ATMO Tuning

I think news of Tony's 360bhp has leaked out



http://www.pistonheads.com/news/default ... ryId=15385

My ATMO has 160bhp and i love every single one of them.

Saying things like changing the cams, carbs and putting a free flowing exhaust on are easy to say as they are just generic mods that will work on any engine,


No drama..

Sure, but when we built the first engine with high lift cams, it lasted 200 miles. Went like stink, but not a very good result - and that was following the book to the letter. I had to combine the advice of more than two "experts" to achieve what we finally got (which is still working fine some 3000 miles later and still going). It wasn't as good as it could be if it were running carbs or EFI, so I'm itching to do it "properly" for someone . Unfortuntely our DeLorean freinds tend to like keeping things looking the same, so I'm stuck with that god-orrible K-Jet sensor plate

I think news of Tony's 360bhp has leaked out



http://www.pistonheads.com/news/default ... ryId=15385

Hmmmm??? I went to the MPH show and it weren't there that I saw... Prolly appearing at the Earls Court one.

Yes, with the K-Jet you can't make simple mods..

For comparison between engines for instance, way back when on a standard atmo, I ditched the weak standard fuel pump for a Facet high pressure one, binned the original fuel regulator and flow meter for an adjustable bowl type so I could alter fuel output instead of having to play around with rejetting the twin choke carb, cut open the standard GTA silencer and diverted the internal baffling to straight through, modified the air intake into the filter and played with more advance on the dizzy.

From a standard engine dyno'd at 159bhp it made a gnats whisker under 180bhp, just from slight fueling and ignition mods. The grand sum of parts was under £150, though I spent nearly a grand over the space of a couple of months every week down my local dyno, trying different combinations. It could have squeezed probably another 5-10 bhp with a set of manifolds and a proper exhaust system..

..and a power curve to die for...The 3 lines are 3 seperate runs with different advance, but its a standard engine with a gain of nearly 20bhp, and I got it to pull much harder in the midrange at 3500-4k..Top end would pull to about 6400 before dropping off..

I'd be interested in the mixture you were pushing at those power o/p's? I've got a DeLorean to go a LOT quicker on standard cams by running more advance and kicking more fuel through it. The snag is the fuel economy suffers and 100 miles down the road you start losing oil pressure because it's soaked in fuel. Something I suspect that did for Rob's.

When you start getting into cam changes, we had horrendous trouble with the rocker arms getting eaten. You lose up to 15% from the uneven cam timing too, so straitening that out helps a lot.

After solving that little conundrum, we started playing with the exact advance on the cams and found a real "swings and roundabouts" situation - more advance equalled better power but worse bottom end torque and trouble with knocking at high RPM. Retard it a bit and the throttle response massively improved and gave neck-snapping off-the-line accelleration, but it ran out of puff at top. I suspect we inadvertantly hit on the reason for VVT!

The result we settled on gave the owner a car that isn't as ball-bouncingly rapid as Rob's was, but it's a thoroughly well mannered peppy car to drive. It has two noticable power bands, one at 3000 and a second at 4500 which pulls to the red like a train with the fuelling upped, but we set it at a level where his fuel economy on a long run exceeds a stock engine.

I'll freely admit that carbs are not my strong point, but I don't really need to be considering I have Alex Canini round the corner who can set up even the most complex systems to tick like a swiss watch (eg Aston Martin V12). I'd take care fo the internals, and he can do that bit

I'd be interested in the mixture you were pushing at those power o/p's? I've got a DeLorean to go a LOT quicker on standard cams by running more advance and kicking more fuel through it. The snag is the fuel economy suffers and 100 miles down the road you start losing oil pressure because it's soaked in fuel. Something I suspect that did for Rob's.

No, if anything I had it leaner at top end, hence why it gained in power. Only the midrange needed more fuel, the top end relied on the advanced. The car was run like that for about 2 years, and is still running this setup with the current owner. On long extended full throttle, the manifolds could get to the point of faintly starting to glow, hence why it would be more efficient and better for the engine to fit the tubular manifolds and draw the heat away from the heads to the collectors.

