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MG Midget and Sprite Technical - high speed knock


I do have a frogeye:

I use it for holidays abroad (France to Estonia) where the octane is 95 or even less, so I use to take tetraboost with me. Sometimes I use the car for track days (I use much more, 32 degree, total advance there). It gives much more power especially in low refs!

Problem is low octane high-speed low throttle highway use. When I am driving 70 MPH for more than 3 minutes I hear high speed knock, later it even hesitates. It can only be avoided when I go to curve 4 (see below) or lower. Disconnecting vacuum or driving full throttle also helps. Traffic, road conditions and speed traps make FT not always possible….

Can a Peter Burgess treatment of the head (lower compression) help to avoid this problem?
What treatment: Fast road or eco?


Flip

948 cc
2A629 head, lead free.
AE flat pistons first overbore
H1 carburettors GC needle (mixture controlled on rolling road)
Maniflow small bore exhaust system
123-ignition curve 4 (8 degrees at idle, makes 28 degrees at 4500 rpm)
Flip Brühl 948 frog 59

You should check your mixture isn't weak for the LT/HS cruising (most RRs focus on WOT); does richening up a flat or 2 cure the knocking?

If not, you could always back the static timing off say 4 deg for low octane/throttle high-speed cruising, and put it back for normal use.


A
Anthony Cutler

Anthony: Good question, we controlled the mixture at low throttle.
always good (between 1:12 and 1:13)
Yes I can live with backing the timing wit 4 degrees. I even think about buying a programable 123 ignition, so I can change the curves with a switch. It wil cost me €240,--....
I get much more HP (with high speed knock) when I set the total ignition on 32. The question is can flowing the head and diminishing the CR change the combustion so that I get more HP without high speed knock?

Flip
Flip Brühl 948 frog 59

The lower CR will also lower the low torque and total power....
Onno Könemann

Yes indeed. Will the shape of the combustion chamber make any difference? And what will the squeeze with flat pistons do?

Flip
Flip Brühl 948 frog 59

Flip, I have the same problem I think. I (partly) solved it with another vacuum cannister with a higher vacuum range. It was 4-9-8 (IIRC) and now have 10-15-9 or something like that. In my case it's big carbs, 2 HS4's, so little throttle needed on cruise and thus high vacuum.

I'm now contemplating to change the first spring of the centrifugal advance system for a slightly heavier one.

Anyone happen to know where I can get a assortment of springs?
Alex G Matla

Sure shape will make some difference
But in a recent discussion with PB he stated that he now supported the theory that power gains are made with:
higher CR
good port work

He therefore rather not deshround to mutch (is wat reshaping the chamber does)but rater increase CR to gain power.

This is just for the principle of getting more power

You have a sort of reverse question which PB best answer him self.
Very friendly and helpfull throug mail contact so i suggest you try the source.
Onno Könemann

This thread was discussed between 19/05/2011 and 25/05/2011

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