Last edited by Graham; 25-10-2015 at 11:28.
Okay, carbs off today and they are running 38mm chokes.
Good or bad?
Also, the head will be coming off soon to check the piston to block clearance. Will have this information soon.
38mm chokes are great for top end power, 36's much better suited to midrange and low speed flexability/sensible road cams
Okay. So how much top end will it cost me by switching to 36mm chokes to make it more driveable?
Also some more info on the bottom end.
We made 130psi cold.
140psi hot
145psi with oil.
Average on all cylinders pretty close <5psi discrepancy.
Does this point to a low compression bottom end rather than worn rings? Would a block skim to the correct height solve the issue without breaking the whole bottom end apart?
36mm chokes Will make it more drivable and at teh power level your at it probably wont help teh top end.
might be a low comp bottom end, but you cant skim the block with it still assembled.
No such luck in just setting the Pistons to half height and just skimming the top off the required amount?
in theroy you could skim the block with the crank rods and pistons in it, but few will for a number of reasons.
when you skim a block you have to lift it onto a resurfacing machine and carefully shim it up to make sure you skim it level, not easy with teh weight of a short pinto, its hard enough with a bare block, so depending on how the block is supported it may not be possible with the crank sticking out the ends, lastly the process chucks a lot of fine swarf everywhere, how you going to be sure none gets where it shouldnt, either trapped in a ringland, or possibly worse still ending up in an oil way, which you cant blow out properly because the block is still assembled.
The swarf would get everywhere. Pull it apart put New shells, rings and rehone it yourself along with a block skim would up the power nice along with the 36mm chokes, i doubt it would affect your top end but will greatly increase how well the engine drives/pulls from down low which is really important for a road engine and you would get your monies worth out of the engine then.
what you really need to is work out where you are in terms of compression ratio FIRST. meassure ACCURATLY how far the pistons are down the bore at tdc, cc the head, meassure head gasket thickness and work out what CR you actually have.
if the ratio is low as much as possible correct with piston hieght, even on high comp pintos you would skim the block and run the pistons a minimum of flush with the top of the block, quick ones i usually go 10 thou out the top of the bore, more if im not using a thin gasket
It will not cost you top power unless you are going to pass 180 Bhp in most cases. And even than it will be small. I got a 189 Bhp Pinot and made the same power with 36 as with 38 mm chokes.
Okay. So how much top end will it cost me by switching to 36mm chokes to make it more driveable?
I would a least try 34 mm and even smaller. Carb calibration will also be easier.
Last edited by Graham; 25-10-2015 at 19:17.
Good luck seeing any decent top end power with 34's.
That's the point, this guy is not making big power on top, he don't need the 38mm, maybe not even 36mm.
Last time I was testing a VW T4 2,6 Ltr 4 cylinder engine. It made 3 Bhp less on 44 IDF with 36 mm chokes compared to the 48 mm with 42 mm chokes. The torque in the middle and bottom raised like there was nothing to compare anymore. And if you pushed the throttle, I know this is feeling, not measuring, but it felt like you hurt yourself by the "bang" of the engine. We are talking about 200 and 203 Bhp.
For 160 Bhp Pinto power, you can do the test between 34 and 36 mm, for numbers like 150 Bhp and lower in this case, I don't think you will have to try the 36mm but you can always push your luck.
Well I can't find any 34mm chokes so I have ordered some 36mm chokes as it needs to be back to the tuners this weekend. We found an issue with air leakage on the manifold after the set up so the carbs were set up with the air leakage 😡.
We will see what sort of improvement it gives.
In any case with regards to the bottom end. I'm so annoyed with not having it on the road I'm going to run it as it is for now.
I've managed to blag another 205 bottom end which I'm going to build while mine is on the road with the stipulation I give him my block once it's all back together.
So the question is, how far do I go with it to get the best out of the head and cam? I can get the cc of the combustion chamber in the head from Eric at Jem engines and then skim the block to suit, what compression ratio is advised, what about forging components? If I'm going to do this on I want to do it right.
