larby

Hi everyone,

I'm in guernsey, channel islands (between uk and france) and I've got a revenger 25 with a single BBC, currently a 454.

I'm just building a 502 for her on a budget, and need to have it sorted for racing over here in 2 weeks. It's fairly standard, just merc hi-perf heads, different intake manifold and stainless marine headers.

The 454 has a holley 750cfm carb on it, and i'm told a 750 will be fine for the 502? Is this the case? Some people have told me i need an 850?
If i can use the 750, can i just bolt it on as-is and adjust it to suit, or do i need to be looking at different jets etc? Any suggestions on jet sizes?

Cheers, James

Griff

A 750 will work if tuned correctly. An 800 would be ideal. You could send your 750 to Nickerson and have the cfm increased.

Ghostrider

If you still have the choke tower you could mill that off to help increase air flow. Your problem would be that your motor will run fine all over the place but if you plan on racing as you said, you'll run short on fuel when you start to maintain higher rpms in a race condition, that could lead to a lean condition and bad things happen when leaned out.
A 750 can be modified as Griff suggested, you could put a Proform body in it and keep your ancillaries, increase your jet 2 - 4 from whatever sizes you have in there but it's probably quicker and easier to get a 4150 DP / HP 800 - 850 off Ebay or something, you'll save money in the long run trying to make the 750 work when it really never will. I know, I tried to make a 750 BG Demon work on my 468 and I think I've run out of tuning ability!

You didn't mention whether your carb is a DP or Vac Secondaries, if it's a vac, don't bother trying to fix it. Get a DP or HP.

Good Luck

larby

Hi all,

Thanks for the great replies, it's got vacuum secondaries. Modifying it is right out of the question, so it looks like i'd better get myself an 850!
If it changes anything, I'm only planning to run it up to 5k revs tops, this is only basic local racing...

Thanks again,

James

mrhorsepower1

I run a bunch of 825 mighty demons on mild 502 builds and love them. That would be my first choice. The 750 is too small.

ezstriper

I run a 750 DP on my mild 509, works well and gets good fuel economy, tried a 850 DP not much diff....Rob

1BIGJIM

iTrader: (1)

One of the best in the business has spoken, take free advise

larby

Looks like it's gonna be an 850 then! There's only one guy in the UK so far who's told me to run a 750 so I'll take the advise on here and stick with 850. More cost, oh well!
Thanks very much for the help guys

James

Griff

You don't need an 850. You need an 800-825 max and get mechanical secondaries.

larby

Just as i thought i knew what i was gonna do!

So with it only being very mildly tuned (stainless marine headers, rectangular heads, different intake manifold), would you say a 750 could suffice up to 5000rpm ok?

What's the big difference with mechanical secondaries opposed to vacuum? excuse my ignorance, I always used to be an outboard guy, only just seen the light!

James