40hp and duel port heads

Who is the best person to rebuild your engine? You...

Moderator: sparkmaster1

Post Reply
madst3
Posts: 2
Joined: Thu Dec 24, 2009 10:37 pm

40hp and duel port heads

Post by madst3 » Tue Nov 08, 2016 5:55 pm

Can duel port 1600 heads work on a stock 40hp

User avatar
FJCamper
Moderator
Posts: 2690
Joined: Wed Nov 14, 2007 5:19 pm
Location: Birmingham AL

Re: 40hp and duel port heads

Post by FJCamper » Thu Nov 10, 2016 3:11 pm

Hi Madst3,

No, the 40-horse 1200cc cylinder size and spacing won't take dual port heads. The 1300/1500 cases will take the 1600cc cylinders and dual port heads.

FJC

User avatar
Marc
Moderator
Posts: 23731
Joined: Thu May 23, 2002 2:01 am
Contact:

Re: 40hp and duel port heads

Post by Marc » Thu Nov 10, 2016 5:05 pm

The answer is YES...but with some caveats. The cylinder spacing of a 40HP is identical to that of a 13/15/1600 - it's the 36HP which is tighter. 36HP cylinder spigots are only 81.8mm OD; 40HP are 87mm, and 13/15/1600 are 90mm. The stud spacing is the same on the latter two.

The factory made dual-port 1300cc engines (also with the 77mm bore and 89.8mm upper spigot OD of the 40HP). The heads were essentially identical to 1600DP heads except for the smaller cylinder register and somewhat smaller chamber volume (necessary to maintain a serviceable compression ratio with the smaller displacement). They also used end castings with a smaller ID, and an intake manifold center section made from smaller tubing with a smaller carb flange for the 31PICT-3 carb - had they used "normal" 1600DP components there, the infamous dualport flat spot would've been made even worse...the smaller intake tract was used to keep flow velocity and vacuum up.

You asked if DP heads could be used on a stock 1200. The chamber volume of a typical 1600DP head is ~51.5cc. so even at a relatively tight .040" piston deck height the compression ratio would be 6.3:1. To get it up to the ~7.64:1 of a stock 1600DP you'd need to flycut the heads about .120". That'd provide an opportunity to "step-cut" them from the stock 93.8mm diameter down to the 89.8mm of a 1200 cylinder, but it'd also be bringing the intake valve that much closer to hitting the cylinder so that clearance would need to be checked. If you were to use stock 1600DP endcastings and (shortened) center section the diameter would be too great and it'd have a bad stumble...with the 1300DP parts it shouldn't be too bad, but personally I'd go with a pair of 32PDSIT Type III carbs. 1967 Type III manifolds would fit but they're too short for Type I sheetmetal; you'd need to use a spacer, or make a trench in the overcylinder tin on one side to clear the accelerator pump linkage. Taller manifolds for the 32PDSITs exist but are extremely rare (they were used briefly in Brazil); the more-common tall manifolds made for 34ICT Webers have a wider carb stud spacing and would have to be modified. You might be able to run 34ICT carbs and manifolds but I think they'd be a little big for 1200cc.

40HP heads have 32x30mm valves; 1300SP have 33x30. But the 1300DP heads used the same 35.5x32 valves as 15/1600 heads, and they won't fit inside a 77mm bore. With a stock cam and uncut heads, the intake valves did not open far enough to contact the cylinders - but if you increase the lift and/or flycut the heads it will become an issue.
35.5x32 valves will just fit inside .040"-over cylinders, but those are rare as hens-teeth anymore.

My first 1300cc race engine used a set of Euro-spec 1300DP heads, flycut clear to the sparkplug hole (18½cc). The pistons had to be notched for the electrodes as well as for the valves - that yielded a 12½:1 C.R.
Valve lift was .425", and in order to provide intake valve clearance I had to grind back each cylinderwall slightly, being carefully to not get into the zone that the top piston ring reached at TDC. The valve shrouding was horrendous, of course, but rules limited the bore to 78mm so that just had to be lived with.

All of these valve-to-cylinder clearance concerns would go away if you were to use 82mm or 83mm "Big-Bore 40HP" pistons and cylinders; the 1352 or 1385cc displacement would change the math favorably on the compression ratio, and be large enough that 34ICTs should work fine. You'd still want to flycut the heads ~.050" to raise the C.R. and provide the proper diameter to center the smaller-OD cylinders, but that shouldn't be enough to introduce issues with pushrod length and exhaust system fitment that you'd need to deal with at .120".

https://aapistons.com/collections/vw-pi ... g-bore-kit

User avatar
FJCamper
Moderator
Posts: 2690
Joined: Wed Nov 14, 2007 5:19 pm
Location: Birmingham AL

Re: 40hp and duel port heads

Post by FJCamper » Thu Nov 10, 2016 7:05 pm

Hi Marc,

Yes, it was the 36hp I was thinking of.

Maybe Madst3 has a chance after all.

FJC

madst3
Posts: 2
Joined: Thu Dec 24, 2009 10:37 pm

Re: 40hp and duel port heads

Post by madst3 » Sat Nov 12, 2016 12:44 pm

Thanks for the information we have 2 original 40 hp motors and many later duel port heads . We have one of motors right now in a tub buggy with an early turbo set up from the 60s and looking for options to add a little more fun

User avatar
Marc
Moderator
Posts: 23731
Joined: Thu May 23, 2002 2:01 am
Contact:

Re: 40hp and duel port heads

Post by Marc » Sat Nov 12, 2016 2:58 pm

I forgot to mention - L-Jetronic heads (`75-`79 Type I) are dualport, but have the smaller 33x30mm valve sizes of a 1300SP - that would eliminate all of those valve-to-cylinder clearance worries. They have a ~1½mm step where the 1600 cylinder seats that would provide more of a centering ring once they were flycut to the smaller diameter of the 77mm cylinders. The smaller intake would suffer less from shrouding, and if you're going to run a turbo it shouldn't be a liability. The one thing I've never checked is if the chamber wall on the intake side is laid back any further than on a "normal" DP head, which could prevent their use with smaller-O.D. cylinders. Even on normal DP heads the chamber overhangs the cylinderwall by about half of its width, but they still seal OK - it just leaves an ugly edge hanging out if you don't blend it back.
Those heads do use unique exhaust valves, however (30mm head but 9mm stem). They're still available, or you could use conversion guides and run 8mm-stem valves. Berg used to sell those for use in the `74 043 35.5x32 heads which also used a 9mm-stem exhaust valve, although I don't see them listed at their web page.
I don't recall the specifics but I think there're some Type IV intake guides which have the same O.D. as the 9mm guides...if you found some which had too large of an O.D. that could be rectified on a lathe.

Post Reply