[erikgto]

Hi Guys,

After I pulled the Original 400 engine out of my '70 GTO convertible back in 2008 I finally had time to take the engine to our local machine shop while the body is almost ready for paint and the chassis just came back from the powder coater's.
Even though she ran great without issues back when the Goat was on the road (2006-2008) I didn't want to just put the engine back in without checking everything out.

Turn's out she's still pretty much orginal and hadn't been messed with much in the past.
The heads look pretty much ok and the crank is within specs too. Only the bores have some wear to them (a little over 0.10) so an 0.20 overbore would be sufficient.

Now I'm kinda stuck on what to do next.
My idea is to build a slightly restomodded GTO convertible with a little more punch than stock. Nothing crazy, no racing or autocrossing, just a potent cruiser. It'll get a modded 4R200 trans and it's factory 3.55 safe-t-track rear with SC&C suspension.

Now what to do with the engine...At first a 460 stroker build crossed my mind, but that doesn't go well with my original heads (13's? which I'd like to keep on the engine) as it gives a CR that's way too high for our european unlead gasoline. I have a set of old 6X-8 heads lying around, but I don't like the look of newer heads on my original engine...might's as well get another engine if I do that.
(On a side note; since the GTO was in pieces I bought a 70 Bonneville 455 vert and it has issues with our gas too if I get on it and she has a 10.25 to 1 cr).
So I need to lower the cr a bit but keep the performance up.

The only things that come to mind are getting dished 0.20 pistons with basic eagle rods and possibly replacing the stock manifold with RA manifolds. Plus I sent the factory Q jet to Cliff's to work his magic.
What else should I do to it to create a potent street 400?!? Plus which cam would work best here...

Thanks!
Erik

[steve25]

Put as much money as you can on rebuilding a strong short block as in new rods and forged .030" over Pistons , new 60 psi oil pump, HD oil pump drive rod and a latter baffled oil pan.
Your stock heads at best gave the 400 a 9.7 to 1 compression so only a small dish in the new pistons should be needed to get you to a euro fuel livable 9.2 to 1, so you can rebuilt the 13s and use those , some bowl porting work on the heads and a cam of 225 @ .050" with some .470" lift should net you a easy 380 to 400 hp and drive fine on 3.55 rear gears.

[erikgto]

Just asking; Is there a reason to go straight to 0.30 over as opposed to 0.20 over?

I thought the CR was actually higher. 9.7 to 1 doesn't sound that bad as a starting point.

Erik

[cdrookie]

a 0.30 overbore is impossible, 0.20 is also impossible. it's 0.030, or 0.020, etc... i know everyone knows what you mean, but as a machinist, that's a HUGE difference. we once had some new tooling made that was suppose to save tons of set up time, but the guy who did the prints put +/-.005 on a hole when it should've been +/-.0005. it was a BIG mistake.

[steve25]

All of the compression ratios list in for example the Chiltons manuals for that era where 1/2 a compression point higher than actual !

[geeteeohguy]

If you can go .020" and have it clean up, go for it. You will need special pistons, as .030" is a more common overbore size. Here, less is better. Less overbore means thicker cylinder walls and more meat for future rebuilds.

[68WarDog]

Like you, I wanted to spare my nm engine but wanted a bit more performance, which I realistically realize would become a play toy since my 68 was for cruising. The 400 went. 30 over and a H/R installed. Then changed my mind and putting that engine a side. Starting with a 455 race ported e's and solid roller.Put your nm engine up,because you'll regret it if you destroy it while playing around. JMO

[ponyakr]

Just for kicks, I ran some numbers thru the CR calculator.

http://www.wallaceracing.com/cr_test2.php

400 + .030, 3.75 stroke, 72cc heads, 6.7cc valve reliefs, .039 x 4.3 Fel-Pro gaskets, .000 deck height.

The calculator says: CR = 10.45

Some say those 72cc heads are many times closer to 75cc.

So with 75cc heads, the calculator says: CR = 10.14

I think where the lower factory CR comes in is when the top of the pistons are maybe .020 or more in the hole. Most today recommend zero deck height. But .005 will leave a little for another small cut should it be needed on a later build.

