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esde Samba Member
Joined: October 20, 2007 Posts: 5969 Location: central rust belt
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Posted: Thu Dec 29, 2022 8:02 pm Post subject: 36hp, domed pistons or smaller chamber? |
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I am looking at my pile of parts, contemplating how to squeeze a little more out of my next 36hp rebuild. I have a set of nice nos Kolbenschmidt domed pistons, and a set of newer flat top pistons. My last 36hp engine I used flat top pistons, and cut each head several mm to get compression to 8.1:1, here's the cut head compared to stock
the downside to this was shortening the intake manifold, muffler, and pushrods, which was a bit of work.
So, now that I look at this in retrospect I wonder what (if any) real world difference there might be between using the domed pistons, or running the flat top pistons and a smaller chamber? Using the domed pistons sure would save a ton of other work, but it seems like the flat top piston/ smaller chamber has to be better.
Any thoughts appreciated.
SD _________________ modok wrote:
Bent cranks are silent but gather no moss. I mean, ah, something like that. |
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Robert Chambers Samba Member
Joined: August 11, 2005 Posts: 216
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Posted: Fri Dec 30, 2022 10:30 am Post subject: Re: 36hp, domed pistons or smaller chamber? |
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The VW factory used domed top pistons on the 36 HP bus engine for more compression to achieve more torque and power. I assume production cost was the biggest contributing factor building millions of units?
Also with your builds, dual carbs with domed pistons would solve the problems of engine width while increasing compression. The problems could be engine originality and cost of dual carb sets. _________________ '63 OG 117 Herbie Replica
'56 Euro Oval Deluxe
'52 Standard Zwitter
'63 OG 117 |
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Bub Samba Member
Joined: June 10, 2004 Posts: 1154 Location: Central Washington
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Posted: Fri Dec 30, 2022 10:40 am Post subject: Re: 36hp, domed pistons or smaller chamber? |
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I honestly think a decent 36hp can go higher in compression and still be super safe and reliable. When I was building my big-bore i found some people running over 9:1 with no complaints. I built a 1.4L w/ 40hp / 83mm big bore and set it up at 8.6:1 and put nearly 3500 miles on it last year. Amazingly have had no problems- I mean I really had to butcher those cylinders to get them to fit, and they already have thin walls. Many of those miles were in the heat of the summer over 100* and it runs HOT, but really great.
Not sure if domed or flat would matter honestly? I like the idea of the domed top because they're less common, and I guess the purpose of the dome is to increase compression?
But I'd consider slightly higher compression myself. _________________
hitest wrote: |
Had a girlfriend once who shall we say, nearly arrived at the mere sight of a semaphore in action- easy to please she was... |
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petrol punk Samba Member
Joined: August 21, 2007 Posts: 975 Location: Las Vegas, NV
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Posted: Fri Dec 30, 2022 11:18 am Post subject: Re: 36hp, domed pistons or smaller chamber? |
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The problem with dome top pistons is finding them. As far as I know nobody makes them so you're stuck with searching for NOS parts. How much do they bump the compression? It doesn't look like much to me.
Are you running a stock 36hp with cut heads for more compression? Did it make it noticeably more powerful? _________________ 36hp '56 European DeLuxe oval, '70 bug 1835cc dual 36DRLA, and a '98 4x4 5spd single cab Tacoma |
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esde Samba Member
Joined: October 20, 2007 Posts: 5969 Location: central rust belt
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Posted: Mon Jan 02, 2023 6:30 am Post subject: Re: 36hp, domed pistons or smaller chamber? |
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petrol punk wrote: |
The problem with dome top pistons is finding them. As far as I know nobody makes them so you're stuck with searching for NOS parts. How much do they bump the compression? It doesn't look like much to me.
Are you running a stock 36hp with cut heads for more compression? Did it make it noticeably more powerful? |
I have a set of the factory dome topped 77mm pistons, which is what started me thinking. I haven't done any math to verify, but research suggests that they only bump the compression to 7.7:1, I'd certainly like to be at least 8.1
The 36hp that's got cut heads an d 8.1:1 is in my project split, and has only been down the block and across the yard so I don't have real world seat time in it yet. That said, it feels very strong compared to a stock 36, especially off the line. I have a new oval coming in the next few weeks and hope to install it so I can get some real extended driving time. _________________ modok wrote:
Bent cranks are silent but gather no moss. I mean, ah, something like that. |
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