by dr20t » Mon Sep 07, 2015 2:54 pm
So my headers were leaking at literally every flange. The ticking sound and hesitation / lean out I've described in the past on this thread turned out to be from leaking flanges. I narrowed it down by having all the header surfaces machined in the following order:
- manifold to uppipe connection on manifold side
- uppipe to manifold connection on uppipe side
- stock 3 bolt Subaru turbo flange on uppipe side
- bottom of twist mount adapter 3 bolt flange
- t3 flange on top of twist mount adapter.
This eliminated 50% of the leaks and improved spool slightly. However the ticking and slight hesitation was still there as soon as I reached positive boost (although a little better).
This then led me to the header to heads flanges - these are by far the worst leaks and warped. I'm going to get these resurfaced soon but in the meantime I decided to try the stock single scroll, unequal length manifold from a post facelift GT including stock uppipe.
Pulled timing out of the tune and some fuel (knowing the engine was not going to breathe as easy with the stock header as the twin primary and secondary of the aftermarket headers).
Went for a drive, and as soon as you accelerate the engine feels extremely choked. Keep in mind i only changed avcs setttings above 5500rpm (reduced overlap - my thinking here based on my anticipated higher backpressure higher in the rev range). This I have put down to two things, one having more impact than the other:
1. Increased backpressure due to reduced exhaust header volume. This I believe has the most impact. Will come back to this.
2. Unequal length nature of the manifold resulting in irregular exhaust pulses.
On the increased backpressure, the impact of this is substantially more noticeable on my engine for many reasons:
- huge avcs overlap that I run on my engine - 52 degrees inlet advance and 50 degrees exhaust retard - more than any motor I've seen or heard of
- with this overlap, and with increased backpressure, the engine is obviously pulling back hot air into the chamber.
Without having a gauge to measure backpressue in the exhaust manifold, this tells me the backpressure is ALOT higher in the stock headers than I initially anticipated.
This will sound like a wank, but I suppose my engine and turbo combo is best placed as a test of this, given the substantial change in breathing capacity. Again I know I don't have a egbp gauge to measure, but the significant change in afr and airflow in g/second is a pretty good indicator.
All that said, I am not writing off the stock headers. Whilst it would seem the engine is alot more choked with these, I believe I can still optimise for the stock header.
Remembering a turbo spools off exhaust pressure and not exhaust flow. Yes that's correct. Pressure is a function of, amongst other things, pre turbo exhaust volume. I would guestimate the volume on the stock single scroll header is less than 50% of the aftermarket equal length tubular header (tomei / psr / gt spec etc). This is good and bad.
Good : helps build exhaust pressure for better spool
Bad: can choke up top and limit the amount of overlap able to be run
For my engine, ill be reducing overlap substantially to optimise inlet to exhaust pressure ratio values (so where it goes above 1:1 in favour of exhaust pressure, completely eliminate valve overlap). Reason forthis is grounded in fluid dynamics. Matter flows to lowest pressure point. If exhaust backpressure higher than inlet (up until inlet valve), and inlet and exhaust valves open simultaneously (overlap), then exhaust wil flow back to inlet (egr). Given the massive amount of overlap I'm running on my engine, this would be a ridiculous amount of egr happening for a very long time.
The trade off is less overlap which will increase lag, but higher exhaust pressure which will improve spool. Cross point of inlet to exhaust pressure ratio will then dictate when I can start easing off overlap and as long as ratio stays below 1:2 then I can hold boost to redline. If ratio increases beyond 1:2 (eg 1:2.2) then boost needs to be tapered, which will hurt power of course
My gut tells me I'm going to be limited to 24/25psi on this engine with stock exhaust at redline due to backpressure, even with the large 0.82 gtx3076 I'm running.
Spool should still be improved with higher backpressure down low but need to eliminate overlap.
Once my psr headers are fixed, I will definitely put them back on pending killerb holy headers arriving.
This exercise has shown the value of strong breathing through big headers with a trade off of spool which is made up for in a big way by the avcs capabilities of my engine.
Overlap is awesome on a well thought out turbo setup where pre- turbo exhaust backpressure is up to 1:1 in absolute terms. Soon as this goes above 1:1 then overlap is downright dumb and thus why my engine seem s choked with the stock headers at the moment.
With the very large 42.7mm runners of the psr equal length headers, combined with the length of the runners and the large 63mm uppipe, and on my tiny 2.0 litre engine, there is a hell of alot of exhaust volume to fill pre turbo. This causes lag but is more than made up for by the huge overlap, when it can be run. I dare say I wouldn't cross 1:1 until probably 7500 rpm on my setup wth the headers.
If my headers weren't leaking (and seem like they always have been as this ticking sound has been around from day one this engine combo went on), I dare say I'd be spooled even earlier
Mick