There is an Icon piston with a 14cc dish, which has a pin height .020 more than stock, which will take up most of the factory deck height.

http://www.summitracing.com/parts/ue...w/make/pontiac

http://www.cnc-motorsports.com/icon-...-150-bore.html

http://www.rpmmachine.com/shop/index..._detail&p=6218

But even plugging in the 14cc dish and .005 deck height, the CR calculator comes up with 9.36 CR.

If you wanna go any lower, you'll either have to open up the chambers a bit, use thicker Cometic head gaskets, or by custom pistons with more dish.

Sub .045 x 4.3 gaskets and reduce the CR to 9.25. Is that close enuff ? Of course, you must know the exact cc of your heads.

http://www.jegs.com/i/Cometic-Gasket...ductId=1288713

[ponyakr]

Don't know what you have to do for shipping. It may be best to buy everything from Summit. I have no idea.

But for the benefit of those in the states who might need this info I'll post it.

RPM rods from CP are good quality and cheaper than Eagle.

http://www.competitionproducts.com/R.../#.VI5hP9LF_0w

But, if you insist on Eagle rods, lots of places sell 'em. Unless you can catch somebody with a stock reduction sale, Summit has about the best shipped price I can find right now.

http://www.summitracing.com/parts/es...w/make/pontiac

You mentioned a cam. At 9.25 CR I'd go with the Summit 2801. A Voodoo 262 would increase cylinder pressure and power, but would also be worse for your pump gas situation.

http://www.summitracing.com/parts/sum-k2801

[erikgto]

Sorry for my 0.20 vs 0.020 mistake....

It think a nice performing 400 might be enough for me... I really don't hammer my cars that much. Just an occasional traffic light sprint or a very light burnout would be the maximum I'll ever do to it. No real bruising...
Plus a bit of fuel economy comes into play as well with gas prices close to $ 10,- a gallon.

I might be considering building or buying another engine, but cost is a factor. Can't really go overboard on it right now. Got way too many bills coming from all other aspects of the frame off build.

Ideally I'd like to buy all the parts from one supplier and then have the entire lot shipped to the docks so the parts can hitch a ride in a container heading this way.
Having everything shipped USPS or UPS / FedEx is waaaaay too expensive.

[Evil Stu]

If the 70 Bonneville you bought has 64 heads on it those are almost ideal for you 400 + 0.030 pump gas motor. Chambers are 87 cc nominal, good heads. Put the 6x on the B-vill and both will run well

[77 TRASHCAN]

Building a 400 would be cheaper. A stock 400 crank (even though it's cast) can take a lot of punishment. Up grading to a forged crank in either a 4.25 or 4.21 stroke engine is cheap insurance, but not needed for a 400 (3.75 stroke)

Pontiac folks are spoiled. A 455 has the same physical dimensions as a 400. Building for the most cubes is the only way to go, but doesn't have to be... A 400 can be made to run VERY WELL!!!

What is the octane of gas you have available to use???

A 10:1 engine can be built to run on 91 octane, sometimes even less. There's a lot of fine details that need attention, none of them can be skipped to make the deal work though. Most here do not recommend these combo's though.

Cliff Ruggles posts here often. Follow his post, or send him a message, he can help!!!

Your 6X-8 heads would help your Bonneville. A cam change could make a difference also.

I used to ship parts to a customer in England. I used the big name shipping folks the first few times, sending essentially a box the size of 2 loaves of bread. Those shipping clusters cost $250-$350.00 each. The last time he ordered he advised me to try the US mail, crap it was about $50.00

[gtofreek]

Originally Posted by cdrookie

a 0.30 overbore is impossible, 0.20 is also impossible. it's 0.030, or 0.020, etc... i know everyone knows what you mean, but as a machinist, that's a HUGE difference. we once had some new tooling made that was suppose to save tons of set up time, but the guy who did the prints put +/-.005 on a hole when it should've been +/-.0005. it was a BIG mistake.

Yep, in our world, .001" might as well be a mile! Either way, it's not close enough.