8 cylinder front engine iconic vehicle
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By Herr-Kuhn
#151832
We have been doing some head and valve train work on the 928S4 heads and I have come to the conclusion that any moderately boosted 928S4 should have a valve spring upgrade. We have identified three viable twin spring options and as well, some retainer options for these engine that are mostly "off the shelf". All of these options stem from the spring options which are available for Audi I5 (AAN,3B, 7A) and include springs from Supertech, Ferrea and Autotech. There are some tricks to making it all work properly which in my case will likely include custom spring perches. For the record, on the black twin turbo car we ran the PAC1223L springs on top of custom made perches with over 90 lbf on the intake seats and over 100 lbf on the exhaust seats. While this may seem high that engine was spun all the way to 8,000 RPM and never missed a beat. Prior to this we were running with about 68 lbf on the intakes and that was inadequate for manifold pressures over about 16 psig where the engine would nicely lay out 725-740 RWHP on pump gas. With the upgraded seat forces and more manifold pressure that engine put down 850 RWHP on 21 psig, again on pump gasoline. That engine was running an enhanced S3 profile done by Elgin. I think the key here is to really know where you plan to run the engine and how much manifold pressure is going to be run before you select a cam and spring setup. The spring forces become real important to know when higher lift cams are utilized. In this regard we can learn a lot by looking at what sort of springs are being run on the I5 and I4 stuff from VW/Audi. I'm embarking on a full engine build for my personal car and we will be doing some mild porting, potentially a semi custom set of cams and probably a bit on intake work. I've learned over the years that a pair of GT3576R turbos can really get things breathing super heavy and one has to be careful on the setup. One of the most overlooked areas on any high performance engine build is the valve train. More information as it becomes available.
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