Dealing with the restrictor rule
Posted by vbares
Vittorio Bares Professional Moderator Location: Londonderry, NH Join Date: 04/10/2007 Age: Ancient Posts: 413 Rally Car: Audi 4k (3b conversion) |
Last event - we had some challenges (blew up a turbo and cooked the motor) - so the opportunity presented itself to build it better than it was
Subject: 2.2L 5pot Audi motor w/20v turbo head J&E Rods Weisco 81.5mm flat top pistons coated bearings Balanced rotating assembly 3-angle on the head light skim guides and seals CC'd the head - looks like we're at 9.48:1 CR - I wanted to get to 10:1 due to the 34mm restrictor on the turbo. Restrictor will limit flow to ~29 lb/in - so we're looking to produce torque at the bottom end and give up hp at the top (basically limited to a theoretical 290chp) - we saw 240whp on the dyno with a GTX2862 @3400 rpm with stock pistons and stock CR (9.3:1). I have a guy looking into a custom turbine starting from a GT2871 - from what I've read/studied having close to zero trim on the turbine wheel should allow quicker spool at lower rpm and thereby more torque at the low end, giving up top end...which is restricted anyway. But, I digress - that said, I'm looking for the vast wisdom on this forum with regards to cams... It has been suggested that if I were to use 7a cams and advance the timing on both exhaust and intake I could pop more exhaust into the turbine, thereby getting boost sooner as well as suck more fuel/air into the chamber - oh yeah, and I'd like to run pump gas... I found this nifty piece of data in the pic below... Many Audi folk will add the 7A exhaust cam for high end power enhancement - I'm not convinced with my application, restrictor and goals this is the best way to go - but I am thinking that using the stock 3B cams and adjustable cam gears could give some benefit in the low end without compromising my restricted power limit on the top - what say you? |
Vittorio Bares Professional Moderator Location: Londonderry, NH Join Date: 04/10/2007 Age: Ancient Posts: 413 Rally Car: Audi 4k (3b conversion) |
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John Vanlandingham Junior Moderator Location: Ford Asylum, Sleezattle, WA Join Date: 12/20/2005 Age: Fossilized Posts: 14,152 Rally Car: Saab 96 V4 |
Vittorio Bares Professional Moderator Location: Londonderry, NH Join Date: 04/10/2007 Age: Ancient Posts: 413 Rally Car: Audi 4k (3b conversion) |
Well you're both right (I'd rather see pump gas as the only allowed fuel) - now with fluctuating restrictor requirements one would want to build for flexibility.
Lets review - money spent on optimizing for restrictor so far in the motor: uhm zero... I've also decided to leave the HG alone as the 0.5 CR gain really isn't worth the effort, dollars or potential reliability. On the cams - Audi made several motors with the same bottom end but different heads. The cams in two of the motors ares described in the chart above. The 7A is a 20v NA head and the 3B is a 20v turbo head. 7A has cams that have a longer duration (not sure about lift) and start a bit earlier than the 3B - These cams can be acquired for a couple hundred bucks. If we add another couple hundred bucks for adjustable cams - would it make sense to: a) Go with the 7A cams and longer duration b) index the cams for more advance to improve spool My thought is that this could provide for a very flexible platform that can be adjusted for different intake limiting rules. (if we could stay focused on the technical merits of this approach it would be appreciated )
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Paulinho Ferreira Infallible Moderator Location: Charlotte, NC Join Date: 03/15/2007 Age: Midlife Crisis Posts: 721 Rally Car: Honda Civic |
Vitorio, more duration and lift isnt particularly preferable on a restrictor car. It tends to raise the powerband in the rev range and decreases port velocity. So on the exhaust it will hurt spool up in the lower rpms. Also it will effectively decrease your dynamic compression ratio. I'm sure those stock cams are still small enough not to hurt the performance but I'm sure John will know more about this. |
Vittorio Bares Professional Moderator Location: Londonderry, NH Join Date: 04/10/2007 Age: Ancient Posts: 413 Rally Car: Audi 4k (3b conversion) |
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