SBC build- top end advice

[sbc75seriesAU]

G'day, fuel forum.

I'm putting together an engine for my daily and need.... well let's say a LOT of advice.

As mentioned this is a daily (this includes 2km of offroad & regular 4x4 fun, minimal towing) but i still want to get the most out of it as i can, Built it once build it right, the bottom end should be pretty bullet proof it's the top half thats a little daunting, limited experience with LPG/propane

Guys i really need help with cam spec's, and advice on heads, valves and rocker gear. tried to give as much information as possible.... and this is actually first post on a forum ever.

I'm from SW of Brisbane, Australia.

RIG
4X4 75 series Landcruiser ute
SBC 350 (tired) dual fuel (90% LPG) want to keep dual fuel.
impco 425
140L tank with views to put another tank under tray.
700r4 moderately improved 1900 stall.
block huggers to short balanced pipes
750cfm vac.
31in tires

NEW ENGINE (so far)

350 small block
1pc rear main seal
4-bolt main
#638 block casting number
ROLLER cam
Molly rings
HYPEREUTECTIC Flattop 4-Valve Relief Pistons
Double row timing
Hardened bolts throughout

Intentions

aluminum heads (11-1cr)
850cfm mech secondaries

would love to get 400+ @ engine

as mentioned at the start build once built right... i think with some honest, experienced advice this can be possible.

Kindest regards

[storm]

Is this vehicle engineered for a chev? The 75 series came out in 1985 so ADRs (and NCOP) require the engine to be from an unleaded vehicle (for 1986 and younger vehicles) of a comparable age or younger if you are running petrol.

Specs seem good but you won't get 400+ out of 1 Impco 425. To get 400 HP you will need a minimum 600 cfm carb on an engine pulling over 90% volumetric efficiency. That means 1.5 cfm per HP and to get that you will also need to get more than 1 HP per Cubic Inch (not difficult but the restriction of the Impco will stop it from happening). At 1.5 cfm per HP the Impco flows enough air to supply 300 HP but to feed that and not lean the engine out at full throttle you will need 2 regulators which will then supply enough fuel to feed 600-650 HP. I would also suggest an 850 Mechanical secondaries is overkill for a street engine, it will give you power but also use more fuel than a 750 or 800.

For the cylinder heads any of the aftermarket manufacturers can supply heads that will get you 400 HP. Dart, World, Edelbrock, etc, all make good heads so the only limiting factor is your wallet cause the exchange rate sucks right now. Edelbrock are probably the easiest simply because you can get them with roller rockers supplied from Edelbrock.

Your combination, so far, seems more at home in something like a Torana or a HQ-WB than a 75 series. Don't get me wrong I'd like to own such an engine myself but (and this is coming from someone who has a HSV 350 stroker in a 75 series Troopcarrier) instead of looking for high HP you would be much better off looking for stump pulling torque. It will make for a more enjoyable vehicle that uses less fuel.

[sbc75seriesAU]

Thanks for the feedback storm

I picked up the cruiser dirt cheap it already had gas and was engineered and complianced for the SBC, it started life as a diesel. An 86 model with styleside tray (don't see them much)

I'm currently building my own house on part of a large property that has fun access at times. It has been perfect as a work horse but now the major building is finished and the drivline tired I figure it's time for a birthday. It gets used off road fair bit for fun the rest is highway work.

The engine short block is being built in the states, lucky to be part of a small private family import business.
I agree with you on the torque vs HP... Torque is more important. It's a big heavy truck probably around 2.5t when I'm finished with the whole thing.

The carb I was going to purchase from there new but now you've enlightened me a little on the math I might go for a 750cfm.
Heads will be purchased from the states but final assembly will happen in Brisbane with a top builder but he has minimal lpg experience. I take it the rocker gear is all pretty similar??

Is there a single unit or alternative I can use to replace the impco to get the performance/torque? I can see forsee fitting 2 units and keeping the dual fuel a bit of a mission possibly?

I guess the rest comes down to cam?

You also picked it pretty well mate, all my early street cars were H series holdens... Old habbits die hard I guess. The 4x4 seems to save me loosing too many points.

