underpowered pump?

Post by **Peetee** » Fri Sep 11, 2009 1:00 pm

What is the potential of a standard replacement fuel pump? I ask as recently I had my car rolling road tuned and the tester - a very experienced hand in tuning minis - struggled to get more than 60bhp from the old thing. Given the spec and condition of the 1275 motor in Albert that was a bit low. A standard A+ should push this out and mine has a K&N, alloy inlet and large bore LCB and system. I would expect that to loosen up another 10 bhp. Could the weak link be the fuel pumps capacity to deliver? It is a brand new standard replacement item.

Post by **bmcecosse** » Fri Sep 11, 2009 2:56 pm

Doubt it! But test it's output by timing how long it takes to fill a gallon container.
The '60' bhp - was that achieved after carb needle change and dizzy tweaking ? Also - have you compression test figures for the engine ? And what size carb is it ? If HIF 38 then 60 will be about right - especially if it has not been 'Vizardised'.

Post by **autolycus** » Fri Sep 11, 2009 5:59 pm

As a very rough guide, an old-technology petrol engine will probably give around 10-15 bhp for every gallon per hour of petrol at full throttle and high revs. So 60bhp at the wheels, or, say 65 at the flywheel, might need around 5-6 gallons per hour. Single SU pumps are typically rated at 7-8 gallons per hour, so not a huge amount in reserve.

Kevin

Post by **PSL184** » Fri Sep 11, 2009 6:01 pm

I would say that if the engine was running without mis firing or cutting out then it was getting enough fuel - just pumping more fuel in won't give you more power !! Where was the 60 hp - at the wheels or flywheel?

Post by **Peetee** » Fri Sep 11, 2009 8:47 pm

The compression was low at 155 due to work on the head. I intend to fit a worked large valve 12G940 head but wondered f I needed to address anything else too.

Post by **bmcecosse** » Fri Sep 11, 2009 8:56 pm

Large valve head and decent compression ratio will obviously help. You are looking for ~ 170/180 psi compression figures (200 would be even better!) - an MG Metro camshaft will help - and you really need an HIF 44 carb to get the best from a 1275.

Post by **MarkyB** » Fri Sep 11, 2009 9:48 pm

Don't they use a CO2 meter or similar to keep an eye on the mixture?
Or were bigger jets making no difference?

If the operator was "very experienced" what was his opinion?

Post by **bmcecosse** » Fri Sep 11, 2009 10:08 pm

You don't change the 'jets'! Assuming SU carb anyway!

Post by **Peetee** » Fri Sep 11, 2009 10:40 pm

If the operator was "very experienced" what was his opinion?

He was stumped. I didn't offer an expectation of bhp with my set up. He probably hadn't ever seen that spec with a standard cam so didn't know what to expect either. He suggested that compression was low but wouldn't be drawn on whether raising the CR would address the shortfall.
It does have a HIF44 btw.

Post by **rayofleamington** » Fri Sep 11, 2009 10:47 pm

Single SU pumps are typically rated at 7-8 gallons per hour, so not a huge amount in reserve.

Running from a battery (~12.6v) you'll typically get a gallon in 6 or 7 minutes. This should run a V8 in normal tune with some capacity to spare. If you've got an alternator, running voltage will be typically 14 to 14.5 volts which will speed it up but only a few % as the limiting factor is the return spring.

Post by **bmcecosse** » Fri Sep 11, 2009 10:51 pm

Good that it has the HIF 44 carb - do you know what needle ? Did he try a variety of needles ? When it was running flat-out on the RR - was the carb piston right up inside the carb ? This is something you can only see on a RR with the Minor. On a Mini - the speedo can be removed and the carb watched on the highway!! So - I'm interested to know if it was all the way up !

Post by **Peetee** » Sat Sep 12, 2009 1:25 am

He did try a variety of needles but no idea if the piston was all the way up.

Post by **bmcecosse** » Sat Sep 12, 2009 9:26 am

Ahh - well that's critical - if it only goes half way up - then the air flow is horribly obstructed, and obviously the needle is working in the wrong range of it's length! The piston MUST go all the way up for decent performance - if it doesn't (assuming it's not sticking) then the spring is too strong and a weaker spring should be fitted (or the existing spring shortened).
I guess he wasn't 'experienced' at all! This is very basic stuff!!
No great harm if the piston gets all the way up before full revs - the needle is then working in the very end range of it's length, and fuel flow will simply be regulated by the amount of air rushing through the venturi. The idea is to use as weak a spring as the engine will reasonably take - without gasping/stumbling - and of course the viscosity of the damper oil comes into play too during acceleration.

Post by **Peetee** » Sat Sep 12, 2009 12:52 pm

Oh I wasn't meaning to infer he hadn't checked - only that I wasn't there to observe if he did either. He is very experienced having been a main player in race tuned minis for many years. I guess the point of bringing this up is to establish if there any inherant issues with tuning a Minor that are not commonplace with Minis and he may have overlooked.
Perhaps it is just a shortfall that can be attributed to the compression.

Post by **bmcecosse** » Sat Sep 12, 2009 9:02 pm

Much the same engine - but with the 'small' valve head and standard cam - 60bhp is not unreasonable.
" main player in race tuned minis " - not Calver - I hope !!!!!!!!

Post by **MarkyB** » Sun Sep 13, 2009 8:05 am

Which K&N ?
Some of the smaller cone shaped ones have end plates rather close to the carb inlet.
It's not as if space is very limited in the Minor engine bay

.

Post by **Peetee** » Sun Sep 13, 2009 8:55 am

It was one the RR operator fitted himself.

Post by **bmcecosse** » Sun Sep 13, 2009 10:27 am

Hopefully - it's a nice deep cone type - and is the hole large enough to match the HIF44 inlet ? It's picture time ! If not new - was the element cleaned correctly - and re-oiled with the K&N special fluid?

Post by **Peetee** » Sun Sep 13, 2009 7:58 pm

The air filter cone is the correct size for a HIF44. not only is it matched but the head is matched to the manifolds too by the magic that is 'spare gaskets'.

Full spec is:
1275 A+ (in excellent, leak-free order)
Ported, standard valve 12G940
1.75" Peco (long) water heated inlet
3-2-1 large bore manifold and two box stainless system.
HIF44
K&N cone

That's got to add up to more than the Ital's 60bhp in my book

Post by **Mogwai** » Sun Sep 13, 2009 9:14 pm

All the ingredients are there for more power
a few extra things to check would be the valve timing, can be done with a dti & degree disk so will only need the rocker cover removing
another thing could be a slightly restrictive silencer would need to do a backpressure test (welding an adaptor to exhaust required). all this could also be checked with a vacuum gauge fitted to the inlet manifold.
another thing is the inlet definitely a 1.75" as ones intended for 1.5" carbs can look identical apart from the inlet hole & throat sizes