Valve springs

[Peter T]

After having my freshly built steel X-flow re- mapped and dynoed i am a bit miffed at the results that it produced. Both HP and Torque graphs were consistant and smooth with no large drops or peaks, it resulted in 180hp at 6850rpm and 160lbft, good i thought, but could not understand why peak power was so low rpm as the cam i fitted was a kent 254 power band 4-8500rpm. After further investigation cam timing was checked and found to be ok. Further dyno work revealed another problem after 7350rpm we experienced severe valve float/bounce and was not able to record any power gains after this rpm, the springs i have are double isky springs supplied by vulcan engines and all of my valve train is steel. Can any one shed some light on this matter or offer good advice before i strip it down to find the cause.

Thanks.

 

C7 PWT X-Flow all Steel

Life begins at 40(00rpm)

[Mike Bees]

Cam manufacturers quoted "power bands" are an extremely vague notion. Other than as a rough guide to comparing one cam with another one from the same manufacturer I think the only conclusion you can draw is that the power will probably peak somewhere between those two numbers.

 

The only xflow-powered Caterham I've ever driven (no really) had a Roger King xflow with a 254 cam. Being used to a 200bhp K-series I was fully prepared to be somewhat disappointed, although actually I was rather impressed. It wasn't at all peaky despite coming 'on cam' a bit harder than the K, and it redlined at just over 7000 IIRC (cast iron crank), all of which leads me to doubt that you'd expect the power to be holding up at 8500 with that cam.

 

Mike

[Casbar]

Just had my xflow rebuilt and put a 244 cam in. On the rolling road it showed 135 - 140bhp at the wheels at 7000 rpm. It hadn't reached its peak, but I lost my bottle and wouldn't let them rev any higher.

 

Don't have a steel crank!!

 

Don't know how accurate the rolling road was, as there can be a lot of different readings from various roads.

[Revin Kevin]

Peter,

Why are you using isky springs, my first thoughts are these the correct weight springs for the cam.Why not use kent valve springs recommended for your cam?

NICE TO SEE PEOPLE INVESTING IN PROPER ENGINES , not metro ones

 

CHEERS

CHRIS

 

PS is your car live axle, which one , any reliability problems.

[Peter T]

Chris, its 91 de dion, and reliability it is faultless, i think the x-flow engine is a great engine to tune because it gives good rewards for power to £ value. Also these modern engines (metro) are far to quiet for my liking. Its good to see that there is still a lot of people who appreciate the x-flow.

 

C7 PWT X-Flow all Steel

Life begins at 40(00rpm)

[Peter T]

PS, forget to mention why i am using isky springs, they were recomended to me by an engine builder and yes we knew that there was a possibility that the springs might be a tad to soft for that cam, but rather than load a new vavle train with tremendous strain, we opted to stick with the isky springs untill we had bedded the engine in. However this particular engine is only temporary and will be for sale soon, as the real monster is being built as i speak! So i am reluctant to spending more money to replace the springs given that the hp it is producing.

Thanks.

 

C7 PWT X-Flow all Steel

Life begins at 40(00rpm)

[oilyhands]

I'd double check that the installed height and over the nose pressure is as you would expect, the poundage may be correct when the installed height is right, but if the heights are wrong it will all be lime jelly.

 

Oily

[Roger King]

6800rpm is about right for the peak power on a 254, assuming the test was on an engine dyno. If this is from a rolling road it seems a bit high because increasing transmission/tyre losses tend to outrun engine power increases as revs rise. 160ftlb of torque is a monster figure though for a Crossflow - we don't normally reckon to see more than 140ftlb on an 1800.

 

I agree with Mike Bees comments about quoted power bands.

 

The Isky would be our spring of choice for this cam (certainly over the 'official' spring), but there is a lightweight version available for OHC engines. This might give problems as described, but we've never tried it. For the record the Isky (which is orange) is good for 8500rpm on a 264, so your usage shouldn't be a problem. Have you got roller rockers, because these will not work properly with this cam? Doesn't sound like it from your description on steel valve gear.

 

Spring height is important and so is the mass of the valve gear - valves, retainers, etc - but your use doesn't strike me as being particularly difficult for the springs. Are they the real thing?

 

[Peter Carmichael]

Oi! There's rampant engineism on this thread. Stop it!

 

Peter

The orange one with the noisy metro engine

[Peter T]

Thanks for the advice Roger, they are orange but i am going to strip the head down and double check anyway, and while its of i am going to install some Titanium valve spring retainers to try and reduce some valve mass.

 

C7 PWT X-Flow all Steel

Life begins at 40(00rpm)

[Graham Perry]

Valve bounce at 8500 on a steel cross flow, Eeeek, I had better cut my rev limit a tad, although I must admit I have never noticed this. A recent, ahem 'accidental excursion' to 10,000+ at a sprint didn't reveal it either, but I was probably too busy trying to find a gear to notice !

 

As usual, excellant advice from Roger.

[Revin Kevin]

ROGER

Just out of interest why don't roller rockers work with 254 cams.

 

CHRIS

[puremalt]

Why don't you expect to see over 140 ish lb/ft on a cross flow? Mine dynoed at 138 after a recent rebuild. My ignorance lead me to think that a X flow would have greater torque per cc than the shopping trolley, sorry Metro, engine.

Incidentally the K series in my mates Elise sounds really pitiful after my snorty monster.

 

Dave Robertson

[Roger King]

Blimey! Lots of questions.

 

I didn't meant that you would get valve bounce over 8500rpm with Iskies, merely that their life expectancy would be reduced at very high revs with a 264. We used to use Hart 420R springs, but sadly they haven't been available for a long time. If you run an A8, you'll get away with much higher revs with Iskies - well out of the power band though.

 

All the family of cams that the 254 belongs to suffer from problems to a greater or lesser degree with rollers. It appears to be related to the effective mass of the rollers coupled with the valve train accelerations the cam produces. You can encounter a period of surge where the spring can't control the valve, but the engine will run cleanly again at higher revs if you force it through the nasty bit. This effect is worst with a 264, but I've seen engines in trouble running a 244. This is not a roller criticism, but an illustration that parts have to be compatible. I am not the only engine builder to have encountered this problem - and yes there will always be someone who has such an engine that runs well.

 

With regard to torque Puremalt, your figure of 138ftlb sort of backs my argument up. As a very rough rule of thumb, once you give an engine a decent exhaust system and a free flowing induction system it's max torque is determined simply by it's capacity. I fully admit that this is a GROSS oversimplification and does not take into account resonant tuning, 2 valve versus 4 valve heads, injection versus carbs etc. If you tune the same engine for more power, you will still see roughly the same maximum torque, but it will be higher up the rev range. The trick is to keep the shape of the torque curve as flat as possible if you want the engine to be pleasant to drive. Whereas you might be able to double an engine's power output with tuning, the peak torque gains will be much less spectacular. Forced induction is a different matter entirely of course.