"690" heads on a 348??

[RCE1962]

Just a thought.

A 348 stroker with 690 heads + (2X4) 409 intake???

I know the heads are interchangable, but they would require a compatible 409 intake. I've searched for various threads hoping for some help but my search found that the appropriate threads are basically inconclusive. Maybe I'm searching under the wrong prompts.

The guy I bought the heads from said they would work on the 348 but I would experience the engine running ±20 degrees hotter than normal. Something to do with water jackets and welding etc.

If this set up would work OK..I suppose a larger oil pan, larger exhaust manifolds, would be in order too. I'd like to keep it looking "stock" if I can. But I'd like the engine to have some "attitude" as they say. I don't mind shooting rabbits with a bazooka.

I don't think I'm going to be too successful in finding a decent 409 block...so maybe back to the original plan with the engine that is already in the car. If so...hope to get Aubrey's lightened pistons, good rods and get a decent cam.. If this works..I'm sure I'll know, soon after, where the weakest links are in the drivetrain.

Ron

 

[Impalaguru]

The heads will for sure bolt up, but I'm not sure if the smaller 348 bore will handle the larger 690 valves. I guess it depends on what your bore is. For some reason I think that the exhaust valve my hit your cyliner wall. I'm sure others, more knowlegeable than I, will chime in. Best of luck!!
Ross

 

[MK IISS]

In the 70s a friend of mine put 690 heads on a 348. When he started the engine it bent all the exhaust valve pushrods. The valves were hitting the cyl wall. This what he told me...I didn't actually see it happen.

 

[SteveD409]

I have an 817 head with 690 size valves installed and the valves seem to just barely clear on an .060 over 348 block. But I don't know if the valve spacing is the same between the two heads and I have only checked on ONE cylinder.

SteveD

 

[models916]

valve spacing?

I thought I read somewhere that the valve spacing is different on the hipo heads for the 409 to fit larger valves. If that is so, exhaust valve cut in block might have to be enlarged. 690 heads on a 348 will have the same problems as the BOSS 302 had with drivability and low speed. Low intake velocity at sub-7,000 rpm. Valves were too big. You could spin a BOSS 302 to 8,5000 and run a 4 series gear, but a 348 is not going to be happy past 6,000. Port size needs to compliment the the size and speed of the engine. Just my thoughts on it.

 

[Ronnie Russell]

First, for the original question--Let me say " I dont know". I cant see the valve spacing being different. W heads are interchangable , if the spacing was different then the pushrods would not line up in pushrod slots centered. The big problem Isee is the ex. valve to cyl problem. A rough mock up would quickly show where the valve hits. Probably bottom of ex. notch. Seems that this could be easily relieved with die grinder. I have to agree that 690s on a 348 would not be a great street motor. You would want to use a decent cam to take advantage of the 690s . All this adds up to good hp at high rpm, but not so good at low to mid range. No doubt that Aubrey has at one time or other, use 690 on 348. Im sure he will jump in and provide info.

 

[models916]

still not clear

I am not clear on the valve spacing thing. If the 379 heads and the 690 heads have the same valve spacing, it would mean you cound in therory put the biggest 409 valves in the smallest 348 head. I was thinking the spacing change was at an angle up and down more than sideways, just to clear the larger valves. I don't know where I read that, maybe I'm thinking about somethin else altogether. Anybody got a solid answer on this, heads to measure? Thanks

 

[Firepower354]

A couple thoughts from an airflow standpoint:
If the walls need notched to clear, it's likely that no air is flowing on the shrouded side, thus killing any imagined benefit. Also adding big valves to a small port will hurt flow unless the bowl below is opened to about 85% of head diameter(without getting in to the wet part of the head) and the transition to the port, especially the short side is smoothed out. With a mild lift cam, the port area is usually greater than the "curtain area". The flow isn't through the center of the valve, just the perimeter. Even though I won't have the new flowbench done until mid summer, I'd bet that a set of low perf heads with an 8 hour port job, necked down valves, and a good blended valve job would make the best "under the curve" flow and power on anything short of a big incher. Admittedly I don't have a lot of W flow experience yet(picked up some scrappers to chop up and grind on), but flow quality is as importaint as quantity: my Vortec head SBCs make 425-450hp with 175cc ports and 1.94 valves, over 500 on a 496BBC with 2.06s in oval ports, and the torque curve to shove a heavy ride down the track on a tight converter and mild gears or tow a heavy trailer like those smelly smoky compression ignition things.

