1600 cc engine with 1130cc Volumex, detonation

[tmvolumex]

I was just browsing the photo section and noticed the neat Spitfire with the 1600 cc Volumex engine belonging to TS131Volumex. I don’t know what supercharger drive ratio he will be running but if he uses the stock VX pulley diameters (crank and supercharger, 1.3 drive ratio) he will be running a fairly high boost pressure for pump gas. Too high, in my opinion, for detonation free operation depending on the engines compression ratio and cams. I built and drove, for 150,000 miles, a 1756 engine with the stock VX supercharger drive ratio. On that engine with a 40 DCOE carb, 8.0 to 1 compression and 40-80 cams, the boost level was 8.5 PSI. The engine ran detonation free and was very fast indeed. That same setup on a 1600 will generate over 10 lbs of boost because of the smaller engine displacement. As a minimum, I would install a boost retard ignition system. IE the ignition system senses boost pressure and retards the ignition timing in proportion to boost pressure. This is the simplest and least expensive way to attempt to keep detonation under control. I briefly tried an engine knock sensor, because I was considering increasing the boost pressure, but found that the supercharger noise confused the knock sensor and retarded the ignition timing when it was not needed. This was one of the very first knock sensors, 1980’s design; maybe they are better these days. Today I run a 2 liter VX with cams, DCOE carb, GC head, 8.5 to 1 compression ratio, big exhaust, increased supercharger drive ratio etc and an ignition retard CD ignition. I have no detonation issues even on California’s lousy 91 octane gasoline but it is close.
tmvolumex

[Guy Croft]

Some good points Tom,

it's a GC built low comp unit (7/1), big valve, fully flowed, GC comp inlet cam and with stock pulley ratios. Should be OK, but thanks anyhow.

GC

[tmvolumex]

Guy,
Yes, with the low 7 to 1 compression and UK high octane gas it should be OK.
I wish we could get high octane gas like that sold in the UK here in the US. Worse still is the fact that gas sold in California is one point lower than what is available in other states in the US.
tmvolumex

[TS131Volumex]

Yes , thank you for your concern tmvolumex , you are clearly knowledgable .
I am about to sell a more standard MK 3 Spitfire to help fund completion of MK 4 131 V Spit . After 7 years being built by Kev , Guy and Dave it is more than due for a maiden fire up and test run ( hopefully with all involved included ) - bring on the summer ! Matt .

[tmvolumex]

Matt,
Let us all know when your Volumex (VX) Spitfire is up and running.
I see your board name is 131volumex. Do you have a Volumex in your 131 or is it a Volumetrico? Got any pictures of it / the engine?
tmvolumex
Fiat 124 Sport Coupe
Fiat 131 2 door Coupe
Lancia Scorpion / Montecarlo
Lancia Scorpion / Montecarlo VX

[TS131Volumex]

No tmvolumex , no 131 , just the Spitfire . But yes , will keep progress posted . Thanks . Matt .