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Vw 8v normally aspirated stroke to bore size

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Hi

I am going to build a normally aspirated vw 8v as my first build ever. I have worked on cars before but never built a engine on my own.

I am going to run the car on methanol.

What will be a good compression to run?

Also I'm struggling with the bore to stroke combination.

I can go with a 80mm stroke crank and 84.5mm piston, which will give me just about 1800cc. But will be the only over square option I have. I don't know if I can get a original crank machined to give me something between 82 to 85mm to get the displacement up, as a aftermarket crank is out of my budget . My question here is will the sacrifice of displacement for the smaller stroke to get the better stroke to bore ratio be worth it. I also will be able to run a longer conrod with the smaller stroke. Will the engine be to underpowered am i putting to much time and energy into the stroke to bore ratio as well as the conrod to stroke ratio. I want to use the engine for drag racing.

Regards

Pravs

Hello,

I guess it comes down to budget and what you can afford, I prefer not to think outside the square too much on this type of thing because of the time it takes to fix things when they go wrong if you stick to a standard bore stroke combination and spend the extra money in cylinder head work the benefits will be far better is it turbo or not will make a large difference to compression ratio required, also the piston dome/dish shape plays a major part.

Regards Ross

Hi Ross

Thanks for the reply.it's a naturally aspirated build.

So for the bottom end these are my options.

Option 1

84.5mm bore and 80mm stroke over square

Conrod to stroke ratio : 1.987 (159mm conrod)

Engine cc: 1794

Option 2

84.5mm bore and 86.4mm stroke under square

Conrod to stroke ratio: 1.840 (159mm conrod)

Engine cc: 1938

Option 3

84.5mm bore and 92.8 stroke under square

Conrod to stroke ratio : 1.713 (159mm conrod)

Engine cc: 2081

I could get a longer conrod if it's worth it.

There is also a 95.5mm crank but for my application I don't think it will work. The current fastest normally aspirated vw 8v in South Africa runs a 11.9sec at the coast.

I want to try and give the record a go.

Also what piston top design will work? Or how do I determine the design?

So which option should I go for and why?

Regards

Pravs

Option 1 would allow the engine to rev more but you can not beat cubic inches and you are certainly giving away a bit if you take this option.

Option 2 is the closest to square and in my opinion, the best option allowing you to rev highest and with minimal stress on the pistons being pushed into the side of the bore, it will most likely have less bottom-end torque than option 3 but will make up for it with reliability and top-end HP.

option 3 will produce torque but limit being able to turn it hard in my opinion,

in drag racing, there is so many factors to consider when choosing pistons and heads like what is available, what is achievable and budget are just a few, i recommend trying to contact some of the UK Vw guys and having a chat about what they are using in there 9-sec cars would be the best idea. you can find their videos from Santa pod drag racing all over the net

Regards Ross

Thanks again.

Will using a shorter piston and a longer rod on option 2 make a big difference to the engines ability to make power or just to Rev better? I can possibly run a 6 to 8mm longer rod. Giving me a rod to stroke ratio of 1.909 compaired to a 1.840 with standard set up.

Well what I'm asking is, is the small increase in the ratio going to make a noticeable difference.

And on the compression ratio can you advise on what is a good range.

Regards

Pravs

Regards

Pravs

As Ross has implied, there are many, many variables to consider.

You don't actually say what the base engine is, but as there are several different versions of many VAG engines for different brands I'd suggest getting familiar with that is available and what is interchangable - you seem to be already looking at that.

It seems you're determined to build the engine around an 8 valve head, I'd suggest planning around that. The head and camshaft is going to be the limiting factor on airflow, even with correct porting, so extreme capacities and/or rpm will probably be wasted as the head won't flow enough to feed the engine demand. It will, however, make it easier to build compression.

The higher rod:stroke ratio will, I guess, help a little as the piston will have a slightly more even travel and it will reduce friction at the thrust face of the piston.

As Ross suggested, you may be best checking out what's currently being done around the world on these engines - I think you'll be surprised what people are getting out of them, and how much it's costing!