OK, it's cold as heck out. All we need to solve the valve to bore clearance issue for sure is a junk block, a few Sawzall blades, and some grown-up beverages..........

 

[Mr Goodwrench]

firepower you made good points.... while I am no expert I did work with a guy that had a flow bench and was really into cylinder heads. he used to bring in heads that had been ported by "hot rod magazine cylinder head specialists" and show us how a little bit of material removed from the wrong locations KILLED the flow. his words of wisdom were bigger is not always better. I've read some posts here about some members having valve seats cut on a serdi. we got a new serdi and it was an excellent machine, but would not hold the valve seat concentric by as much as .015 to .020. finally figured out the old steel pilots we were useing were flexing opposite the cutter. bought some new carbide pilots (very stiff) and fixed the problem.

 

[Ronnie Russell]

MrGoodwrench, You are correct about " hurting air flow instead of helping" when port work is done haphazardly. Much good can be done porting "little" heads if you work on the roofs and only port match the entries. Little can be done to 690s in the port area to improve flow. Only the most experienced ( not me ) with a flow bench can improve 690 ports. Basic port matching (gasket) is all that should attempted by the in-experienced. However, I think all the heads can be improved by bowl-blending, This is tedious, time consuming work, but by smoothing entry and exit from bowl area , I think only good can come as long as you dont hit water. Thats the trick , how far is too far? As far as firepowers post, Have you ever wondered why 817 and 690 share the same size exhaust valve? Looks like the designer would have increased valve size for the 690 if they could. Ive heard the "big boys" say that anything over 1.75 is not good because of valve being shrouded, not enough room between valve and cyl for breathing, just as firepower said. Probably the reason ex. ports in 690 is huge compared to the 817(same size valve). Modes, Im sure the valve spacing for the heads are the same , valve stem center to valve stem center. However, Ive never thought about valve angles. Interesting question.

 

[Firepower354]

Yep, gotta use carbide pilots to get 'em round! And they gotta fit just right. Pricey little suckers, but they last till you tunk one and break it.

As far as size, we filled nearly 20% of the floor of SOHC 427 F-word heads to pick up a ton of cfm(still could stuff a dead cat through them), same with 426 hemi exhausts, F Cleveland 4v intake and exhausts, BBC exhausts, Olds exhausts, 292 GMC lump-port, and so on... The velocity probe said some of those spots had air going in circles or practilly stalled. The short turn is the most critical part. The air will follow a gentle curve, but fly right over a sharp edge and head for the opposite side of the bowl, just like my worn out old van...

 

[Mr Goodwrench]

ronnie you are dead on on the bowl blending, the guy I was talking about used to map out all those diff seat angles and order that cutter. That alone improved the flow on most heads without even touching the ports.

 

[dq409]

MrGoodwrench, You are correct about " hurting air flow instead of helping" when port work is done haphazardly. Much good can be done porting "little" heads if you work on the roofs and only port match the entries. Little can be done to 690s in the port area to improve flow. Only the most experienced ( not me ) with a flow bench can improve 690 ports. Basic port matching (gasket) is all that should attempted by the in-experienced. However, I think all the heads can be improved by bowl-blending, This is tedious, time consuming work, but by smoothing entry and exit from bowl area , I think only good can come as long as you dont hit water. Thats the trick , how far is too far? As far as firepowers post, Have you ever wondered why 817 and 690 share the same size exhaust valve? Looks like the designer would have increased valve size for the 690 if they could. Ive heard the "big boys" say that anything over 1.75 is not good because of valve being shrouded, not enough room between valve and cyl for breathing, just as firepower said. Probably the reason ex. ports in 690 is huge compared to the 817(same size valve). Modes, Im sure the valve spacing for the heads are the same , valve stem center to valve stem center. However, Ive never thought about valve angles. Interesting question.

Interesting info !!!
I think you will find that Rich (alum heads) has worked very hard on correcting this problem on his new heads.
I won`t get into it as Rich is the one to tell us what he has done.
I`ll be working with him shortly on getting on the forum here and explaining whats coming down the pike with his new stuff ,,,dq