8 replies to this topic

[Gaz66]

How do I find out recommended compression ratios for 8 port cams?
Piper details list everything bar suggested CR
I seem to remember Mr Vizard giving suggested CR's for the megadyne range back in the day but can't seem to find anything about compression ratios anywhere now, or probably I'm not looking in the right places as usual 🙁

Edited by Gaz66, 11 February 2025 - 05:28 PM.

[DeadSquare]

How long is your piece of string ?

 

With a quality head like that, are you competing, how hot is the cam and what Octane can you get ?

 

I had a 7 port and ran 11.75 on 105 octane in a race engine.

[Spider]

If I can suggest if you are doing this kind of a build, work to Dynamic Compression Ratios are I feel they are WAY more relevant than just (Static) Compression Ratios.

I start this by establish what Octane fuel is being used, then Street or Track use, then looking at the Cam and it's characteristics and how I'd time that in to place the power band where I feel it needs to be.

 

I work to 8.4:1 DCR for the street and 8.6:1 DCR for the track, these are both on Iron Heads and is a little higher for Alloy.. While these numbers to someone who used to working with SCRs may appear low, the 60's Cooper S has a DCR of 8.4:1 and a SCR of 9.75:1.

The longer the duration of the Cam, the more SCR you need to end up with.

 

<EDIT: The DCR Numbers I've given here are for 97 Octane Fuels >

Edited by Spider, 12 February 2025 - 08:21 PM.

[Gaz66]

The engine will be a race engine for track use.
Cam durations which I am looking at are 310-320
It looks like the highest octane fuel I could possibly get is 105 (with octane booster) so I'm presuming that will be the limiting factor.
From Vizards seminar he said he was running 14.1 in the 310 megadyne but I'm not sure if this was in the old days of 4* fuel + booster.
This seems very high to me but maybe wasn't with the old fuel.

Edited by Gaz66, 11 February 2025 - 07:06 PM.

[imack]

He mentions a minimum of 13.5:1 cr for a 310 megadyne in the last edition of his book. Looking at the timing figures it may well need 14ish to 1 static cr to achieve a decent dynamic cr.
In my experience, the only way to work out dcr is to physically measure it during a dummy build. But it changes significantly depending on how you time the cam in and what valve clearances you use. It's also not particularly easy to determine exactly the moment the valve has closed even using a dti. Very easy to be off by a couple of degrees at the crank and then the pistons at a different height! It's fairly time consuming to get consistent, accurate results.

[Shooter63]

If you are serious about a 8 port set up, you need to put a deposit down A.S.A.P, there is a batch being made at the moment but its heavily subscribed, something in the region of 80%

Shooter

[Gaz66]

If you are serious about a 8 port set up, you need to put a deposit down A.S.A.P, there is a batch being made at the moment but its heavily subscribed, something in the region of 80%

Shooter

Deposit with who? I thought it was just a case of waiting till they were in stock then buying one.
To be honest I've never seen them in stock over the last two years or so at med or minispares.
So it's a case of putting a deposit down then they order it?
I think it's going to be a case of what's available when I have the money available as I still need a crank and rods too.
It's going to be an expensive year 😁

Edited by Gaz66, 11 February 2025 - 09:50 PM.

[Shooter63]

If you are serious about a 8 port set up, you need to put a deposit down A.S.A.P, there is a batch being made at the moment but its heavily subscribed, something in the region of 80%

Shooter

Deposit with who? I thought it was just a case of waiting till they were in stock then buying one.
To be honest I've never seen them in stock over the last two years or so at med or minispares.
So it's a case of putting a deposit down then they order it?
I think it's going to be a case of what's available when I have the money available as I still need a crank and rods too.
It's going to be an expensive year

As you have said MS or MED, demand out strips supply hence the need for a deposit, the heads are very nice, excellent ports etc but there aren't a bolt on job, I doubt you would see much change out of 10K to support the heads potential.

Shooter

[DeadSquare]

The engine will be a race engine for track use.
Cam durations which I am looking at are 310-320
It looks like the highest octane fuel I could possibly get is 105 (with octane booster) so I'm presuming that will be the limiting factor.
From Vizards seminar he said he was running 14.1 in the 310 megadyne but I'm not sure if this was in the old days of 4* fuel + booster.
This seems very high to me but maybe wasn't with the old fuel.

 

 

The engine will be a race engine for track use.

It looks like the highest octane fuel I could possibly get is 105 (with octane booster) so I'm presuming that will be the limiting factor.
 

I know it cost a lot to fund a retirement in the "Gulf of America", but unless you are simply, good enough to win, may I suggest that you go to someone like Richard Longman, who has the skill and history of getting the last few horses out of his expensive cup-winning engines.

 

I started motor racing with the Olympian ideal of 'taking part' in a home prepared car, but when I got the chance of an ex-works Downton engine, I'm very glad that I spent the money on it.