CR versus FI

[r_towle]

At what point (If any) does the compression ratio become to high for the stock Djet FI system?

9:1?
Anyone gone higher than that?

Rich

[bd1308]

oh snap. this is intresting.

[Bleyseng]

No, I think its the question of cam. Djet can use only such a mild cam for the proper vacuum signature so raising the CR beyond 9to1 becomes a HEAT problem due to the cam.
I am using a Raby 9550cam with 9t1 CR and its working good so far. Still tuning the Djet.....

[KWales]

Hunh?

Compression ratio and D-jet systems are not related. Compression ratio and KNOCK (destructive detonation) are. Up the compression ratio, you up the chances of knock and damage to your engine. Has nothing to do with the fuel system type.

Think of it this way. Piston moves up in cylinder to compress fuel air mix a certain amount. For a given stroke of the piston, to increase the compression, one of 2 things need to happen.
1. Add material to the top of the piston
2. Reduce the combustion chamber size in the head.

Ya can up the CR and screw with the distributor timing. Max power is right at the hairy edge of knock so there is a little gain there. Go too high on CR and changing the timing wont help with knock.

Ken

[Bleyseng]

Ken, you are thinking in broad terms. Yes, of course this is true but there are limits to Djet as its a hard wired FI vs todays modern FI.
The cam is one limit as too much overlap and the vacuum sig is too lumpy and the MPS can't read it properly as the diaphram pulses.
HEAT from the higher CR (higher burn temps) is what you need to address, usually by cam change so you are back to square one with Djet and its limits.
A larger exhaust valve (38mm) helps too to bleed off the heat out the exhaust.

[r_towle]

So really we are dealing with a heat issue.

9:1 seems to be a tested limit..has anyone gone higher than that?

Rich

[lapuwali]

IMHO, if you want to start playing around with CR, then you need to start playing around with cams, too. Dynamic CR is strongly affected by the cam you use, so you can use more CR with a wilder cam. CR and detonation are also affected by combustion chamber shape and size. Smaller chambers are inherently less detonation-prone, so you can usually run a higher CR (which is good, since a smaller chamber will, by itself, raise CR).

To do all of this, switch to PEFI. You'll get a lot more performance this way, and you won't be playing within D-Jet's narrow limits. No reason to stick with it if you want to build a performance engine.

[Dave at Pelican Parts]

Originally posted by KWales
Max power is right at the hairy edge of knock...

Not universally true. Grassroots Motorsports (or their sister mag, Classic Motorsports) did a "dyno testing ignition modifications" on an MGB a year or two ago. Max power was actually produced by timing somewhat retarded from "the hairy edge of knock".

Of course, this varies from engine to engine--and an MGB tractor engine is not a 914 Bus engine.

--DD

[r_towle]

I see your point, and it is a song I have heard before.
The question is what is the limit of the STOCK FI system.
I know I can increase CR by reducing the combustion chamber volume, that is a given.
I am curious how far I can go...
I guess I will be the test mule for this question, and of course timing will play a major role here, but the FI system is so tied to the distributor position I am sure there is not alot of wiggle room there.

So Bleyseng has done 9:1 CR and he has a running system.
The only way for me to know, unless someone has acutally done this is to go a bit higher and test the results.

I will be dyno tuning this setup, and if it becomes impossible, I can always ad base shims to decrease the overall CR.

Basically the goal is to push the FI to its limit, in stock form. I have yet to see a documented failure above 9:1 CR.

Rich

Originally posted by lapuwali
IMHO, if you want to start playing around with CR, then you need to start playing around with cams, too. Dynamic CR is strongly affected by the cam you use, so you can use more CR with a wilder cam. CR and detonation are also affected by combustion chamber shape and size. Smaller chambers are inherently less detonation-prone, so you can usually run a higher CR (which is good, since a smaller chamber will, by itself, raise CR).

To do all of this, switch to PEFI. You'll get a lot more performance this way, and you won't be playing within D-Jet's narrow limits. No reason to stick with it if you want to build a performance engine.

[Dave at Pelican Parts]

A friend of mine ran his 2.0 at (I think) 10:1 CR, with D-jet. He claimed it ran fine, at least until it sucked a plastic grocery bag into the fan... Sadly, that happened pretty quickly after the first rebuild.

BTW, this car ("CA POP E") lives on now as an electric-powered monster.

--DD

[r_towle]

it seems that anything over 9.5:1 will require race fuel.

Rich