X-Flow - A Brick Wall

[greg]

But in simplistic terms reving to 7K means that you change gear less, hence more time that foot is on loud pedal, and therefore bigger grin:

 

grin = time on loud pedal / time

 

Seriously my non standard Xflow will rev to 7K quite happily, and then hits a wall, so I change below this.

 

From what I've read / been told / could be completely wrong / the standard pistons are of variable quality (get some forged ones!) and the valve train is simple driven (is this right ?) and the springs are a little weak then high (read near 7K) is probably a bit over kill, easpecailly as peak power is arround 6K

 

I would assume that you should change just after peak power, so that you in the next gear you are just below. Someone correct me if I am wrong. Where this is depends on your engine and gearbox.

 

Greg, Q 880 RAE (Green/Ali XF)

[Casbar]

Before I the re-build of my xflow, it was at peak power around 6000. Now with forged pistons, 244 cam etc it will rev to 7500, but I have it limited at 7200.

 

So for standard pistons, 234 cam etc, around 6000 is all you should expect

[hugh]

does anyone have a bench mark for what a caterham supplied 1700 x/f engine should rev to and the power it produces, as it would be intresting to compare it to my 1650 bitsa crossflow which is balanced lightened 234 cam, lotus twin cam pistons and well put together head built by gemini engineering, this engine revs freely past 7000 rpm but does not have much power. would there be any benefit in changing the cam to a 244 with vernier (rally cam ?) and unrating my ancient carbs to weber 45,s p.s. can the cam be changed with the engine in the car.

[cdg]

Hugh,

A cam swap on a x-flow is very difficult without removing the engine from the car as it's buried deep in the block, and I would imagine space to be at a premium.

 

Also, and more importantly, as soon as you remove your present cam the followers will drop down, and so to insert the new cam necessitates inverting the engine to hold them in situ. I have heard of people who manage to do it without inverting the engine using an array of rubber suckers to stop the followers from descending, but it sounds a little "Heath Robinson" to me!

 

Best to take the engine out and then it's all so much easier. It would also be much easier to assertain the engines general health before undergoing upgrading.

 

I would certainly think about an upgrade to 45s, but if you're planning on road and sprint use I see no point in going for chokes any larger than 36mm, which is what the carbs come with. To be honest, the power you can expect depends on many factors, but I would have a good look at how well the head has been ported, valve sizes, inlet manifold, exhaust manifold.etc.

Using "twink" pistons means you should have plenty of clearance to run a 254 cam which gives a little more "oomph" than a 244, and yet allows the car to remain driveable at lowish engine speeds.It's a great cam for spirited road and sprint use. Make sure you've got some decent big-end bolts to stop your rods making a bid for freedom.

 

If your head's any good you should see between 150-160 bhp at the flywheel, and good mid-range torque.

 

Hope that helps.

 

Chris.

[MARK WARREN]

cskip,

Did you sort the under reving problem?

Mark.

[hugh]

thanks for the advice chris the engine had new rings and cossie big end bolts 5000miles back and the head has been modified (new guides and inserts)by burtons 120miles ago to take unleaded the engine is in reasonable nick so hopefully it will cope with a new cam and carbs.

[ashaughnessy]

Well, a bit of a late reply but I haven't been surfing for a few weeks.

My standard supersprint likes to rev to about 6300-ish and then feels rough. Peak power (according to an old rolling road session) is about 5800 or 5900. Power starts to come in strongly between 3000 and 3500. It revs very freely up to my self-imposed limit of 6000 (or 6200 when the red mist comes down :-) and within that limit never feels like it's unwilling.

Just another random data point in the survey.

Anthony

[cskip]

Anthony,

 

I have decided that the only way to truely sort this is to take the car to a rolling road and see what is happening.

[Andy G]

Having contributed to this posting I make no apologies for reviving it and adding what may be the final word but knowing Blatchat probably isn't.

 

As will be seen from previous postings I encountered valve bounce at about 5950 RPM,an examination by Clive, who maintains my car, lead to a severe ticking off. Valve bounce in this case caused broken collets, a session on the rolling road sorted the timing out and balanced the carbs.

 

Result oodles of power up to 6500 RPM, I daren't go any higher in case I get valve bounce again and Clive tells me off, but I am fitting a rev limiter.

[bstark]

Hmmm... they do seem to be a bit variable, don't they. Mine still sings to about 5500rpm and then it just seems right to change up rather than force it.

 

At the rolling road session last year, the guy who tuned it mentioned that the compression test figures were very high on a couple of cylinders and that could cause pinking - which would also limit the revs. Any thoughts on this?

 

Bob Stark

Supersprint first-timer