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MG Midget and Sprite Technical - 45 degree seat vs 3 angle

Ages ago Bob asked me if I had ever tested 45 degree seats in a modified head, the answer from me was no as I could not see any reason to do it.
This week I has a race 1275 midget in it lead to the following I posted on PH....
People often ask what is the difference between 45 seats and 3 angle and are they really needed? I have always thought the 3 angle seats are the first 10-15% increase in power.
We had a restricted class MG Midget in. Restricted to the small standard inlet valves 1.33" (1.401" Cooper S /MG Metro inlets always seem to be worth 10 or so more bhp but we can't use them in this class), a pair of 1 1/4" SUs, air filter free and cam free.
The one today showed 92 at the wheels and sounded soft. It was running a Swiftune SW23T cam. The last similar car we ran up was on our old rollers and showed 103 bhp at the wheels.
I asked the owner and he had done the head himself but had not had three angle seats cut. He had also left a small lip in the inlet ports on the short side as he had inserts fitted to save valve seat sinkage.
With the head back on we re ran the tuning, having optimised fuelling and ignition as was.
We had to retard the ignition about 2 degrees to get the best from the re done head, fuelling remained the same. We ended up with 99 bhp at the wheels. Customer happy and we were happy to test before and after. I was feeling kind today and the customer did all the work removing. stripping, rebuilding and refitting the head.

Peter Burgess Tuning

Here is a pic of the 45 degree seat and the 3 angle seat I cut

Peter Burgess Tuning


Thanks for posting this article. It's great to learn the intricacies of A Series tuning and, especially, to see before and after comparisons. I am always surprised that attention to small details makes such a difference. According to the PO, my Midget has a Metro head that came from an early 80's Halford's Metro Racing Series car. This could explain why it seems to go quite well although I notice it most from its ability to pull easily from tick over in top (4th!)gear.
I look forward to more tuning gems.

Ray Rowsell

Thanks for the feedback Ray. It was too good a chance to miss and it also helped the owner as he now has more power.
As I said I never would have put the time in to try that off my own back as no reason to do so but it makes it interesting to see the 3 angle philosophy is the right one to follow.
When I first started off I worked on a TR4 that won it's championship first year with my headwork. The other heads were running 45 seats very narrow by making the throat really wide. I went 3 angle smaller throat and larger inlet valve (much smaller ex valve) We pushed 148 bhp at the wheels on SUs, the full race Weber ones were pushing 120 ish at the wheels, our guy, on intermediates gave the slick tyred full race ones a good run for their money and dominated his class.....1987 was a long way back :)
Peter Burgess Tuning

I am almost at the point of first start of my 1950 supercharged engine with a PB Supercharger head. Can't wait to see how it goes after I have bedded in the camshaft.
Dominic Clancy

Hopefully like a rocket Dominic
Peter Burgess Tuning

Good stuff Peter
It's nice to have some actual figures/proof
Usually we just roll along with the latest trends and hope for the best,without really knowing
Multi angle, 3 and 5 angle seat jobs have been the go for quite a while here but every now and then someone pulls something out of the bag with just a 45deg. job
My mate ,Simon does a lot of headwork for the local Harley troop and has a carbide cutter that just does a rounded seat and then he 'just' touches it with a 45 cutter, the seats look fantastic done like this, all smooth and rolled in--how much difference it makes- don't know, but he's pretty popular with the leather clad guys so must be getting the results they're after-
William Revit

We have had some fun with Harley heads. We came to realise, once around 2" inlet valves, we needed a complete rethink on how we did the flowing. Using our tried and tested techniques we lost flow! It turned out the Harley needs wide seat on the valve and wide seats in the head, a sort of magnified version of what we do. It lead us to play with an extra bottom cut to keep the valve bowl/throat narrower. We ended up with something like 38 degrees top cut, 2 mm 45 seat, 60 degree and a 75 degree cut. It sort of flows like a perfect smaller valve if that makes sense. The work on the Harley heads has lead us to improve the work on medium sized valves such as Bs. On the small valve stuff we find we need as wide a throat as we can get whilst still retain around 1mm bottom cut/blend into the throat, a juggling act!
I have another before and after in the pipeline. I may have to change the control system and software for my rollers which would bal*s up the testing as we would end up with different sampling, display etc. The car in question is a B. It was fitted with a Fast Road head of ours around 4 years ago, nice and sharp. The owner rebuilt the engine last year and fitted a 285 cam. We tuned it and it was very soft. The CR was too low for the cam. It made less power than the std cam until 4000 rpm then only a little more! The owner came in on a Thursday. Friday 8am he was at our unit so we could skim the living daylights out of the head! He refitted the head same day and sent this email.
Hi Pete!
Just been out in the Mg, WOW!!! Itís a different beast, I can not believe the difference in acceleration. It keeps on progressively pulling through the revs and feels so much faster. I really appreciate you taking the time to do it and Iíll be up in 4 weeks if ok with you.

Let's hope we get the after so we have std cam, CR too low for a hairy cam then CR better for hairy cam.

Peter Burgess Tuning

This thread was discussed between 22/03/2019 and 23/03/2019

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