1970 350 questions

[Chris_oz]

Hi all,

I have the factory 350 in my Longhorn, it has 70k on it and runs very well, a small amount of smoke on start if it’s been sitting for a week or so, but otherwise it runs like a new one.
I plan on doing a few things to it to increase its HP without going crazy.

Plans include:

Alloy heads, not sure which, but will have 185cc runner, 2.020/1.60 valves, 64cc chamber, screw in rocker studs. Looking at Pro Maxx or Edelbrock E street.
Edelbrock rpm performer intake.
Comp Cams XE268H-10 cam, with double row timing chain and new lifters.
1-5/8” tri Y headers.
Sniper EFI
I currently have Pertronix ignition and it works well.

I will decide if I am going to freshen up the bottom end when I pull the heads, but given how well it runs, I am hopeful it won’t be required.

I would like to know what sort of pressure I should get from a compression test, it’s a stock engine, 255hp with 9:1 CR
I was thinking 145-150 psi would be pretty close, but would like to hear what others have gotten. I could not see anything in the manual.

If I run 64cc head on the stock bottom end with a .015 gasket, I am hoping the CR will be 10:1 or close, like to keep it under 10.5:1
If anyone knows what the head chamber volume is on stock 1970 heads that would be great, I don’t have a casting number, but can get one if required.

I am happy to listen to recommendations on cams if you know of a better cam that will work with my planned parts.
Truck will be auto, stock converter (for now) and 4.56 rear end on 30" tyres

I would love to build a 383 but wary of doing it on a stock 2 bolt bottom end and a fresh top end on my 350 will do everything I want.

Thanks in advance

[Sparky dave]

My 69 has 991 heads. Also the 9-1 compression 255hp motor and are 76 cc. Hope this helps.

[Already Gone]

f you do the heads on an engine with 70k miles and don't touch the bottom end from past experience you could have problems with crank etc. within a few thousand miles.

[Chris_oz]

Originally Posted by Already Gone

f you do the heads on an engine with 70k miles and don't touch the bottom end from past experience you could have problems with crank etc. within a few thousand miles.

I have been thinking this for a while, my main issue is if I go the whole way through the engine the cost will be so close to a good 383 crate engine, I might as well buy one of them and be done with it, store my engine away.

Edelbrock heads are just on $3000
Rebuild kit with decent parts $1200
Cam, lifters and pushrods $ 450
Machining $1000

I can get a new blueprint 4 bolt 383 with roller cam for $7k to my door, this is a NEW block, all I need is the bolt on bits, most of which I have.

No matter what I do to my engine, it will always be a 2 bolt.

I was trying to talk myself out of a new 383, but I think that you and I both know that there is no point in doing what I had planned.

I don't see much point in putting the best part of $6k into a 48 year old 2 bolt, better off putting it in the shed and sticking a new engine in its place.

Thanks for making me admit to myself what I was trying to avoid.

[Chris_oz]

Originally Posted by Sparky dave

My 69 has 991 heads. Also the 9-1 compression 255hp motor and are 76 cc. Hope this helps.

Thanks for the information, I did some more searching and found from several places that 76cc is indeed the chamber size on the 255hp 350 with 9:1 CR

I did some calculations using an online calculator, if I retained the same deck height, piston design, head gasket etc, the 64cc would give a static 10.29:1 CR
which is right where I wanted to be.

I have however given in to the fact that a crate engine will be my best bet, given what I want to achieve.

Thanks

[WorkinLonghorn]

I've always heard that these trucks got the 4 bolt mains.

Also, this isn't what you were asking but a Longhorn is a heavy truck with the One Ton frame. My '69 with all options and a few tools weighs 5300 Lbs. This is not the ideal platform to make a quick or sporty truck in my opinion. I had a stripped down 1/2 ton with a modified engine that would hold it's own against most cars (back in the 90's).
Sometimes I wish for more power in my Longhorn but I usually think about TORQUE, maybe a torque 383 build.
Honestly though, if your engine is that low mileage, I would check the vacuum, compression, ignition advance settings, carb function and make a few changes like jetting, cold-air intake (not the open element crap that sucks in hot air from the radiator!) as a start. -BA

[toolboxchev]

Originally Posted by Already Gone

f you do the heads on an engine with 70k miles and don't touch the bottom end from past experience you could have problems with crank etc. within a few thousand miles.

I could not agree with you more. I did a complete top end 78 k ago. I got lucky and skill played to my favor. I use Valvoline VR-1 Motor Oil. Turns out I was very wise after doing a ton of research. Still no smoke and good compression, just lacking in power.

Will be doing things a bit different in the future.

[Chris_oz]

Originally Posted by WorkinLonghorn

I've always heard that these trucks got the 4 bolt mains.