There was no step in the powerband as you can see from the plot.

There was no step in the powerband as you can see from the plot.

Yeah, but the cams were still at 105/107 which is probably there to provide a smooth power band, given our experience of running them both at just about every figure from 105 to 110. PLus the new cams make the engien a very different animal. I had to run 6 degrees at idle and no vac advance just to stop it knocking at high rpm/light throttle. I suspect this might have more to do with being K-Jet than anything else though.

All SOHC engines respond the same with advancing and retarding the cam, either revving off the clock or good low down spread, but they don't really gain much power, it just 'feels' like its going fast due to the big step in the power curve when it comes on song, and the added borewash low down in the rev range.

Why do you think most manufacturers engine's are set around 106-108 degrees. 1 or 2 degrees at most, and its hard to judge what is really better with a fixed management, as one way will make better use of the factory ignition and fueling settings than the other, but on standalone, it may be a different matter. Ive found on a twin cam, such as a T16 Turbo where you can dial in the inlet cam seperately to the exhaust cam, it made big power gains on the rollers even on standard management. IIRC, my 800 made 297bhp at 13psi, previously it made 272 at 14psi with out the vernier pulleys.

But -why- are the odd fire engines set at 105 left bank and 107 right (from memory!) ? Nobody, even Kevin at GTO , knows the answer, and it acocunts for a lot of lost HP when you start tuning them. I reckon it smooths out that power band.

Its to smooth out the off beat idle..

You have to think of the oddfire as two 3 cylinder engines (sort of next to each other) but one is rotating the other way as the inlet and exhaust are 'swapped over', so even though the best scenario would be identical cams as the 'two' 3 cyl engines should be the same, one sides vibrations does not cancel out the others due to the oddfiring nature, so they used different cam timing to compensate..On the even fire it is not a problem as the firing order is equal, and the engine pulses are regular.

This is far more noticeble at low revs due to the fact that the valve intervals take longer due to the engine spinning slowly - at high revs its not noticable due to every thing happening so fast..

Wrong way around.... actually the odd fire engines are smoother - regardless of how the cams are set up. This is why the late 3 litre engines ended up with a balance shaft.

Wrong way around.... actually the odd fire engines are smoother - regardless of how the cams are set up. This is why the late 3 litre engines ended up with a balance shaft.

Thats good logic...

Maybe they went from oddfire to evenfire to make it smoother, then fitted a balancer shaft to make it 'even' smoother...not to make the evenfire as 'smooth' as an oddfire...

Oddfire is lumpy at idle, smooth from 1500rpm upwards.

[stephendell]

This is a strange one. The idea is that using a crank with journals offset by 30° of rotation should result in smooth, even firing.

(The downside of course is that the crank is then weaker than a straight one)

However in a GTA the Atmo (odd fire) engine always feels smoother and revs better (easier) than the Turbo (even fire). Most turbos I have owned or driven have a lumpiness or vibration at the low end of the rev range and just don't like to rev. By contrast the atmo is round to 6K before you know it and smooth as anything.

However this could just be a strange GTA only trait. The 24V engine seemed better but that had the extra balance shaft as well.

I think the move to even fire could have just as much to do with the fact that this was the easy way the engine could be made to run EFI with early 80's technology.

Pretty sure the uneven cam timing on the Atmo was deliberate to make engine smoother though and therfore possibly detrimental to power.

I'm going to press on with the odd fire EFI so will let you know how I get on.

This document is always worth a read: http://www.renaultalpineownersclub.com/ ... manual.pdf

Yes, the same..In all the atmos they are smooth from 1500 upwards whereas in the GTA turbo there is always a slight shudder around 1500, but at idle it is rock solid..

This may be a GTA trait though as I never had this in V6 Renault 25's i've had...

But, the cam differences on the Atmo are to eleviate any inbalance from the FIRING sequence, not from any mechanical inbalances. An evenfire will sit quite happily at say 600rpm, but an oddfire will shudder a bit at low revs.