Alstrup
Samba Member
Joined: July 12, 2007
Posts: 7262
Location: Videbaek Denmark
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 Posted: Tue Apr 30, 2024 8:25 am Post subject: Re: 1915 Build Advice Wanted |
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| clockworkbox wrote: |
Going nuts trying to figure out how the cr can possibly work on this 1915…
I have ordered a set of Tims stage 1 heads which Greg tells me have 60cc chambers. As I understand it, I’m looking for about .05 deck height. When I punch the numbers into the CB engine calculator, this gives me a cr of 8 which is too low. If I have Greg cut the heads to give me 55cc, that gets me to 8.5cr but I think this may still be too low. I’m also unsure of how to get the deck height to .05, there is a chart here that says even with a 1.5mm decking of the case I would still be at .230… If I bump the crank up to a 74mm and drop the head cc to 55 I can hit 9cr but I don’t understand how you guys make a 69mm crank hit a reasonable deck height and cr. I’m selling off as much stuff as I can to fund this snowball. Can I run a 74mm crank without clearancing the case? Will the dual 40 Webers still be ok? Am I heading out of the engine reliability zone? There is a lot of conflicting information…
Here is where I’m getting the theoretical deck height projections from:
https://www.thesamba.com/vw/forum/viewtopic.php?t=612050&highlight=deck+height+projection |
As we have discussed about a million times already, a tight deck height is paramount for getting an engine which works well, no matter what the old school people will tell you. ANYTHING above 1,3 mm deck (0,051") and your mixing "ability" or quality decreases rapidly and so does the engines overall efficiency. so that is point No 1 to be aware of, hence my earlier note about decking the case.
Getting the correct volume in the chambers is merely a question about flycutting the heads (within reason) If the heads are 60 cc´s to begin with and you need say 52 cc in the chambers then the heads need to be flycut something like 1,5-1,7 mm. No big deal.
I just had a discussion with Brian about this the other day, and I know it is not mainstream thinking, but i would definitely recommend you to get the Web 218/119 variant for several reasons. 1. If you intend to use the 1,25 rockers you will bleed off way too much DCR with a srraight 218, so you would need towards 9,5 static to maintain a decent "real" compression when the engine is running. 2. Most heads on the market apart engine specific ported heads, flow too much on the exhaust side, so on mild engines like your aim it is beneficial to reduce exhaust flow a tad in order to get more usable power in the driving rpm on the cost of a little rpm power (if any) after peak. Splitting the valve lift has so many advantages in engines like ours, but it is rarely used.
In cases like Challomer´s, the heads are being ported to the engine, and the use of a relatively small, but well designed exh. valve- and port will flow plenty, and still achieve something like 75% I/E ratio, maybe a hair less. This is good for plenty of power in the driving rpms and also good off idle behavior. With the 1955 cc displacement you - do - have a good deal of cc per cylinder, so there the relatively high lift of the exh valve serves as if the cam had a "cheat" built in, meaning as if the cam were flat on max lift. Here we just lift it a little above where the port peaks, which will be around 11,5 mm (0,460") in order to get the burnt mix out in the given time of the cam duration. - In Challomer´s case the carbs will be the limiting factor, but with a little tickle here and there 36 Dells can support a solid 130 hp so it wohnt be a dawg thats for sure.
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https://www.thesamba.com/vw/forum/viewtopic.php?t=435993 |
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