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MG MGB Technical - Making CR compatible with cam

Further to my mention of this on the Midget threads....

About four years ago we sold a Fast Road MGB head to a customer and dynoed the standard engine with K&Ns and nice exhaust at 86 bhp at the wheels, standard cam and around 9.4:1 CR. Customer rebuilt engine +20 thou" and fitted a Piper HR285 cam, he did not increase the CR to match and we reckon he was about 9.5:1, we tell folk to aim for 10.5:1 to match the 285 cam. The engine was flat on the rollers and maxed out with a dismal 90 at the wheels.with less power than the standard based engine below 3500 rpm. We dynoed on a Thursday and the next day at 8 am the owner was on our doorstep so we could skim the living daylights off the head! We raised the CR to 10.3:1 as we couldn't skim any more off the head! Today (Friday 5th April) we ran the higher CR engine, excellent results, engine sounds very sharp and maxed out at 96 bhp! Blue line std based engine, dark red line 285 cam with low CR, lighter red line Cr up to 10.3:1


Peter Burgess Tuning

Really interesting Peter, how much do you think is just down to upping the CR?
Allan Reeling

Peter
I've got an old formula here somewhere for working out required increases in compression ratio compared to the amount of compression stroke piston travel for various exhaust valve closing points---all I have to do is find it
willy
William Revit

sorry- should read inlet valve closing points
William Revit

As with most things in life it's all about getting the balance right (but in this case also maths and engineering niether of which I know about).

My mate took his car on a (charity) RR single graph run and because it couldn't do the run above a certain revs because of a misfire basically got told what I've been telling him for the last two years, sort the basics out (in this case plugs. leads, coils, etc.).

He's had the car 5+ years and it's still on whatever it came with despite being MoT'd and 'serviced' (oil & filter now and again perhaps) each year. He only bought that particular car (not a MG) because it was chipped to give (an unbelievable) increase in power and I'm sure it ran at well below standard power (as was just about proven on the rollers).

Peter,
just so that you know, I wasn't there but I believe it was running at a murder of planks bhp and a pride and a half sealions torque, it's a Japanese engine (in a 'british' car) so those figures are probably metricified.
Nigel Atkins

Hi Allan
The cam will not work efficiently without the increased CR and the increased CR would be a bit much for road use (and fuels) with the standard cam. In a nut shell the 285 pushes the power band up a 1000 rpm or so.

Nigel never trust planks and sealions :)
Peter Burgess Tuning

Peter I kept well away, as you know, I'm not interested in ego graphs and dial-your-own-figures runs but I wasn't sure if planks and sealions were metricifacted or not but I'll not refer to that system again thanks to your advice.
Nigel Atkins

Maybe we will return to Imperial units once we get the Empire back .....

on a more practical note, the timing & overlap of the 285 reduces the effective running compression ratio so the static ratio has to be increased to compensate.

The last time I did a calculation it showed that a 9 to 1 static ratio was around 8.1 to 1 when running.

I usually aim for 9.5 to 1 on a standard cam so that would equate to Peter's suggestion of 10.5 to 1 with a 285.

I think a better measure is compression psi - I normally look for 180 - 200 psi on a tuned engine depending on fuel octane.
Chris at Octarine Services

Peter-
Yep, that's engineering for you. There are no "change one thing and the engine will come alive" solutions. If one thing is changed, then something else has to be changed in order to go along with it. It's always a total package which is the sum of its parts.
Stephen Strange

Hi Peter
How is it going?
I have found similar with engines I have built, with wilder cams liking even more.
On my own engine, originally set up for 310-320 deg cams so CR 11.5-12:1, I have on occasion reverted to std cam for particular events and had it running ok on pump fuel (98 octane) by reproducing the advance curve. Pretty similar to std rubber bumper advance curve, slow to start and not peaking till 6000, at about 28 deg. Goes very well! But set-up needs to be careful so I don't do anyone else's car like that.

For std cam I usually go for 10:1 with an advance curve going through 20 deg at 2000 and on to a Max of 30 at 4500-ish. Works a treat but sometimes the vac unit needs scaling back too.
Paul Walbran

Hi Paul
We are ok thanks, plodding on getting older but no wiser! Interesting reducing timing with a B timing curve at lower rpms to make the cam 'behave'. I suppose it is why programmable ignition is so good.
Peter Burgess Tuning

You and me both!
Programmable makes it easy now, but I have done plenty in the days of weights and springs. Amassed a big collection of 25D units from varying cars to provide a suitable supply of a good range of springs. Like most things, you get an eye for it after a while and settle on the right combo quite quickly.
Paul Walbran

Funny you should say about getting the knack of it, we are training a new 22 year old employee to do all the work we do. I think he is amazed at times how we can 'guess' jets and needles, alter jets and needles, play with timing and achieve good results, we explain this knack has taken 34 years, 57 combined for Keith and me...and still learning new things and ways things can go wrong!
Peter Burgess Tuning

A new 22 yo employee Peter, I hope he knows how lucky he is--------
On the compression matching subject, I've had a little remembering that I would like to add

A few years back I had my camshaft grinder mate grind up a pair of sporty cams for the Elan 290 In 285 Exh---All excited, put them in and set them up and was warned it would probably idle a bit rough--No such luck smooth as a baby's but, didn't go any better at all except from about 4500 onwards and then only minimaly better
Had the engine out fixing oil leaks 6 months later and decided on a set of rings while at it. One piston was a bit dicey so went for a new set-couldn't get the std 9.5 pistons and went for a set of 10.3---The result was unbelievable, idled strongly with a bit of an angry roll and is as strong as an oxx right through the rev range--which is how it was supposed to be originally
Only 8 points rise in comp.ratio but that's what was needed---
---backs up your result in the original posting
Keep em tuned
willy

By the way, how's progress with the engine dyno room - up and going yet---
William Revit

This thread was discussed between 06/04/2019 and 17/04/2019

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