generalmopars

hey every one, im getting amped up to swap my 350 inplace of the 307. I finally got all the parts to finish it. so heres whats going on, its a 1990 307 single exhaust and its going to be true duels for the 350. I just got thornton 350 jrs shipped in for the new exhaust. I have a new cam in the 350 that needs broken in and I don't want to run it without an exhaust. Im thinking since my 7a exhaust ports are smaller than the 350 ports I could get away with ripping the whole exhaust off the car and put the torntons on the 7a heads so I could drive it over to the exhaust shop down the st and they could build the head pipes to fit to the new manifolds and they could also build the whole exhaust in one shot. after its done I could put the thorntons on the 350 when its ready to go into the car and they would already be lined up to the new head pipes. I know it sounds dumb but im not towing a car to the exhaust shop because a cam isn't broken in yet. also driving to and from the shop are the only miles the car would be driven this way. what im not sure of is, are the bolt locations in the same place for the 7as and the older type heads? if they are I should be fine because the big ports on the manifolds should completely cover the little ports on the heads...right???

svnt442

Nope...nope....nope. If you have a flat tappet cam you MUST break it in before you drive it or you run a major risk of destroying it.

redoldsman

Tow the car or ruin a new cam. Seems like a pretty simple choice to me. You only have to run the car for 15 minutes to break the cam in.

generalmopars

No your not hearing me. I bolt the Thorntons into the 307, I get the exhaust installed, then I put the 350 In.. This way the pipes are bent to the 350. So I can brake the cam in on a complete exhaust. Will the thorntons bolt into my 7a 307 heads so I can take it to the exhaust shop??? I know they have different ports and I know the set up will be incorrect but I want to use the 307 as a template to anchor the thorntons where they need to be for the 350. Please read what I am saying before you jump the gun.

svnt442

OOOHHH...sorry, I misunderstood. I'm just making a guess here, but the listing for exhaust gaskets include the same number for a 90 307 and a 76 350 sooo.....go for it I say.

joe_padavano

There is a good possibility that the early style manifolds won't seal on the 7A heads. The 7A heads have material missing where the early manifolds would seal. I don't think this is going to work. They will bolt up and will be in the right place for the exhaust to be fabricated, but they will leak at the head.

joe_padavano

By the way, a "duel" exhaust means the pipes are shooting at each other. A "dual" exhaust means that there are two pipes.

generalmopars

If they will bolt up I'll be in the good
If they leak I'll be ok wi h that, as long as they are in the right position. Also they will be separate pipes so I guess a dual exhaust.

joe_padavano

The position will be correct. I guess the leak doesn't matter if there are no pipes attached to them on the drive over.

generalmopars

Thanks joe, thats all I was unsure of. I thought the holes for the bolts were in the same location but wanted to double check. It should be a fun drive over btw lol. In case anybody's wondering I'm using 2.5 dynomax ultraflo welded mufflers with no cats, should sound pretty good... I hope. Also using tails, I'm tired of axle dumps I've had on previous cars.

olds 307 and 403

They will bolt on but will need gaskets to seal. A 86 442 owner on G just got his original 7A head 307 rebuilt. He did all sorts of no no's. He used an Edelbrock 500 cfm carb, 2711 Performer intake and Thornton manifolds with dual exhaust. Professionally rebuilt and tuned. He can not believe how much power it has now, go figure.

jetfire88

Just a comment for future reference.

If you're having an engine rebuilt and the shop has a dyno, have them do the break-in on it.
I had my 455 done that way, then when it got in the '92 we could fire it up and dial in the TBI without worrying about break-in issues. Saved a lot of anguish.

Good luck on the swap!

generalmopars

Just wanted to follow up, I had the manifolds and exhaust installed a week ago. It worked perfectly with no gaskets. I pulled the motor and trans yesterday and left the manifolds attached to the pipes. I can see clearly looking at the mating surfaces from manifold to head there was absolutely no leak lol. I say SUCCESS !!!

olds 307 and 403

Awesome. The other 307 guy needed gaskets to seal.