Also, this isn't what you were asking but a Longhorn is a heavy truck with the One Ton frame. My '69 with all options and a few tools weighs 5300 Lbs. This is not the ideal platform to make a quick or sporty truck in my opinion. I had a stripped down 1/2 ton with a modified engine that would hold it's own against most cars (back in the 90's).
Sometimes I wish for more power in my Longhorn but I usually think about TORQUE, maybe a torque 383 build.
Honestly though, if your engine is that low mileage, I would check the vacuum, compression, ignition advance settings, carb function and make a few changes like jetting, cold-air intake (not the open element crap that sucks in hot air from the radiator!) as a start. -BA

I will check that with the next oil change, I have a bore-scope with light I can put into the sump through the drain hole, everything I can find points to 2 bolt mains, so who knows, would a casting number give it to me?

I understand they are a very heavy truck, hence my desire for more torque via a 383, I was just hoping to short cut it a little, but I know there is no short cuts.......

The tuning is pretty good, I will do a compression test on the weekend, vacuum at idle is 18-19 in-hg at 700 rpm.

It has late electronic ignition, working vacuum advance, the carb seems fine, it feels like it’s a little rich down low, blows some black smoke when taking off, but clears pretty quick, I know nothing about the Q Jet, hope to learn, On the 500 mile drive home from where I bought the truck, it returned 15 mpg for the whole trip, sitting on 60 mph, 80% was on the highway, but there was a solid 2 hours in crawling traffic on the "Gold Coast Highway", sitting on 5-10 mph for ever.

In general it drives really well, I do plan to tow with it occasionally however, so just looking at upgrades.
Maybe I should just be happy with it and drive it, put the $10k I would spend on upgrades into tyres and fuel.

Thanks to all for the advice, looks like I will have to decide on where I want to spend my money.

I think I am just happy to have a vehicle I can buy aftermarket stuff for, after 30 years of owning AMC Javelin, early Cadillac's, Studebakers and alike, with next to zero hot up parts available, owning a Chevy is weird, I can buy parts, lots of parts, easily.............what a time to be alive.

[Coley]

The longhorn is a heavy truck and needs torque....not hp, this is very important to keep in mind because a lot of guys have missed this in their engine build/spec.
If I was doing a change similar to what you are doing I would go with the 1.94 intakes and a 1.6" exhaust on the heads.
You will get better low end response with the 1.94" intake valves.
Bigger is not always better for low end performance....it shows up for high end, higher winding stuff....ie: over 4500+ rpm.
As well, when it comes to a cam try to emphasize 'lift' more than 'duration'. A lot of guys with trucks end wanting the wumpity wumpity lope (ie: long duration cam)....which although may be cool when you pull into parking lots, sacrifices low end power and torque.
All of this is not a problem if you are targeting your performance 'zone' for 40-50 mph and above...such as when merging onto the freeway. However, your in town stop light performance will suffer with the wumpity wumpity cam and the 2.02's.

All good
Coley

[Chris_oz]

I am all over the whumpy cam lope. Been there before.
The XE268h is about as big as I would ever go. I want decent vacuum and ability to run a standard stall TC.
I am waiting to hear back from blueprint engines on the cam / vacuum and recommend stall TC for thier roller 383 engine.
I think it may be a bit big, given the hp they quote and it's running EdelBrock rpm alloy heads, it may be a bit top end orientated.

I have the option of buying a short block and building it as I want, but I will wait to see what they come back with.

[Marine-58]

I'm a little late coming into this conversation but may have a little input that will help. From personal experience, I always like to rebuild if possible when replacing heads. Your compression will go up a little and not knowing what shape the bottom end is in, you are taking a chance. Saying that I have known people to swap heads and have both good and bad results.

When you pull the old heads, it is easy to check the cylinders for wear or any ridge. Not to be too obvious but either push you towards a rebuild. If you can pull the pan, pulling a couple mains will show you the wear on the bearings.

From what you said above, it sound like the engine is in pretty good shape. Unless there is something unforeseen, you should be able to save the crank, rods and pistons (assuming you don't need to bore the cylinders).

Heads - many choices out there. 64cc Combustion chamber. Here is a link to Summit for some decent Edelbrocks. Cost - $879
https://www.summitracing.com/parts/e...make/chevrolet

Cam/Lifters - Also many choices. Here is a set for $175. https://www.summitracing.com/parts/c...make/chevrolet

This is what us old guys used to call a 3/4 cam. Very streetable but will increase torque and HP an ok amount. You will be able to idle at about 700 rpm and have good vacuum.

Since you aren't building this for a lot of power, don't worry about getting forged rotating assembly. Total over kill. The 2 bolt mains that you have are also fine as it is doubtful you will be turning 6K+ RPM.

Other things I would use are true roller rockers, a good dual plane intake. 650 cfm carb, HV/HP oil pump. Decent 2 1/2" dual exhaust as well. If you don't have it swap the ignition to a HEI. Points suck and not many people know how to adjust them any more...

This setup should get you close to 350 - 400 HP. Much better than stock.

So this is just my opinion, I'm sure many others will correct any mistakes made. Good luck on this...

[Sparky dave]

Was reading this today and thought it might interest you

https://www.hotrod.com/articles/hrdp...k-chevy-build/

Particularly at the bottom of the article mentions options for getting a 400 hp Chevy 350 brand new crate engine for under 3k or a set of heads that will give you 400 hp for 650 dollars.