Twin 520's

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Mar 26, 2004 | 11:28 PM

scottie2hottie

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From: Honkytonkville

Twin 520's

I have 2 520 engines(a '520' is a 460 bored to .030 out and stroked to 4.3") that I have built to put into a boat. They were built identical...from oil pan to air scoop and from water pump to flywheel. They each have a 12-71 Weiand supercharger strapped on top pushing approx. 18 psi, along with two Holley 750 cfm carbs each. The problem I am running into is that when dyno'ed one engine is giving around 1075 fwhp, and I only get about 950 out of the other. Before I bolted on the blowers I had them tweaked to 676 and 662. For this reason I feel the problem lies within the new parts(carbs & blower primarily...also pulleys and an additional alt. for each, everything is still the same though).

I would like to boost the power of the second motor a little to match the first but w/o makin drastic changes, as I already have a lot of money wrapped up in this ordeal. What recommendations can I get to bring these motors closer together, as I feel a difference of 125 ponies is fairly significant, and may drag one side of the boat.

I've never tried to synchronize two complete engines, so any advice is much appreciated!!!!!

Mar 27, 2004 | 03:06 AM

77'F-150Mudder

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From: Boise, ID

Sorry, I can't help you.

Here's a great source for big horsepower 385 tech though.
http://www.network54.com/Hide/Forum/85220?it=1&fromad=1

Mar 27, 2004 | 04:04 AM

DeenHylton

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From: Crooked River Ranch, OR

I would do a search for some of the performance boating type sites and ask your question there.
Also I think your 750 Holleys are too small for that amount of cubic inch and that big of blower. I will be using twin 750's on my 380 c.i. small block with a large bore 671 blower...

Deen

Mar 27, 2004 | 01:42 PM

bigsnag

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From: Pryor

What all have you done to make sure they are both properly tuned?? Timing needs to be identical. I would suspect that one engine is not getting as much fuel. Have you checked A/F ratios on both engines?? Throughout the RPM range?

Mar 27, 2004 | 11:25 PM

scottie2hottie

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From: Honkytonkville

Snag: To keep timing similar, I first set them to OEM specs and then retarded them to account for the S/C's.
I have not checked air/fuel ratios....what is the best way to do this???

Hylton: What size carb would you recommend??

also....what level of octane would be best for this combo?

Mar 27, 2004 | 11:43 PM

bigsnag

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From: Pryor

You would need an A/F meter. They are available from Jeg's or Summit. Dyno shops have them and have the ability to hook them up to your exhaust and measure as you make your pulls.

Apr 10, 2004 | 11:09 PM

Hired Gun

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From: Oregon Coast

A/f ratio is best measured on the dyno. They have air bells that measure the airflow and flow meters on the fuel lines to measure the pounds of fuel consumed. The dyno computer kicks out the fuel curves on a graph.

The octane requirement is going to be very high for 18 pounds of boost. Way over regular 110 octane race gas. I'm thinking

VP C16
Used in turbocharged engines, blown engines and nitrous applications with CRs up to 17:1. Recommended by the top nitrous oxide companies. Spec Fuel for NHRA Comp Eliminator.

• Color: Blue
• Motor Octane: 117
• Specific gravity: .735 at 60° F

To be sure I would call someone here.
http://www.vpracingfuels.com/vp_12_c...B7C8C06B5D3A66