Before you start I would advise reading graham s threads on his engine builds if nothing else it will give you a grounding on what goes into an engine build 😉
Will do 👍🏻
what kind of inlet manifold is he running with the bike carbs? if its rubbish, that could be the sole loss of the 10bhp at 7k. remember bike carbs (mostly) are variable venturi carbs, and most bike bike carbs are 38mm throttles.
if they are 38mm bike carbs, theres no way its a carb restriction thats causing the drop off up top. on WOT with the venturis completely open a bike carb will flow as much air as a DCOE with a 38mm choke there or there abouts. Dave walker did this on his flow bench many moons ago in an edition of CCC.
has he the bike carbs jetted correctly?
Finally, that manifold will be doing no engine any favours, period.
If it helps my pintos compressions were 175psi across when warm with coil disconnected
In any case. Making 130-140psi isn't any good, especially as the head was ported and designed to run at a 10.8-11.1 ratio. This is obviously the first problem to address.
The bike carbs, although giving more torque lower down, never felt correct. Cold starting was a nightmare, they were really tricky to set up, he even had to polish the needles to the correct sharp to try to lean out the top end, was still running rich as you can see from the air/fuel on the graph. It spat back through the carbs all the time, and just really felt odd.
Runs 100x smoother with the webbers, although making less torque. I have 36mm chokes ready to go in, but would 34mm restrict too mush if we are aiming for 160bhp with decent compression? and hopefully picking up a new bottom end this weekend. I'll keep you guys updated when it's back together and running again to see the improvements.
the needles have nothing to do with top end - thats 100% down to the main jet. if its too rich at full throttle your main jet is too large. if you have that size main jet trying to compensate for a leaning mid range, then you need to raise the needle and drop a main jet size.
Tbh it looks like a lack of knowledge from the RR operator of how CV carbs actually work that's causing your running problems on that front.
for cold starting bike carbs you have to use the choke and NO THROTTLE. as soon as it fires then feather the throttle.
Last time i had my xflow on the RR the operator commented that the closest carb setup hes ever seen to EFI from an AFR point of view. So it can be done
Totally agree. I'm not very familiar with bike carbs (Mikuni CV) but it's for sure, bike carbs are a lot further developed as the Webers we are using. Bike carbs, if correct setup can come very close to the best fuel injection setup. And because they where build for street use of semi race engines with some serious valve lap at TDC, they can run with fair hot cams without the problems we encounter with Webers. The trick is to let you believe your run full throttle but as with SU's the piston is not at full throttle. Something today car manufacturers are doing with drive-by-wire.
The needle height on my carbs weren't adjustable. Maybe this is what he was on about?
In any case after removing the head we defiantly have 1.5mm clearance between the top of piston and the deck of the block. I've now got a standard compression block so will be reassembling and trying again.
Yes I know. Sorry mis worded it.
Should be running this week. I'll get the graph up once it's tuned.
Yes I was referring to my old one.
Old one = 1.5mm
New one = 0.2mm
That's more like it, your method of measuring is impressive lol....a vernier or depth mic will be more accurate bud
Feeler gauges work
better with a properly machined straight edge, but good enough to work out low or high comp
Watch this space.
Engine is in. Just need to get the head and bits back on. Change the chokes in the carb and were away. Lack of light when I get home from work is halting progress. Should be running the weekend...
You should get a significant difference in performance with the high compression engine. It will probably feel like night and day! I bet your rolling road session will probably be back up to what we all expected it should be (140bhp or so).
Okay so new bottom end in. Got it all back together. First on the list was a compression test. We hit 160psi cold and 170psi warm. 175psi with a dash of oil. Exactly the same on all cylinders.
Car also feel much better down low with the new 36mm chokes. Can say much on top end power differences yet, as I don't want to push it untill it's set up, but it is running quite rich which is fine until it goes off for dyno tuning and jetting on Saturday. Will report findings and the new graph when done.
sounds promising
Sounds like a great result! I'm going out to the garage now! To check the mrs capris compression as her car is down a good few bhp from what it should be. The car is off the road for winter now so it can have a new bottom end too.
Can't believe we have to wait 6 days to find out if it has improved the car any more though. Lol.
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