A.k.a.: Col
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posted on April 22nd, 2003 at 11:32 AM
Adz, the single-port heads only create more power at low revs because they have a smaller port cross-section (ie. they keep the inlet mixture velocity
higher which helps to keep the fuel in suspension and aids mixture). At higher revs, single ports suffer because there is a limit to how fast the
inlet mixture can move into the cylinders, caused by turbulence, cavitation, blah, blah blah.
Twin-port heads have a larger intake cross-section, which reduces the intake velocity, allowing more air to flow before reaching the limit of flow. Of
course, this loses power at lower revs because there isn't enough velocity to get the necessary mixture effect.
Basically, anything you do to increase the flow of your single-port heads is going to have a similar effect to replacing them with TP heads.
Ps. While it is true that SP heads are leass prone to cracking than TP heads, this will not necessarily be the case when you've finished
removeing metal from the SP's ports to make them behave more like TPs.
Kruizin Kol
aussiebug
A.k.a.: Rob Boardman
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posted on April 22nd, 2003 at 02:36 PM
The other problem with sp heads is that you can use only the smaller 30 series carbs on it - the larger 34 won't fit on the smaller flange
manifold.
The 30PICT/2 etc works OK up to 1600cc (the 68/69 Kombi used this combo, and the 1970 USA bug had a 1600sp with 30PICT/3 carby), but if you go higher
capacity than that it starts getting seriously undercarbed, which means you start to loose the advantage of a larger capacity anyway.
So if you are thinking "cheap power" about the cheapest you can get is a set of 1776 P&Cs (yeah I know - they aren't cheap!), with
twin port heads and a stock 34PICT/3 carby. That way you get a noticable lift in power over the stock 1600dp engine, and then have the dp heads
already for when you want even more - the dp heads are much better for twin carbs and such than the sp heads.
But I would not be doing that with a 1500 H series case. It's has the older metalurgy - the high magnesium content makes the case more prone to
going brittle as it ages (cracked cases), and it's a single relief case too. It also has the non-doghouse cooler which would need converting to
doghouse if you are going over 1600cc, and then you still have the 8mm oil galleries; where the 71+ cases have 10mm galleries, dual relief and
doghouse fans - AD series or later). And the 73+ cases are better again - they have an AS21 or AS41 stamp on the right side of the case - this means
they have a higher aluminium content (about 4% v 1% for the older cases) which makes the case less prone to cracking and more suitable for higher
hp.
For the H series case, the best increase is a set of stock 1600 P&Cs and keep everything else stock - sp and 30PICT/2 carby. The hp increases
from 53 to about 56-57 - certainly enough to notice but not too much stress on the case.
I have this combo on Bertie Beetle, and that engine has 248,000 miles on it (just the one rebuild - when it became a 1600).
[Edited on 22-4-2003 by aussiebug]
Rob
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posted on April 23rd, 2003 at 02:43 PM
1500 engines with 1600 B&C's
The engines in my Ghia and 65 Splitty are ex-industrial engines which had 1500 heads on a 126 case with 1600 Barrells and Pistons. I run a standard 30
PICT carb, and these engines run sweet. Lots of low down torque to get the kombi moving, and they buzz along fairly unstressed.
In this state they would serve more miles than a 1600DP as the heads have more meat on and are less likely to crack.
Hey Rob, Any idea what the fuel performance figures/power output figures are for this combo versus a stock 1600DP?