If it were me, and this is making MANY assumptions as I'm only VAGUELY familiar with the engines - I'd be looking at having the head reworked by a professional, with O/S S/S valves and about as much camshaft as I can get. I'd look at the tallest block available (some manufacturers use blocks that are identical except for height for different strokes), forged crankshaft if available OEM (there may be a diesel engine variant that uses one) or aftermarket if there's a known weakness with an OEM cast crank. There should be several options for forged pistons, I'd be looking at the largest bore that was compatible with the block, with as much nominal compression as I could find with the valve pockets (if needed) for V2P clearance - depending on the engine configurations, you may find the same 'dome' available for different engine strokes and/or block heights, this suggests different pin ehights and the higher the better as you can use a longer connecting rod, of, and a slipper design should cost less frictional losses. For connecting rods, you may find a suitable length diesel variant which 'should' work with decent bolts but aftermarket rods will be lighter, stronger, and can be made to the exact length required for a small premium.

That's only part of it, though, and it's going to get expensive. There's going to be the oiling - sump, pump, etc. There's going to be the whole intake side and exhaust. The flywheel, clutch, gearbox and dif' setup. Probably springs, dampers and replacing bushings, etc. Don't forget the safety side, too.

Thanks Gord

I know it's going to cost me.

I have not gotten anything as yet. I'm just set on the 8v as I think it's the simplest vw engine to start with. And there is a lot of planning before I get any parts. I am going to get a long block now and then the crank. Then will look at the conrod and piston dimensions. I am still not 100% set on a 8V.

I also considered a 16v even a 20v. The 20v seems to be the fastest na vw engine. Well guys are running the best times with them na.

Also if you would know what's a good compression to run. Be it 8v or 20v but na on meth?

Regards

Pravs

At this point you're still in the planning stages, so I'd suggest holding fire on any purchases, including the block or any other parts, until you know exactly what you are going to need. It'll also give you a chance to find the best price options for the parts you decide on.

I would expect - others will better advise you - you can run as much compression as you can while still having good flame propogation.

Hi

its been a while since this post.

I have now gotten an engine and have gone with the Vw FSI engine. So a 16v motor and the best naturally aspirated platform for a vw from what I have seen and researched.

I am building the engine for drag racing only 400m. Will run meth or alcohol.

so I want to just confirm that now being a 16v is the 86.4mm stroke with a 84.5mm bore still the best option. In a long block 236mm.

these are the options I have.

Option 1

84.5mm bore and 80mm stroke over square

Conrod to stroke ratio : 1.987 (159mm conrod)

Engine cc: 1794

Option 2

84.5mm bore and 86.4mm stroke under square

Conrod to stroke ratio: 1.840 (159mm conrod)

Engine cc: 1938

Option 3

84.5mm bore and 92.8 stroke under square

Conrod to stroke ratio : 1.713 (159mm conrod)

also a 98mm stroke crank available.

There is also a short block 220mm height and the long block is 236mm height.

Regards

Pravs thanks

Hello some help!!!!

try it, test it, change it, repeat.

Or...

Find a pro engine builder for these engines and take their advice (and buy parts / services from them).

I think Ross's advice of Option 2 is still the best choice to start.

#2 may be the best for a first build, #3 if you have the money for light rods made for the tall block. 98mm stroker if you have the time and patience to do a custom build - and it's a proper forged steel item instead of the more common cast/nodular iron used for crankshafts - it will definitely require a proper harmonic damper, though, and properly built you may be able to take it to 9-10k, if the head, etc, can flow the air - regardless, it will almost certainly be cast iron

However, that's a 'best guess', and you should follow the others' advice and see exactly what others running the VAG engines are doing - it won't be cheap!

I would also remind you that you're looking to beat people who will have been playing around with different engine configurations, head work/porting, camshafts, EFI/carb'setups, exhaust dimensions, etc. and the chances of you hitting on something none of them have is most unlikely.

I would also remind you that the engine is only one part of what you need - there's the clutch, transmission, suspension, lightening of the vehicle, gearing - and, not least, learning to drive the car properly.

Hi

Thanks for the feedback.

regards

pravs

You should be on the vortex.

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