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Post by VW Toast on Feb 6, 2019 13:40:01 GMT
Mines got a bigger thingy in it
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Post by Zed on Feb 8, 2019 11:47:25 GMT
I needed 0.090” cylinder base spacers to get a 0.062” deck height and with a 57cc chamber volume I have a 9.4:1 static CR. What do you think Zed, is 9.4:1 too high for a C25 cam, what does it give me as a dynamic CR? I don’t want to increase the deck but I could open up the chamber a bit, if I have to. By the way I’m just about to purchase a set of new valves (41mm and 34mm), when Robert built your engine did he use standard valves, or stainless steel, or sodium filled? Or none of the above. Ah, well, you know...so many variables. So many different opinions on CR. If I could ask anyone I would ask Alstrup on Samba. Air-cooled.net recommend 7.5-8.1 as I'm sure you know. I was "off on one" earlier in the thread but I can't raise the enthusiam to go back calculating...the moment has passed.
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Post by Zed on Feb 8, 2019 12:08:42 GMT
I've just read back a few pages and...it all comes down to what you believe is the valve closing point. What does worry me about these theoritial DCR numbers is that maybe they could be correct at very low revs, but when the engine is "on cam" the DCR could be a lot higher. Too high? Therefore I would err on the safe side and dish your pistons ..or something to get the CR down. Me? I'd want 8-8.3:1 static, or even less. But I don't actually KNOW of course.
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Post by 77westy on Feb 8, 2019 20:13:54 GMT
I've just read back a few pages and...it all comes down to what you believe is the valve closing point. What does worry me about these theoritial DCR numbers is that maybe they could be correct at very low revs, but when the engine is "on cam" the DCR could be a lot higher. Too high? Therefore I would err on the safe side and dish your pistons ..or something to get the CR down. Me? I'd want 8-8.3:1 static, or even less. But I don't actually KNOW of course. I know air-cooled.net is too conservative, John always recommends very low CR but a normal engine works best with around 8:1 dynamic CR and you can’t get that from 7.5:1 static. A standard cam with no overlap low 8:1 will work but a cam with overlap needs more static CR to get 8:1 dynamic – you know that. The trick is guessing how much more static is required. Anyway, I’ve opened up the chamber to 60cc for 9.1:1, I’m probably happy with that but I’ll open them up a bit more to get 9:1 or just under. I don’t want to dish the pistons as they’re nicely balanced and I don’t want to reduce the crown thickness. I think it will work – my all alloy V8 in the MGB runs more static CR with a similar cam profile, valve size and bore/stroke ratio but of course it’s water cooled. But if the air cooled doesn’t work I’ll make another engine.
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Post by Zed on Apr 29, 2019 20:29:14 GMT
I've just read back a few pages and...it all comes down to what you believe is the valve closing point. What does worry me about these theoritial DCR numbers is that maybe they could be correct at very low revs, but when the engine is "on cam" the DCR could be a lot higher. Too high? Therefore I would err on the safe side and dish your pistons ..or something to get the CR down. Me? I'd want 8-8.3:1 static, or even less. But I don't actually KNOW of course. I know air-cooled.net is too conservative, John always recommends very low CR but a normal engine works best with around 8:1 dynamic CR and you can’t get that from 7.5:1 static. A standard cam with no overlap low 8:1 will work but a cam with overlap needs more static CR to get 8:1 dynamic – you know that. The trick is guessing how much more static is required. Anyway, I’ve opened up the chamber to 60cc for 9.1:1, I’m probably happy with that but I’ll open them up a bit more to get 9:1 or just under. I don’t want to dish the pistons as they’re nicely balanced and I don’t want to reduce the crown thickness. I think it will work – my all alloy V8 in the MGB runs more static CR with a similar cam profile, valve size and bore/stroke ratio but of course it’s water cooled. But if the air cooled doesn’t work I’ll make another engine. Did I read something about an engine being fitted and cam run in?
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Post by 77westy on May 3, 2019 7:49:50 GMT
You did. I can’t believe how long it has taken me to build this engine but I guess other things got in the way and there wasn’t really much wrong with the 2.0l – the light knock at idle was a worn cam thrust bearing. Anyway, the 2316cc is finally in the bus and running. I drove it about 360km the day after I ran in the cam; it’s been on motorways and up single track mountain roads to above 1300 metres. And it’s bloody good, I knew it would pull from low revs because the 2.0l with the same cam did and the extra capacity has made it better. Head temperatures are no more than the 2.0l - but it certainly needs an external oil cooler. I like the .82 4th gear that ‘nobody’ fitted in the 091 box, sensible revs on the motorway are nice and the bigger engine pulls it easily, fuel consumption is maybe slightly better than the 2.0l but there’s not much in it, a couple of MPG perhaps. I ended up with 61cc combustion chambers, 0.061” deck and 9:1 CR. The carbs are the same 40 IDFs from the 2.0l and I haven’t changed the jets or vents (30mm).
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Post by Zed on May 3, 2019 8:55:17 GMT
It's hard to beat lots of ccs. Actually, it's impossible to beat lots of ccs.
I have the same 4th and would rather he hadn't fitted it TBH. It's made it less comfortable to drift along at 30mph in 4th and slightly slowed acceleration from 50-70 in 4th. It was fine before but he didn't mention changing it until after the deed was done.
You say your engine pulls from idle (actually I see you've modified that to "low revs") - what gear would that be? What happens if you boot it in 4th 1500rpm? I chose 1500rpm because it's cruel to boot it below that.
