supercharging a realy high comp RV8

disco-v8 · Post by **disco-v8** » Mon Oct 20, 2008 2:55 pm

no offence taken...

its been a confused post from the start, i was intreasted in supercharging my RV8 which ive had loads done to it to produce 15BAR on crancking pressure, which i think is very impressive and maybe abit to high for charging..... the confusion came about when i stupidly put 15 PSI

so i think alot of you thought i was prducing 15psi BOOST with out a charger

im sorry for this confusion that ive coused....

so do you recon it can be done with a small charger????

the work ive carried out to get 15bar crancking pressure was, having the complete engine rebuilt about 5000miles ago, ive had both heads skimmed, and i believe they have already been skimmed once before, stuck the thinner tin head gaskets on, ive ported the heads quite abit, put uprated valvespring on (one less coil) rhoads lifters, adjustable pushrods, crower torquer camshaft (will get the figures for this later), arb head stud set, msd ignition, msd coil, K&N airfilter, larger 4.6 airflow meter.......... thats all i can think of at the moment but im sure ill remember sumthing later.....
....... the heads have been lowered that much to increase the CR i had to enlarge the bolt holes on the inlet manifold just to get it to fit

will be getting a programable ECU when i got the funds sorted so will be able to remap it all

Post by **Rossco** » Mon Oct 20, 2008 3:13 pm

Ok, I'm now on board with this.

First off a static compression test of 220psi would indicate that forced induction is probably not a good idea, but there's always a but.

I ran a 3.8 (4.2 crank in a 3.5 block) at around that mark and still put 7psi on top.

Two reasons why I could do that.

1. I ran only LPG so the extra octane helped prevent detonation.
2. The cam was a custom profile with a very late closing inlet valve.

Google "Miller Cycle".

Until the intake closes the engine makes no compression and on a supercharged engine the supercharger is far more efficient at compressing the inlet charge in the first 90 degrees of the compression stroke than the piston is. The charger will not add as much heat to the charge as the more inefficient piston will.

So if you have a cam that leaves the inlet open until about 90 or even 95 degrees ABDC the charger will have done the initial compression of the inlet charge leaving the piston to do the bit its good at, that being the last 90 degrees or so.

The result is you can run high static compression ratios and still use boost.

Hope that gives you some ideas

Allan

chodjinn · Post by **chodjinn** » Mon Oct 20, 2008 3:20 pm

or you could run comp gaskets since the head has been skimmed twice already which would lower the comp ratio by approx 0.7. You could then run a 'charger, which with a small intercooler would be ok at low boost.

disco-v8 · Post by **disco-v8** » Tue Oct 21, 2008 8:49 am

so why is it a bad idea to boost an already high comp engine????

from my understandings is that the boost is pressurising the cylinder to what ever the boost is limited to, so having a high CR is sort of just helping it to build up the pressure!!!!!

ive seen old 80/90's small engines with boost of 60psi and producing 1500HP, just like the old bmw 1.5 turbo engine.... so i would of thought that boosting at normal pressures wouldnt be to bad....... so with my extra 5psi or so that ive added by just increasing the CR i could take off the final boost the charger produces i.e if i want to run 1.5BAR boost (22psi) i take off the extra 5psi and just boost it to 17psi????

or is there alot to more to it than im understanding oat the moment?????

the 1.5bar boost i mentioned is not an actual figure im hoping for just a figure i used for explantions

kiwicar · Post by **kiwicar** » Tue Oct 21, 2008 9:17 am

Hi Disco
With a petrol engine you aim to burn as much fuel and air as you can as at a controlled rate to give as large an average pressure above the piston as you can, this gives you maximum power. If you create too much pressure and therefore heat in the cylinder it explodes (often called pinking) which is not good, the shockwave of this explosion breaks pistons, the extra heat generated by this explosion melts pistons and causes the alluminium of the heads and pistons to burn, also not good.
Lower compression ratio with external boost has three effects, one the chamber is bigger so more fuel and air can be stuffed in the space for a given initial cylinder pressure secondly as the chamber is bigger the flame front takes longer to cross it so post ignition cylinder pressure rises more slowley when the mixture is ignited, finally as the CR is lower the rate of drop of cylinger pressure is lower as the piston goes down the cylinder, again the average cylinder pressure is higher therefore more power.
Mike

CastleMGBV8 · Post by **CastleMGBV8** » Tue Oct 21, 2008 9:24 am

In a word BANG.

A supercharged engine reqiuires reduced static compression to compensate for the additional boost being applied and the higher the boost level the more complex it becomes to control combustion and avoid detonation.

In high boost applications it is usual to have an intercooler, possibly water injection and sophisticated ignition control with knock sensors to detect and avoid meltdown of the engine, and in the case of the BMW F1 engine a very special blend of fuel to allow the engine to obtain that level of power delivery.

Most otherwise standard engines will only tolerate 6-8 psi of boost unless other modifications are done.

Kevin.

kiwi303 · Post by **kiwi303** » Tue Oct 21, 2008 9:29 am

It can be pretty much explained as the higher the CR when boosted, the sharper and harder the combustion. to hard and fast and bits break.

Most turbo engines run at 7:1 to 8:1, whereas a modern high performance NA engine is often up to 11 or 12:1 and uses some careful ECU routines and knock sensors to adjust timing to avoid knock.

chodjinn · Post by **chodjinn** » Tue Oct 21, 2008 9:33 am

Read up on static and dynamic compression ratios, then that might shed a bit of light.

Basically, don't boost your engine unless you lower the comp ratio by a fair amount, or you'll need a new engine pretty quickly.

In my twin turbo set up, my comp ratio works out about 8.2:1. That is achieved using standard 9.35 pistons, with comp gaskets, unskimmed heads, and by having the combustion chambers fettled to increase their size, ARP bolts throughout. I'm running a cosworth intercooler, and even then I'm only going for 7-8psi max just to be safe.

You want to run more boost, then you'll need forged pistons etc. and that is when it starts getting expensive.