[storm]

The carb I was going to purchase from there new but now you've enlightened me a little on the math I might go for a 750cfm.

For torque you need good air speed which the 750 will provide much better than the 850. My personal preference would also be vacuum secondries and a quadrajet but I am biased and freely admit it.

Heads will be purchased from the states but final assembly will happen in Brisbane with a top builder but he has minimal lpg experience. I take it the rocker gear is all pretty similar??

I don't know of any modern cylinder head for a chev petrol engine that isn't setup to take LPG. Hardened valve seats are installed already as far as I am aware. The engine builder should know this. Depending on the heads you buy some rockers will work while others may not. Your best bet is to purchase the valve train recommended by the heads manufacturer.

Is there a single unit or alternative I can use to replace the impco to get the performance/torque? I can see forsee fitting 2 units and keeping the dual fuel a bit of a mission possibly?

You need to think about how you will fit the 425 to work with an existing Holley. You can get carb hats that a 425 will bolt to. If you could get a carb hat to fit 2 425s you are home and hosed.

I guess the rest comes down to cam?

You can't really pick a cam until you know what heads you are fitting. A good flowing head will allow a bigger cam than a head that doesn't flow as well.

You also picked it pretty well mate, all my early street cars were H series holdens... Old habbits die hard I guess. The 4x4 seems to save me loosing too many points.

I know the "type" because I am the same. I had a genuine 1977 Pontiac Trans Am Bandit before my Troopie. I went for 350 HP to 80 HP, doubled the weight not to mention the drastic reduction in aerodynamics. Cruisers really benefit from a well thought out engine.

Take your time and get your heads sorted, once you know how much they will flow you'll be able to make a decision on a cam based on good data.

[sbc75seriesAU]

Again storm cheers for the speedy reply.

So I just got an email from the old man who's in LA at the moment.

"owns jerry's performance engines an Australian Italian he has been here 20 years. Has raced at Bonneville salt flats . I showed him your quote. He has a 350 he rebuilt it blue printed and balanced and has worked vortex heads on it has a hot cam which he will change for one for better torque. his recommendation is a MSD plug and go disturber and a 670cfm Holley (adjustable secondary's) 750cfm has no more benefit. carby and dizzy we can have at his cost has harmonic balancer and flex plate. It will be a turn key engine test run in his shop. I spent a hour min. with him. nice honest guy that loves to build engines and really knows his stuff."

So it looks like I won't be putting it together here but that gives us carb and the heads?? I have asked for a little more information but that might help go a bit closer for the cam, I'm sure he has access to pretty much anything.

My current setup is single with hat to carb... Probably wouldn't be too hard to fab really and I do have intentions for dual snorkles but will it be necessary now that I might not be going quite as crazy as intended.

Grew up on Aussie muscle, switched to American because of the family business had a Jap phase and now I guess it's muscle 4x4 hahaha
Just love the small blck V8 it's a beautiful thing.
The bandit sounds like it was pretty sweet.

I'm yet to be hassled in the cruiser even with the obnoxious noise... They're too worried bout the falcons and commodores..

[storm]

"owns jerry's performance engines an Australian Italian he has been here 20 years. Has raced at Bonneville salt flats . I showed him your quote. He has a 350 he rebuilt it blue printed and balanced and has worked vortex heads on it has a hot cam which he will change for one for better torque. his recommendation is a MSD plug and go disturber and a 670cfm Holley (adjustable secondary's) 750cfm has no more benefit. carby and dizzy we can have at his cost has harmonic balancer and flex plate. It will be a turn key engine test run in his shop. I spent a hour min. with him. nice honest guy that loves to build engines and really knows his stuff."

So it looks like I won't be putting it together here but that gives us carb and the heads?? I have asked for a little more information but that might help go a bit closer for the cam, I'm sure he has access to pretty much anything.

Sounds like a good combination, 670 cfm is about right, it will give power but also air speed through the crab barrels that will provide good fuel atomisation and good torque. The only thing you really need to know now is head flow specs but I think with it being set up in a specialised shop the builder may be wanting to keep them close to his chest. What you really want is the highest flow from the smaller port and valve sizes. That keeps air velocity up and helps the engine make good torque from low revs.

Let us know how you go.