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Post by 77westy on May 3, 2019 11:59:00 GMT
On the flat in 4th it’ll take full throttle at 1500 revs, it just pulls away and it will pull from idle in 4th being kind with the throttle (idle is about 850 revs on the old Smiths MGB rev counter). There’s no replacement for displacement.
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Post by 77westy on May 3, 2019 16:39:38 GMT
So, which cam to choose?
I’m happy with the Scat C25 but I have never driven a bus with an unmodified engine so I can’t compare the performance between a standard cam and a non-standard. All I know is that a C25 in a 2.0l is flexible enough to be easy to drive; it doesn’t need lots of clutch slip to get going or lots of gear changes to keep it going. It has enough torque at idle to crawl along in town feet off the pedals and enough power to keep up with the traffic on open roads. Engine temperatures are acceptable but it needs an external oil cooler for sustained high speeds (for a bus). The 2.0l had 1800 heads with 41mm x 34mm valves, twin 40 IDF carbs with 30mm vents. The crank, pistons and rods were all standard.
The 2.3l (2,316cc) engine has the same heads but with 41mm martensitic stainless steel inlet valves and 34mm sodium filled exhaust valves and the same carbs with the same vents and jets. The crank is a counterweighted 80mm and the pistons are 96mm in Biral cylinders, rods are H-beam with oil spray grooves (per the VW Tech bulletin). I’ve used swivel foot valve adjusters, solid rockers shaft spacers and HD Aluminium pushrods. The crankcase is a completely standard single relief CJ Type 4. And the cam is a Scat C25 with lube-a-lobe lifters.
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Post by Zed on May 3, 2019 18:10:42 GMT
I also have ltd experience. C35 in 2l type-1 based, or stock in every form. Stock VW engines are actually all pretty similar. There's quite a jump from stock to C35 in terms of where on the rev range they perform (in my very limited experience). In terms of duration @050 Stock 214° C25 234° C35 245° So for me there looks to be quite a leap from stock to C25 too. I bet your engine is better than mine though, I do like a big stroker.
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Post by deefer66 on Sept 22, 2019 0:41:23 GMT
Well to revive an old thread.. been through this as considering the c25 as an option to refresh my 1800.
As I understand it..but I am a civil not mech engineer..dynamic compression comes into it more with the longer durations cam having a bigger overlap. Got an excel sheet to work out the numbers, think it's about right, but, was the valve closing point ever decided. Most calculators seem to ask for the 0.05' figure but texts suggest adding another 10-20°.
I get Mr Westy 2316 at a SCR of 9.01 and a DCR of 8.1 at the 45° (234° duration) ABDC but drops to 7.7 for a 55° ABDC and 7.2 by 65°. Is this where you both got to?
Keeping mine 66 crank and 96 barrels I'd struggle to push a DCR of 7.1 SCR 8.3 even with a deck down near 1mm and adpting 55° ABDC close.
Going up to 71 crank with 96 barrels I'd get a DCR at 7.5, SCR 8.8
I think this partially down to the estimated chamber volume of the 1800 heads at 58cc.
Can't see how the 8:1 can be achieved without going large on the displacement or a lower duration cam.. am I missing something?
... And at the end of the day, I'm not a boy racer just like to keep up with traffic on hills, hence being drawn to the C25, so is the quest for more cam duration really worth it?
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Post by Zed on Sept 22, 2019 8:57:58 GMT
Mr Zed he say NO
What are you trying to achieve? More duration puts the max torque further up the rev range. Since HP is torque per second more revs in that second = more HP. At lower revs there will be less torque hence less HP.
My 1700 Westy I had for 10 years came alive at 40-45mph in 3rd with a stock cam. To push that up 500-1000rpm wouldn't have been good, I'd hit the max revs just as it started to go. In 4th where it was good above 60mph so with more cam I'd never have got to the engine's sweet spot at all.
If you like high revs and constantly changing down in search of power cam it up.
Now the opposite... My 2413 stock cam has max HP 100 at 3850rpm. Nothing to write home about. BUT at 2000rpm it has 70hp because the torque peak is at 1850rpm. That's why stock cam is good.
Yours with cam I'd suggest would have 20hp at 2000rpm. I made that up but it's close. Have a search and you'll find cammed up engines, they don't even start the RR run below 3,000rpm because there is no power of interest to record.
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Post by deefer66 on Sept 22, 2019 10:40:47 GMT
..I like the "stock" webcam we got for the first build in the tintop, think it's actually a 914 grind but the mildest they do anyway ... well I like the lack of bother trying to figure out the repercussions on build and other parts of the system to be honest. Perhaps the modest uplift to 1911 and the 36 Dell's will give the doka it a similar performance to the 2.0ltr stock we have. If I had more confidence in assessing the existing cam I'd even consider going with that!
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Post by Zed on Sept 22, 2019 11:42:12 GMT
My cam is 40 years old. People sling the type-4 cams because the wear pattern looks ominous, but if they are not actually worn, just have that pattern I see no reason not to reuse with refaced stock followers. I guess I was lucky and Rob Parry, who used to reface 100 followers on a Saturday morning made the judgement and sorted the followers.
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Post by 77westy on Sept 22, 2019 14:04:14 GMT
Mr Westy he say YES! But you need more capacity. As cam duration increases engine efficiency improves, although maximum torque is higher up the rev range, but if you increase the capacity it makes up for the loss of torque at low revs. Torque is linked to cc’s and Zed’s 2.4 will make more torque than my 2.3 but only at low revs because a C25 moves the torque curve up and my 2.3 will still be increasing when Zed’s 2.4 is decreasing. And if the crossover point is at, or below, normal cruising revs… A Scat C25 is still a very mild cam and a 914 cam isn’t the same as a standard bus cam.
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