650 coffman exhaust on 760 cylinders

[edog395]

So I have a 1990 superjet with a 61x single carb bottom that I grinded out to accept some 64U 760 cylinders. Ive been running it like with stock 61x exhaust for years now and its been ok, but Im going for more power and I have a full coffman exhaust off a 650 and want to put it on the 760. From the research I have done, it seems like I have an early 650 coffman manifold, the ports are small and narrow. Even if I got a 701 manifold I think I will still have to get it ported to match. I compared the exhaust gaskets from the 61x stock exhaust to the 760 exhaust gasket and 760 is still bigger. Just trying to see if anyone has done this, and what my options would be. Will I be able to have the 650 manifold ported to match the 760 cylinder? Besides the ports, is the rest of the 650 coffman MANIFOLD the same size internally as the 701, or is it bigger? Thanks in advance

 

[kawa js550]

Pretty sure you would have to drill out the manifold bolt holes to accept the larger 760 exhaust manifold bolts.

 

[edog395]

Yeah I drilled out the stock manifold to 10mm bolts back when I did the conversion. I just bought thread inserts to go down to 8mm so I dont have to drill out the coffman manifold.

 

[Storbeck]

I have not done what you are looking at doing, but I have put a 61x factory pipe manifold on a 760 cylinder (I think you mean 64x, but I'm not sure). I used helicoils in the exhaust manifold holes. Very easy, you can just screw 8mm helicoils right in with no modification, the holes in the cylinder are the same pitch.

I just "ported" the manifold slightly to match the cylinder, basically gasket mathched it, only took a little bit off of the top of the port in the manifold, maybe about 1/8 inch or a bit less, but if you have a 650 manifold it is probably going to be a bigger mismatch than a 61x, and I don't know how different the Coffman is compared to a Factory Pipe.

Another consideration is the size of the hole from the exhaust manifold to the head pipe. Old Factory pipe manifolds are 44mm, newer are more like 47 or 48mm. There are people who say the larger opening is better for 701+cc motors (some even say to go bigger, ~50mm). I don't know if the Coffman can/should be opened up, but it is another thing to look into, you're putting it on a much bigger more powerful motor than the 650 it was designed for.

 

[Storbeck]

Relevant:

http://www.x-h2o.com/index.php?threads/b-pipe-manifold-diameter.9561/

 

[edog395]

Interesting thread, thanks for the response and tips. Im going to take the manifold and head pipe to my machinist tomorrow and see how much he can machine out to match.

 

[Quinc]

Port match and open the headpipe and manifold up to 47mm.

 

[edog395]

Port match and open the headpipe and manifold up to 47mm.

Thanks, I measured it and its only 42mm on the coffman. 47mm might be a squeeze with how close the water jackets are but Im about to see this morning when I take it in

 

[Storbeck]

I would be a little careful because it's not just that point that needs to be bigger, if the whole rest of the manifold/head pipe is small in diameter and you just open up the end I don't think that is doing you much good.

I'm not an expert on the subject but that's my $.02 for what it's worth.

 

[TheBuzzard]

So I have a 1990 superjet with a 61x single carb bottom that I grinded out to accept some 64U 760 cylinders. Ive been running it like with stock 61x exhaust for years now and its been ok, but Im going for more power and I have a full coffman exhaust off a 650 and want to put it on the 760. From the research I have done, it seems like I have an early 650 coffman manifold, the ports are small and narrow. Even if I got a 701 manifold I think I will still have to get it ported to match. I compared the exhaust gaskets from the 61x stock exhaust to the 760 exhaust gasket and 760 is still bigger. Just trying to see if anyone has done this, and what my options would be. Will I be able to have the 650 manifold ported to match the 760 cylinder? Besides the ports, is the rest of the 650 coffman MANIFOLD the same size internally as the 701, or is it bigger? Thanks in advance

there aint no such thing as a 701 manifold for that 650 sizzler pipe. just put a 62t gasket up to the manifold, mark it and grind it to match. thats all that needs to be done. it'll add a bunch of power without spending any money at the machine shop. by the way, who's your machine shop? meticulous metals or somebody else?

 

[edog395]

there aint no such thing as a 701 manifold for that 650 sizzler pipe. just put a 62t gasket up to the manifold, mark it and grind it to match. thats all that needs to be done. it'll add a bunch of power without spending any money at the machine shop. by the way, who's your machine shop? meticulous metals or somebody else?

Gotcha I read a post somewhere that said the later manifolds were bigger for a 701. I guess not. But yeah thats what im going to do, try and match it. Its worth a shot since the pipe was basically free. My machine shop is a local old timer in my area who does mainly car motors, and my pops is friends with him so he hooks me up pretty good.

 

[MedicFF252]

Hey @edog395. Curious what did you end up figuring out with your manifold? Just took my Coffman pipe off and I have the same issue. From the research I’ve done, I think that might be why I have always struggled with a low speed hesitation I’ve never been able to fully tune out.