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Practical Engine Building

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Hello have a few questions, engine c20let 2litre 4 cylinder turbocharged, are arp bolts reusable many times. If i crush my new MLS headgasket to check valve clearance and have to remove head to alter valve clearance can i reuse the gasket or what is the best way around this i don't want to but two gaskets if its unnecessary. Is arp main bolts recommended in 500 + bhp 8500rpm . What valve clearance do u think id need .? What fuel is best to run will be boosting between 2.8 bar absolut and 3.5 bar absolut. Are itb's with fabricated plenum a good choice for power. If i move my peak torque up higher in the rpm and spin the engine faster to get more bhp will it be faster without dropping my gear ratios down to take advantage of the higher rpm and bhp

Sorry if some of this is 'granny - egg'.

If you haven't done so, check out some of the Vauxhall, Vectra and Calibra club sites. There is plenty of info' there if you use the 'search' options and they should give you plenty of info' on best practices there. I use the Mig-Web site.

Bear in mind I've XEs, not LETs, but as they're basically the same engine, some comments.

ARP bolts are re-usable many times, as long as they're not overstressed/tightened. MLS gasket is supposedly re-usable - check for damage, though - I would use old gaskets when checking but lightly torquing the head nuts/bolts enough to seat the head is going to be close enough as gaskets only compress a small amount. if in dobt buy a new one and mark the one you've used as "for clearance checking only", or similar, for further use like that. Those gaskets can be a bit fussy for surface finish - make usre the block and head are FLAT and the finish is as required.

When you say 'valve clearance' do you mean valve to piston clearance, as you can just remove the camshafts, with the head in place, to access the 'buckets' for adjustment?

Later blocks are stronger, but at a minimum use studs and a girdle is STRONGLY recommended.

Can't give specific V-P clearance, but the exhaust will be a little more for safety, as the piston is chasing the exhaust valve and if it's a little tardy...

The best fuel you can get - you're in Ireland and, IIRC, you can't get 98 or E85? You may need to drop the compression a little from what you'd like and you'll need to cool the charge as best you can.

The stock inlet manifold is quite thin walled, which restricts port work, and can be a little restrictive, epecially over 6k, or so. On the NA XE, a decent set of TBs (I use Jevney) is worth10-20% more power, depending on the rest of the engine spec'n, so I would expect it to have a similar affect on forced induction engines. For your application, a single butterfly on the plenum entry should be fine and a LOT cheaper than individual butterflies.

On manifolds, a GOOD banana manifold (AKA "EVO") feeding the turbo' is important!

Power>torque - you can always drop a gear for torque multiplication. What affect the engine upgrading has on the optimum gearing is going to depend on what you're going to be using the car for. If it's a 1/4 monster, you're going to gear it so it's hitting the limiterat the end, or a little over the power peak, for pure top speed, gear it so it's at, or very slighly past, peak power. however, it isn't hat simple, you also have to look at the bottom end - if the car is going to be used for a lot of town driving, a lower 1st gear - stock - might be best for driveability from stops, if it's a 1/4 miler, you may want a taller tyre so you can get more work out of 1st/2nd gear with less chance of blowing the tyres off (a lot of people start in 2nd for that reason), a track day toy may work best with a tall 1st and cloe ratio box.

On gearboxes, what are you looking at using? The F20 will be killed in short order, especially with high grip tyres! The F28 (AWD) used OEM with the LET is getting like hen's teeth for a half decent one and parts are getting scarce. Some use the F35 (IIRC) gearbox and there areflywheels and adaptors for them, but a bit of work is involved and you need the correct one for the starter location. You may need to go to the aftermarket ($$$s) for something reliable, and you can option the gear ratios, within reason - I know Quaife seems to be well regarded?

On flywheels, is it a set up for a flat or pot type - an aftermarket steel one would be wise either way and will be needed for a F35. Again, you'll need to consider what it's used for - a light one can be more responsive for faster driving but can be trickier when moving off, while some people like a heavier one as it makes starts easier, especially with peaky engines at the strip, supposedly it also slows down the wheelspin peaks a little. You will also need to select a flywheel that works with the clutch and pressure plate you decide to use. Pot and flat use different pressure plates and the friction plate will need to match the gearbox input spline - F20 is different from the F28 and F35, not to sure about aftermarket options.

On clutches, you're going to be looking at around 500lbs.ft of torque, so you will probably need a dual plate with uprated cover to match, and the flywheel will need a compatable bolt pattern and friction surface.

Forgetting, The 'German' head has a different casting process from the original 'Cosworth' head (easist way to confirm is by casting mark under exhaust port) and can crack internally between an oil-way and the water jacket. If you have one of those you can buy kits if it hasn't been done - worth doing as they usually fail when torquing the head, so may not show up beforehand. The OEM MAHLE pistons are good, but not up to that power level, there are several good sources in a small range of nominal compressions, they also have deeper valve pockets, so less chance of interference. Connecting rods are usually considered marginal around the 300 lbs.ft/300hp/8k rpm point (with ARP rod bolts, or equivalent, OEM have been known to fail as low as 6k8) - there are plenty of manufacturers offering them - but confirm they are suitable for forced induction as the critical loadings are quite different.

Are you looking at a dry sump or OEM style wet? If wet, the stock oil pump gears are sintered and have been known to fail during normal use - spend the dosh for steel upgrades as it is very cheap insurance when considering a serious failure can take out the entire engine...