Single or Twin Plugs?

[North Coast Cab]

I have a 3.0 with 10.2:1 RSR pistons, Webcan 120//104 cam and PMO 46''s. I am looking for a few mkore HP.
It is a single plug and presently I am mixing 93 & 100 fuel.

Is this set-up ok?
Should I go to straight 110?
Would twin plugging this set-up make a difference?

TIA, John

[Tom C.]

Been there, done that . . . Rennsport Systems has a excellent write-up on this topic. This should help with your decision.

http://www.rennsportsystems.com/2a.html

Regards,

Tom

[Steve@Rennsport]

John:

A few questions,....

1) What venturies do you have in those carbs?

2) Exhaust system details?

3) Details about your heads & valves?

Have you put this on an engine dyno?

[North Coast Cab]

Steve, first let me say that that information is awesome, unfortunately, most of it is over my head.
Heres' my carb set-up
Carb Size 46
AV's 4.5
Venturis 38
Main Jets 150
Idle Jets 55
Idle Airs 130
Airs 180
Emulsion Tubes F11
FBVCV's 50
I've got 39.5mm intakes. 1 5/8" headers with a custom M&K w/3" outlets. It is running a little rich and I have some parts on the way from PMO to play with that. My dyno shows a very horizontal torque curve from 5k-7k with RWHP around 195. It's seems a little low to me and my Nasa GTS class would allow me up to about 215RWHP so that is why I am contemplating some of these issues.

Thanks for the help, John

[Steve@Rennsport]

John:

First, Your main jets are a little lean for a 3.0 w/120-104 cams, headers, and 38mm venturies.You should be using 160's.

Second, if you really are at 10.2:1, thats not much compression for those cams and this displacement. You should be around 11.2-11.4:1 for a race engine to make power. This is one reason why this is not making the HP you expect.

Does your dyno graphs show air/fuel ratios????

Call me sometime,..............

[cstreit]

Steve,

I was thinking the same thing, 150 seems a little low on the mains, but the air-correctors are pretty small too, which would richen up the high-end quite a bit.

John FWIW I was running 160 mains with a 170 air-corrector on a similar engine...

Steve, could you suggest a better cam while you're at it?

[North Coast Cab]

Thanks for the response Steve. I am out of town, but when I return I'll get a current dyno run and give you a call.
Chris, I know we spoke at Putnam about some of my issues and I appreciate your input.
The car idles and at low RPM runs lean, 14's, and at temp under load in the higher RPM bands, 5-7k, runs rich at around 11.5. I spoke to Richard Parr at PMO and relucatantly he sent me 145 & 140 mains as well as 190 & 200 airs since the richness is at the high end.
I guess I'm not surprised by the piston comment. I bought this longblack already assembled with lots of goodies, but no real documentation. It was a very good deal and I took the chance. Well, after I realized the huge oil leak upfront was from it having the 2.7 smaller nose bearing I split the case to reapair it. That was the only time I've actually seen the internals and where I got the info on the pistons etc. It's been running solid, but I'm not happy with the AFR or the power I'm getting. I guess ordering a set of custom JE's would solve some of my problems.

I will post a dyno next week.

John

[Steve@Rennsport]

Originally posted by cstreit
Steve,

I was thinking the same thing, 150 seems a little low on the mains, but the air-correctors are pretty small too, which would richen up the high-end quite a bit.

John FWIW I was running 160 mains with a 170 air-corrector on a similar engine...

Steve, could you suggest a better cam while you're at it?

Chris:

I'd be using 160's with either 175 or 180 air correctors,.....the final say will come from dyno tests.

Its hard to recommend camshafts unless I know the vehicle's weight, gearing, expected engine longevity, and desired power output. Plus, I like to know something about the driver's skills since more cam narrows the powerband and requires more from the driver.