Don Terrill’s Speed-Talk

I bought a set of Tri Y headers a few years ago from a "friend." I was told the headers were for a 4-7 swap so I ordered a 4-7 cam and put a new engine together, it was a sponge. Just didn't have any ballz. I eventually pulled it out in favor of another combination that had 4-1 headers and it was fast. After the fact I looked at the Tri Y pipes and they were for standard firing order, giving me a couple cylinders firing back to back into the first y/collector. How bad did this hurt me? I want to try that engine again with 4-1 pipes if it is obvious I was losing a fair amount of power with this screw up. I assume it cost a good bit, does anyone know for sure how bad? Thanks

A lot of cheap v8 tri y's are made with 1st and 2nd cylinders paired then 3rd and 4th cylinders paired on each bank because it makes the headers easy to fit and quick and cheap to make.
Better than most cast manifolds , yes
,, hurt performance with stock to mild low overlap stuff , not really
,,, change note, yes
,,, hurt performance with big cam overlap and restricted exhaust system ,YES!
Depends on your engine spec and exhaust system as to how much it hurt having even a good set of tri y's built right but for the wrong firing order.

I noticed with Doug's tri-ys for a SBC, standard firing order was good (obviously), 4/7 swap was no bueno like the op mentions, and you're right back in business with an LS firing order.

Try tri-Y's on current engine wouldn't take as long as changing engine.

Thanks, it is a 355 with a 260/266 @.050 106 roller. Shoenfeld dirt track headers. Not going to run the tri y this time hopefully it acts much better and will be a good backup

You can assume a loss of around 20-30hp on a 500-600hp engine, easy.
If you are trying to use a 4-7 swap with a dual plane intake, it is even worse.

Can you install a conventional firing order cam in an LS firing order engine and then use regular Tri-Y's and produce good power?

pdq67

Thank you Calvin, that was what I was looking for. I appreciate the response

exhausted wrote: ↑Mon Dec 10, 2018 6:37 pm You can assume a loss of around 20-30hp on a 500-600hp engine, easy.
If you are trying to use a 4-7 swap with a dual plane intake, it is even worse.

It could sure influence a guy's conclusions re the value of 4-7 swap if he was running Tri Ys!

exhausted wrote: ↑Mon Dec 10, 2018 6:37 pm You can assume a loss of around 20-30hp on a 500-600hp engine, easy.
If you are trying to use a 4-7 swap with a dual plane intake, it is even worse.

There are three common firing orders for bent crank v-8. You pair cylinders for the firing order;
The standard SB chevy firing order , What I call the double swap or the LS firing order and the single swap or the 4-7 swap.
Cylinder pairing is the same for the STD and the LS (double swap). The 4-7 is all its own 421.
You need to pair cylinders properly for the firing order. 421's work great for all firing orders. You can not mismatch though and have to be using a spyder or open intake manifold. You also can not use a dyno that has the exhaust coming off the front of the engine. Bill Jenkins dyno was like that and he could not test his V8 ngines with a 421 header, HA.

A dual plane manifold is designed for a given firing order. There are a lot of Drag race stockers that use a 4-7 swap on their dual plane manifold engines. This combination does not work if you use a 421 paired for either. The dual plane intake functions the same way as the 421 header as it separates cylinder pulsing and it is adversely affected with both intake and exhaust working against each other.

The main reason racers go away from the STD firing order is because it unevenly loads the rear rod throw leading to overstressing the rear of the crank and block. I do not think it is wise to go away from the LS firing order to a STD order on a LS motor.

Ah, hadn't thought of the potential effect of a 4-7 swap on a dual plane!

Re the 'forward-facing header' dilemma, good point: swapping headers L to R to point them up and forward to match the dyno plumbing would screw up the Tri-Y pairings, but rotating each by 180°(if mechanically possible) would not. For those who feel strongly about rotational firing with 4-1 collectors the solution might differ...

To tell the truth I would have never thought of it except when I bought the headers I was told they were 4-7 swap. I actually had to replace the cam but it was a rush to get together in 2 days and it was faster to order a cam than a set of pipes. After I ran it a few times it was pulled and stuck in the garage corner. I began to look at the pipes to see what the difference was and it didn't quite make sense to me to pair 2 back to back. When I figured the normal firing order it seemed to fit......

Unless I'm missing the obvious, I'm still seeing the intake pulses alternating between the upper and lower plenums with a 4/7 swap. I'm I looking in the wrong spot for the problem?

RevTheory wrote: ↑Wed Dec 12, 2018 6:42 pm Unless I'm missing the obvious, I'm still seeing the intake pulses alternating between the upper and lower plenums with a 4/7 swap. I'm I looking in the wrong spot for the problem?

Worked this out with Daryl Wikle. 8 O2's on the dyno and number 7 cylinder was dead. never bothered to try and figure out why. About 30hp on a 625hp engine.

exhausted wrote: ↑Wed Dec 12, 2018 11:50 pm
Worked this out with Daryl Wikle. 8 O2's on the dyno and number 7 cylinder was dead. never bothered to try and figure out why. About 30hp on a 625hp engine.

There's a name I ain't heard in a while! Don't think I've talked to Wikle on about 4 or 5 years. I assume this was on his stocker?