First cobra related post in ages!
The poor old cobra has been sat in the garage for 5 months and not even started. I pushed it about 2ft to access some tools but that's it. I never intended to leave it so long but trips abroad for work and then a 5 week holiday over Christmas meant I just never got round to driving it.
Anyway, last weekend we fired it up to attend a cobra club meeting. One pump of the throttle, turn the key and literally the first revolution of the engine it fired, as if it was impatient to get back outside. Drove to Corby (around 40 miles) to meet up with some other Cobra owners, which was nice.
The car was happy but I got it booked in for a rolling road session just to see if it could be improved. I have documented my low speed running problems on here in great detail and I was slowly coming round to thinking that its simply because of the engine combo and the fairly big cam and there wasn't much I could really do about it, apart from change my driving style and stay above 1800rpm.
I went to Wilshires Garage in Wimpole and met super-experienced Pete Baldwin who has about 50 years experience of Holley type carbs. Now - remember that being a young person, I've never owned a car with a carburettor before so I didn't know what was normal. Once I'd described the engine spec to Peter he agreed that if I want to chug around smoothly at low rpm then I should swap the cam for something softer. He said we would get it set up as best it could be but warned me not to expect miracles - its a limitation of the engine components. I was happy with that so we got stuck in.
Peter twiddled around with the timing while I held the revs steady to work out what the best all-in + vac cruise figure was. This was 45 degrees. With my mechanical + vacuum I was getting 42 so he recommended I give it 3 more degrees vac. We did a run where I held it at high revs and he adjusted the timing to find the best mechanical all in value. I had it at 34 based purely on knowledge that roughly 36 is the limit so I backed it off 2 degrees to be safe. We could see the HP on the screen and either side of 34.5 degrees the HP started to drop off so he agreed that it was set fine for the road. If I was racing and every single HP was critical then he would just nudge it up half a degree but it was perfectly acceptable for the road with a little margin to allow for poor quality fuel etc.
My IFR's and idle mixture were ok, perhaps a little rich on the IFR. Peter suggested I try the next size down but I explained that I had already tried that and it extended the light acceleration lean spot and made it quite noticeable so I was running it rich on purpose. He said that was ok as in those situations with micro-throttle movements the accel pump isn't doing much so that's the only solution. The reason behind all of this is the component combination. Big cam with lots of overlap means some of the fresh charge is wasted. At low engine speeds the big carb means that the air velocity is slow so the fuel metering isn't as good as it could be. The fuel comes through in globs rather than a nice even mixture. The third main problem is the single plane intake. Its a very large volume so the engine has to eat through all of the existing air/fuel charge in the plenum before it can get a richer charge when you crack the throttle open slightly.
On to power - we did a couple of power pulls. For reference, the old Edelbrock performer achieved 376bhp and 396lb.ft torque after A LOT of fiddling.
For power, all I've done with the Quick Fuel 750 as far as full power is concerned is jet down a couple of sizes across the board to get the WOT AFR to 12.5. This is what "felt" fastest to me on the road. It turns out that this was spot on. Peter suggested that for racing I should run slightly richer so maybe the out of the box setup of the carb was spot on but for the road I was ok. Here's the interesting bit - unfortunately I don't have a direct comparison between this carb and my previous one as I used a different rolling road but the power was 420bhp and torque was up to 430lb.ft. Peak torque was coming in a little lower in the rev range too, at 3400rpm.
From my research into annular boosters, I was expecting the peak torque to come in sooner which is good. I also have never heard of anyone swapping from Eddy to Holley and going slower so that was good too. I am still a little suspicious of such a large gain from 376 to 420 but there will be differences between different rolling roads so my conservative estimate is that I've probably gained somewhere around 25-30hp from going to Holley.
It was really interesting to talk to someone with extensive experience of driving engines like mine with hot cams.
Basically I need to adjust my driving style and stop expecting perfection in all driving conditions!
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