200 4R, D Body (Fleetwood/Brougham)
    7.5" axle, THM200 4R, D Body (Fleetwood/Brougham)
    8.5" axle, THM400, D Body (Fleetwood/Brougham)
    8.875"/8.75", THM400, D Body (Fleetwood/Brougham)
    8.5" axle, THM200, D Body (Fleetwood/Brougham)
    7.5" axle, THM200, D Body (Fleetwood/Brougham)
    8.5" axle, THM700 R4/4L60/4L60E, D Body (Fleetwood/Brougham)
    9.5" axle, THM700 R4/4L60/4L60E, D Body (Fleetwood/Brougham)

    (of course, you can add to the list is replace D with B body and sub out the 9.5" and you have the shorter shaft cars...)

    IIRC, the THM350 was never installed in a D Body. The exception might be the 4.1L V6 cars. I honestly haven't checked....

    Yes, the mounting are all the same. So interchange is ok. But JB is correct, driveshaft if the real issue. Some minor issues of brake line fitment too. But minor....

    Search car-part.com and you can get a good idea of interchange.

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    Re: 5A heads on my 307

    Are these 20/40hp gains with the 5A heads dyno-proven? The reason I ask is the '84 307 engine was rated at about 145 hp and 245 lbs. of torque. The '87 307 engine was rated at about 140 hp and 255 lbs. of torque. Having driven both (in an '84 Cutlass and an '87 Regal), I can say that the '84 engine seemed to have a LITTLE more juice in the "upper" RPM range (if you can call 4500+ "upper"), but the '87 was way more torquey, and was much smoother.

    In other words, the 5A headed 307 was technically rated at about 5 more hp, but with less torque, compared with a 6A or 7A headed 307. So I was wondering about those claims of at least 20 hp just from the head swap alone. The 5A headed VIN 9 307 was rated at 180 hp, and the 7A headed VIN 9 307 was rated at 170 hp. I wouldn't think the heads alone would be worth more than the 10 hp rating difference, but I won't argue with a dyno either.

    So do 5A heads really turn an '87 VIN 9 engine from 170 to 190-200+ hp?

    By the way, it's true that you do need new exhaust manifolds. Tip: the 350 diesel manifolds are exactly the same as the older 307 manifolds...just without the O2 sensor bung drilled and tapped. If you were going to all the work to swap heads, I'd just slap a 403 in there. I swapped a 403 in for the 307 in my '87 Regal and I guarantee it did more for that car than different heads ever would have.

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    Re: 5A heads on my 307

    (But you know this right Jason? Welcome back!)

    GM spec'd the 1985-1990 307 7A head with "Y" cam as 140hp
    GM spec'd the 1983-1987 307 7A head with "9" cam as 180hp

    Yes..... GM dyno proven.... But that hp number was always the same with 5A and 7A heads. My bet is 180 hp is still with 5A's and less with 7A's.

    The 1981-1985 307 "Y" was rated at 150 hp (didn't you have this listed somewhere from Flash?). The 1985-1990 307 "Y" was rated at 140 hp. The 7A heads to 5A heads are worth a good 10hp, probably more if you ask me! The flow is significantly better on 5A's. I put a 1982 intake on my 3A headed 350 and it was a sweet OEM swap. Got it for $15 at a boneyard. Don't waste $$ on the aftermarket intakes unless you are going to hop it up more. Just not worth the $$. But it might help some, maybe with a "9" cam more. But the Edelbrock intakes won't line up to a 7A head, ever.....

    Considering 180hp out of a 307 and the 350 in late 70's was only 170 that isn't too bad for a 5L. But my 350 smoked my 307 by a long shot, stock to stock.... And when I put in the aftermarket cam, even more so....

    I still say a 307 can be built. People don't give it much, but I say yes. The LS1 V8's don't have a large bore, neither does the 307. Sure, you will lose bottom end some, have to, but you aren't gonna be running 2.56's with it either. Gear swap is mandatory. 3.42's and a 307 would do well, with at least the 442 cam ("9" cam), decent dual exhaust, older cast iron manifolds or headers, setup the Q-Jet like a 442. This alone should get you into the 250 range. Look at the 4.8L Chevy V8. 290 hp from that tiny engine. Any engine can do it, just need to do the work on it.

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    Re: 5A heads on my 307

    Oh yeah, I know the CAMSHAFT change (VIN Y to 9) is worth 40 hp, but I was questioning the head swap alone. I read above where someone stated that the 5A heads alone should be worth 20+ hp, and I just don't see it.

    I really like the 307 engines. The one in my '87 Regal (a roller engine) was VERY smooth, even though it had a lower end knock (wrist pin I think). The older gen 5A 307 in my '84 Cutlass felt a bit "rougher"...and it should have, without the high turbulence heads and non-roller cam. I actually preferred the newer engine, due to better smoothless and better low-end torque (but I know the 3.08 gears, vs. 2.56s in my Cutlass helped with that). I know that with a buildup, the small square ports of the 6A/7A heads won't cut it.

    I still question the 20+ hp claim from heads alone.

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    N0DIH's Avatar
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    Re: 5A heads on my 307

    20hp, no, 10, sure. I agree completely. I always wanted to build up a 455 with 7A heads. I know it is crazy sounding, but my goal was monstrotious torque and 25 mpg not overall massive power. I never did get to do it. Maybe someday... The velocoty on those heads is fantastic. I think with some good head work the 307 7A heads could do wonders.

    BTW, was a set of 6A's ever seen? I have never seen a set of them. Just wondered what they really were different from the 7A's. AIR maybe?

    I have never driven a 5A headed 307. I wanted to get a set for my 350 to bump up compression and toss in some larger valves.

    My 307 with 7A's are very smooth even at low rpm with 2.14's nearly lugging the engine. It did work well for low end torque.

    Take some 7A's and modify them with tall ports and put on EFI and you have a good street head.

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    Re: 5A heads on my 307

    I think 6A heads were 1985 only...at least that's the rumor, from what I remember. I can't remember actually laying eyes on a 6A head, but I guess I never questioned their existence either. I understand that they're also small port, high velocity heads...very similar to the 7As. I don't know about any specific differences.

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    N0DIH's Avatar
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    Re: 5A heads on my 307

    I had always thought 6A's were 7A's without the AIR passages and rumored to be seen in Canada only. Which would explain why I haven't seen one.

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    Re: 5A heads on my 307

    Quote Originally Posted by N0DIH View Post
    ...and rumored to be seen in Canada only...
    Yes, I think I do remember that.

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